Luiz C Cotovicz

Bastiaan A Knoppers

Carolina R Régis

Daniel Tremmel

; Suzan Costa-Santos

Gwenaël Abril

Renan Cardoso

Marcelo Muniz

Eutrophication overcoming carbonate precipitation in a tropical hypersaline coastal lagoon acting as a CO2 sink (Araruama Lagoon, SE Brazil)

Keywords: carbonate chemistry, coastal eutrophication, hypersaline waters, climate change, atmospheric CO2 sink

The carbonate chemistry was investigated in the semiarid eutrophic Araruama Lagoon (Brazil), one of the largest hypersaline coastal lagoons in the world. Spatial surveys during winter and summer periods were performed, in addition to a diurnal sampling in summer. The hypersaline waters have higher concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) than the seawater that feed the lagoon, due to evaporation. However, TA and DIC concentrations were lower than those expected from evaporation. Calcium carbonate (CaCO3) precipitation partially explained these deficits. The negative correlation between the partial pressure of CO2 (pCO2) and chlorophyll a (Chl a) indicated that DIC was also consumed by primary producers. The uptake by photosynthesis contributes to 57-63% of DIC deviation from evaporation, the remaining credited to CaCO3 precipitation. Marked pCO2 undersaturation was prevalent at the innermost region with shallow, confined, and phytoplankton-dominated waters, with a strong enrichment of heavier carbon isotope (δ 13 C-DIC up to 5.55‰), and highest pH. Oversaturation was restricted to an urbanized region, and during night-time. The lagoon behaved as a marked CO2 sink during winter (-15.32 to -10.15 mmolC m -2 d -1 ), a moderate sink during summer (-5.50 to -4.67 mmolC m -2 d -1 ), with a net community production (NCP) of 93.7 mmolC m -2 d -1 and prevalence of net autotrophic metabolism. A decoupling between CO2 and O2 exchange rate at the air-water interface was attributed to differences in gas solubility, and high buffering capacity. The carbonate chemistry reveals simultaneous and antagonistic actions of CaCO3 precipitation and autotrophic metabolism on CO2 fluxes, and could reflect future conditions in populated and semiarid coastal ecosystems worldwide.

Introduction

Carbon is continuously displaced laterally and vertically along the land-ocean aquatic continuum comprising freshwaters, estuaries, and coastal areas [START_REF] Regnier | Anthropogenic perturbation of the carbon fluxes from land to ocean[END_REF]. Compared to the open ocean, the coastal ocean represents an important carbon sink [START_REF] Cao | Diagnosis of CO2 dynamics and fluxes in global coastal oceans[END_REF]) and plays a disproportionate role despite its modest surface area [START_REF] Gattuso | Carbon and carbonate metabolism in coastal aquatic ecosystems[END_REF]. According to the latest global estimates, estuaries emit ~0.10 GtC yr -1 of CO2 to the atmosphere [START_REF] Chen | 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], whereas continental shelf waters absorb ~0.20 GtC yr -1 [START_REF] Roobaert | The spatiotemporal dynamics of the sources and sinks of CO2 in the global coastal ocean[END_REF]); however, these fluxes are still highly uncertain owing to the complexity of processes involved in these air-sea CO2 fluxes [START_REF] Borges | Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?[END_REF]. One of the main issues to be solved is the intrinsic spatiotemporal variability related to the heterogeneity of coastal ecosystems in different climatic domains [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. In addition, important temporal variabilities at the tidal, diurnal, seasonal, annual and inter-annual time scales should be addressed [START_REF] Borges | Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?[END_REF]. Furthermore, anthropogenic impacts of global change and land and water uses have increased in coastal zones and have modified the biogeochemical cycles, particularly through nutrient pollution and eutrophication [START_REF] Howarth | Coupled biogeochemical cycles: eutrophication and hypoxia in temperate estuaries and coastal marine ecosystems[END_REF]. Coastal lagoons cover about 13% of the coastal areas [START_REF] Kjerfve | Coastal Lagoons[END_REF]. They are very productive ecosystems, sustaining high rates of benthic and/or planktonic primary production (Knoppers et al. 1999;[START_REF] Maher | Carbon budgets for three autotrophic Australian estuaries: Implications for global estimates of the coastal air-water CO2 flux[END_REF]. Coastal lagoons present ideal conditions for human settlement, hosting ~14.4 % of the global population [START_REF] Kjerfve | Coastal Lagoons[END_REF]), but suffering the increasing symptoms of demographic expansions mainly via urbanization and sewage discharge [START_REF] Knoppers | Coastal lagoons of Southeastern Brazil: Physical and biogeochemical characteristics[END_REF]. Few studies have investigated carbonate chemistry dynamics and air-water CO2 fluxes in coastal lagoons. Findings in tropical and subtropical coastal lagoons indicate that they can act both as net CO2 sources or sinks depending on hydrological, chemical, and biological characteristics which are affected by natural and anthropogenic-derived disturbances, especially the inputs of nutrients and dissolved carbon from the watershed [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Ribas-Ribas | Effects of upwelling, tides and biological processes on the inorganic carbon system of a coastal lagoon in Baja California[END_REF][START_REF] Mc | Air-Water CO2 fluxes and net ecosystem production changes in a Baja California coastal lagoon during the anomalous North Pacific warm condition[END_REF][START_REF] Maher | Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon[END_REF]. Furthermore, [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF] postulated that physical settings (mixed versus stratified water column) play an important role modulating air-water CO2 fluxes.

In productive coastal waters, the carbonate chemistry is strongly influenced by the processes of primary production and microbial respiration of organic matter [START_REF] Maher | Carbon budgets for three autotrophic Australian estuaries: Implications for global estimates of the coastal air-water CO2 flux[END_REF][START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Maher | Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon[END_REF]. In estuarine and coastal ecosystems, the acid-base properties of waters are driven by ecosystem primary production and respiration, CaCO3 precipitation, CaCO3 dissolution, air-water CO2 fluxes and thermodynamic equilibration during mixing of freshwater with seawater [START_REF] Cai | Natural and anthropogenic drivers of acidification in large estuaries[END_REF][START_REF] Abril | Thermodynamic uptake of atmospheric CO2 in the oligotrophic and semiarid São Francisco estuary (NE Brazil)[END_REF]. Ecosystems that are net sinks of CO2 are generally autotrophic (gross primary production, GPP > community respiration, CR), and ecosystems that are net sources of CO2 are generally heterotrophic (GPP < CR) [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF].

However, many exceptions exist where autotrophic systems are CO2 sources and heterotrophic systems are CO2 sinks [START_REF] Borges | Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean?[END_REF]Cotovicz et al. 2020;[START_REF] Abril | Thermodynamic uptake of atmospheric CO2 in the oligotrophic and semiarid São Francisco estuary (NE Brazil)[END_REF]). In addition, the diurnal variability of carbonate chemistry parameters and associated air-water CO2 fluxes are significant in productive ecosystem [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]. Estuarine systems and near-shore coastal waters are considered as significant sources of CO2 to the atmosphere due to their heterotrophic character and inputs of CO2-enriched waters from rivers and intertidal vegetated areas [START_REF] Cai | Oxygen and carbon dioxide mass balance for the estuarineintertidal marsh complex of five rivers in the southeastern U[END_REF][START_REF] Cai | Acidification of subsurface coastal waters enhanced by eutrophication[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. However, net uptake of atmospheric CO2 has been found in coastal ecosystem located in tropical [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[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] and subtropical [START_REF] Maher | Carbon budgets for three autotrophic Australian estuaries: Implications for global estimates of the coastal air-water CO2 flux[END_REF][START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF] regions. The sink behavior is often associated to humaninduced coastal eutrophication stimulating high levels of phytoplanktonic primary production [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]. Capture of atmospheric CO2 also occurs in coastal lagoons as the result of vertical haline stratification that favors phytoplankton development [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF]. However, the enhancement of primary production by eutrophication is associated to a concomitant enhancement of respiratory processes and emissions of CO2 to the atmosphere [START_REF] Cai | Acidification of subsurface coastal waters enhanced by eutrophication[END_REF]. These processes can be tracked by analyzing the stable isotope signatures of the DIC (δ 13 C-DIC) due to the preferential uptake of the lighter stable carbon isotope ( 12 C) during photosynthesis, turning the residual DIC enriched in the heavier carbon isotope ( 13 C) [START_REF] Mook | Environmental Isotopes in the Hydrological Cycle: Principles and applications[END_REF]. For that reason, the δ 13 C-DIC of phytoplankton-dominated coastal waters show very positive values [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]2020). The ecosystem-level responses of carbonate chemistry in face to eutrophication are antagonists and site-specific exerting both positive and negative feedbacks on the process of coastal and ocean acidification by increasing or decreasing the aquatic levels of CO2.

Among tropical lagoons, hypersalinity is a common characteristic in arid and semiarid regions [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The aridity is increasing worldwide as the result of climate change [START_REF] Huang | Dryland climate change: Recent progress and challenges[END_REF]. Observations and modelling of future scenarios suggest an increase of evaporation compared to precipitation in arid/semiarid regions; in other words, arid and semiarid regions tend to be larger and drier [START_REF] Chou | Evaluating the rich-get-richer mechanism in tropical precipitation change under global warming[END_REF][START_REF] Huang | Dryland climate change: Recent progress and challenges[END_REF]. Currently, semiarid regions cover ~15 % of the Earth's continental surface [START_REF] Safriel | Dryland systems[END_REF].

