Luiz C Cotovicz
email: luizc.cotoviczjr@id.uff.br

Luciana O Vidal

Carlos Eduardo De Rezende

Marcelo C Bernardes

Bastiaan A Knoppers

Rodrigo L Sobrinho

Renan P Cardoso

Marcelo Muniz

Roberto Meigikos

Antoine Anjos

Biehler

Roberto Meigikos Dos Anjos

Antoine Biehler

Gwenaël Abril

Carbon dioxide sources and sinks in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing

Keywords: carbon dioxide, carbonate chemistry, thermodynamics, tropical coastal deltas

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The concentration of carbon dioxide (CO2) in Earth's atmosphere has been increasing at fast rates, and the current CO2 level is unprecedented over the past 3 million years [START_REF] Willeit | Mid-Pleistocene transition in glacial cycles explained by declining CO2 and regolith removal[END_REF], changing approximately from 277 ppmv in 1750 [START_REF] Joos | Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years[END_REF] to 414 ppmv in April 2019 (NOAA, 2018), as a result of fossil fuel burning and land-use changes. According to recent estimates, the oceans and the land absorb approximately 23 and 32% of these total emissions, respectively, whereas the remaining 45% accumulate in the atmosphere (Le [START_REF] Quéré | Global Carbon Budget 2018[END_REF]. This CO2 accumulation in the atmosphere is causing global warming, with CO2 as the most important anthropogenic-derived greenhouse gas [START_REF] Ipcc | Climate Change 2013: The Physical Science Basis[END_REF]. In addition, penetration of CO2 in the ocean is causing acidification, increasing the surface ocean partial pressure of carbon dioxide (pCO2), with a net decline of pH [START_REF] Gattuso | Contrasting futures for ocean and society from different anthropogenic CO₂ emissions scenarios[END_REF]. While carbon sink in the open ocean is currently estimated with a relative high degree of confidence (2.5 ±0.5 GtC yr -1 ; Le [START_REF] Quéré | Global Carbon Budget 2018[END_REF], the air-water CO2 fluxes and controlling processes remain uncertain in the coastal ocean. Indeed, coastal oceans have not been satisfactory included in global carbon budget calculations, despite their importance in terms of global carbon cycling [START_REF] Chen | Reconciling opposing views on carbon cycling in the coastal ocean: continental shelves as sinks and near-shore ecosystems as sources of atmospheric CO2[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF][START_REF] Bauer | The changing carbon cycle of the coastal ocean[END_REF].

Estuaries are aquatic ecosystems characterized by the mixing of fluvial and marine waters, hosting a large diversity of interfaces and gradients, with distinct physical, geomorphological and biological features [START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. Studies have shown that CO2 emissions by estuarine systems and near-shore coastal waters are globally significant [START_REF] Chen | Reconciling opposing views on carbon cycling in the coastal ocean: continental shelves as sinks and near-shore ecosystems as sources of atmospheric CO2[END_REF]. However, in 10 years of global data compilation, CO2 emission estimates have decreased by a factor of 6. The first global estimate of estuarine CO2 emissions calculated a degassing of 0.6 PgC yr -1 [START_REF] Abril | Carbon dioxide and methane emissions from estuaries[END_REF], whereas the two lasts estimates showed degassing of approximately 0.1 PgC yr -1 [START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF][START_REF] Laruelle | Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins[END_REF]. This decline has been attributed to various factors, mainly the disproportional and insufficient global sampling (poor coverage in tropical regions and the Southern Hemisphere), few studies with direct and continuous measurements of pCO2, and low temporal resolution (semi-diurnal, diurnal, seasonal, and annual) [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF]Chen et al., 2013).

In general, the upper estuarine regions with low-salinity waters are important sources of CO2 to the atmosphere. This is normally attributed their net heterotrophy and, to a lesser extent, to inputs of CO2enriched freshwater and lateral inputs from intertidal areas such as salt marshes and mangroves [START_REF] Cai | Oxygen and carbon dioxide mass balance for the estuarine-intertidal marsh complex of five rivers in the southeastern U[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. However, in some estuaries, the riverine contributions of CO2 (allochthonous sources) can be greater than contribution of estuarine heterotrophy (autochthonous sources) at certain times of the year [START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Joesoef | Air-water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability[END_REF]Van Dam et al., 2018a).

Climatological and hydrological characteristics may change seasonally according to river water discharge, temperature, and formation of vertical stratification, which modulate physical mixing and biological production [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Salisbury | Seasonal observations of surface waters in two Gulf of Maine estuary-plume systems: Relationships between watershed attributes, optical measurements and surface pCO2[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF][START_REF] Dinauer | Spatial variability in surface-water pCO2 and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)[END_REF]. Downstream in the seaward direction, estuarine mixing zones are considered as moderate CO2 sources, whereas the marine domain from the river mouth to the shelf break generally behaves as a CO2 sink [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF][START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF]. In general, the pCO2 values and CO2 emissions are much higher in river-dominated estuaries than in marine-dominated estuaries [START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]. In addition to the marked and complex natural variability, the coastal ocean hosts approximately 37% of the human population [START_REF] Cohen | Estimates of coastal populations[END_REF], creating human-induced modifications in the metabolism of aquatic ecosystems associated with nutrient enrichment and eutrophication [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Zhai | Carbonate system and CO2 degassing fluxes in the inner estuary of Changjiang (Yangtze) River, China[END_REF][START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF][START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF].

Historically, studies addressing estuaries have identified biological processes as predominant drivers on pCO2 distribution and CO2 fluxes along salinity gradients [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Cai | Oxygen and carbon dioxide mass balance for the estuarine-intertidal marsh complex of five rivers in the southeastern U[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. However, the theory of carbonate chemistry predicts drastic abiotic changes in proportions of dissolved CO2, bicarbonate (HCO3 -), and carbonate (CO3 2-) during the mixing of freshwater with seawater. The influence of thermodynamics in the CO2 system in estuarine waters was first described by the pioneer studies of [START_REF] Mook | Chemistry of dissolved inorganic carbon in estuarine and coastal brackish waters[END_REF] and [START_REF] Whitfield | The carbon dioxide system in estuaries -An inorganic perspective[END_REF]. These authors showed that modification of the carbonate equilibria in estuarine waters during the mixing of freshwater with seawater results in changes in the pCO2 and, in turn, in the CO2 fluxes at the air-water interface [START_REF] Whitfield | The carbon dioxide system in estuaries -An inorganic perspective[END_REF][START_REF] Hu | Estuarine acidification and minimum buffer zone-A conceptual study[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF]. In particular, the displacement of acid-base equilibrium during the mixing of weakly buffered freshwater (low total TA concentration) with well buffered seawater (high TA concentration) can generate pCO2 values far below that of the atmosphere, with no need for biological uptake [START_REF] Whitfield | The carbon dioxide system in estuaries -An inorganic perspective[END_REF]. In this regard, thermodynamic processes would be potentially significant compared with biological processes in oligotrophic/mesotrophic estuaries with short residence times as, for example, estuarine deltas.

Thermodynamic changes as the main driver of carbonate chemistry changes along the salinity gradient in estuaries were have been demonstrated in theoretical studies (Hu and Cai, 2003;[START_REF] Salisbury | Coastal Acidification by Rivers: A Threat to Shellfish?[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF], but rarely validated using field data. In the Amazon River plume, pCO2 undersaturation has been attributed mainly to phytoplankton productivity [START_REF] Ternon | A seasonal tropical sink for atmospheric CO2 in the Atlantic ocean: the role of the Amazon River discharge[END_REF][START_REF] Körtzinger | A significant sink of CO2 in the tropical Atlantic Ocean associated with the Amazon River plume[END_REF]; however, recent findings showed that, from a salinity value of approximately 10, surface water becomes undersaturated as a result of the mixing effect [START_REF] Lefèvre | Net heterotrophy in the Amazon Continental Shelf changes rapidly to a sink of CO2 in the outer Amazon Plume[END_REF].

The geographical position also exerts influence over CO2 concentrations and emissions across different latitudes. Estuaries located at low and intermediate latitudes present the largest flux per unit area, whereas those located in regions north of 50° N and south of 50° S present the lowest flux per unit area [START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF]. These differences are attributed to local/regional characteristics of the coastal zone, including the mixing of freshwaters (presenting variable levels of pCO2) with low-pCO2 shelf waters, water temperature, and biogeochemistry complexity [START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF]. However, it should be highlighted that studies on carbon cycling conducted in tropical coastal regions are overlooked compared with those in temperate and boreal regions. In addition, tropical rivers present lower TA concentrations than temperate/boreal rivers [START_REF] Cai | A comparative overview of weathering intensity and HCO3-flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi Rivers[END_REF], thus the impact of thermodynamic processes during river-ocean mixing can be especially important in tropical estuaries.

Estuarine deltas can locally/regionally be the main estuarine type through which significant terrestrial carbon is transported to the ocean [START_REF] Laruelle | Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins[END_REF]. Deltas are mostly located in tropical and subtropical regions [START_REF] Laruelle | Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins[END_REF].

The present study aims at investigating the carbonate chemistry and air-water CO2 exchange in the Paraiba do Sul River estuary (PSRE), southeastern Brazil. This estuary is a tropical mesotrophic ecosystem that has been suffering with increasing eutrophication and construction of dams in its watershed. This ecosystem has a short residence time and low TA concentrations in freshwater, potentially generating pCO2 undersaturation conditions during mixing with seawater as a result of thermodynamic changes -a fact that is still disregarded globally. Based on high resolution monitoring of carbonate chemistry parameters along the entire salinity gradient, we could differentiate between the biotic and abiotic contributions to water pCO2 distributions and CO2 fluxes at the air-water interface; we also analyzed the metabolism of the main channel using pCO2 distributions and stable isotope data.

Material and Methods

Study Area

The Paraiba do Sul River is an important water resource located in the southeast region of Brazil. The river watershed has an area of approximately 54,400 km 2 , with a length of 1,145 Km, traversing the most industrialized and urbanized region of Brazil (the states of São Paulo, Minas Gerais and Rio de Janeiro). According to previous studies, the river basin is divided into three regions [START_REF] Ovalle | Longterm trends in hydrochemistry in the Paraíba do Sul River, southeastern Brazil[END_REF]: 1. the upper basin (7,300 km 2 ), with water sources at 1,800 m of altitude until 600 m in a valley of crystalline rocks; 2. the medium basin (27,500 Km 2 ), with average elevation of 500 m; 3. the lower basin (22,500 km 2 ), comprising coastal plains, with extensive meanders and islands until the river mouth into the Atlantic Ocean (Fig. 1).

The PSRE, located at the north region of the state of Rio de Janeiro (21°37′ S; 41°01′ W), is composed of a main river channel and a minor secondary channel (Fig. 1). The secondary channel harbors a mangrove forest of approximately 8 km 2 [START_REF] Sterza | Distribution and abundance of cladocera (brachiopoda) in the Paraiba do Sul River Estuary, Rio de Janeiro, Brazil[END_REF]. The coast is microtidal with high energy of waves, forming an estuarine delta. According to the Köppen's classification, climate in the region is Aw, with wet summers (November to January) and dry winters (July to September) [START_REF] Cotovicz | Assessment of the trophic status of four coastal lagoons and one estuarine delta, eastern Brazil[END_REF]. The river discharge in the estuary follows a seasonal cycle, with high water flow in summer and early spring and low water flow in winter and early autumn, with discharges averages of 1158 and 444 m 3 s -1 , respectively. The river discharge modulates the residence time of waters in the mixing zone [START_REF] Souza | Use of multitracers for the study of water mixing in the Paraíba do Sul River estuary[END_REF] and the extension of the river plume in the adjacent coastal ocean [START_REF] Rudorff | Spectral mapping of the Paraıba do Sul River plume (Brazil) using multitemporal Landsat images[END_REF]. During low water flow, the plume is reduced and it takes approximately 10 days for the water to reach open ocean (salinity 35), whereas during high water flow, the plume is extended along the shelf and the residence time decreases to 6 days [START_REF] Souza | Use of multitracers for the study of water mixing in the Paraíba do Sul River estuary[END_REF].

