Thaís Erbas

Aguinaldo Marques

Gwenaël Abril

A CO2 sink in a tropical coastal lagoon impacted by cultural eutrophication and upwelling

Keywords: partial pressure of CO2, autotrophy, carbon dynamics, tropical shallow lagoon

Tropical coastal lagoons are recognized as very productive ecosystems. However, very little is known on the direction and magnitude of their CO2 exchange with the atmosphere. We report on air-water CO2 fluxes and carbon concentrations in a tropical shallow lagoon impacted by cultural eutrophication. During two contrasting field surveys, the first in Nov. 2019 during dry conditions and the second in Feb. 2020 during wet summer condition and intrusion of coastal upwelling waters inside the lagoon. At both seasons, spatial in situ monitoring of temperature, salinity and partial pressure of CO2 (pCO2) reveal the presence of undersaturated pCO2 values (between 60 and 300 ppmv), uptake of atmospheric CO2 (between 0.9 and 9.2 mmol m -2 h -1 ) inside the lagoon, at salinities between 10 and 32. Temporal monitoring data at the mouth of the lagoon throughout tidal cycles revealed the entrance of cold South Atlantic Central Waters (SACW) from the Cabo Frio's upwelling during flood tides in Feb. 2020 but not in Nov. 2019. The cold SACW with temperatures down to 16°C were supersaturated in CO2 (pCO2 about 520 ppmv) and a CO2 source to the atmosphere (between 0.2 and 1.4 mmol m -2 h -1 ). During mixing of this SACW in the most marine compartment of the lagoon, the biological CO2 uptake and enhanced autotrophy overcame the effect of degassing SACW waters by heating. For both cruises, a consistent landward trend consisted in a decrease in pCO2 and dissolved inorganic carbon (DIC) mirrored by an increase of dissolved organic carbon (DOC) (between 4.6 and 19.8 mg L -1 ), particulate organic carbon (POC) (between 3.3 and 12.3 mg L -1 ), and chlorophyll a (Chl a) (between 7.6 and 217 μg L -1 ). POC, DOC and Chl a and the POC:Chl a ratio suggest that, organic carbon in the lagoon is a mixture of phytoplankton biomass produced in the lagoon, domestic wastewater loaded from multiple point sources and pulsated inputs from small rivers. Our study in the Saquarema lagoon confirms recent works in other densely populated tropical coastal ecosystems, revealing that eutrophication enhances the autotrophy of shallow tropical lagoons, making them act as efficient factories for atmospheric CO2 absorption and organic carbon storage in sediments and export to the ocean.