However, the area of global drylands will expand ~10 % by 2100, from inland regions to coastal zones [START_REF] Feng | Expansion of global drylands under a warming climate[END_REF]. In such ecosystems, the water balance during dry months is normally negative, when the water loss by evaporation exceeds the water inputs by rainfall and riverine contribution. During evaporation of seawater, the concentration of dissolved salts increases in the remaining water, which become hypersaline (some authors call this remaining aqueous solution as "brines"; Babel and Schreiber, 2014). The negative water balance has the potential to change the carbonate chemistry, increasing the concentrations of TA and DIC compared to the initial concentration of seawater [START_REF] Golan | Controls on the pH of hyper-saline lakes -A lesson from the Dead Sea[END_REF][START_REF] Yao | Responses of carbonate system and CO2 flux to extended drought and intense flooding in a semiarid subtropical estuary[END_REF][START_REF] Mccutcheon | Characteristics of the Carbonate System in a Semiarid Estuary that Experiences Summertime Hypoxia[END_REF]. If the weather conditions remain dry and warm and evaporation continues at high rates, salts can precipitate, CaCO3 being the first salt to form (Babel and Schreiber 2014). The precipitation of CaCO3 releases CO2 in the water, while TA and DIC are consumed in a ratio of 2:1 for every mole of CaCO3 precipitated (Dickson 2010). To the contrary, CaCO3 dissolution can occur in areas of organic matter degradation, absorbing CO2 from the water and releasing TA and DIC [START_REF] Abril | Carbonate dissolution in the turbid and eutrophic Loire estuary[END_REF]. The CaCO3 precipitation consumes DIC from the water in the form of carbonate (CO3 2-), which is the DIC form that has the heavier carbon stable isotope signature, turning the remaining DIC pool slightly depleted in 13 C relative to the initial seawater [START_REF] Zhang | Carbon isotope fractionation during gas water exchange and dissolution of CO2[END_REF][START_REF] Isaji | Biological and physical modification of carbonate system parameters along the salinity gradient in shallow hypersaline solar salterns in Trapani, Italy[END_REF].

These increases in TA and DIC concentrations affect the saturation state of CaCO3 minerals (Ω), the buffering capacity of the ecosystem, and the air-water CO2 fluxes [START_REF] Frankignoulle | A complete set of buffer factors for acid/base CO2 system in seawater[END_REF][START_REF] Frankignoulle | Marine calcification as a source of carbon dioxide: Positive feedback of increasing atmospheric CO2[END_REF][START_REF] Egleston | Revelle revisited: buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity[END_REF]. Hypersalinity may also lead to biological changes in the ecosystem in terms of abundance and diversity (Breaux et al., 2009) and, consequently, changes in the carbonate chemistry. Furthermore, the ongoing long-term sea-level rise is expected to have a significant impact on the water chemistry of coastal lagoons in some regions [START_REF] Atwood | Coastal lagoons and climate change: ecological and social ramifications in U.S. Atlantic and Gulf coast ecosystems[END_REF], which will interact with the impacts of increasing eutrophication and aridity in a near future.

As noted above, eutrophication and hypersalinity drive the carbonate chemistry in specific pathways. The primary productivity consumes DIC (and CO2) from the water, and makes the δ 13 C-DIC signatures heavier. In contrast, the CaCO3 precipitation also consumes DIC (but releases CO2), and this process makes the δ 13 C-DIC lighter. To our best knowledge, the effects of eutrophication on carbonate chemistry of hypersaline coastal waters are almost undocumented. Araruama Lagoon, located at the Southeast coast of Brazil, is one of the largest hypersaline coastal lagoon in the world [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The system has been suffering from increasing anthropogenic disturbances along and within the watershed, especially urbanization receiving high amounts of domestic effluents. Here we report on carbonate chemistry in the Araruama Lagoon, were both processes of primary production and CaCO3 precipitation are intense and competing. We investigated the main controls on the air-water CO2 fluxes and carbonate chemistry considering the spatial and temporal variabilities.

Material and Methods

Study Area

The Araruama Lagoon (Fig. 1), located in the state of Rio de Janeiro (Brazil), is one of the largest permanent hypersaline lagoon in the world, covering 220 km 2 , and composed of seven elliptical-shaped cells [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF][START_REF] Knoppers | In situ measurements of n benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE, Brazil[END_REF]. The average depth is 3m, with a maximum of 17m in the unique channel that connects the bay to the ocean. The lagoon is 40km long and 13 km wide at maximum, with a total water volume of about 0.62 km 3 [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The climate in the region is semiarid, with a mean annual rainfall of 800 mm (Barbiére and Monteiro1974). The lagoon exhibits an excess of evaporation compared to precipitation and drainage from its very small freshwater basin. The lagoon waters are hypersaline [START_REF] Barbiére | Ritmo climático e extração do sal em Cabo Frio[END_REF][START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. Historical data indicate that salinity in the lagoon is on average 52 and oscillates seasonally in response to the balance between evaporation and precipitation [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The wind velocities in the region are high, with annual averages > 6 m s -1 [START_REF] Amarante | Atlas Eólico, Estado do Rio de Janeiro[END_REF]. The flow of most incoming rivers is quite low (except during episodes of local heavy rain), with a mean annual surface runoff estimated at about 1 m 3 s -1 [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. Owing to the restricted connection with the open ocean, the time necessary to renew 50% of the water in the lagoon with seawater is estimated at about 83 days [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF].

In the 70's-80' years, the embayment was classified as oligotrophic [START_REF] Knoppers | In situ measurements of n benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE, Brazil[END_REF]. At that time, studies have documented blue and crystalline waters with healthy beaches, mangroves, sandbanks and dune areas [START_REF] Bertucci | Tourism and urbanization: environmental problems of the Araruama Lagoon, state of Rio de Janeiro, Brazil[END_REF]. The primary production was dominated by microphytobenthos, which greatly exceeded the planktonic primary production [START_REF] Knoppers | In situ measurements of n benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE, Brazil[END_REF]. However, intensive urbanization in the 80's and 90's, with absence or inefficient wastewater treatment, have led to a dramatic increase in organic matter and nutrients concentrations in the lagoon waters and sediments [START_REF] Moreira-Turcq | Impact of a low salinity year on the metabolism of a hypersaline coastal lagoon (Brazil)[END_REF][START_REF] Souza | Nutrient budgets and trophic state in a hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. In addition, the settlement of an industry of carbonates extraction in the lagoon (Anomalocardia brasiliana shells for calcareous manufacture) in the 60's has increased the sediment remobilization [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF][START_REF] Bertucci | Tourism and urbanization: environmental problems of the Araruama Lagoon, state of Rio de Janeiro, Brazil[END_REF].

This industry was decommissioned in 2006. All these factors have contributed to environmental degradation, turning the waters turbid, with an increase of planktonic primary production and a shift from oligotrophic to mesotrophic / eutrophic conditions (Knoppers et al. 1999;[START_REF] Souza | Nutrient budgets and trophic state in a hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF][START_REF] Laut | Organic matter compounds as a tool for trophic state characterization in a hypersaline environment: Araruama Lagoon, Brazil[END_REF]).

Sampling Strategy

Two sampling campaigns were conducted in the lagoon, in winter (from 17 to 20 of July-2017) and summer (from 19 to 22 of February-2018) conditions. The sampling consisted in spatial surveys covering the major area of the lagoon using a medium-size boat, with continuous real-time measurements and discrete sampling (Fig. 1).

Continuous measurements included partial pressure of CO2 in the water (pCO2), pH, salinity, temperature, dissolved oxygen (DO) concentrations and geographical position. Discrete sampling of surface waters was performed for determination of TA, stable isotopic composition of DIC (δ 13 C-DIC), and photosynthetic pigments (chlorophyll a and pheopigments). During the sampling of February-2018, we also performed a diurnal sampling at an anchored station located at the central region of the lagoon (Fig. 1), also with continuous measurements and hourly discrete sampling during 22 hours. We could not achieve a complete 24 hours of diurnal sampling due to logistical limitations.

Continuous measurements

The continuous measurement system allows real-time (every minute) record of surface water pCO2 and ancillary parameters [START_REF] Frankignoulle | A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments[END_REF]. A water pump positioned at depth of ~0.2 m supplies water continuously to an equilibrator system at a rate of ~3L min -1 . The equilibrator consists of an acrylic cylinder (diameter of 8 cm, height of 100 cm), where the water flows from top to down. Marbles inside the equilibrator promotes an increase in the surface area, which reduces the equilibration time between gaseous and aquatic phases. A closed air circulation flows from the bottom to the top of the equilibrator at a rate of ~ 1 L min -1 . The air passes through a desiccant, and then the pCO2 in the dry air is measured by a non-dispersive infrared gas analyzer (NDIR; LICOR® 820 type). Before each field cruise, the LICOR® was calibrated using three gas mixture standards (pCO2 of 410, 1007, and 5035 ppmv; White Martins Certified Material, RJ, Brazil). N2 was used to set the zero, and the standard of 410 ppmv to set the span. The standards of 1007 and 5035 ppmv were used to check for linearity. The precision and accuracy of pCO2 measurements were estimated at about ±3 and ±5 ppmv, respectively. The system is ideal for analyses of pCO2 in coastal waters because the equilibration between air and water phases is promoted in about 3 minutes [START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquatica 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]. Another part of the water flow (~1L min -1 ) was directed to a home-made acrylic chamber where the ancillary parameters were continuously measured with probes. Water temperature, salinity and DO concentrations were measured with a calibrated YSI® 6600V2 multiparameter sonde inserted in the acrylic chamber. The salinity probe (0.01 of resolution) was calibrated before the cruise with IAPSO standard seawater and deionized water; we checked the linearity of the sensor along the entire 35-70 salinity range, by comparing the values observed in situ with the probe with those measured in the laboratory on five discrete water samples using a refractometer (slope = 1 with R 2 = 0.95). The DO probe (optical optode) was calibrated every day of measurements through a 1-point calibration in watersaturated air. The accuracy was estimated at about ± 0.2 mg L -1 or 2% of reading. The pH was determined continuously by the potentiometric method at the NBS scale (pHNBS), with a glass electrode sentix 41 connected to a WTW® 3310 pHmeter. The calibration of the pHmeter was made prior to and after each sampling campaign, using three NBS traceable buffer solutions: pH 4.00, pH 7.01 and pH 10.00 (WTW® buffers). NBS refers to the National Bureau of Standards (now called NIST, National Institute of Standards and Technology).