The river has undergone increasing anthropogenic influences. The total human population on its watershed is estimated at 5 million inhabitants. Considering only the estuarine region, the surrounding population is estimated at approximately 600,000 inhabitants (IBGE, 2018). In addition, the lower basin presents extensive farming, particularly sugar cane production [START_REF] Ovalle | Longterm trends in hydrochemistry in the Paraíba do Sul River, southeastern Brazil[END_REF]. The coverage of wastewater treatment plans is still very small in the region, less than 50% of the total households. Therefore, the estuary receives a large amount of industrial, agricultural and domestic effluents, with large inputs of nutrients to the waters, thus presenting mesotrophic characteristics [START_REF] Cotovicz | Assessment of the trophic status of four coastal lagoons and one estuarine delta, eastern Brazil[END_REF]. Reservoirs and hydroelectric dams have been constructed along the drainage basin (forty-seven different hydroelectric power plants; [START_REF] Ovalle | Longterm trends in hydrochemistry in the Paraíba do Sul River, southeastern Brazil[END_REF], which have changed hydrological patterns and potentially altered carbon cycling in the estuary.

Sampling Strategy

Three extensive spatial surveys were conducted in the estuary on February 17, October 17 and March 18. The Feb 17 sampling, historically considered as the rain season, presented an accumulated monthly precipitation of 40 mm, which is 60 mm lower than the historical average, with a 7-day accumulated precipitation before sampling of only 1 mm. The Oct 17 sampling presented an accumulated monthly precipitation of 13 mm (80 mm lower than the historical average), with a 7-day accumulated precipitation before sampling of 4 mm. The Mar 18 sampling presented an accumulated monthly precipitation of 376 mm, which is 267 mm higher than the historical data. Thus, the Feb 17 and Oct 17 samplings were considered dry conditions and the Mar 18 sampling was considered wet condition. These climatological characteristics influenced the hydrological pattern. The Feb 17 and Oct 17 samplings were conducted during low river flow, with average discharges of 288 and 178 m 3 s - 1 , respectively (Fig. 1c,d), whereas the Mar 18 collection was performed during high river flow, with average discharge of 1,240 m 3 s -1 . The mean historical river water discharge (reference period of 95 years, 1923-2018) presented higher values than the discharge verified on the sampling days (Fig. 1c,d). The regression line of the historical data shows that the river discharge has been decreasing at a rate of ~5 m 3 s -1 per year, which means that the mean discharge has already decreased by 475 m 3 s -1 .

In each sampling campaign, continuous and discrete aquatic measurements were performed.

Continuous and online measurements were conducted for pCO2, dissolved oxygen (DO), pH, salinity, and temperature. Surface water was sampled in the mangrove creeks and in the main channel in discrete points distributed regularly along the whole salinity gradient (0 to 35) with approximately three salinity unit increments. Discrete samples were used to analyze TA, Chl a, and δ 13 C-DIC. The spatial surveys covered the freshwater domain (upper estuary), the mixing zone (complete salinity gradient), and the external estuary (river plume adjacent to the coastal ocean). The spatial surveys started in the morning, at around 8:30 am, and finished in the early-afternoon, at around 4:00 pm. In addition to the spatial survey screening, an Eulerian time series (24 h of sampling) was conducted in the mangrove tidal creek located in the secondary channel on March 2018 (Fig. 1), combining continuous measurements and discrete water sampling within 2 hours of interval for the same water parameters previously described.

Continuous Measurements

Continuous samplings were performed from a boat, with real-time measurements of position, pCO2, DO, pH, salinity, and temperature, at a frequency of 1 min [START_REF] Frankignoulle | A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments[END_REF][START_REF] Abril | Amazon River Carbon Dioxide Outgassing fueled by Wetlands[END_REF][START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuários tropicais contrastantes: o estuário eutrofizado da Baia de Guanabara (RJ) e o estuário oligotrofico do Rio São Francisco (AL)[END_REF]. Briefly, a water pump placed at a depth of ~0.5 m provided continuous 6 L min -1 water flux to the boat. A part of this flux (~3 L min -1 ) was directed to an equilibrator to measure the pCO2. The marble equilibrator consisted of an acrylic tube (60 cm in length; 8 cm in diameter) filled with glass marbles where the water flows from top to bottom and a closed air circuit with a flow of ~1 L min -1 in the opposite direction, from bottom to top. The air is dried using a desiccant (Drierite™) and pumped into a non-dispersive infrared gas analyzer (Li-820 NDIR, LI-COR). Before and after each sampling campaign, the NDIR was calibrated using gas standards with air pCO2 values of 410, 1007 and 5035 ppmv (White Martins Certified Material, RJ, Brazil). Fresh soda lime was used to set the zero and the standard at 1007 ppmv was used to set the span. The air standards of 1007 and 5035 ppmv were used to verify the system stability. The boat speed was 8-10 km h -1 during the spatial surveys. The equilibration time of the pCO2 measurement system is 2-3 min [START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuários tropicais contrastantes: o estuário eutrofizado da Baia de Guanabara (RJ) e o estuário oligotrofico do Rio São Francisco (AL)[END_REF]. This gas equilibration device is ideal for pCO2 analysis in estuarine waters due to its fast response time [START_REF] Frankignoulle | A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments[END_REF][START_REF] Abril | Amazon River Carbon Dioxide Outgassing fueled by Wetlands[END_REF][START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuários tropicais contrastantes: o estuário eutrofizado da Baia de Guanabara (RJ) e o estuário oligotrofico do Rio São Francisco (AL)[END_REF]. This time is sufficient to guarantee the equilibration along the estuarine mixing. In the steepest part of the salinity gradient, between salinities 10 and 20, the velocity of the vessel was reduced to less than 2 km h -1 before all discrete sampling, this allowed a complete gas equilibration for pCO2 measurements.

Accuracy of the pCO2 measurements was estimated at ±5 ppmv. Temperature, salinity, and DO were measured using a calibrated YSI Professional Plus multiparameter meter, and pH was measured using a WTW 3310 pH meter equipped with a Sentix 41 electrode, calibrated with a three-point standard (pH 4.01, pH 7.00 and pH 10.01) according to the National Institute of Standards and Technology (NIST), before and after each sampling campaign. The precision of the pH measurements estimated after seven verifications against standards before and after each sampling campaign was approximately 0.01 NBS units. For the anchored continuous sampling (Eulerian approach), the total duration of sampling was 20 h, encompassing one complete tidal cycle plus one ebb tide period. Two tidal cycles of measurements were not completed due to logistic problems.

Discrete Measurements and Laboratory Analysis

Discrete samples were collected at a depth of ~0.5 m using a Niskin bottle and then stored for further laboratory chemical analysis (i.e., fixed and/or kept on ice in the dark). The water was filtered with Whatman ® grade GF/F glass microfiber filters (pre-combusted at 500 °C for 6 h), followed by determination of TA and Chl a. The filters used to analyzed Chl a were kept at -18 °C prior to analysis. TA measurements were performed according to the classical [START_REF] Gran | Determination of the equivalence point in potentiometric titrations-Part II[END_REF] titration in an automated titration system (Metler Toledo model T50) in filtered water samples. The accuracy of this method was better than ±5 μmol kg -1 (inferred from certified reference material -CRM; A. G.

Dickson from Scripps Institution of Oceanography). Chl a was extracted from the filters with 90% acetone and quantified by spectrophotometry according to [START_REF] Strickland | A practical handbook of seawater analysis[END_REF]. For δ 13 C-DIC measurement, water was sampled and transferred directly to 150 mL serum vials, which were poisoned by adding 0.2 mL of a HgCl2 saturated solution, and then sealed. In the laboratory, δ 13 C-DIC was determined following the protocol by [START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF]. A headspace of approximately 40 mL was created with Helium, and 0.2 mL of ultrapure concentrated H3PO4 was introduced to convert all inorganic carbon to CO2. The samples were shaken and kept in the dark at controlled temperature of 25 °C for 12 h. The δ 13 C of CO2 in the headspace was determined by injecting 0.5-1 mL of headspace gas in an isotopic ratio mass spectrometer (IRMS) equipped with a manual gas injection port. δ 13 C-DIC was calibrated using laboratory standards, which were prepared by adding 45 mg of Na2CO3 in a sealed vial flushed with helium and dissolved with 3 mL of 85% H3PO4, as described in [START_REF] Deirmendjian | Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed[END_REF]. This standard was calibrated against a certified standard (NBS 19, -1.96‰) using a dual-inlet IRMS. The isotopic value of the Na2CO3 standard was -4.5 ±0.2‰. The obtained δ 13 C values were corrected for the partitioning of CO2 between the gaseous (headspace) and water phases in each sample using the algorithm of [START_REF] Miyajima | Determining the stable isotope ratio of total dissolved inorganic carbon in lake water by GC/C/IIRMS[END_REF]. Repeatability of the analysis was ~0.1‰.

The δ 13 C-DIC concentrations are reported in ‰ relative to the standard Vienna Pee Dee Belemnite (V-PDB) scale.

Calculations

Carbonate System

DIC was calculated from pCO2, TA, temperature, and salinity using the CO2calc 1.2.9 program [START_REF] Robbins | CO2 Calc: a user-friendly seawater carbon calculator for Windows, Max OS X, and iOS (iPhone)[END_REF]. We used the dissociation constants for carbonic acid proposed by [START_REF] Mehrbach | Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure[END_REF] refitted by [START_REF] Dickson | A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media[END_REF], the borate acidity constant from [START_REF] Lee | The universal ratio of boron to chlorinity for the North Pacific and North Atlantic oceans[END_REF], the dissociation constant for the HSO4 -ions from [START_REF] Dickson | Thermodynamics of the dissociation of boric acid in synthetic seawater from 273[END_REF] and the CO2 solubility coefficient of [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. The estimated uncertainty was better than ± 6 μmol kg -1 for DIC [START_REF] Orr | Routine uncertainty propagation for the marine carbon dioxide system[END_REF].

2.5.2 Excess DIC (E-DIC) and Apparent Utilization of Oxygen (AOU)

The excess of DIC (E-DIC, μmol kg -1 ) was calculated according to [START_REF] Abril | Carbonate dissolution in the turbid and eutrophic Loire estuary[END_REF]:

E-DIC = DICsample -DICequilibrium (1)
Where DICsample represents the measured concentration of DIC (μmol kg -1 ) and DICequilibrium is the theoretical DIC at atmospheric equilibrium (µmol kg -1 ). DICequilibrium was calculated from the observed TA and the measured atmospheric pCO2 during the cruises.

The apparent oxygen utilization (AOU, μmol kg -1 ) was calculated as proposed by [START_REF] Benson | The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere[END_REF]:

AOU = DOequilibrium -DOsample (2)
Where DOsample is the measured DO, and DOequilibrium is the DO saturation.