Introduction

The growing concern about the impact of anthropogenic carbon dioxide (CO2) emissions has intensified research of organic and inorganic carbon fluxes through and within ecosystems. The coastal environments, require particularly precise quantification as they are highly dynamic components of the global carbon cycle and budget [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][START_REF] Bauer | The changing carbon cycle of the coastal ocean[END_REF]. Although the global estuarine area is modest, these ecosystems have the capacity to emit a quantity of CO2 that substantially offsets the of CO2 sink on the continental shelves [START_REF] Borges | Do We Have Enough Pieces of the Jigsaw to Integrate CO2 Fluxes in the Coastal Ocean? Estuaries[END_REF][START_REF] Borges | Budgeting sinks and sources of CO2 in the coastal ocean: Diversity of ecosystems counts[END_REF][START_REF] Cai | Air-sea exchange of carbon dioxide in ocean margins: a provincebased synthesis[END_REF][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, owing to their strong intrinsic heterogeneity, coastal ecosystems are still poorly documented in terms of CO2 exchange, especially in the tropics. Despite their modest surface area, compared to the ocean, coastal environments such as estuaries, mangroves, lagoons and bays, play a fundamental role in marine primary production, remineralization and sedimentation of organic carbon, and global air-sea CO2 exchange [START_REF] Wollast | The coastal organic carbon cycle: fluxes, sources and sinks[END_REF][START_REF] Sabine | The Oceanic Sink for Anthropogenic CO2[END_REF][START_REF] Borges | Do We Have Enough Pieces of the Jigsaw to Integrate CO2 Fluxes in the Coastal Ocean? Estuaries[END_REF][START_REF] Cai | Estuarine and coastal ocean carbon paradox: CO2 sinks or sites of terrestrial carbon incineration?[END_REF]). In most estuaries, i.e. in semi-enclosed water bodies where freshwater mixes with seawater [START_REF] Pritchard | Observation of circulation in coastal plain estuaries[END_REF], total ecosystem respiration generally exceeds gross primary production. As a result, estuaries are generally net heterotrophic and sources of atmospheric CO2 [START_REF] Gattuso | Carbon and Carbonate Metabolism in Coastal Aquatic Ecosystems[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. This conclusion was reached on the basis of intensive works in estuaries with geographically well-defined salinity gradients. These "funnel-type estuaries" receive significant inputs of terrestrial organic matter from the rivers that fuel heterotrophic respiration, while primary production is limited by the availability of light due to the high turbidity, and despite a large availability of nutrients [START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. In general, low salinity regions are highly heterotrophic, oxygen depleted and strong CO2 sources (Frankignoulle et al. 1998), whereas at high salinity, estuarine and marine phytoplankton can grow, reduce CO2 degassing flux and, in some cases, generate uptake of atmospheric CO2 [START_REF] Körtzinger | A significant CO2 sink in the tropical Atlantic Ocean associated with the Amazon river plume[END_REF][START_REF] Jiang | A comparative study of carbon dioxide degassing in riverand marine-dominated estuaries[END_REF][START_REF] Borges | Carbon dioxide and methane dynamics in estuaries[END_REF]. Some recent works in tropical regions have reported different patterns of CO2 fluxes, with strong sinks of atmospheric CO2 induced by eutrophication and/or by thermodynamic mixing of poorly buffered freshwaters with seawater [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]2020;2021;[START_REF] Abril | Thermodynamic uptake of atmospheric CO2 in the oligotrophic and semiarid São Francisco estuary (NE Brazil)[END_REF]. Estuaries, lagoons and tropical bays have their specific characteristics, and will also present specific properties with respect of carbon cycling and CO2 fluxes, very different from those from temperate and boreal estuaries.

Tropical coastal lagoons are particularly under-sampled for CO2 fluxes [START_REF] Chen | Air-sea exchanges of CO2 in the world's coastal seas[END_REF], and only a couple of studies document the carbon cycle, carbonate chemistry and CO2 fluxes, in these types of estuaries [START_REF] Koné | Seasonal variability of carbon dioxide in the Rivers and lagoons of Ivory Coast (West Africa)[END_REF][START_REF] Ávila-López | Air-Water CO2 Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition[END_REF]. Tropical coastal lagoons are characterized by shallow depths, important intrusion of ocean water with tides, variable inputs of freshwater, and high productivity, both planktonic and benthic [START_REF] Knoppers | Coastal lagoons of southeastern Brazil: physical and biogeochemical characteristics[END_REF]. In the most developing countries, the eutrophication is generated by anthropogenic nutrient inputs from untreated wastewater from sewage and industry [START_REF] Nixon | Coastal marine eutrophication: a definition, social causes, and future concerns[END_REF]). An example is lagoons in the tropics that suffer from anthropogenic pressure, and particularly from cultural eutrophication induced by sewage loads (Knoppers et al. 1999;[START_REF] Cerda | Nutrient budgets in the Piratininga-Itaipu lagoon system (southeastern Brazil): effects of sea-exchange management[END_REF]. Eutrophication can create conditions for strong CO2 uptake, as in densely populated tropical coastal embayments [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]). In addition to biological activity, drastic changes in temperature occur in tropical lagoons, induced by the intrusion of cold marine waters and strong surface heating [START_REF] Chen | Air-sea exchanges of CO2 in the world's coastal seas[END_REF][START_REF] Cotovicz | Sources and sinks of CO2 in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing[END_REF].