The reproducibility of pH measurements was typically better than 0.01. The geographical position was determined with a GPG model Garmin® 60CSx. All parameters measured continuously recorded the data at a frequency of 1 minute.

Discrete sampling and laboratory analysis

Surface waters were sampled at a depth of approximately 0.2 m with a Van Dorn bottle (Alfakit®, 2L). Water was filtered through GF/F Whatman® filters. The filters were used for the analysis of photosynthetic pigments, and the filtrate was used for the titration of TA. The filters were frozen at -20°C until chlorophyll a (Chl a) and pheopigments analysis were done few days after the cruise. In the lab, filters were inserted in assay tubes containing 90% acetone and kept for 20 hours on the dark at 4°C. The pigments were quantified spectrophotometrically before and after acidification of the samples, with formulations and corrections proposed by [START_REF] Lorenzen | Determination of chlorophyll and pheo-pigments: spectrophomometric equations[END_REF]. The total pigments content was considered as the sum between Chl a and pheo-pigments.

The analyses of TA were carried out with an automatic titration system (Mettler Toledo® T5) using HCl 0.1N.

The equivalent point was calculated by linearizing the Gran function [START_REF] Gran | Determination of the equivalence point in potentiometric titrations-Part II[END_REF]. Measurements were compared to certified reference material (CRM, provided by A. G. Dickson from Scripps Institution of Oceanography) and corrected according. The precision and accuracy of the measurements were estimated at ±3 and ±5 µmol kg -1 , respectively.

The analytical procedures for δ 13 C-DIC are described in [START_REF] Gillikin | Determination of δ 18 O of water and δ 13 C of dissolved inorganic carbon using a simple modification of an elemental analyzer-isotope ratio mass spectrometer: an evaluation[END_REF]. Samples for δ 13 C-DIC were collected in 120 ml of serum glass bottles. 100 µL of saturated mercuric chloride was added to the samples to inhibit microbial activity. In the laboratory, a headspace of 40 mL was created by adding helium gas. After creating the headspace, samples were acidified adding H3PO4 to convert all inorganic carbon to CO2. Samples were shaken and let stabilize at room temperature for 5h. δ 13 C of CO2 in the headspace was determined with an isotopic ratio mass spectrometer (IRMS, Micromass IsoPrime), equipped with a manual gas injection port. The δ 13 C-DIC 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]. δ 13 C-DIC measurements were calibrated against certified standard (C2, -8.25 ‰). The reproducibility of the analysis was approximately 0.2‰. The δ 13 C-DIC signatures are reported in ‰ relative to the standard Vienna Pee Dee Belemnite (V-PDB) scale.

Calculations

Carbonate System

DIC was calculated from the pair TA-pCO2, at in situ seawater 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]. The dissociation constants for carbonic acid were those 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 -ion from [START_REF] Dickson | Thermodynamics of the dissociation of boric acid in synthetic seawater from 273[END_REF], and the solubility coefficient of CO2 from [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. The dissociation constants of carbonic acid in natural seawater are available for limited ranges of temperature (0-50 °C), and salinities (0-50) [START_REF] Chen | The marine carbonate system of the Arctic Ocean: Assessment of internal consistency and sampling considerations, summer 2010[END_REF]. The upper values of the salinity ranges measured in Araruama Lagoon (39.0-56.5 in winter; 33.7-65.9 in summer) are above this limit. We extrapolated linearly the values of these constants as a function of salinity, in accordance with findings in other evaporative environments with similar salinity ranges where the dissociation constants were measured [START_REF] Sass | The carbonate system in hypersaline solutions: Dead Sea Brines[END_REF][START_REF] Burke | Measurement of total alkalinity in hypersaline waters: values of fH[END_REF].

Air-water CO2 fluxes

Diffusive CO2 fluxes at air-water interface (FCO2) were computed according to:

FCO2 = k α ΔpCO2 (Eq. 1)
where k is the CO2 gas transfer velocity, α is the solubility of CO2 in seawater [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF] The gas transfer coefficients were normalized to a Schmidt number of 600 obtained with the three parameterizations and converted to the gas transfer velocity at in situ temperature and salinity, following the procedure of [START_REF] Jähne | On parameters influencing air-water exchange[END_REF]. Fluxes were computed for each sector of Araruama Lagoon (inner, central and outer sectors; Fig. 1), using water pCO2 values and wind conditions representative for each day of sampling campaign. The annual budget was spatially and temporally integrated. Wind speed data were provided by the National Institute for Space Research (INPE), in a meteorological station located in the Cabo Frio city (at the mouth of the lagoon). The wind velocities in the region are strong and homogeneous due to the flat relief [START_REF] Amarante | Atlas Eólico, Estado do Rio de Janeiro[END_REF].

Net Community Production (NCP)

The NCP can be calculated by considering the changes in DIC concentrations from the diel-moored station, and corrected for CaCO3 precipitation/dissolution and air-sea CO2 flux, according to the following equation:

NCP = ((DIC1-DIC2)ƿd)/Δt -((TA1-TA2) ƿd)/Δt -FCO2 (Eq. 2)
Considering the fact that we did not observe significant differences of TA concentrations at the diel timescale, the CaCO3 precipitation/dissolution was assumed to be negligible along this period of sampling. Thus, the NCP was calculated according:

NCP = ((DIC1-DIC2)ƿd)/Δt -FCO2 (Eq. 3)
where NCP is in mmol m -2 h -1 , DIC1 and DIC2 are the salinity-normalized concentration of DIC (mmol kg -1 )

during two consecutive discrete sampling with one hour of interval, ƿ is the seawater density (kg m -3 ), d is the average depth (m), Δt represents the time interval (hour) and FCO2 is the carbon dioxide flux (mmol m -2 h -1 ) across the air-water interface. The vertical density, salinity and temperature stratification were considered negligible in the lagoon [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. Values of NCP were hourly-averaged for nigh-time period (from dusk to dawn; NCPnight-time), and for daytime period (from dawn to dusk; NCPdaytime).

Evaporation carbonate chemistry model

Araruama lagoon is hypersaline and the freshwater discharge is very small compared to the entire water volume of the lagoon [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]). In addition, the time necessary to renew 50% of the water in the lagoon with seawater is estimated at about 83 days [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF], favoring evaporative processes. Taking into account these characteristics, we adapted an evaporative carbonate chemistry model from a previous mixing model used by [START_REF] Jiang | A comparative study of carbon dioxide degassing in river-and marine dominated estuaries[END_REF]. The principle is that precipitation and evaporation create no salt flux across the air-water interface. Thus, in estuaries and lagoons with low or negligible freshwater inputs, the conservations of DIC and TA are written as: DICconservative = Smeasured/Socean*DICocean (Eq. 4)

TAconservative = Smeasured/Socean*TAocean (Eq. 5)

Where Smeasured is the measured salinity, Socean the salinity of the ocean end-member, DICocean and TAocean the DIC and TA concentrations of the ocean end-member. This model adapted to estuaries assumes that DICconservative is the DIC concentration after the ocean end-member is diluted by a zero DIC freshwater. In the hypersaline lagoon, we applied this same approach for an evaporative environment, assuming that the ocean end-member is linearly concentrated by evaporation, salinity being conserved.

The deviation from evaporation path (ΔDIC) is defined as the DIC addition or loss relative to the theoretical DIC concentration during evaporation of seawater:

ΔDIC = DICmeasured -DICconservative (Eq. 6)
Where DICmeasured is the measured DIC. Deviation of TA (ΔTA) from the evaporation path can be calculated in the same way. As we could not measure the ocean end-member during the winter sampling, we use the value from [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF], which measured these parameters in an adjacent coastal region with similar salinity and water temperature and during the same month.

According to the approach of [START_REF] Yang | Treated wastewater changes the export of dissolved inorganic carbon and its isotopic composition and leads to acidification in coastal oceans[END_REF], the difference between the δ 13 C-DIC of the sample (δ 13 C-DICmeasured) and the δ 13 C-DIC of the marine end-member (δ 13 C-DICocean) represents the stable isotopic deviation from the marine end-member (Δδ 13 C-DIC), as follows:

Δδ 13 C-DIC = δ 13 C-DICmeasured -δ 13 C-DICocean (Eq. 7)

If we consider only the process of seawater evaporation, the δ 13 C-DICmeasured is equal do the δ 13 C-DICocean assuming that evaporation creates no carbon stable isotope exchanges across the air-sea interface (during evaporation of seawater the 12 C mass and 13 C masses are conserved). In addition, the relationship between ΔDIC/DICocean and Δδ 13 C-DIC are tightly linked in coastal waters with limited and/or absent river inputs [START_REF] Yang | Treated wastewater changes the export of dissolved inorganic carbon and its isotopic composition and leads to acidification in coastal oceans[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]). We used this assumption for hypersaline waters, considering the ratio ΔDIC/DICocean and the values Δδ 13 C-DIC will be null considering only the evaporation, whereas positive and/or negative deviations are attributed to CaCO3 precipitation/dissolution, photosynthesis/respiration, and/or air-water CO2 exchanges [START_REF] Samanta | Dissolved inorganic carbon (DIC) and its δ13C in the Ganga (Hooghly) River estuary, India: evidence of DIC generation via organic carbon degradation and carbonate dissolution[END_REF].

Statistical analysis

We applied the Shapiro-Wilk test to verify the normality of data distribution. As the data set showed nonparametric distributions, we used the non-paired Mann-Whitney test to verify the statistical differences between averages of two groups for spatial (differences between sectors), and temporal (seasonal, diurnal) investigations.

To compare the differences between three or more groups, we applied the Kruskal-Wallis test. Linear and nonlinear (second order polynomial) regressions were also calculated. All statistical analysis were based on α=0.05 and were performed with the GraphPad Prism 7 software.