Diffusive air-water CO2 fluxes

Diffusive fluxes of CO2 at the air-water interface were calculated according to the following equation:

FCO2 = kCO2,T . k0. ΔpCO2 (3)
Where FCO2 is the diffusive flux of CO2, kCO2,T is the gas transfer velocity of CO2 at a given temperature (T), k0 is the solubility coefficient of CO2 calculated from in situ temperature and salinity [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF], and the ΔpCO2 is the difference between the measured water pCO2 and the measured atmospheric pCO2 . The atmospheric pCO2 was measured every sampling day at morning, noon and afternoon. The gas transfer velocity of CO2 kCO2,T was computed as follows [START_REF] Jähne | On parameters influencing air-water exchange[END_REF]):

kCO2,T = k600 x (600/ScCO2, T) n (4)

where k600 is the gas transfer velocity normalized to a Schmidt number of 600 (Sc = 600 for CO2 at 20°C), ScCO2, T is the Schmidt number of CO2 at a given temperature [START_REF] Wanninkhof | Relationship Between Wind Speed and Gas Exchange[END_REF], and n being equal to 2/3 for wind speed <3.7 m s -1 and equal to 0.5 for higher wind speed [START_REF] Jähne | On parameters influencing air-water exchange[END_REF][START_REF] Guérin | Gas transfer velocities of CO2 and CH4 in a tropical reservoir and its river downstream[END_REF].

To compute the k600 values, we used three different parameterizations proposed by [START_REF] Wanninkhof | Relationship Between Wind Speed and Gas Exchange[END_REF], [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF] and [START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF], according to respectively: k600(W92) = 0.31(U10^2)

(5)

k600(RC01) = 1.91exp0.35U10 (6) k600(A09) = 1.80exp-0.0165v + [1.23 + 1.00LOG(S)]U10 (7)
where k600 is the gas transfer velocity normalized to a Schmidt number of 600 expressed in cm h -1 , v is the water current velocity in m s -1 , S is the surface area of the specific estuarine section considered in km 2 , and U10 is the wind speed at 10m height in m s -1 . Water-to-air CO2 fluxes were calculated using the daily-average pCO2 for each estuarine domain: low salinity domain (salinities 0-5), mixing domain (salinities 5-25), marine domain (salinities > 25), and mangrove domain. We also differentiated the gas transfer velocities at different moments (morning, afternoon and nighttime). We divided the gas transfer velocities for these periods because the region is strongly impacted by marine breezes, wind speed being stronger during midday/afternoon than during the night/early morning [START_REF] Amarante | Atlas Eólico, Estado do Rio de Janeiro[END_REF][START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]. Wind speed wind velocity (U10), accumulated precipitation and atmospheric temperature were recorded in the meteorological station of Campos dos Goytacazes City and were provided by the National Institute for Space Research (INPE). The water current velocities were taken from [START_REF] Ovalle | Longterm trends in hydrochemistry in the Paraíba do Sul River, southeastern Brazil[END_REF]. [START_REF] Rudorff | Spectral mapping of the Paraıba do Sul River plume (Brazil) using multitemporal Landsat images[END_REF] calculated the spatial coverage of the Paraiba do Sul River plume applying a spectral linear mixture analysis. The authors proposed a linear regression to estimate the surface area of the estuarine plume in the coast as a function of the historical river discharge. We applied this general linear regression to estimate the river plume spatial coverage for our sampling period of maximal river discharge (Mar-18), when the river discharge averaged 1,240 m 3 s -1 , and the extension of river plume was calculated at about 90.75 km 2 . Accounting for the salinity values along the boat's trajectories, we constructed maps of surface salinity applying the data-interpolating variational analysis (DIVA gridding) using the Ocean Data View Software. The spatial coverages of the salinity zones for each sampling campaign were used to calculate the area-averaged CO2 flux (Fig. S1, Supplementary file).

Mixing Models

Two types of mixing models were applied, one for TA, DIC and δ 13 C-DIC in the discrete samples, and one for temperature, salinity and pCO2, which were recorded continuously.

For DIC and TA, the model assumes conservative mixing according to

Cmix = CfreshwaterCfreshwater + Cmarine(1 -Cfreshwater) ( 8
)
Where Cmix is the concentration of a given solute during conservative mixing (in our case TA and DIC), and the subscripts freshwater and marine indicate the endmember concentrations in the river and the ocean, respectively. The freshwater fraction (Ffreshwater) is calculated as:

Ffreshwater = 1 -salsample/salmarine (9)
Where sal refer to the salinity, and the subscript sample refers to the in situ values for each station.

Concerning stable isotope ratios, we calculated the conservative mixing of δ 13 C-DIC (δ 13 C-DICmix) according to [START_REF] Mook | Stable carbon isotopes in rivers and estuaries[END_REF], as follows: The TA and DIC concentrations measured in the discrete samples (see result section) were close to the conservative mixing line. Thus we could compute these two parameters for each salinity obtained with the continuous measurements. The theoretical pCO2 conservative mixing curve (pCO2mixing) was obtained by solving the carbonate system using the in situ temperature and salinity, and the computed conservative TA and DIC for each salinity value obtained with the continuous measurements from the estuary [START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Joesoef | Air-water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability[END_REF]. Because the pCO2 was variable in the freshwater endmember, we calculate the average and standard deviations for each sampling campaign.

In order to allow a full comparison with the observed pCO2 distribution, two corrections of the conservative mixing pCO2 model are necessary: a correction for gas exchange and another for the effect of heating or cooling in the estuary. In order to correct the mixing pCO2 curve for gas exchange, an iterative calculation was applied for each salinity increment. The loss or gain of DIC as CO2 evasion or invasion between each salinity increment "sal" was calculated as:

DICgain(sal) = -FCO2(sal).RT.H -1 (11)
Where FCO2(sal) is the flux (mol m -2 h -1 ) for every salinity unit, RT is the residence time (h) during mixing between salinity (sal) and ( sal+1) and H is the water depth (m). FCO2(sal) is calculated with Eq.3, and the conservative mixing pCO2 (sal). The new corrected DIC concentration at sal+1 is obtained by adding the DICgain(sal+1) to the DIC(sal+1) previously deduced from the conservative mixing of DIC(sal) with DICmarine:

Corrected DIC(sal+1) = DIC(sal+1) + DICgain(sal) (12)

Then we recalculate the corrected pCO2(sal+1) from a new mixing lines between corrected DIC(sal+1)

and DICmarine, and TA(sal+1) and TAmarine. The next iteration consisted in calculating the FCO2(sal+1) from the corrected pCO2(n+1) and the DICgain(sal+2) using Eq.3. The difference between pCO2gas exchange and pCO2mixing (ΔpCO2gas exchange) is attributed to gas exchange.

The thermal influence on the pCO2 (cooling or heating), was calculated from the TA and DIC conservative mixing, and this time by considering not the in situ measured temperature, but the temperature calculated as linearly conservative between the two endmembers. The difference between pCO2mixing (calculated with in situ temperature) and the pCO2 calculated with the endmembers interpolation is attributed to the thermal pCO2 along the mixing zone (ΔpCO2cooling/heating).

Statistical analysis

We applied the Shapiro-Wilk test to investigate the normality of the data set. As the data did not follow normal distributions, we applied non-parametric statistics. To compare the differences between averages for the concentrations of biogeochemical parameters in water, we applied the Mann-Whitney test, which compares the distributions of unmatched groups. Linear and non-linear regressions were calculated to analyze the correlations between variables, providing the best-fit slope, the intercept and the goodness of fit (R 2 ). All statistical analyses were based on α = 0.05. The statistical tests and calculations were performed with the Graph Pad Prism 6 program.

Results

Spatial and temporal variations of carbonate chemistry and ancillary parameters

Spatial and temporal variations of pCO2 and the main parameters analyzed in this study are presented in Table 1 for the four domains. Surface water temperature showed a marked seasonal trend between 26.2 and 31.8 °C in summer (Feb 17 and Mar 18 samplings) and between 23.4 and 26.2 °C in winter (Oct 2017 sampling). The highest temperatures were found in freshwaters, with a cooling trend seaward. This decrease in water temperature was slight, with less than 2 °C difference, on average, between the river and marine zones. However, this seaward temperature decrease trend was not linear during estuarine mixing. Intermediate salinities (values from 20 to 30) were generally sampled close to midday, when the solar irradiance is maximum. These intermediate salinities exhibited temperatures higher than that expected for the conservative mixing because of the heating effect (>0.8 °C, on average), reaching 2.2 °C during sampling on March 18. In contrast, waters with temperatures lower than that expected for the conservative mixing (cooling effect) were verified only on the Feb 17

sampling in low salinities.

The spatiotemporal salinity distribution in surface waters was governed by the river water discharge (Fig. 2; Fig. S1 Supplementary information). This figure shows the boat track for the three sampling campaigns. Note that the boat tracking was not the same for the sampling campaigns. The principal reason for these differences is that the main estuarine channel navigated in the first two trips (Feb 17

and Oct 17) became very shallow and dangerous for navigation. For safety reasons, we decided to navigate along the secondary channel during the trip of March 18. During the period of lowest freshwater discharge (178 m 3 s -1 ), the mixing zone was located close to the river mouth, with an intrusion of brackish water (salinity ≤20) until approximately 4 km inside the internal estuary at high tide. Consequently, at low river discharge, the river plume covered a small area (~30 km 2 ) in the adjacent shelf waters, with the marine zone (salinity >30) located closest to the river mouth (between 3 and 6 km from the river mouth) (Fig. S1). Contrastingly, during the Mar 18 sampling at high river flow (1,240 m 3 s -1 ), the mixing zone was located outside the limit of the coast, and no salinity intrusion occurred in the internal estuary (Fig. 2c). During this high flow period, the mixing zone covered the highest surface area (~63 km 2 ) in the adjacent shelf (extending to approximately 12 km from the river mouth), with the marine zone located far from the internal estuary (>10 km) (Fig. S1).

Surface water pCO2 in the estuary was strongly related to the salinity and followed a distribution close to that predicted for the mixing curve between the river and marine endmembers, with marked differences between the dry and rainy seasons in the low salinity region (Figs. 2 and3). During low river flow, the low salinity regions (0-5) presented undersaturated pCO2 conditions compared with the equilibrium with atmosphere in all sampling campaigns, with mean values of 157 ±47 ppmv and 192

±52 ppmv for the Feb 17 and Oct 17 samplings, respectively (Fig. 2a,b). In the seaward direction, pCO2 increased with salinity, following a general trend close to the thermodynamic mixing curve (Figs. 2a,b and 3a,b). During the rainy period, the river zone presented high pCO2 values with permanent oversaturation (mean of 1,946 ±252 ppmv). However, pCO2 showed a sharp decline between salinities 0 and 5, and quickly reached undersaturated conditions at salinity higher than ~10, with a mean value of 380 ±116 ppmv in the mixing zone (Fig. 2l and3h). The marine zone (salinity >30), during the rainy period, showed pCO2 values close to those found during the dry period, 439 ±17 ppmv, on average. Mangrove-dominated waters (mangrove creek) presented pCO2 values much higher (up to 11,514 ppmv) than that found in the waters of the main estuarine channel in all sampling campaigns.

Strong relationships with salinity were also verified for carbonate chemistry parameters, including TA, DIC, and δ 13 C-DIC (Fig. 4). The TA and DIC concentrations for the freshwater endmember presented small but statistically significant seasonal variation depending on the intensity of river flow (Mann-Whitney test, p<0.0001). The TA and DIC concentrations in the freshwater endmember were 446 ±89

(n = 4) and 435 ±77 µmol kg -1 (n = 3) during the dry season and 428 ±101 and 469 ±86 µmol kg -1

during the wet season (n = 4), respectively. In the mangrove waters, the TA and DIC concentrations were much higher than those in the conservative mixing, and did not follow the linear pattern found in the main channel (Fig. 4a,b,c,d,e,f). Concerning the δ 13 C-DIC signatures, the freshwater endmember showed important seasonal variation according to river discharge (Fig. 4). During low river flow (Feb 17 and Oct 17 samplings), freshwater δ 13 C-DIC values showed a mean of -3.55 ±0.43‰, and the distributions along the salinity gradient showed slight 13 C depletion compared with the conservative mixing curve (Fig. 4). During high river flow, the stable isotope signature mean was -7.38 ±1.49 ‰, and the δ 13 C-DIC distributions along the salinity gradient showed only a minor deviation from the conservative mixing curve. δ 13 C-DIC values in the mangrove creek were more negative than in the main channel during the two dry periods assessed, with mean and minimum values of -10.96 ±1.23‰ and -11.82‰, respectively (Fig 4g,h). During the rainy period, the δ 13 C-DIC values in the mangrove creek presented a mean of -6.43 ±1.79‰, with enrichment in 13 C compared with that of the waters of the main channel, and above the conservative mixing curve (Fig. 4i).