Results

Spatial and Seasonal Variability

Air was colder during winter, with temperature averaging 19.9 ± 1.9°C, while in summer temperature was 26.1 ± 1.2 °C, with a maximum of 31.2 °C. The accumulated precipitation during 7 days before sampling was higher for the summer campaign (35 mm) than winter (1.6 mm). Water parameters analyzed in the inner, central and outer lagoon sectors, in summer and winter are shown in Table 1. The inner sector represents the most confined region of the lagoon with shallow depths (< 3 m), comprising the four westernmost "elliptical cells" (Fig. 1). This sector is the only one that receives some freshwater runoff from two rivulets (annual average of 1 m 3 s -1 ; Knoppers et al., 1999); this sector also receives the sewage effluent discharges from the Araruama city. The central sector includes the two longer and wider "elliptical cells" with a mean depth of 3 m, a maximum depth of 17 m, and the highest water volume and surface area. Due to the high surface area, this sector experiences strong northeasterly winds year-round, producing wind-driven currents and significant waves with heights frequently exceeding 1 m [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The outer sector includes the 14-km long entrance channel where the mixing between seawater and waters from the lagoon occurs. This region includes two very shallow "lagoon cells", and a narrow 4 m deep channel close to the lagoon mouth [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. This region receives direct inputs of domestic effluents from the Cabo Frio city. The wind velocities were about the double in summer (averaging 8.0 ± 2.5 m s -1 ), compared to winter (averaging 4.1 ± 2.9 m s -1 ). The water temperature followed the same seasonal tendency, with higher temperatures in summer. The highest measured water temperature was 32.0 °C, whereas the lowest was 21.2 °C, both in the inner lagoon. During winter, the spatial variability of water temperature was low, whereas in summer the spatial difference was pronounced, with a landward increase of ~4°C. Salinities were also higher in summer than in winter (Mann Whitney test; p < 0.001). In general, inner and central sectors presented similar salinities, averaging 63 in summer and 54 in winter. The outer sector showed the highest spatial variability and lowest values, due to the connection with the ocean.

The distributions of pCO2 values in the lagoon showed marked spatial and temporal variability ( Concerning the sector-averaged values, the winter sampling presented lower pCO2 compared to summer for all sectors (Mann Whitney test; p < 0.0001). The plot of pCO2 versus salinity showed a significant and inverse relationship (Fig. 2). The spatial variability of DO concentrations showed opposite trend compared to pCO2 during summer, but not during winter. The waters were more oxygenated in winter, exhibiting always oversaturated conditions. During summer, the levels of oxygen saturation decreased significantly (Mann Whitney; p < 0.0001).

In general, the correlations between pCO2 and DO were variable and changed according to the day of sampling.

In particular, we observed a drastic change in the DO versus pCO2 relationship from one day to the other, before and after a heavy storm in winter (when wind velocities exceeded 15 m s -1 ). For a similar pCO2 value, the DO concentration was significantly lower after the passage of the storm, with DO levels approaching the equilibrium with atmosphere whereas pCO2 levels remained well-below the atmospheric levels (Fig. 4). Temporally and spatially, water pCO2 was inversely correlated with Chl a concentrations (R 2 = 0.69, p < 0.0001 for winter; R 2 = 0.76, p < 0.0001 for summer; Fig. 5); however the linear regressions showed different slopes and intercepts considering the different sampling campaigns. The highest Chl a concentrations were found during summer, and at the innermost region of the lagoon (up to 151 µg L -1 ). Following an expected pattern, the values of pH were higher in summer and at innermost regions, showing significant and inverse correlations with pCO2.

Overall, the variabilities for TA, DIC and its stable isotope composition (δ 13 C-DIC) showed different spatial trends during the sampling campaigns (Table 1). During winter, TA and DIC concentrations showed lower values in the inner region (p < 0.001), averaging 2626 ± 62 µmol kg -1 and 1820 ± 117 µmol kg , 2269 µmol kg -1 , and 5.55‰, respectively. TA distributions followed a positive trend with salinity (R 2 0.90; p < 0.001) (Fig. 2a). This trend was not verified for DIC concentrations and δ 13 C-DIC, which did not vary with salinity (Fig. 2b,c). TA and DIC values were well below the calculated evaporation line, indicating a net loss of inorganic carbon in the lagoon (Fig. 2). For δ 13 C-DIC, the values were overall above this line, with highest positive deviations during summer in the inner region of the lagoon, associated with low pCO2

values, and high water temperature and Chl a concentrations (Fig. 6). In winter, all data points were located close to the evaporation line (within the analytical error of ±0.2‰). In summer, one point in the outer region was below the line, those in the central region were on the line and those in the inner region were well above. For winter and summer, the δ 13 C-DIC presented lowest signatures in the outer region when the pCO2 values were at maximum.

Air-water CO2 flux (FCO2) and ancillary parameters are shown in Table 2. Data for wind velocity and k600 were obtained for each day of sampling, and then multiplied by the sector-averaged air-water pCO2 gradient. The airwater CO2 fluxes were integrated for the entire surface area of the lagoon for winter and summer periods. The wind velocities were higher in summer compared to winter, with averages of 8.0 ± 2.5 m s -1 and 4.1 ± 2.9 m s -1 , respectively. For both sampling campaigns, peaks of wind velocity overpassed 10 m s -1 . Gas transfer velocities averaged 7.7 ± 9.0 cm h -1 in winter and 17.9 ± 8.3 cm h -1 in summer. Generally, k600 values calculated from the equation of [START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean revisited[END_REF]W14) were systematically lower than those calculated from the relationships of [START_REF] Van Dam | Parameterizing Air-Water Gas Exchange in the Shallow, Microtidal New River Estuary[END_REF]VD19) and [START_REF] Jiang | A comparative study of carbon dioxide degassing in river-and marine dominated estuaries[END_REF]J08). However, at wind velocities higher than 9 m s -1 , the parameterization of W14 gave higher k600 values than that of VD19. During winter, FCO2 exhibited always negative values, indicating a permanent sink of CO2, with fluxes ranging from -19.5 ± 2.5 mmolC m -2 d -1 at the inner sector, to -8.2 ± 1.9 mmolC m -2 d -1 at the outer sector. During summer, the inner and central regions remained a net sink of CO2, with fluxes ranging between -19.3 ± 9.8 mmolC m -2 d -1 to -1.5 ± 1.9, while the outer region was a weak source of CO2, ranging from 6.8 to 7.9 mmolC m -2 d -1 . Time-integrated CO2 fluxes, accounting for seasonal variations, revealed that the lagoon behaved as a net CO2 sink during the considered period of sampling.

Diurnal Variability

The diurnal variations of carbonate chemistry during the summer cruise are presented in Fig. 7

Discussion

Evidence for CaCO3 precipitation in the Lagoon

Araruama Lagoon is a hypersaline coastal lagoon, with a negative water balance along the year associated with low freshwater inputs and high atmospheric temperatures [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF]. The historical data shows that the salinities in the lagoon are normally in the range of 52-67 [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF], which are quite close to our measured values. The hypersaline conditions are reflected in the higher concentrations of TA and DIC in the brines of Araruama Lagoon, compared to the initial seawater. These increases of TA and DIC in the brines illustrate the general pattern of chemical evolution of seawater during the evaporation process in hypersaline systems [START_REF] Krumgalz | The study of natural hypersaline lagoons in desert areas (the Bardawil Lagoon in northern Sinai)[END_REF][START_REF] Lazar | The carbonate system in hypersaline solutions: alkalinity and CaCO3 solubility of evaporated seawater[END_REF]). Higher concentration of TA in the water increases the buffering capacity, i.e., the ability to resist changes by transferring protons [START_REF] Millero | The Marine Inorganic Carbon Cycle[END_REF]Dickson 2010;[START_REF] Middelburg | Ocean Alkalinity, Buffering and Biogeochemical Processes[END_REF]. This means that the changes in carbonate chemistry (like variability of pH, pCO2) related to additions/losses of CO2 in the ecosystem will be lower in waters with higher buffering capacity compared to waters with lower buffering capacity [START_REF] Frankignoulle | A complete set of buffer factors for acid/base CO2 system in seawater[END_REF][START_REF] Middelburg | Ocean Alkalinity, Buffering and Biogeochemical Processes[END_REF]). However, measured TA and DIC concentrations are still well below those predicted by the evaporation model (Fig. 2). TA and DIC do not behave conservatively and a net loss of TA and DIC occurs in the lagoon, creating negative deviations in the values of ΔTA and ΔDIC (Fig. 8a). Multiplying ΔTA and ΔDIC values by the water volume of the lagoon (0.618 km 3 ), dividing by the average water depth (7m) and considering the residence time of 167 days, the estimated net losses of TA are on the order of 0.08 to 0.10 gC m -2 d -1 for winter and summer, respectively, whereas for DIC these losses are 0.11 to 0.12 gC m -2 d -1 . These losses of DIC are lower than the global-averaged community CaCO3 production in coral reefs (140 to 334 gC m -2 yr 1 ), similar of CaCO3 production in seagrass meadows (30 to 125 gC m -2 yr -1 ) and well above the precipitation rates found in carbonate If the weather conditions remains dry and warm and evaporation continues at high rates, the solution evolves from undersaturated to oversaturated conditions and some solid phases will precipitate (Babel and Schreiber, 2014; [START_REF] Marion | Precipitation of solid phase calcium carbonates and their effect on application of seawater SA-T -P models[END_REF]. Carbonate minerals (aragonite, calcite, and high-magnesium calcite) are usually the salts that precipitate earliest during seawater evaporation [START_REF] Mccaffrey | The evaporation path of seawater and the coprecipitation of Brand K + with halite[END_REF][START_REF] Millero | The Marine Inorganic Carbon Cycle[END_REF][START_REF] Morse | Calcium carbonate formation and dissolution[END_REF]; Babel and Schreiber 2014). For example, CaCO3 mineral may start to precipitate at the volume reduction ratio of about 50% from the original seawater [START_REF] Isaji | Biological and physical modification of carbonate system parameters along the salinity gradient in shallow hypersaline solar salterns in Trapani, Italy[END_REF], which in the Araruama lagoon is equivalent to a salinity of ~53. For each mole of CaCO3 formed, TA decreases by two moles and DIC by one mole [START_REF] Gattuso | Measurement of community metabolism and significance in the coral reef CO2 source-sink debate[END_REF], which partially explains the deficits of TA and DIC (Figs. 2 and8a). We must also point out that the saturation state of CaCO3 (Ω) increases with temperature (Mucci 1983), and heating of water in the lagoon also favors the CaCO3 precipitation during summer conditions.

The precipitation of carbonate minerals can occurs spontaneously following the abiotic reactions of chemical thermodynamic equilibrium, or it can occurs mediated by organisms (biogenic calcification) [START_REF] Morse | Influences of T, S and pCO2 on the pseudo-homogeneous precipitation of CaCO3 from seawater: implications for whiting formation[END_REF][START_REF] Morse | Influences of T, S and pCO2 on the pseudo-homogeneous precipitation of CaCO3 from seawater: implications for whiting formation[END_REF][START_REF] Marion | Precipitation of solid phase calcium carbonates and their effect on application of seawater SA-T -P models[END_REF]. A theoretical study applying a chemical thermodynamic model (FREZCHEM) found that carbonate precipitation begins at salinity of about 73 under pseudo-homogeneous nucleation [START_REF] Marion | Precipitation of solid phase calcium carbonates and their effect on application of seawater SA-T -P models[END_REF]). However, an experimental study investigating the evaporation path of seawater at the Morton solar salt production (Great Inagua Island -Bahamas) revealed the occurrence of CaCO3 precipitation at concentrations of about 1.8 times that of seawater, equivalent to a salinity of 63 [START_REF] Mccaffrey | The evaporation path of seawater and the coprecipitation of Brand K + with halite[END_REF]. This was attributed to the heterogeneous nucleation in shallow waters that are probable to contain preexisting mineral solid phases (Mcffrey et al. 1987;[START_REF] Marion | Precipitation of solid phase calcium carbonates and their effect on application of seawater SA-T -P models[END_REF]. Accordingly, the presence of relevant carbonate and organic solid phases including planktonic micro-organisms in the water of the Araruama lagoon may favor the occurrence CaCO3 at early stages of evaporation path. Our data confirms this process in Araruama Lagoon, which is a shallow carbonate-rich and productive environment containing suspended carbonate grains in the water column as the results of intense sediment resuspension by wind and anthropogenic activities [START_REF] Kjerfve | Hydrology and salt balance in a large hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF][START_REF] Knoppers | In situ measurements of n benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE, Brazil[END_REF], creating conditions for the occurrence of high pH and spontaneous CaCO3 precipitation. Indeed, in addition to carbonate precipitation, the primary productivity is also an important driver of inorganic carbon removal from the water column (see section 4.2). It is important to point out that the high rates of primary production may contribute -together with evaporation -to rise the supersaturation of CaCO3, thus locally counter-acting the process of ocean acidification. Indeed, CaCO3 nucleation from seawater have been associated with plankton blooms at shallow depths and hypersaline environments [START_REF] Morse | Influences of T, S and pCO2 on the pseudo-homogeneous precipitation of CaCO3 from seawater: implications for whiting formation[END_REF].

Fig. 8a allows to evaluate the relative contribution of primary production and CaCO3 precipitation to the deficits of TA and DIC in the brines of Araruama Lagoon. Indeed, the slopes of ΔTA and ΔDIC are consistent with simultaneous photosynthesis and CaCO3 precipitation.

The carbonate precipitation can be written as:

Ca 2+ + 2HCO3 -à CaCO3 + CO2 + H2O (Eq. 8)
This reaction involves consumption of TA and DIC in a ratio of 2:1. The process of primary production, can be written as:

CO2 +H2O à CH2O + O2 (Eq. 9)

This process has an almost negligible effect on TA, but a strong effect on DIC. In our calculation, we consider that TA is affected only by carbonate precipitation, whereas DIC is affected both by carbonate precipitation and primary production.

The effect of carbonate precipitation on DIC concentrations (ΔDICCaCO3), was calculated as:

ΔDICCaCO3 = ΔTA / 2 (Eq. 10)
ΔTA is the deviation of TA concentration from the evaporation model, and was assumed to be influenced only by CaCO3 precipitation.

The effect of primary production on DIC concentrations (ΔDICpp) was calculated as follows:

ΔDICpp = ΔDIC -DICCaCO3 (Eq. 11)
ΔDIC is the deviation of DIC concentrations from the evaporation model. The effect of primary production and carbonate precipitation accounted for, respectively, 63% and 37% of the DIC deficits in the brines of Araruama during winter, and 57% and 43% during summer. This calculation reveals that even occurring CaCO3 precipitation (that is a net source of CO2), the lagoon remains a CO2 sink because of the photosynthesis occurring simultaneously. For the calculations exposed above, we considered that photosynthesis has a negligible effect on TA concentrations. However, in fact photosynthesis slightly increase TA (in case of preferential NO3 -assimilation) or decrease TA (in case of preferential NH4 + assimilation) [START_REF] Abril | Nitrogen -alkalinity interactions in the highly polluted Scheldt Basin (Belgium)[END_REF]) (Fig. 8a). NO3 -assimilation is probably more significant than NH4 + because NO3 - concentrations are about 10-times higher than NH4 + concentrations in the lagoon (Ramos-Régis 2020). However, during summer and under rainy conditions, the concentrations of NH4 + increase in the outer sector of the lagoon, close to a sewage outlet (Ramos-Régis 2020), and potentially changing the TA-DIC stoichiometry in this particular region.

CO2 uptake driven by enhanced biological activities and eutrophication

The eutrophication of estuaries is a widespread environmental problem and particularly enhanced in densely populated regions [START_REF] Nixon | Coastal marine eutrophication: A definition, social causes, and future concerns[END_REF][START_REF] Cloern | Phytoplankton primary production in the world's estuarine-coastal ecosystems[END_REF]. Despite the recent efforts to implement wastewater treatment plans in developing countries [START_REF] Tong | Improvement in municipal wastewater treatment alters lake nitrogen to phosphorus ratios in populated regions[END_REF], the delivery of nitrogen and phosphorus is continuing to grow with urbanization and population increase [START_REF] Larsen | Emerging solutions to the water challenges of an urbanizing world[END_REF]). This situation is more critical in the tropics where are located most low-income economies. The water quality of Araruama Lagoon reflects this tropical scenario of environmental degradation, and long-term eutrophication (Knoppers et al. 1999;[START_REF] Souza | Nutrient budgets and trophic state in a hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF][START_REF] Laut | Organic matter compounds as a tool for trophic state characterization in a hypersaline environment: Araruama Lagoon, Brazil[END_REF].

During photosynthesis, the primary producers assimilate high amounts of DIC from the water column, decreasing the levels of dissolved CO2. The uptake of CO2 is high in warm and shallow waters, and enhanced by the high availability of light and nutrients sustaining phytoplankton blooms [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], even in hypersaline environments [START_REF] Morse | Influences of T, S and pCO2 on the pseudo-homogeneous precipitation of CaCO3 from seawater: implications for whiting formation[END_REF][START_REF] Pages | Distribution of carbon in a tropical hypersaline estuary, the Casamance (Senegal, West Africa)[END_REF]. High biological production enhanced by domestic nutrients was found in different estuarine types, including large river plumes [START_REF] Borges | Distribution and air-water exchange of carbon dioxide in the Scheldt plume off the Belgian coast[END_REF][START_REF] Huang | The carbon dioxide (CO2) system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico -I: Distribution and air-sea CO2 flux[END_REF], coastal embayment's [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], and coastal lagoons [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Maher | Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon[END_REF]. The lowest value of pCO2 of 70 ppmv in Araruama Lagoon occurred within a dense bloom with the highest concentration of Chl a of 152 µg L -1 (Fig. 5). However, for the same amount of phytoplankton biomass, the values of pCO2 are higher in summer than winter. This seasonality can be attributed to four main reasons: i) during summer, the water temperature was about 8°C higher, on average, than winter, meaning that temperature alone contributes to a seasonal pCO2 increase by 80 ppmv due to water heating. A similar seasonal effect of heating was reported in the Paraiba do Sul River Estuary, located close to Araruama Lagoon (Cotovicz et al., 2020); ii) inner and central sectors presented salinities averaging 63 in summer and 54 in winter, which create an additional theoretical increase of about 45 ppmv of pCO2 in summer compared to winter due to changes in water solubility [START_REF] Millero | The Marine Inorganic Carbon Cycle[END_REF]; iii) the levels of CaCO3 calcification (ΔTA values) are higher in summer, when water salinity and levels of evaporation were highest (Fig. 8), consuming DIC from the water column but releasing CO2 (Eq. 7); iv) finally, we observed the highest pCO2 peak (up to 844 ppmv) in the outer sector in summer (Fig. 2), when the population of the city of Cabo Frio more than doubles [START_REF] Bertucci | Tourism and urbanization: environmental problems of the Araruama Lagoon, state of Rio de Janeiro, Brazil[END_REF], discharging more domestic sewage in the lagoon (Laut et al. 2016). In summary, half of the winter to summer pCO2 increase can be attributed to changes in thermodynamics due increasing temperature and salinities (~125 ppmv). The other half is attributed to the balance between changes in DIC and TA driven mainly by autochthonous processes (primary production, respiration, CaCO3 calcification) and allochthonous sources (sewage discharge).