During the dry period, Chl a concentrations were higher in freshwaters (between 3.86 and 21.30 µg L - 1 ) and showed a marked decrease in the seaward direction (Fig. 4j,k). This decrease in Chl a concentrations with increasing salinity is coincident with an increase in pCO2. Chl a and pCO2 were negatively correlated in the main channel (R 2 = 0.59 on Feb 17; R 2 = 0.60 on Oct 17; p<0.001) (Fig. 5).

On the other hand, for the rainy period, the freshwaters presented the lowest Chl a concentrations, and the distributions did not follow a clear pattern along the salinity gradient, not presenting significant correlations with pCO2 (Figs. 4 and5). The Chl a concentrations in the mangrove tidal creek were high for all the sampling campaigns, reaching a maximum value of 26 µg L -1 on February 17.

Tidal and diurnal variations of carbonate chemistry in the mangrove creek

Results of diurnal variations of the carbonate chemistry parameters in the tidal creek are presented in and7). In this way, pCO2 showed strong negative correlations with DO (R 2 = 0.99; p<0.0001) and δ 13 C-DIC (R 2 = 0.97; p<0.0001) (Fig. 7d). Hypoxic and anoxic conditions occurred from the end of the nighttime period until sunrise (between approximately 00:30 am and 07:30 am) (Fig. 6c). DO concentrations were also strongly positively correlated with δ 13 C-DIC values (R 2 = 0.96; p<0.0001) (Fig. 7c). During the diurnal cycle in the mangrove creek, δ 13 C-DIC values varied between -8.79‰ (nighttime ebb tide) and -3.27‰ (daytime flood tide). The TA and DIC distributions followed a nearly linear relationship with salinity (Fig. 7a). Increased TA concentrations followed decreased DO concentrations linearly, but only under hypoxic/anoxic conditions (Fig. 7b). As expected, E-DIC and AOU were well correlated in the mangrove creek, presenting positive concentrations and a high deviation above the line 1:1 in all sampling campaigns, which were different from the main channel pattern (Fig. 8a). The TA and DIC concentrations also showed strong deviation from the conservative mixing (ΔTA and ΔDIC), and different from the pattern observed in the main channel (Fig. 8b).

Gas transfer velocities and CO2 exchanges at the air-water interface

The k-wind speed parameterization is one of the most critical parameters to compute the air-sea CO2 fluxes between the pCO2 gradients in the surface ocean and lower atmosphere [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF][START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean revisited[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF]. Diverse formulations of k-wind parameterizations are available to estimate the main driving forces of turbulence at the air-sea interface, based on theoretical results from several studies that showed that their performance is more appropriate under certain conditions and environments [START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean revisited[END_REF]. Thus, we computed the air-sea CO2 fluxes using three different k-wind parameterizations: two appropriate for estuarine environments [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF][START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF] and one appropriate for open ocean waters [START_REF] Wanninkhof | Relationship Between Wind Speed and Gas Exchange[END_REF], providing ranges of estimates (Table 3).

The calculated gas transfer velocities varied between 0.44 and 21.26 cm h -1 over the morning, afternoon and night periods (Table 3). The k600 values calculated from the equation of [START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF] for estuarine waters, which account for wind velocity, estuarine size and current velocity, were higher than those calculated using the parameterizations proposed by [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF] and [START_REF] Wanninkhof | Relationship Between Wind Speed and Gas Exchange[END_REF], except for wind speeds >6 m s -1 , when the equation of [START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF] provided lower gas transfer values. The inner estuary current velocities estimated for each estuarine region, considering the water discharge and the cross-sectional area, ranged from 0.1 m s -1 , during low river flow, to 1.2 m s -1 , during high river flow. For the outer estuary (river plume), the current velocities were measured according to [START_REF] Knoppers | The coast and shelf of East and Northeast Brazil and material transport[END_REF]. The Oct 17 sampling presented higher gas transfer velocity values compared with those measured on February 17 and March 18 (p<0.0001; t test) as a result of the higher wind speed averages. Higher computed gas transfer velocities occurred during the afternoon, followed by night and morning (Table 3).

Calculated CO2 fluxes showed high spatiotemporal variability (Table 3) related to salinity and distance from the mangrove forest. The freshwater endmember was a CO2 sink during low discharge and a CO2

source during high discharge. The highest values of air-water CO2 uptake in the estuarine zone (between -26.23 and -37.80 mmol C m 2 d -1 ) were found during the afternoon, when wind speed was strongest. The mixing zone was also a CO2 sink, but with smaller intensity compared with fresh and low-salinity waters. The air-water CO2 fluxes in the mixing zone varied between -0.34 and -18.41 mmol C m 2 d -1 . The marine zone acted as a slight CO2 source, with emissions ranging from 0.12 to 5.78 mmol C m 2 d -1 . The mangrove creek waters were a strong CO2 source, with values ranging from 1.97 to 216.14 mmol C m 2 d -1 .

Considering all the estuarine areas and time of sampling (morning, afternoon and night), the estuary was a slight CO2 sink under dry conditions, with weighted surface fluxes ranging from -1.34 to -5.26 mmol C m 2 d -1 (Table 3), representing an uptake between 1. Oct 17 and Mar 18 samplings, respectively. Consequently, the DIC inputs from the river to the estuary were between ~11 and 32 times higher than the carbon influx or efflux at the air-water interface.

Discussion

4.1. Predominance of thermodynamic processes on pCO2 distributions in the main channel and CO2

air-water fluxes Oversaturated pCO2 conditions are usually observed in river-dominated estuaries as a result of the net heterotrophic metabolism sustained by inputs of organic carbon from the catchment [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Dai | Diurnal variations of surface seawater pCO2 in contrasting coastal environments[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF], river input of dissolved CO2 (Abril et al. 2000;[START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Joesoef | Air-water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability[END_REF]Van Dam et al., 2018a), and of tidal pumping [START_REF] Maher | Methane and carbon dioxide dynamics in a subtropical estuary over a diel cycle: Insights from automated in situ radioactive and stable isotope measurements[END_REF][START_REF] Santos | Carbon outwelling and outgassing vs. burial in an estuarine tidal creek surrounded by mangrove and saltmarsh wetlands[END_REF]. The overall trend of pCO2 decreasing seaward has mostly been described in rivers with moderate to high TA concentrations (≥1000 µmol kg -1 ) and moderate to long water residence time [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. However, in the case of many tropical river deltas, TA concentrations are generally low at the riverine endmember [START_REF] Cai | A comparative overview of weathering intensity and HCO3-flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi Rivers[END_REF], and these estuaries have relatively short water residence time [START_REF] Ludwig | Atmospheric CO2 consumption by continental erosion: Present-day controls and implications for the last glacial maximum Global Planet[END_REF]. Under such conditions, thermodynamic changes during estuarine mixing generate strong pCO2 undersaturation in the low salinity region [START_REF] Whitfield | The carbon dioxide system in estuaries -An inorganic perspective[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF] that can exceed CO2 production and consumption by biological processes. Low TA concentrations in the river endmember provide low buffering capacity in fresh and low salinity waters [START_REF] Egleston | Revelle revisited: Buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity[END_REF][START_REF] Hu | Estuarine acidification and minimum buffer zone-A conceptual study[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF]. We calculated the Revelle factor (γDIC = ∂ln(CO2)/∂DIC) -1 along the mixing zone of the PSRE to investigate its buffering capacity [START_REF] Revelle | Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO2 during the past decades[END_REF][START_REF] Egleston | Revelle revisited: Buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity[END_REF]. The results corroborate this hypothesis, showing that waters with salinities <10 presented the lowest buffering capacity in all sampling campaigns (Fig. S2; Supplementary information).

In the PSRE, pCO2 distributions along the salinity gradient were very close to that predicted by theoretical thermodynamic changes during conservative mixing between the river and marine endmembers, including a pCO2 minimum much lower than the atmospheric value (Fig. 3). During low river discharge (Feb-17 and Oct-17), the river endmember already presented CO2 undersaturation with relation to the atmosphere (due to phytoplankton uptake). During high river flow, pCO2 was high in freshwater, but showed a sharp decrease between salinities 0 and 5 and remained below the atmospheric value along the mixing zone (salinity 0-30). This marked pCO2 decrease occurred in poorly buffered estuarine waters. The mixing of river water and seawater generates strong equilibrium displacements between species of the carbonate system and strongly alters water pCO2 values and airwater CO2 fluxes, creating pCO2 undersaturation by inorganic processes [START_REF] Whitfield | The carbon dioxide system in estuaries -An inorganic perspective[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF]. In the case of acidic freshwater with low TA and eventually high pCO2, dissolved CO2 is absorbed by the buffering capacity of the seawater and immediately incorporated into the HCO3 -and CO3 2-pools rather than totally emitted to the atmosphere. Because the displacement of chemical equilibriums is an almost instantaneous process, it can occur completely even with the short residence time of waters of few days in the estuary.

A previous study discussed these acid-base processes in a theoretically, suggesting that the Amazon River presents a CO2 deficit along its mixing zone with relation to atmospheric conditions as a result of thermodynamic changes, and the pCO2 variations are quickly dominated by seawater, sustaining a CO2 sink along the river-ocean mixing [START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF]. In the PSRE, these acid-base processes are confirmed along the mixing zone, showing strong agreement between the theoretical model and in situ data. For the three sampling campaigns, however, the pCO2 values in the salinity gradient remained close but slightly above the theoretical dilution curve (pCO2 mixing). The slight deviations of measured pCO2 compared with pCO2 mixing are attributed to the influence of biological activities, gas exchange, and thermal variability during estuarine mixing, as discussed in the following section.

4.2 Origin of the observed pCO2 deviation from conservative mixing Deviations of measured pCO2 compared with pCO2 mixing are potentially related to gas exchange, thermal effect (heating/cooling), and ecosystem metabolism (estuarine autotrophy/heterotrophy plus tidal pumping) during estuarine mixing [START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Joesoef | Air-water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability[END_REF]Van Dam et al., 2018a). In order to account for these processes, first we binned the measured pCO2, calculating the means and standard deviations for each salinity increment. Then we calculated the effects of gas exchange and thermal variability on pCO2 mixing as described in material and methods (section 2.5.5)

for the same salinity intervals, assuming a constant and homogeneous residence time along the entire salinity gradient. This way we could calculate different pCO2 curves during estuarine mixing: measured pCO2 (pCO2 measured), conservative pCO2 (pCO2 mixing), conservative pCO2 corrected for gas exchange (pCO2 gas exchange), and conservative pCO2 corrected for thermal effects (pCO2 heating/cooling) (Fig. 9). The difference between pCO2 gas exchange and pCO2 mixing (ΔpCO2 gas exchange) was attributed to gas exchange, whereas the difference between pCO2 heating/cooling and pCO2 mixing (ΔpCO2 heating/cooling) was attributed to thermal effects. Finally, the residual difference between pCO2 measured and pCO2 gas exchange + pCO2 heating/cooling (ΔpCO2 residual) was assigned to ecosystem metabolism (Fig. 9).

During low river flow, the values of ΔpCO2 gas exchange were positive along the entire salinity gradient in all sampling campaigns, with higher values at low salinities (<10), indicating strong CO2 invasion that increases the values of pCO2 in the water (Fig. 9d,e). During the rainy period, the low salinities also showed the highest ΔpCO2 gas exchange deviations, but with negative values, indicating the influence of CO2 evasion that decreases the values of pCO2 (Fig. 9f). This shows that the maximal influence of gas exchange on pCO2 variability during estuarine mixing occurs at low salinities. The influence of gas exchange on pCO2 showed a marked decrease tendency seaward, with nearly negligible influence from salinities >10, as a result of higher buffering capacity and lower air-water pCO2 gradient. In addition, in intermediate and high salinity waters, pCO2 values remain very close to air pCO2 values, minimizing the effect of atmospheric gas exchange. Consequently, during estuarine mixing in the PSRE, the process of gas exchange has a significant impact on DIC concentrations, particularly in the low-buffered waters of the river domain (Fig. S2; Supplementary information). The highest influence of CO2 invasion was verified on October 17, when the highest wind and gas transfer velocities were observed (Table 3) and ΔpCO2 gas exchange reached a value of 50 ppmv (Fig. 9b,e). The highest influence of CO2 evasion was verified on March 18, when the ΔpCO2 gas exchange reached a value of -150 ppmv (Fig 9f). For intermediate and high salinity waters, the influence of gas exchange on pCO2 variability was minor and nearly negligible, close to that of the marine endmember. Indeed, previous studies have reported lower buffering capacities in regions dominated by freshwaters when the freshwater endmember present low TA concentrations [START_REF] Hu | Estuarine acidification and minimum buffer zone-A conceptual study[END_REF][START_REF] Cai | Carbon dioxide dynamics and fluxes in coastal waters influenced by river plumes[END_REF].

The values of ΔpCO2 cooling/heating were almost always positive, indicating that the water in the estuarine zone was warmed compared to expected behavior for two endmembers mixing. The highest values of ΔpCO2 cooling/heating occurred in mid-to high-salinity waters (salinity values of 20-30). These salinities were generally sampled around midday, when solar irradiance is highest, thus causing this thermal effect. For these salinities of the PSRE, the increase of 1 °C in water temperature tend to increase pCO2 by ~15 ppmv. As the difference between the temperature measured in situ and that predicted by the conservative mixing mean of approximately +0.8 °C, the influence of heating was 12 ppmv, on average; however, the heating effect could reach up ~30 ppmv of pCO2 under conditions of high intensity solar irradiance during sampling in summer, particularly on the Mar 18 sampling (Fig. 9c,f).

An exception occurred during sampling on February 17, when water cooling was observed at low salinities (Fig. 9a,d). In addition to local thermal effects occurring at the time scale of estuarine mixing, seasonal variation in temperature induced variations in pCO2 mainly in waters with salinity >30, resulting in approximately one half (30 ppmv) of the pCO2 difference between summer and winter [START_REF] Takahashi | Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects[END_REF]. Indeed, pCO2 variability in the oligotrophic surface waters of the continental shelf of the Southwestern Subtropical Atlantic Ocean is mainly driven by temperature [START_REF] Ito | Distribution of surface fCO2 and air-sea fluxes in the Southwestern subtropical Atlantic and adjacent continental shelf[END_REF]. The continental shelf of southeastern Brazil, where the river plume of the PSRE is located, is influenced by the Western Boundary Current, which flows southward along the Brazilian shelf as a section of the South Atlantic Ocean subtropical gyre [START_REF] Campos | Shelf break upwelling driven by Brazil current cyclonic meanders[END_REF]. The Brazil Current carries the Tropical Water in the upper layers, the South Atlantic Central Water at intermediate depths, in addition to the Coastal Water in the inner portion of continental shelves, which compose the water masses in the region [START_REF] Castro | Physical oceanography of the western Atlantic continental shelf located between 4°N and 34°S[END_REF].

In general, the differences between pCO2 measured and pCO2 mixing were positive for all sampling campaigns, with deviation means of +45 ppmv, +30 ppmv and +32 ppmv for the Feb 17, Oct 17 and Feb 18 samplings, respectively (Fig. 9d,e,f). Together, the gas exchange and thermal effects accounted for nearly half of this difference (with gas exchange and water heating corresponding to approximately 18% and 36%, respectively), whereas the other half (residual difference, ΔpCO2 residual) was attributed to the ecosystem metabolism along the mixing zone. These positive deviations reveal a CO2 production along the mixing zone that is not linked to conservative mixing, gas exchange and thermal variability, but rather to net heterotrophy during estuarine mixing. However, some negative ΔpCO2 residual values were also observed in low salinity waters under low river flow conditions, which shows a net biological CO2 uptake by phytoplanktonic primary production. At low salinities in summer, some autotrophy decreases pCO2 measured values in relation to those of pCO2 mixing.

Impact of phytoplankton on the river and low salinity region

Phytoplankton had an important impact on pCO2 in the low salinity region of the PSRE, as revealed by the Chl a concentrations (Fig. 4j,k,l), the δ 13 C-DIC signatures (Fig. 4g,h,i), and by the mixing model presented in the previous section (Fig. 9). During low river flow, the PSRE presented an atypical pattern of pCO2 variability along the salinity gradient. In that period, Chl a concentrations were higher in the fresh and low salinity waters, with a strong negative correlation with water pCO2, showing an uptake by planktonic primary production (Fig. 5). This inverse correlation between Chl a and pCO2 occurred only during low river flow, when the lower hydrological model forcing provides favorable conditions for phytoplankton growth in freshwaters because of the high availability of nutrients, high incidence of photosynthetically active radiance, increased water residence time, and formation of vertical water column stratification [START_REF] Souza | Use of multitracers for the study of water mixing in the Paraíba do Sul River estuary[END_REF][START_REF] Cotovicz | Assessment of the trophic status of four coastal lagoons and one estuarine delta, eastern Brazil[END_REF]. The freshwater discharge contributes to maintaining low dilution and flushing potentials, changing the trophic status in the estuary from oligotrophic to mesotrophic [START_REF] Cotovicz | Assessment of the trophic status of four coastal lagoons and one estuarine delta, eastern Brazil[END_REF]. Uptake of CO2 by phytoplankton is also evidenced by the negative and significant relationship between pCO2, DO, and δ 13 C-DIC signatures (Fig. S3, Supplementary information). During low river flow, pCO2 values were lower, whereas DO concentrations and δ 13 C-DIC signatures were higher, indicating prevalence of net autotrophy. During photosynthesis, plants preferentially utilize the lighter carbon isotope ( 12 C) rather than the heavier carbon isotope ( 13 C), making the remaining DIC pool 13 C enriched [START_REF] Finlay | Stable isotope tracing of temporal and spatial variability in organic matter sources to freshwater ecosystems[END_REF][START_REF] Cotovicz | Sources and sinks of dissolved inorganic carbon in an urban tropical coastal bay revealed by δ 13 C-DIC signals[END_REF]. A previous study showed that the stable isotope composition of the dissolved CO 2 (δ 13 C-DOC) also presented a marked seasonal signal in the PSRE, with more 13 C depletion in the rainy season (minimum of -25.7 ‰) than in the dry season (maximum of -20.0 ‰) [START_REF] Marques | Dissolved Black Carbon in the Headwaters-to-Ocean Continuum of Paraíba Do Sul River, Brazil[END_REF]. These authors suggested that algae could be a potential source of isotopically heavier DOC to the river zone during the dry season, as our data of low pCO2 values and high δ 13 C-DIC signatures and Chl a concentrations also suggest.

The Paraiba do Sul River receives large amounts of domestic and industrial effluents along its course, which drains one of the most urbanized and industrialized regions of Brazil [START_REF] Cotovicz | Assessment of the trophic status of four coastal lagoons and one estuarine delta, eastern Brazil[END_REF].

Low pCO2 levels due to nutrient enrichment and phytoplankton blooms development has been previously shown for other coastal bays surrounded by large urban areas, e.g., the tropical Guanabara Bay [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF] and in the temperate Tokyo Bay [START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF]. However, this is very uncommon in river-dominated estuaries, which are considered important CO2 sources globally [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. To the best of our knowledge, this pattern of low pCO2 in freshwater was only reported in the temperate Merrimack Estuary, located in a highly industrialized region of the United States, which experienced a rapid population growth [START_REF] Salisbury | Seasonal observations of surface waters in two Gulf of Maine estuary-plume systems: Relationships between watershed attributes, optical measurements and surface pCO2[END_REF].

Large phytoplankton biomass is observed in the freshwaters of the PSRE during low river flow, but it shows a marked decline seaward (Fig. 4j,k,l). Indeed, estuarine deltas with short residence times are characterized by rapid salinity changes in water masses, which hinder the installation of phytoplankton communities because of osmotic stress (Lancelot and Muymaert, 2011). Salinity and turbulent mixing are often reported as the major factors determining phytoplankton distribution [START_REF] Cloern | Phytoplankton bloom dynamics in coastal ecosystems: a review with some general lessons from sustained investigation of San Francisco Bay, California[END_REF][START_REF] Lancelot | Trends in Estuarine Phytoplankton Ecology[END_REF]. In addition to osmotic stress, the community structure, biomass production, species succession, and magnitude of the phytoplankton bloom along the estuary are also related to small availability and short residence time [START_REF] Roubeix | Diatom succession and silicon removal from freshwater in estuarine mixing zones: from experiment to modelling[END_REF][START_REF] Flöder | Dominance and compensatory growth in phytoplankton communities under salinity stress[END_REF]. Thus, sharp salinity changes, high turbulence, turbidity, and short residence time create conditions unfavorable for phytoplankton production.

During high river flow, the phytoplankton community is probably flushed and dies in the main channel. The river discharge is stronger than the tidal prism, and the mixing occurs offshore.

According to [START_REF] Lucas | Why are diverse relationships observed between phytoplankton biomass and transport time?[END_REF], the phytoplankton biomass can increase in an estuary only if the net specific growth rates exceed the water residence time and flushing time in the estuary. In general, development of phytoplankton blooms in estuaries is inversely correlated to river water discharge [START_REF] Strayer | Hydrology and grazing jointly control a large-river food web[END_REF]. However, during the flood on May 18, part of the phytoplankton was apparently trapped in the low salinity mangrove creeks, where the observed Chl a concentrations reached ~70 µg L -1 (Table 2). Mangrove creek provided ideal conditions for phytoplankton refuge due to low salinity stress. This created a special case in this estuary -a low salinity mangrove creek enriched with phytoplankton. Phytoplankton activity and the influence on DIC and δ 13 C-DIC concentrations are evident in the mangrove creek within the diurnal variations (Fig. 6). During the diel sampling in the mangrove creek, phytoplankton biomass was high, and managed to drawdown pCO2 during the afternoon compare with nighttime, making δ 13 C-DIC less negative due to a preferential uptake of 12 C from the DIC pool during photosynthesis. Similar processes have been described in a hypertrophic lake (Lake Taihu, China;Van Dam et al., 2018b) and in a tropical eutrophic coastal embayment (Guanabara Bay, Brazil, Cotovicz Jr. et al., 2019), but not in mangrove creeks. During the rainy period, the mangrove creek presented more 13 C-enriched signatures than the expected for conservative mixing (Fig. 4g,h,i) and related to this phytoplankton activity. However, this pattern in the mangrove creek was not verified during the dry period, when Chl a concentration do not exceed 14.6 µg L -1 , and δ 13 C-DIC signatures were 13 C-depleted more than would be expected for conservative mixing, a distribution generally found in mangrove-dominated waters because of enhanced respiratory processes [START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF][START_REF] Miyajima | Export of inorganic carbon from two Southeast Asian mangrove forests to adjacent estuaries as estimated by the stable isotope composition of dissolved inorganic carbon[END_REF][START_REF] Sippo | Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect[END_REF].