The lowest values of pCO2 (70 ppmv) found in the brines of the lagoon are comparable to those found in eutrophic coastal ecosystems; for example, 22 ppmv in Guanabara Bay, an eutrophic coastal embayment in Brazil dominated by phytoplankton blooms [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]; 43 ppmv in the Cheasapeake Bay (USA) due to high nutrient input stimulating large phytoplankton blooms [START_REF] Chen | Seasonal and spatial variability in surface pCO2 and air-water CO2 flux in the Chesapeake Bay[END_REF]. The high primary productivity leads not only to low concentrations of dissolved CO2, but also to an increase of pH, carbonate ion (CO3 2-) concentration and the saturation state of CaCO3 minerals due to the thermodynamic equilibria of carbonate system in seawater [START_REF] Millero | The Marine Inorganic Carbon Cycle[END_REF].

The high levels of primary production are also evidenced by the highly positive δ 13 C-DIC signatures in the most shallow and phytoplankton-rich waters (inner sector). Fig. 6 corroborates this finding, as the δ 13 C-DIC signature increases with increasing Chl a concentration (Fig. 6c), water temperature (Fig. 6b), and with decreasing pCO2

(particularly during summer in shallow warm-waters; Fig. 6a). The highest phytoplankton biomass (up to151 µg L -1 ) occurred in the warmest waters (up to 32.7 °C), associated with the lowest pCO2 values and the most positive δ 13 C-DIC signatures (5.55 ‰). The positive correlations between δ 13 C-DIC, temperature and phytoplankton biomass reveals a control of isotopic DIC fractionation by phytoplankton blooms [START_REF] Van Dam | CO2 limited conditions favor cyanobacteria in a hypereutrophic lake: An empirical and theoretical stable isotope study[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]. CaCO3 precipitation also consumes DIC from the water but in the form of CO3 2-, which is the DIC form that has the heavier stable carbon isotope signature (compared to dissolved CO2 and HCO3 -) [START_REF] Zhang | Carbon isotope fractionation during gas water exchange and dissolution of CO2[END_REF], turning the remaining DIC pool depleted in 13 C relative to the seawater [START_REF] Isaji | Biological and physical modification of carbonate system parameters along the salinity gradient in shallow hypersaline solar salterns in Trapani, Italy[END_REF]). However, the impact of primary production on δ 13 C-DIC is much stronger than that of CaCO3 precipitation. Indeed, the marine phytoplankton-bicarbonate enrichment factor of stable carbon isotope (typically on the order of -22.1‰ to -35.5‰; [START_REF] Wong | Fractionation of stable carbon isotopes by marine pbytoplankton[END_REF] is much more pronounced than calcite-bicarbonate and aragonite-bicarbonate enrichment factors (on the order of 1.0‰ and 2.7‰, respectively; [START_REF] Romanek | Carbon isotopic fractionation in synthetic aragonite and calcite: Effects of temperature and precipitation rate[END_REF]). To our best knowledge, the highest δ 13 C-DIC signature measured in Araruama Lagoon (5.55‰) is the highest value reported in coastal and open waters worldwide.

Fig. 8b shows that the data points of Araruama Lagoon are located between the theoretical vectors representing the processes of primary production/outgassing of CO2 (when DIC decrease and δ 13 C-DIC increase) and CaCO3 precipitation (when both DIC and δ 13 C-DIC decrease) [START_REF] Samanta | Dissolved inorganic carbon (DIC) and its δ13C 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]. As the large majority of pCO2 values in the lagoon are below the equilibrium with the atmosphere, the effect of CO2 outgassing on δ 13 C-DIC was considered as negligible (Fig. 2). The highest positive deviation of δ 13 C-DIC signatures from the evaporation conservative model (up to 4‰) was found in phytoplankton-dominated waters, during summer (Fig. 8b). The highest negative deviation of δ 13 C-DIC signatures from mixing (up to -0.3‰) was found in low-phytoplankton waters, during winter, meaning that photosynthesis fractionates much more than CaCO3 precipitation.

The pattern of diurnal variability of pCO2 values in the lagoon, with higher values during night-time and lower values during daytime, also illustrates the influence of biological activities on short-time CO2 dynamics (Fig. 7).

Strong diel variations of CO2 concentrations have been reported in productive estuarine systems worldwide as the results of net ecosystem metabolism [START_REF] Yates | Diurnal variation of oxygen and carbonate system parameters in Tampa Bay and Florida Bay[END_REF][START_REF] Zhang | Diurnal and seasonal variations of carbonate system parameters on Luhuitou fringing reef, Sanya Bay, Hainan Island, South China Sea[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]. The biological activities modify not only the daily pCO2 variations in the water, but also the other carbonate chemistry parameters (Table 3). The diurnal variations of DIC concentrations correspond to a net community production (NCP) of 34.2 molC m -2 yr -1 in Araruama Lagoon in summer. This autotrophic metabolism inside the Araruama lagoon generates a diurnal variability of CO2 fluxes, and the sink behavior. In the 90's (1993-1995), [START_REF] Knoppers | In situ measurements of n benthic primary production, respiration and nutrient fluxes in a hypersaline coastal lagoon of SE, Brazil[END_REF] have investigated the metabolism in Araruama Lagoon and found higher rates of benthic net primary production (benthic chamber method, between 5.6 and 12.3 molC m -2 yr -1 ) compared to planktonic primary production ( 14 C-method, 0.6 molC m -2 yr -1 ), and net heterotrophy (community respiration between 12.8 and 23.9 molC m -2 yr -1 ). Within this same period (1989)(1990), but in a "low salinity year", Moreira-Turcq (2000) documented higher rates of planktonic primary production ( 14 C method, averaging 13.5 mol C m -2 yr -1 ) due to this atypical condition of lower salinities. Later, applying the LOICZ (Land Ocean Interaction in the Coastal Zone) nutrient budget calculation, [START_REF] Souza | Nutrient budgets and trophic state in a hypersaline coastal lagoon: Lagoa de Araruama, Brazil[END_REF] reported a slight net autotrophic metabolism (0.9 molC m -2 yr -1 ). These results reveal an important shift in the ecosystem metabolism along decades, from a net heterotrophic benthic-dominated metabolism in the 90's to a net autotrophic planktonic-dominated metabolism afterwards. In addition, the changes in the evaporation-precipitation balance producing lower salinity conditions during atypical years can influence the trophic state of the ecosystem and the levels of primary production [START_REF] Moreira-Turcq | Impact of a low salinity year on the metabolism of a hypersaline coastal lagoon (Brazil)[END_REF]. Our result of NCP falls in the highest range of documented values, suggesting that planktonic primary production is increasing in the lagoon as the result of eutrophication. These values are higher than those documented in other highly productive hypersaline lagoons [START_REF] Cohen | Solar Lake (Sinai) 2. Distribution of photosynthetic microorganisms and primary production[END_REF][START_REF] Javor | Hypersaline environments[END_REF]). The NCP of Araruama Lagoon was also higher than the mean annual phytoplankton primary production found in a compilation of 131 estuaries worldwide (21 molC m -2 yr -1 ; [START_REF] Cloern | Phytoplankton primary production in the world's estuarine-coastal ecosystems[END_REF]. This NCP is close to the threshold of hypertrophic classification (500 g C m -2 yr -1 ) considering the annual phytoplankton primary production proposed by [START_REF] Nixon | Coastal marine eutrophication: A definition, social causes, and future concerns[END_REF].

High oversaturated conditions of pCO2 (maximum of 844 ppmv) were found only in the channel that connects the bay to the ocean (the most urbanized region) associated with urban wastewater discharge. A recent study found the presence of high levels of organic matter from anthropic origin in this region [START_REF] Laut | Organic matter compounds as a tool for trophic state characterization in a hypersaline environment: Araruama Lagoon, Brazil[END_REF].

Mineralization of organic matter in polluted areas close to sewage networks leads to heterotrophy and CO2

outgassing [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] Ramos-Régis | Avaliação do estado trófico da Laguna Hipersalina de Araruama e do Complexo Lagunar de Jacarepaguá (Rio de Janeiro)[END_REF]. The sampling in summer was carried out during a period of high rainfall, which favored CO2 and organic carbon inputs from urban areas.

Apparent decoupling between CO2 and O2 dynamics in hypersaline and productive waters

The relationship between excess of DIC (E-DIC) and apparent oxygen utilization (AOU) illustrates the specific metabolic characteristics of Araruama Lagoon (Fig. 9). E-DIC and AOU were calculated following [START_REF] Abril | Carbonate dissolution in the turbid and eutrophic Loire estuary[END_REF] and [START_REF] Benson | The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere[END_REF], respectively. In general, values of E-DIC and AOU were negative, consistent with net autotrophy in the lagoon. However, some E-DIC and AOU values were positive during nighttime. E-DIC and AOU values in the Araruama Lagoon presented strong deviations below the 1:1 line that represents the quotient between CO2 and O2 during primary production and community respiration (Fig. 9). The deviations were particularly important when values of pCO2 are very low in the water (below 300 ppmv; Fig. 9), associate with large disequilibria of CO2 concentrations between water and atmosphere. Indeed, the cycling of these gases in hypersaline and high-productive waters are not linearly coupled. It is well established that airwater exchange is faster for O2 than for CO2, due to different solubility of the two gases and the buffering effect of the carbonate chemistry [START_REF] Garcia | Oxygen solubility in seawater: better ftting equations[END_REF][START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. The waters tend to equilibrate faster with the atmosphere for DO than for CO2 to the atmosphere because the buffering effect of bicarbonate concentration affects the CO2 concentrations, but not the O2 concentrations.

This difference between air-water exchanges for O2 and CO2 was evident comparing samplings occurred in consecutive days with marked differences in wind speed (Fig. 5). During day 18-07-2018 the wind speed was low (< 5.5 m s -1 ), with high oversaturation for O2 and undersaturation for pCO2. Overnight, the wind speed increased to velocities > 14 m s -1 , and fast gas-exchange occurred, causing DO concentrations to decrease from oversaturation to values close to atmosphere equilibrium; however, the values of pCO2 remained well below 400 ppmv, evidencing the decoupling between O2 and CO2 in these productive and well-buffered waters.