Evidence of heterotrophy in the mixing zone

Multiplication of the residual pCO2 difference attributed to biological activities (ΔpCO2 residual) with gas transfer velocity (Eq. 3) provides an estimate of the community production (NCP) (Table 4). The gas transfer velocity parameterization of [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF], which resulted in intermediate values of air-water fluxes (Table 3), was chosen for this calculation. For the entire estuarine mixing zone, average NCP was -1.70 mmol m -2 d -1 on February 17, -1.13 mmol m -2 d -1 on October 17, and -0.58 mmol m -2 d -1 on February 18. NCP values in the PSRE are in the lower range of NCP and ecosystem metabolism in the European coastal zone [START_REF] Gazeau | The European coastal zone: characterization and first assessment of ecosystem metabolism[END_REF]. These low NCP values observed along the mixing zone of the PSRE confirm the moderate influence of biological activities, with microbial respiration not being sufficient to create CO2 oversaturation in relation to the atmosphere. During low river flow, the entire mixing zone was already undersaturated, and the net heterotrophy was also not sufficient to generate CO2 outgassing. This created a very special condition in the mixing zone of the estuary, with concomitant pCO2 undersaturation and heterotrophic metabolism. During high river flow, the heterotrophic metabolism was also apparent in the freshwater owing to pCO2 values above the atmospheric equilibrium, DO concentrations below the atmospheric equilibrium, and more negative δ 13 C-DIC signatures, all indicating prevalence of microbial respiration over primary production (Fig. S3, Supplementary information). However, as previously shown (Fig 9f ), heterotrophic metabolism was modest in the estuary during high discharge, and thus the river input was a major contributor to the high CO2 concentrations, at least in the region dominated by freshwaters (0-5 salinity). Riverine CO2 inputs represent the major CO2 source in other riverdominated estuaries [START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Joesoef | Air-water fluxes and sources of carbon dioxide in the Delaware Estuary: spatial and seasonal variability[END_REF]Van Dam et al., 2018a).

The covariations of Δδ 13 C-DIC and ΔDIC, considering only the main river channel, also reveal net heterotrophy, with values of δ 13 C-DIC showing slight 13 C-depleted signatures (negative Δδ 13 C-DIC) compared with the conservative mixing (Fig. 10). This graph is divided in four quadrants that represent the processes of primary production/outgassing of CO2 (quadrant PP/OG), carbonate precipitation (quadrant CP), organic carbon degradation (quadrant DC), and carbonate dissolution (quadrant CD) [START_REF] Alling | Degradation of terrestrial organic carbon, primary production and outgassing of CO2 in the Laptev and East Siberian Seas as inferred from 13 C values of DIC[END_REF][START_REF] Samanta | Dissolved inorganic carbon (DIC) and its 13 C in the Ganga (Hooghly) River estuary, India: Evidence of DIC generation via organic carbon degradation and carbonate dissolution[END_REF][START_REF] Cotovicz | Sources and sinks of dissolved inorganic carbon in an urban tropical coastal bay revealed by δ 13 C-DIC signals[END_REF]. The red arrows represent the directions of slopes in which data points would move in relation to a specific process, whereas the grey lines represent the directions in which samples would move in relation to the effects of more than one process (see figure caption for further details). The data points of the PSRE are mostly located in the quadrant of organic carbon degradation (DC), and to a lesser extent in the quadrant of carbonate precipitation (CP). The process of organic carbon degradation increases the DIC concentrations in water (positive ΔDIC) and decreases the isotopic signature of DIC (negative Δδ 13 C-DIC) [START_REF] Alling | Degradation of terrestrial organic carbon, primary production and outgassing of CO2 in the Laptev and East Siberian Seas as inferred from 13 C values of DIC[END_REF][START_REF] Samanta | Dissolved inorganic carbon (DIC) and its 13 C in the Ganga (Hooghly) River estuary, India: Evidence of DIC generation via organic carbon degradation and carbonate dissolution[END_REF]. The respiration of organic carbon produces CO2 with approximately the same isotopic signature than that of the respired organic matter, which is generally 13 C depleted; consequently, the DIC pool tends to be more depleted in 13 C when the organic matter respiration rates are high and prevalent in phytoplankton uptake [START_REF] Kendall | Stable isotope applications in hydrologic studies[END_REF][START_REF] Bouillon | Use of stable isotopes to understand food webs and ecosystem functioning in estuaries[END_REF][START_REF] Borges | Carbon dynamics and CO2 and CH4 outgassing in the Mekong delta[END_REF]. The process of carbonate precipitation is unlikely to occur, because this estuary is not known to host high levels of calcifying fauna and flora. However, as pointed out, DIC and δ 13 C-DIC can be subjected to more than one process at same time. This means that if one sample was subjected to organic carbon degradation followed by CO2 outgassing, it could also be located in the quadrant of carbonate precipitation [START_REF] Samanta | Dissolved inorganic carbon (DIC) and its 13 C in the Ganga (Hooghly) River estuary, India: Evidence of DIC generation via organic carbon degradation and carbonate dissolution[END_REF]. This probably occurs in regions that are sources of CO2 to the atmosphere: the mixing region with very low salinities on March 18 and in offshore waters on Feb 17 and Mar 18 (Figs. 9 and 10). These deviations of δ 13 C-DIC signatures and DIC concentrations from the conservative mixing are thus very consistent with the approach, considering the differences between pCO2 measured and pCO2 mixing (Fig. 9).

The marked seaward decrease of Chl a concentrations along the mixing zone in the main channel suggests that these phytoplankton cells are dying, providing labile organic material for decomposition by heterotrophic bacteria. However, heterotrophy remains weak probably because of the short water residence time in the mixing zone, a crucial factor for bacterial growth in estuaries [START_REF] Del Giorgio | Linking the physiologic and phylogenetic successions in freeliving bacterial communities along an estuarine salinity gradient[END_REF][START_REF] Crump | Microbial biogeography along an estuarine salinity gradient: combined influences of bacterial growth and residence time[END_REF], and because of salinity stress in the bacterioplankton, since that freshwater bacteria could remain active in the estuary, but at a reduced rate [START_REF] Crump | Microbial biogeography along an estuarine salinity gradient: combined influences of bacterial growth and residence time[END_REF]. The ΔTA and ΔDIC values along the salinity gradient in the main channel were low (generally <50 µmol kg -1 ; Fig. 8b), which correspond to less than 5% of the DIC and TA pools, confirming a modest biological influence in the main channel.

Tidal pumping in the mangrove creek

Water pCO2 values in the mangrove creek were much higher than those in the main channel, as well as than those predicted by the conservative mixing curve (Fig. 3). In addition, the mean pCO2 values observed in the mangrove creeks were within the high range of values reported in other mangroves worldwide. The maximum pCO2 value reported here was 22,000 ppmv and, to the best of our knowledge, only one study reported a higher maximum pCO2 value (27,000 ppmv; Australian microtidal system, [START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF]. Our measured pCO2 values are similar to those observed in some micro-, meso- (Koné and Borges, 2008;[START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Rosentreter | Seasonal and temporal CO2 dynamics in three tropical mangrove creeks -A revision of global mangrove CO2 emissions[END_REF] and macro-tidal [START_REF] Call | High pore-water derived CO2 and CH4 emissions from a macro-tidal mangrove creek in the Amazon region[END_REF] mangrove creeks. The mangrove waters presented higher values of ΔTA and ΔDIC compared with those of the main channel (Fig. 8b). The slope of the linear regression of ΔTA vs.

ΔDIC in the mangrove creek was 0.73, a value within the theoretical slopes of organic matter aerobic degradation (0.2), denitrification (0.8), sulfate reduction (0.9), and calcium carbonate (CaCO3) dissolution (2.0) [START_REF] Borges | Atmospheric CO2 flux from mangrove surrounding waters[END_REF][START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF]. Denitrification is likely to be minor in mangrove ecosystems due to the low concentrations of inorganic N-species [START_REF] Alongi | Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam[END_REF][START_REF] Maher | Pristine mangrove creek waters are a sink of nitrous oxide[END_REF]. CaCO3 dissolution is unlike to be important in this mangrove that does not receive important amounts of CaCO3 from adjacent ecosystems. This slope is rather similar to those found in the mangrove tidal creeks of Papua New Guinea [START_REF] Borges | Atmospheric CO2 flux from mangrove surrounding waters[END_REF], Kenya [START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF] and Australia [START_REF] Sippo | Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect[END_REF] and in the secondary channels of the Mekong Delta [START_REF] Borges | Carbon dynamics and CO2 and CH4 outgassing in the Mekong delta[END_REF], suggesting that the main processes responsible for TA and DIC production are sulphate reduction and aerobic degradation of organic matter. TA production occurs when DO presents saturation <30% (Fig. 7b), and it is maximal under hypoxic/anoxic conditions. These results reveal that the mangrove creek in the PSRE is producing and exporting DIC and TA to mangrove surrounding waters, a process recently reported in a number of other mangroves that still needs precise quantification [START_REF] Sippo | Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect[END_REF]and references there in). Important correlations of radon ( 222 Rn) with DIC and TA in several mangrove creeks located at different coastal latitudes have evidenced an important production of TA and DIC in mangrove sediment pore waters with export mediated by tidally induced processes [START_REF] Santos | Carbon outwelling and outgassing vs. burial in an estuarine tidal creek surrounded by mangrove and saltmarsh wetlands[END_REF]Call et al., 2019 and references therein).

The relationship between E-DIC and AOU (Fig. 08a) exhibited important deviations above the 1:1 line that occurred almost exclusively in the mangrove creek. This high deviation above the 1:1 line in the mangrove creek could be associated with anaerobic organic matter degradation in sediments and lateral transfer via tidal pumping. Because the process of sulphate reduction is a dominant mineralization pathway of organic carbon in mangroves [START_REF] Borges | Atmospheric CO2 flux from mangrove surrounding waters[END_REF][START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF][START_REF] Sippo | Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect[END_REF], secondary metabolites such as Fe(II) and sulfides escape re-oxidation by oxygen through the formation of pyrite [START_REF] Alongi | Benthic decomposition rates and pathways in plantations of the mangrove Rhizophora apiculata in the Mekong delta, Vietnam[END_REF][START_REF] Hu | An assessment of ocean margin anaerobic processes on oceanic alkalinity budget[END_REF], resulting in net consumption of protons and net production of TA.

Tidal variation also seems to be important considering the modulation of CO2 production and consumption in the mangrove tidal creek (Figs. 6 and7). On May 2017, CO2 uptake occurred during daytime, making the waters enriched in δ 13 C-DIC, with drawdown of pCO2 and production of DO, even during ebb tide. This explains the low pCO2 values at ebb tide (daytime), which are not common in mangrove waters [START_REF] Bouillon | Use of stable isotopes to understand food webs and ecosystem functioning in estuaries[END_REF][START_REF] Sippo | Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect[END_REF]. The maximal CO2 concentrations were found during ebb tide in early morning, concomitant with the highest concentrations of TA and DIC and the highest 13 C depletion, evidencing the carbon respiration accumulated overnight and the influence of diagenetic processes. Tidal pumping is facilitated, during ebb tide, by the semi-diurnal flushing of crab burrows that are considerably enriched in dissolved carbon due to the diffusion and advection of pore water from the soils [START_REF] Borges | Atmospheric CO2 flux from mangrove surrounding waters[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF][START_REF] Maher | Methane and carbon dioxide dynamics in a subtropical estuary over a diel cycle: Insights from automated in situ radioactive and stable isotope measurements[END_REF]. This pCO2 pattern peaking at the nighttime ebb tide is consistent with findings of previous studies that suggest a pore water source of surface water pCO2 [START_REF] Bouillon | Biogeochemistry of the Tana estuary and delta (northern Kenya)[END_REF][START_REF] Kristensen | Emission of CO2 and CH4 to the atmosphere by sediments and open waters in two Tanzanian mangrove forests[END_REF][START_REF] Linto | Carbon Dioxide and Methane Emissions from Mangrove-Associated Waters of the Andaman Islands, Bay of Bengal[END_REF][START_REF] Call | High pore-water derived CO2 and CH4 emissions from a macro-tidal mangrove creek in the Amazon region[END_REF].

CO2 fluxes in the PSRE compared with those of other estuaries

Estuarine systems and mangrove surrounding waters are sources of CO2 to the atmosphere and significant for regional and global carbon budgets [START_REF] Bouillon | Mangrove production and carbon sinks: a revision of global budget estimates[END_REF][START_REF] Maher | Groundwater-derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?[END_REF]Chen et al., 2013;[START_REF] Laruelle | Global multi-scale segmentation of continental and coastal waters from the watersheds to the continental margins[END_REF]. According to the most recent global compilation of estuarine CO2 emissions propose by [START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF], upper estuaries are CO2 sources of the order of 106 mmol C m -2 d -1 , mid-estuaries emit 47 mmol C m -2 d -1 , and lower estuaries with salinities >25 emit 23 mmol C m -2 d -1 . These values are much higher than those found in the PSRE, which partially acts as a CO2 sink, with annual CO2 emission of only 1.63 to 11.98 mmol C m -2 d -1 (Table 3). The low CO2 fluxes in the PSRE are consistent with those found in the continental shelf of the subtropical Atlantic Ocean al., 2008), in addition to some fjords [START_REF] Takahashi | Global ocean surface water partial pressure of CO2 database: Measurements performed during 1957-2011 (Version 2011)[END_REF]. All these systems are marine-dominated estuaries, different from the PSRE, which is a river-dominated estuary. To the best of our knowledge, only the temperate Kennebec River (-1.36 mmol C m -2 d -1 ; [START_REF] Takahashi | Global ocean surface water partial pressure of CO2 database: Measurements performed during 1957-2011 (Version 2011)[END_REF] and the New River/USA (-0.5 to 5.4 mmolC m -2 d -1 ; [START_REF] Crosswell | Carbon budget of a shallow, lagoonal estuary: Transformations and source-sink dynamics along the river-estuary-ocean continuum[END_REF] are river-dominated estuaries that exhibited CO2 sink at least in certain periods of the year. Recent findings in the mixing zone of the Amazon River estuary also calculated a net CO2 sink (maximum of -9.2 mmol C m -2 d -1 ), attributed to mixing and biological activity, although the Amazon River and the tropical Atlantic are net sources [START_REF] Lefèvre | Net heterotrophy in the Amazon Continental Shelf changes rapidly to a sink of CO2 in the outer Amazon Plume[END_REF]. Considering only the rainy period, the PSRE was a CO2 source of approximately 5.71 to 19.37 mmol C m -2 d -1 . These CO2 emissions are much lower than the average global emissions in estuaries [START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF] due to the fact that, even under high river flow conditions, the mixing zone of the PSRE is a permanent CO2 sink due to the prevalence of thermodynamic processes. However, considering only the river domain during the rainy season, the PSRE presents emission of approximately 60 mmol C m -2 d -1 , a degassing rate close to those reported in other river-dominated estuaries [START_REF] Frankignoulle | Carbon Dioxide Emission from European Estuaries[END_REF][START_REF] Huang | Air-sea exchanges of CO2 in the world's coastal seas[END_REF]. [START_REF] Call | High pore-water derived CO2 and CH4 emissions from a macro-tidal mangrove creek in the Amazon region[END_REF]. These high CO2 emissions in mangrove tidal creeks, in addition to the mangrove production of TA and DIC and transport to surrounding waters, can represent an important fraction of the carbon that is fixed by mangrove vegetation, with high significance in mangrove carbon budgets on a global scale [START_REF] Bouillon | Mangrove production and carbon sinks: a revision of global budget estimates[END_REF][START_REF] Maher | Groundwater-derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?[END_REF][START_REF] Call | Spatial and temporal variability of carbon dioxide and methane fluxes over semi-diurnal and spring-neap-spring timescales in a mangrove creek[END_REF].

Comparison between the amount of DIC delivered by the river and the CO2 exchanges at the air-sea interface in the mixing zone shows that the river contributes ~18 to 520 times more inorganic carbon to the adjacent continental shelf compared with the estuarine carbon sink. This means that the airwater exchange of CO2 is much slower than the acid-base equilibration of DIC during mixing in the estuary, increasing the importance of thermodynamic processes and decreasing the importance of airwater exchanges and biological activities. It is worth highlighting that many of the world's largest river systems are located in tropical regions, which provide ~40% of global freshwater discharge to the oceans (Dai and Thenberth, 2002). Overall, large tropical rivers present low TA concentrations, such as the Amazon, Congo and Orinoco Rivers [START_REF] Cai | A comparative overview of weathering intensity and HCO3-flux in the world's major rivers with emphasis on the Changjiang, Huanghe, Zhujiang (Pearl) and Mississippi Rivers[END_REF][START_REF] Moore-Maley | The sensitivity of estuarine aragonite saturation state and pH to the carbonate chemistry of a freshet-dominated river[END_REF].

Consequently, the CO2 concentrations at the estuarine mixing zone of large and poorly buffered rivers can be largely overestimated due to this still overlooked process.

Conclusions

Overall, the ecosystem showed aquatic sources and sinks of CO2 considering specific estuarine regions, which were modulated by carbonate thermodynamics, gas exchange, thermal effects, and biological activities. The pCO2 and carbonate chemistry parameters in the PSRE showed marked spatiotemporal variations governed by processes somewhat different from those reported so far in the literature:

i) In the freshwater domain, biological effects were prevalent. During high river flow, the waters presented higher pCO2 values and lower DO concentrations and δ 13 C-DIC signatures, as a result of heterotrophic processes in the river. In contrast, under dry conditions, the waters presented low pCO2

values and high δ 13 C-DIC signatures as a result of phytoplankton growth.

ii) In the mixing zone, the thermodynamic processes during estuarine mixing was the most important driver of pCO2 variations, with measured pCO2 following a distribution close to the theoretical conservative curve predicted by the mixing of the river and marine endmembers. During the rainy period, the pCO2 values decreased sharply as a result of rapid absorption by the acid-base buffering capacity of high TA seawaters. During the dry period, the pCO2 values were below the atmospheric pCO2 in freshwater and increased in seaward, also consistent with the theoretical thermodynamic mixing.

iii) The δ 13 C-DIC signatures were slightly below the conservative mixing curve, indicating a heterotrophic metabolism at intermediate to high salinities. The biological activity contributed to approximately half of the deviation of measured pCO2 from pCO2 mixing. The other half was attributed to gas exchange (~18%) and water heating (~36%) during estuarine mixing.

iv) In the mangrove creek, the waters presented high CO2 values in all sampling campaigns, with strong oversaturation attributed to production of DIC and TA in the water and pore waters by aerobic and anaerobic processes, and transport by diffusion and convection via tidal pumping. Interesting to point out that, during high river flow, the mangrove-dominated waters presented higher δ 13 C-DIC signatures than the main estuarine channel, attributed to phytoplankton growth and fractionation of DIC.

Considering the entire area of the PSRE, the estuary was a CO2 sink at low freshwater discharge (airwater CO2 fluxes ranging from -0.34 to -5.26 mmol C m -2 d -1 ) and a CO2 source at high freshwater discharge (5.71-19.37 mmol C m -2 d -1 ). These fluxes are lower and different from those reported in other river-dominated estuaries worldwide, which are normally strong CO2 sources. The construction of dams in the upper sections of Paraiba do Sul River has decreased its water flow and, associated with increased anthropogenic-derived nutrient inputs to the river, can potentially change the functioning of this ecosystem, from an annual CO2 source to an annual CO2 sink. These results also have implications on global carbon budgets, showing that tropical river-dominated estuaries could absorb CO2 along the river and mixing domains as a result of the poor buffering capacity during estuarine mixing. Low TA concentrations are a property of many large tropical rivers (Amazon, Congo, Orinoco), suggesting that this process can be important globally and needs further investigation. Table 3 Calculated mean values for gas exchange coefficient (k600) and CO2 fluxes at the air-water interface in each estuarine regions of the PSRE, as well as, the spatial time-integrated fluxes for the whole system. The periods M, A, and N represents the morning, afternoon and night. W92 is the data calculated according to k600 of [START_REF] Wanninkhof | Relationship Between Wind Speed and Gas Exchange[END_REF], RC01 is the data calculated according to k600 of [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF], and A09 is data calculated according to k600 of [START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF]. pCO2 mixing (blue dots), pCO2 gas exchange (green dots) and pCO2 cooling/heating (red dots) for each salinity unit.

Period FCO2 (mmol C m -2 d -1 ) W92 FCO2 (mmol C m -2 d -1 ) RC01 FCO2 (mmol C m -2 d -1 ) A09 K600 (cm h -1 ) W92 K600 (cm h -1 ) RC01 K600 (cm h -1
Graphs b), d) and f) show the difference between pCO2 measured and pCO2 mixing (ΔpCO2 measured), pCO2 gas exchange and pCO2 mixing (ΔpCO2 gas exchange), and the cumulative difference between pCO2 gas exchange + pCO2

cooling/heating and pCO2 mixing (ΔpCO2 gas exchange + cooling/heating). The green arrow indicates the effect of gas exchange compared with pCO2 mixing (influx or efflux of CO2). The red arrow indicates the effect of thermal variability (cooling and heating). The grey arrow indicates the residual difference between pCO2 measured and pCO2 mixing attributed to biological activities. outgassing (slope = -8.4‰), and organic carbon degradation using the δ 13 C-DIC value of river water (-16.0‰). The grey arrows represent the direction that the samples will follow if they are subject to more than one process (organic matter degradation followed by primary production, which is drawn parallel to vector of primary production; and organic carbon degradation followed by CO2 outgassing, which is drawn parallel to the vector of CO2 outgassing). Details about the formulations and calculations of these vectors can be found in [START_REF] Alling | Degradation of terrestrial organic carbon, primary production and outgassing of CO2 in the Laptev and East Siberian Seas as inferred from 13 C values of DIC[END_REF], [START_REF] Samanta | Dissolved inorganic carbon (DIC) and its 13 C in the Ganga (Hooghly) River estuary, India: Evidence of DIC generation via organic carbon degradation and carbonate dissolution[END_REF], [START_REF] Cotovicz | Sources and sinks of dissolved inorganic carbon in an urban tropical coastal bay revealed by δ 13 C-DIC signals[END_REF].

δ

  13 C-DICmix = salsample(DICfreshwaterδ 13 Cfreshwater -DICmarineδ 13 Cmarine) + salfreshwaterDICmarineδ 13 Cmarine -SalmarineDICfreshwaterδ 13 Cfreshwater / salsample(DICfreshwater -DICmarine) + salfreshwaterDICmarine -salmarineDICfreshwater (10) Where δ 13 C is the isotopic signature of DIC, the subscripts freshwater and marine indicate the endmember concentrations in the river and the ocean, respectively, and the subscript sample indicate the in situ values at each station.