Furthermore, the occurrence of CaCO3 precipitation simultaneously causes additional deficits of DIC concentrations in the water, also contributing to lowering E-DIC values but not affecting AOU (Fig. 9).

However, during diurnal sampling, the data distribution of E-DIC and AOU were located close to the line 1:1, suggesting that at that timescale, primary production and total respiration (autotrophic and heterotrophic) were coupled, and largely dominated diel variations, as verified in other eutrophic and phytoplankton-dominated ecosystem [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF].

Atmospheric CO2 fluxes and net carbon sink in the lagoon

Araruama Lagoon is a coastal lagoon located in a region characterized by strong winds [START_REF] Barbiére | Ritmo climático e extração do sal em Cabo Frio[END_REF][START_REF] Amarante | Atlas Eólico, Estado do Rio de Janeiro[END_REF]. The k-wind speed parameterization is one of the most critical parameters to compute the air-water CO2 fluxes from the gradient between pCO2 in 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]. Several formulations of k-wind parameterizations are available in order to account for the main driving forces of turbulence at the air-sea interface, and applying three k600 formulations allows to provide credible ranges of FCO2 estimates [START_REF] Jiang | A comparative study of carbon dioxide degassing in river-and marine dominated estuaries[END_REF][START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean revisited[END_REF][START_REF] Van Dam | Parameterizing Air-Water Gas Exchange in the Shallow, Microtidal New River Estuary[END_REF].

Classical river-dominated estuaries are considered net sources of CO2 to the atmosphere, with magnitude of emissions decreasing seaward [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF][START_REF] Chen | 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]. The most recent global compilation of estuarine CO2 emissions reports strong sources of CO2 at inner regions of estuaries, on the order of 38.6 mol C m -2 yr -1 , and weak sources at outer regions with salinities >25, on the order of 8.4 mol C m -2 yr -1 [START_REF] Chen | Air-sea exchanges of CO2 in the world's coastal seas[END_REF]. However, other studies in marine-dominated estuaries (coastal embayment's and lagoons)

have reported pCO2 undersaturation and net CO2 sinks annually [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Maher | Carbon budgets for three autotrophic Australian estuaries: Implications for global estimates of the coastal air-water CO2 flux[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], or seasonally [START_REF] Koné | Seasonal variability of carbon dioxide in the rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Maher | Hydrological Versus Biological Drivers of Nutrient and Carbon Dioxide Dynamics in a Coastal Lagoon[END_REF][START_REF] Yao | Hydrologic controls on CO2 chemistry and flux in subtropical lagoonal estuaries of the northwestern Gulf of Mexico[END_REF]). In addition, the coastal plumes of large rivers (Mississippi, Amazon) also behave as CO2 sinks [START_REF] Körtzinger | A significant CO2 sink in the tropical Atlantic Ocean associated with the Amazon River plume[END_REF][START_REF] Huang | The carbon dioxide (CO2) system on the Mississippi River-dominated continental shelf in the northern Gulf of Mexico -I: Distribution and air-sea CO2 flux[END_REF]. The few data existing on semiarid/arid coastal ecosystems with permanent and/or intermittent hypersaline conditions suggest that these ecosystems are net sources of CO2, for example, in the semiarid subtropical region Mission-Aransas Estuary, Gulf of Mexico (12.4 mol C m -2 yr -1 ; Yao and Hu, 2017), because heating and evaporation enhance CO2 degassing [START_REF] Yao | Hydrologic controls on CO2 chemistry and flux in subtropical lagoonal estuaries of the northwestern Gulf of Mexico[END_REF]). In the Araruama Lagoon, the high enclosure and long residence time of waters together with high availability of nutrients and light, promoted conditions for high CO2 uptake, during both winter and summer conditions (Figs. 2 and5), despite the occurrence of CaCO3 precipitation (Fig. 8). The annual sink in Araruama Lagoon ranged between -2.76 and - Lagoon is a net sink of CO2 because eutrophication is enhancing the planktonic primary production, and overcoming CaCO3 precipitation.

mol C m

Conclusions

Prevalence of negative water balance along the year (evaporation higher than precipitation), associated with low freshwaters inputs and low water renewal typical of choked lagoons, create conditions of hypersalinity in the Araruama Lagoon. The residual brines of Araruama Lagoon reflect the evaporation path of seawater. The waters of Araruama have higher concentrations of TA and DIC compared to seawater as the result of salt concentration.

However, the concentrations of TA and DIC are, respectively, approximately 39-44% and 20-28% lower than those expected from the evaporation model. Spontaneous CaCO3 precipitation is occurring in the lagoon and contributed to about 37-43% of the DIC deficits, whereas the remaining 57-63% is attributed to uptake of DIC by primary producers.

The increasing human disturbances, mainly urbanization, have raised the discharges of urban effluents in the lagoon. The high availability of nutrients and the long water residence times have contributed to the development of phytoplankton blooms, particularly at the innermost-shallow region. The vast majority of pCO2 values were well-below the atmospheric values, generating a net annual sink of CO2 as the result of net autotrophic metabolism and overwhelming the pCO2 increase driven by CaCO3 precipitation. Oversaturated conditions of pCO2 were restricted to the region closest to sewage outfall (Itajuru Channel), and during night-time. The thermodynamic effects driven by water heating and increasing salinities were also significant and partially explained the higher values of pCO2 in summer compared to winter. Overall, the brines of Araruama are dominated by phytoplankton blooms with heavier δ 13 C-DIC signatures (due to isotopic fractionation by primary production), higher values of pH and higher buffering capacity compared to seawater. The relationship between E-DIC and AOU presented values well-below the 1:1 line (quotient between photosynthesis and respiration), suggesting that air-water exchanges of CO2 and O2 are apparently decoupled due to solubility differences and high carbonate buffering capacity in these warm, hypersaline, and phytoplankton-dominated waters.

This study suggests that eutrophication locally amplifies the production of organic matter with important CO2 uptake, counter-acting the potential effects of ocean acidification by increasing the pH. Eutrophication and negative water balance (hypersalinity) are modifying the nutrient cycling, plant productivity and carbon budgets in the lagoon. The aridity is increasing worldwide as the result of global warming, whereas at same time the inputs of anthropogenic-derived nutrients have increased in many coastal regions. Therefore, the specific results found in this semiarid coastal lagoon could reflects future conditions in other tropical coastal ecosystems.

Declarations

  , and ΔpCO2 is the difference between pCO2 measured in the water and in the air. Negative values represent a sink of CO2. We used the k-wind parameterization of Jiang et al. (2008; J08) and Van Dam et al. (2019; VD09), which are coefficients specific for estuaries, and the coefficient of Wanninkhof (2014; W14), which is specific for open ocean waters.

  -1 , and higher values in the central region, averaging 2729 ± 33 µmol kg -1 and 2026 ± 40 µmol kg -1 , respectively. The δ 13 C-DIC did not exhibit this clear spatial tendency, with values ranging from 1.29‰ to 1.60‰. During summer, TA concentrations were almost constant in the inner and central regions, averaging 2971 ± 40 µmol kg -1 and 2954 ± δ 13 C-DIC signatures, spatial variability was significant, with lower values of DIC at the inner region (average of 2015 ± 180 µmol kg -1 ), coincident with highest δ 13 C-DIC signatures (average of 4.11 ± 1.09‰). Overall, considering the seasonal variability, TA, DIC concentrations and δ 13 C-DIC signatures were higher in summer compared to winter, with minimum values of 2176 µmol kg -1 , 1678 µmol kg -1 , and 1.29‰, and maximum of 3037 µmol kg-1

(

  daytime). The air-water CO2 fluxes showed emissions during nighttime, ranging from 1.59 to 3.25 mmol C m -2 d -1 , and from-3.99 to -2.05 mmolC m -2 d -1 during daytime, considering the different gas transfer velocities. A net time-integrated flux was -1.01 mmolC m -2 d -1 . The NCP was -91.4 mmol C m -2 d -1 during night-time (heterotrophy), and 278.8 mmol C m -2 d -1 during daytime (autotrophy), with a net time-integrated value of 93.7 mmolC m -2 d -1 .

  rich (3.8 to 5.3 gC m -2 yr -1 ) and poor (0.01 to 0.8 gC m -2 yr -1 ) shelves (Iglesias-Rodriguez et al. 2011; O'Mara and Dune 2019; Champenois and Borges 2021).

Fig. 1

 1 Fig.1Location of the Araruama Lagoon, Brazil. The lagoon was divided into three sectors to analyse spatial heterogeneity: IL (Inner Lagoon), CL (Central Lagoon), OL (Outer Lagoon). The blue line represents the boat track with continuous measurements (nearly the same boat route was made in winter and summer). However, note that the marine end-member outside the channel was sampled on in February. The red circles represent the discrete sampling stations. The green star represents the location of the moored station. The yellow star represents the location of meteorological station.

Fig. 2

 2 Fig. 2 Distributions of TA (a), DIC (b) δ 13 C-DIC (c), and pCO2 (d) along the salinity gradient. The blue dots represent the winter, and the green dots represent the summer. The dashed blue and green lines represent the theoretical evaporation path. The red line (graph d) represents the average values of atmospheric pCO2.

Fig. 3

 3 Fig.3 Maps showing the boat trajectories (black lines) during sampling for winter (a) and summer (f) periods.Below the map on the left side, it is presented the distributions of temperature (b), salinity (c), pCO2 (d) and O2 (f) vs. latitude, for winter. Below the map on the right side, it is presented the distributions of temperature (g), salinity (h), pCO2 (i) and O2 (j) vs. latitude, for summer. The yellow, black, and red dots represent the inner, central, and outer sectors, respectively. The black line represents the value of atmospheric pCO2 and O2.

Fig. 5

 5 Fig. 5 Relationship between pCO2 levels and Chl a concentrations, separated in winter and summer periods.

Fig. 6

 6 Fig. 6 Scatter plots of δ 13 C-DIC as a function of a) pCO2, b) water temperature, and c) Chl a concentration. Blue squares represent the sampling in winter, and green circles represent the sampling in summer.

Fig. 7

 7 Fig. 7 Time series at the mooring station showing the diurnal variation of DO concentrations and pCO2 values. The shaded area symbolizes the period of night-time measurement. The horizontal black line represents the atmospheric pCO2 and O2.

Fig. 9

 9 Fig.9Relationship between E-DIC and AOU. Blue dots are the winter stations, and green dots are the summer stations. The 1:1 line represents the quotient between CO2 and O2 during the processes of photosynthesis and respiration.

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig. 2

Fig. 3

 3 Fig. 3

Fig. 4

 4 Fig. 4

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

Fig. 7

 7 Fig. 7

Fig. 8

 8 Fig. 8

Fig. 9

 9 Fig. 9

Table 1 ;

 1 Figs. 2 and 3). The outer region revealed the highest values, averaging 225 ± 37 ppmv in winter and 460 ± 108 ppmv in summer. Following the landward direction, the pCO2 sharply decreased with intermediate values in the central sector of the bay, averaging 200 ± 16 ppmv in winter and 385 ± 24 ppmv in summer. The inner region of the lagoon presented the lowest values of pCO2, averaging 128 ± 31 ppmv in winter and 233 ± 94 ppmv in summer.

  and Table 3.

	Salinity did not present significant difference between daytime and night-time periods, suggesting moderate tidal
	mixing at the monitoring point (Mann Whitney Test; p > 0.05). Daytime water samples had higher average
	temperature (30.0 ± 0.3 °C) than nighttime samples (29.5 ± 0.3 °C), but with no statistical significance. pCO2
	values averaged 411 ± 11 ppmv during nighttime and decreased to 385 ± 25 ppmv during daytime. The
	maximum pCO2 value was 432 ppmv at dawn, whereas the minimum value was 330 ppmv after midday,

corresponding to a maximal diel amplitude of 102 ppmv. The highest values of pCO2 were coincident with highest concentrations of DIC, and lowest values of δ 13 C-DIC and pH (Table

3

). pCO2 showed strong negative correlations with DO (R 2 = 0.90; p < 0.0001), δ 13 C-DIC (R 2 = 0.80; p < 0.0001), and Chl a concentrations (R 2 = 0.75; p < 0.0001). During the diurnal cycle, δ 13 C-DIC values varied between 2.53‰ (nighttime) and 4.39‰

  -2 y -1 , similar to those reported in other mesohaline coastal lagoons, including Australian Lagoons (-0.4 to -2.0 molC m -2 yr -1 , Maher and Eyre 2012) and the Aby Lagoon in Ivory Coast (-2.7 molC m -2 yr

-1 ; Koné et al. 2009)

. A sink of CO2 has been found in other enclosed and eutrophic coastal ecosystems, particularly in embayment's, for example in Guanabara Bay (Brazil) (-9.6 to -18.3 molC m -2 yr -1 ,

[START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]

and Tokyo Bay (Japan) (-3.2 molC m -2 yr -1 ;

[START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF]

. Even if CaCO3 precipitation occurs, Araruama

Table 1 :

 1 Mean (± standard deviation) and ranges of the principal parameters investigated in the brines of Araruama Lagoon, separated by sectors and sampling campaigns.

			July-2017 (winter)		February-2018 (summer)
		Inner	Central	Outer	Inner	Central	Outer
		Lagoon	Lagoon	Lagoon	lagoon	Lagoon	Lagoon
				(channel)			(channel)
	Salinity	53.4 ± 1.5	55.0 ± 1.3	53.3 ± 2.6	64.3 ± 0.7	62.8 ± 1.8	43.84 ± 8.6
		(49.9 55.5)	(39.0 56.4)	(36.7 56.5)	(62.4 65.9)	(53.4 65.3	(33.7 64.8)
	Temperature	21.8 ± 0.2	21.8 ± 0.3	22.5 ± 0.9	31.2 ± 0.7	29.9 ± 0.8	27.4 ± 1.9
	(°C)	(21.0 22.6)	(19.7 23.3)	(19.0 24.0)	(29.8 32.7)	(27.0 32.1	(24.9 32.3)
	pH	8.69 ± 0.09	8.53 ± 0.02	8.51 ± 0.05	8.49 ± 0.16	8.28 ± 0.05	8.25 ± 0.08
	(NBS)	(8.56 8.88)	(8.42 8.61)	(8.38 8.71)	(8.30 8.84)	(7.94 8.41	(7.86 8.39)
	TA	2626 ± 62	2729 ± 33	2642 ± 111	2971 ± 40	2954 ± 57	2483 ± 285
	(µmol kg -1 )	(2536 2686)	(2660 2772)	2543 2770)	(2931 3037)	(2844 3003	(2176 2925)
	DIC	1820 ± 117	2026 ± 40	1970 ± 43	2015 ± 180	2225 ± 35	2037 ± 112
	(µmol kg -1 )	(1678 1965)	(1936 2058)	1936 2029)	(1795 2220)	(2164 2269)	(1866 2217)
	δ 13 C-DIC	1.50 ± 0.19	1.58 ± 0.12	1.67 ± 0.20	4.11 ± 1.09	2.77 ± 0.26	2.69 ± 0.40
	(‰)	(1.29 1.79)	1.41 1.78)	1.38 1.82)	(2.72 5.55)	(2.43 3.26)	1.94 3.11)
	pCO2	128 ± 31	200 ± 16	225 ± 37	233 ± 94	385 ± 24	460 ± 108
	(ppmv)	(70 182)	(152 304)	(136 406)	(92 366)	(283 435)	301 844)
	DO	123 ± 4	114 ± 6	121 ± 17	103 ± 10	97 ± 7	92 ± 10
	(%Sat)	(109 135)	(90 153)	(62 196)	(80 143)	(56 112)	61 133)
	Chl a	31.2 ± 5.2	21.4 ± 3.8	20.9 ± 7.7	75.7 ± 49.2	33.2 ± 4.7	30.3 ± 24.9
	(µg L -1 )	(25.5 37.4)	(16.3 26.6)	(14.0 30.5)	(26.4 151.9)	(23.8 38.5)	(1.0 71.3)

Table 3

 3 Comparison between night-time and daytime measurements (moored station) in summer period(February-2017) for the main parameters analyzed in this study.

		Night-time	Daytime
	Salinity	64.3 ± 0.13	64.2 ± 0.70
	Temperature (°C)	29.4 ± 0.20	30.0 ± 0.38
	DO (%Sat)	85 ± 6	96 ± 8
	pH (NBS)	8.25 ± 0.01	8.31 ± 0.02
	TA (µmol kg -1 )	2971 ± 9	2990 ± 15
	DIC (µmol kg -1 )	2253 ± 17	2211 48
	δ 13 C-DIC	2.88 ± 0.14	3.05 0.52
	pCO2 (ppmv)	411 ± 11	385 ± 25
	Chl a (µg L -1 )	32.6 ± 1.8	44.7 ± 16.5
	FCO2 (mmolC m -2 d -1 )
	W14	2.67 ± 2.59	-3.99 ± 7.01
	VD19	1.59 ± 1.54	-2.05 ± 3.74
	J08	3.25 ± 3.16	-4.45 ± 8.13
	NCP (mmol C m -2 d -1 )	-91.4	278.8

technical assistance in stable isotopes measurements. Luiz C. Cotovicz Jr. thanks the Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP) for a visiting researcher grant (No.PV2-00125-00405.01.00/21) at the Federal University of Ceara (Marine Sciences Institute /LABOMAR / PRPPG). This is a contribution to the France-Brazil International Research Project VELITROP (Vulnerability of Tropical Littoral Ecosystem facing eutrophication) funded by the CNRS-INEE-France, to the Brazilian Ocean Acidification Research Group (BrOA), the Red Latinoamericana de Acidificación del Océano (LAOCA), and Project FEEDBACKS (CAPES/PRINT/UFF Pr.88881.310184/2018-01).

Funding This work was supported by the Carlos Chagas Foundation for Research Support of the State of Rio de Janeiro (FAPERJ; proc. no. E-26/202.785/2016), and by the Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP; Proc. No. INT-00159-00009.01.00/19).

Conflicts of interests

The authors have no conflicts of interest to declare that are relevant to the content of this article.

Table 2 Summary of calculated mean values for wind speed, gas exchange coefficient (k600) and CO2 fluxes at the air-sea interface (FCO2) in each sector and the whole of Araruama Lagoon. W14 is the data calculated according to k600 of [START_REF] Wanninkhof | Relationship between wind speed and gas exchange over the ocean revisited[END_REF], VD19 is the data calculated according to k600 of [START_REF] Van Dam | Parameterizing Air-Water Gas Exchange in the Shallow, Microtidal New River Estuary[END_REF], and J08 is data calculated according to k600 of [START_REF] Jiang | A comparative study of carbon dioxide degassing in river-and marine dominated estuaries[END_REF].

4.1 ± 2.9 7.7 ± 9.0 10.2 ± 5.0 11.6 ± 9.9 -12.9 ± 1.7 -17.1 ± 2.6 -19.5 ± 2.5

Central (123 Km 2 )

4.1 ± 2.9 7.7 ± 9.0 10.2 ± 5.0 11.6 ± 9.9 -9.2 ± 0.7 -12.2 ± 1.0 -13.9 ± 1.1

Outer (32 Km 2 ) 4.1 ± 2.9 7.7 ± 9.0 10.2 ± 5.0 11.6 ± 9.9 -8.2 ± 1.9 -10.8 ± 2.5 -12.