  45 and 5.72 t C d -1 . During high river discharge (Mar 18), the low salinity region was a strong CO2 source, with emissions ranging from 6.36 to 117.77 mmol C m 2 d -1 , but the mixing zone remained a slight CO2 sink, with fluxes varying between -0.12 and -2.93 mmol m 2 d -1 . The marine zone was a CO2 source, but with pCO2 values very close to the equilibrium with the atmosphere and emissions ranging from 0.05 to 1.27 mmol m 2 d -1 .The mangrove tidal creek was a strong source of atmospheric CO2, with degassing ranging from 9.34 to 358.39 mmol m 2 d -1 . Considering the estuary as a whole, the weighted CO2 surface emissions during high river discharge ranged from 5.71 to 19.37 mmol C m 2 d -1 , representing an efflux between 6.21 and 21.09 t C d -1 .The riverine inputs of DIC to the estuary calculated by multiplying the DIC concentrations at the river endmember by the river flow were 106 tC d -1 , 64 tC d -1 and 488 tC d -1 for the samplings conducted on February 17, October 17 and March 18, respectively. Considering the estuary as a whole, the mean net estuarine fluxes at the air-sea interface were -3.32 tC d -1 , -5.72 tC d -1 and 15.88 tC d -1 for the Feb 17,

  (0.3-9.8 mmol C m -2 d -1 ;[START_REF] Ito | Distribution of surface fCO2 and air-sea fluxes in the Southwestern subtropical Atlantic and adjacent continental shelf[END_REF]. Considering only the dry period, the estuary as a whole was a CO2 sink, with fluxes ranging from -0.34 to -5.26 mmol C m 2 d -1 . These values are close to those reported in the tropical Aby Lagoon (-7.39 mmolC m -2 d -1 ;Koné et al., 2008), subtropical southeast Australian estuaries (-3.28 to -12.05 mmolC m -2 d -1 ; Maher and Eyre, 2012), the tropical eutrophic Guanabara Bay (-26.3 to -50.1 mmolC m -2 d -1 ;[START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF], the temperate Tokyo Bay (-8.8 mmolC m -2 d -1 ;[START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF] and the Liminganlahti Bay (-2.46 mmolC m -2 d -1 ; Silvennoinen et

  The CO2 degassing rate in the mangrove tidal creek (average of 135 mmol C m -2 d -1 ) is above global average of CO2 flux emissions from surrounding mangrove waters (56 mmol C m -2 d -1 ; Rosentreter et al., 2018). Our mean values and ranges of CO2 degassing are close to those found in mangroves in Vietnam (Kiên Vàng and Tam Giang mangroves, 32 to 155 mmol C m -2 d -1 ; Koné and Borges, 2008), Andaman Islands (Kalighat mangrove, 23 to 173 mmol C m -2 d -1 ; Linto et al., 2014), Australia (Moreton Bay mangrove tidal creek, 202 mmol C m -2 d -1 ; Call et al., 2015), and the Amazon region (62 to 437 mmol C m -2 d -1 ;

Figure 1 .

 1 Figure 1. Map 1a shows the localization of the state of Rio the Janeiro. The river basins are shown in shades of grey. Map 1b shows the localization of the PSRE and adjacent shelf waters. The colored lines represent the boat track with online and continuous measurements of pCO2, DO, pH, temperature, and salinity. Note that the boat track varied in each sampling campaign: light green, dark green and blue represent Feb 17, Oct 17 and Mar-18, respectively. Along the boat track, discrete stations were sampled (the exact localization of the sampling stations exhibited variations between sampling campaigns according to the boat track and the salinity gradient). The yellow star represents the localization of the anchoring station (Eulerian time-series data). Graph c shows a comparison between the river water discharge for the year of sampling (2017/2018) and the historical average. The red arrows are the sampling months. Graph d shows a historical tendency of decreasing river flow (average monthly values).

Figure 2 .

 2 Figure 2. Maps of continuous measurements of a) salinity, b) pH, c) DO, and d) pCO2 in surface waters of the PSRE for all sampling campaigns.

Figure 3 .

 3 Figure 3. Distribution of pCO2 along the salinity gradient in the PSRE separated by sampling campaigns. Graphs a), c), e) and g) include all the data, with the main estuarine channel and the mangrove creek. Graphs b), d), f) and h) include only the data in the main estuarine channel.

Figure 4 .

 4 Figure 4. Distributions of TA, DIC, δ 13 C-DIC, and Chl a in discrete samples along the salinity gradient. The black dots are the main estuarine channel and the green dots are the mangrove creek.The dashed blue lines represent the theoretical models of conservative mixing endmembers.

Figure 5 .

 5 Figure 5. Relationship between the concentrations of Chl a vs. pCO2 along the main channel of the PSRE. The different symbols in the figure represent the sampling campaigns, as indicated in the caption. The terms "river" and "ocean" represent the values found in the river and ocean endmembers, respectively, for the different sampling campaigns.

Figure 6 .

 6 Figure 6. Eulerian time-series data of the mangrove creek. The shaded area symbolizes the nighttime sampling.

Figure 7 .

 7 Figure 7. Scatter plots between a) TA or DIC and salinity, b) TA and DO, c) δ 13 C-DIC and DO, d) δ 13 C-DIC and pCO2 in the mangrove creek throughout the 20 h cycle on Mar 18. Red dots characterize hypoxic condition with DOsat% <25% observed in early morning and low tide.

Figure 8 .

 8 Figure 8. Graph a shows the relationship between the excess dissolved inorganic carbon (E-DIC) and apparent oxygen utilization (AOU) in the PSRE main channel and mangrove creek for the three sampling campaigns. Green dots are the stations in the mangrove creek and blue dots are the stations in the main estuarine channel. The 1:1 line represents the quotient between CO2 and O2 during the processes of photosynthesis and respiration. Graph b) shows the deviations from conservative mixing

Figure 9 .

 9 Figure 9. Comparison between mixing model and pCO2 measured in the mixing zone, including corrections performed for gas exchange and thermal variability and deviations from the conservative mixing. Graphs a), c) and e) show the means and standard deviations of pCO2 measured (black dots),

Figure 10 .

 10 Figure 10. Deviations from conservative mixing lines of stable isotope composition of DIC (Δδ 13 C-DIC) as a function of DIC (ΔDIC) for the main channel. Different colors represent the sampling campaigns. The origin represents the conservative mixing with sample values equal to the ocean endmember value. The four quadrants (PP/OG, CP, DC, and CD) in the graph indicate the processes that could influence the DIC and δ 13 C-DIC distributions. Quadrant PP/OG represents the primary production/outgassing of CO2, when DIC concentrations increase and δ 13 C-DIC values decrease. Quadrant CP represents the carbonate precipitation, when DIC concentrations and δ 13 C-DIC values decrease. Quadrant DC represents the degradation of organic carbon, when DIC concentrations increase and δ 13 C-DIC decrease. Quadrant CD represents the carbonate dissolution, when the DIC concentrations and δ 13 C-DIC values increase. The red vectors represent the slopes of specific processes affecting the DIC and δ 13 C-DIC distributions: primary production (slope = -24.0‰), CO2

Figure 1

 1 Figure 1

Table 2

 2

and Figures

6 and 7

. We sampled one ebb tide at daytime followed by one flood tide at nighttime and one ebb tide at nighttime. Tidal amplitude was 0.42 m for the first ebb phase (from ~12:00 pm to 06:30 pm) and 0.64 m for the second ebb phase (from ~00:30 am to 07:30 am). For the first tidal cycle, no statistically significant difference was observed between the ebb and flood tides (Mann Whitney Test; p>0.05); however, the second ebb tide (nighttime) presented higher salinity values than those found in the other sampled periods (Mann Whitney Test; p<0.01), following an atypical pattern. Daytime sampling presented higher average temperature (30.0 ±0.3 °C) than nighttime sampling (28.3 ±0.3 °C).

In the mangrove creek, pCO2 values varied drastically along the diurnal time scale, with means of 7,439 ±1,390 ppmv during daytime ebb tide, 11,191 ±2,264 ppmv during nighttime flood tide, and 18,276 ±3,408 ppmv during nighttime ebb tide (Table

2

). The maximum pCO2 value (21,720 ppmv) occurred during nighttime ebb tide, whereas the minimum pCO2 value (5,318 ppmv) occurred during daytime ebb tide. The highest pCO2 values during nighttime ebb tide were coincident with the highest TA, DIC and salinity concentrations and the lowest pH and δ 13 C-DIC values

(Figs. 6

Table 2

 2 Results of the anchored station at the mangrove creek showing the diurnal variability (mean, standard deviation, minimum and maximum) for the principal physicochemical properties k, separated by time of sampling (ebb tide daytime, flood tide nighttime, ebb tide nighttime).

		Ebb tide	Flood tide	Ebb tide
		daytime	nighttime	nighttime
	Temperature (°C)	30.3 ± 0.3	29.0 ± 0.7	27.6 ± 0.03
		28.8 -31.0	28.1 -30.0	27.6 -27.7
	Salinity	0.38 ± 0.00	0.41 ± 0.02	0.72 ± 0.07
		0.38 -0.42	0.39 -0.44	0.58 -0.80
	pCO2 (ppmv)	7,439 ± 1,390	11,191 ± 2,264	18,276 ± 3,408
		5,318 -11,600	7,366 -13,919	6,358 -21,720
	DIC (µmol kg-1)	1,105 ± 32	1,259 ± 77	2,213 ± 210
		1,071 -1,143	1,103 -1,335	1,791 -2,407
	TA (µmol kg-1)	860 ± 36	931 ± 34	1,582 ± 169
		805 -912	885 -981	1,246 -1,749
	pH (NBS)	7.06 ± 0.06	6.92 ± 0.10	6.89 ± 0.03
		6.84 -7.61	6.81 -7.07	6.80 -6.93
	δ 13 C-DIC (‰)	-4.10 ± 0.95	-5.99 ± 1.12	-8.57 ± 0.15
		-6.00 to -3.27	-7.65 to -4.45	-8.79 to -8.28
	Chl a (µmol kg-1)	66.7 ± 4.0	19.0 ± 3.4	-
		61.3 -72.1	15.5 -22.4
	DO (%sat)	59.8 ± 10.8	39.2 ± 14.2	4.9 ± 4.3
		5.2 -80.3	23.9 -62.0	0.2 -13.1

Table 4

 4 Net Community Production (NCP) estimated for the mixing zone. The NCP was estimated as the difference between (ΔFCO2), (ΔFCO2mixing corrected) and (ΔFCO2cooling/heating). FCO2, FCO2mixing corrected and FCO2cooling/heating were calculated with the values of pCO2measured, pCO2mixing corrected and pCO2cooling/heating. See the section 3.55 in material and methods and 4.3 in discussion for further details.

	Mixing Zone

Acknowledgments

The authors are grateful for the support from the Laboratory of Environmental Sciences and to the Graduate Program in Ecology and Natural Resources of the State University of Norte Fluminense Darcy Ribeiro. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Brazil (CAPES)-Finance Code 001, and by the Carlos Chagas Foundation for Research Support of the State of Rio de Janeiro (FAPERJ; proc. no. E-26/202.785/2016). Luiz C. Cotovicz Jr. is a postdoctoral researcher of the Carlos Chagas Foundation for Research Support of the State of Rio de Janeiro (FAPERJ; proc. no. E-26202.785/2016). Carlos E. Rezende thanks the financial support of CNPq (305217/2017-8) and FAPERJ (E-26/202.916/2017 and 210.883/2016). B.A. Knoppers is a senior scientist of CNPq (proc. no. 301572/2010-0). Luciana O. Vidal is a postdoctoral researcher of the Foundation for the Coordination of Higher Education and Graduate Training (CAPES; proc. no. 88882.314551/2019-01). This research was also conducted with the support of the International Research Project VELITROP (Vulnerability of littoral tropical ecosystems to eutrophication) funded by the French Institute of Environment and Ecology INEE-CNRS.

Table 1 Mean (±standard deviation), minimum and maximum values of the principal physicochemical properties on the waters of the PSRE, separated by sampling campaigns and estuarine zones.