These thermal variations have the potential to generate important changes in the distributions of water partial pressure of CO2 (pCO2) and CO2 fluxes at the air-sea interface [START_REF] Ávila-López | 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] 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] Cotovicz | Sources and sinks of CO2 in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing[END_REF]. The aim of this paper is to present original data of air-water CO2 fluxes and inorganic and organic carbon distribution in a small shallow lagoon in the region of Rio de Janeiro in Brazil, impacted by eutrophication and seasonally influenced by cold marine waters coming from the upwelling of Cabo Frio [START_REF] Castelão | Upwelling around Cabo Frio, Brazil: The importance of wind stress curl[END_REF]. Our study includes high spatial and temporal resolution data of pCO2 and the carbonate system, as well as organic carbon and chlorophyll concentrations, during two very contrasting periods with and without the intrusion of upwelling cold waters in the lagoon. The objectives are, first, to describe spatially and quantitatively the CO2 exchanges between the lagoon and the atmosphere; second, to relate the air-lagoon CO2 fluxes to the variations of other inorganic and organic carbon species, as well as the lateral exchanges with the ocean; third, to discuss on the light our work as well as other recent studies, how eutrophication is becoming a predominant driver of the carbon cycle and air-sea CO2 exchange in densely populated tropical coastlines.

Materials and Methods

Study Area

The Saquarema Lagoon system is located on the southeast coast of Brazil, in the city of Saquarema at 73km of Rio de Janeiro city (Fig. 1), with a population estimated of 89 thousands of inhabitants (IBGE 2020). The lagoon has a total surface area of 21.2 km², with four different compartments called Mombaça (12.6 km²), Jardim (2 km²), Boqueirão (0.6 km²) and Fora (6 km²) and is permanently connected to the sea by a sand bar [START_REF] Knoppers | Coastal lagoons of southeastern Brazil: physical and biogeochemical characteristics[END_REF]. This channel has been artificially opened to minimize impacts of pollution (located in Fora sub-lagoon). The mean depth of the lagoon is 1.2m and the drainage basin total area is 215 km² [START_REF] Oliveira | Análise da hidrodinâmica do sistema lagunar de Saquarema frente às projeções de aumento do nível médio do mar. Programa de Pós-graduação e Pesquisa de Engenharia[END_REF]. Many small rivers discharge freshwater in the lagoon. Among these rivers, the Mato Grosso River that ends in the Urussanga Lagoon (Fig. 1) is the biggest, however the value of the total freshwater discharge by all the small rivers is unknown [START_REF] Alves | Modelagem numérica como ferramenta de gestão ambiental[END_REF]. The tides in the region are microtidal and semidiurnal, averaging 0.5 m and 1.5 m in amplitude during neap tide and spring conditions, respectively [START_REF] Alves | Modelagem numérica como ferramenta de gestão ambiental[END_REF]. In spring and summer, the Fora Lagoon receives cold and nutrient rich waters of the Cabo Frio coastal upwelling of South Atlantic Central Water (SACW, T< 18.0 o C and S 34.4 -36.0). The region has a wet tropical climate, subject to orographic rains that occur most often from October to March. The prevailing winds are from the East and Northeast or winds from the south quadrant during cold fronts.

The residence time of water in the system is 56 to 58 days in the Mombaça compartment, 48 to 56 days in the Jardim compartment, 20 to 48 days in the Boqueirão compartment and from 0 to 20 days in the Fora compartment, which has a connection to the sea [START_REF] Alves | Modelagem numérica como ferramenta de gestão ambiental[END_REF]. The Saquarema lagoon system is undergoing a process of cultural eutrophication, which has been intensified by the accelerated urbanization that occurred since the 1970s and a lack of effective wastewater treatment (Knoppers et al. 1999).

Sampling Strategy

Two sampling campaigns were carried out during two contrasted climatic situations in Nov. 2019 and Feb. 2020. Both sampling corresponded to spring tide conditions (amplitudes respectively 1m and 1.1m). However, Nov. 2019 sampling occurred during the intrusion of a cold front coupled to an extratropical cyclone that enhance humidity in the region, causing strong S-SW winds and intensives rains during the preceding and sampling days. In opposite, Feb. 2020 sampling occurred during well established summer conditions, with high solar radiation and the persistence of strong northeast NE winds, generating intense upwelling of South Atlantic central waters [START_REF] Castelão | Upwelling around Cabo Frio, Brazil: The importance of wind stress curl[END_REF][START_REF] Coelho-Souza | Biophysical interactions in the Cabo Frio upwelling system, Southeastern Brazil[END_REF]. During the two campaigns, the sampling strategies consisted in spatial surveys at daytime covering the major area of the lagoon using a small boat, with continuous measurements and discrete sampling (Fig. 2). Continuous measurements included the pCO2, salinity, temperature, and position were measured in situ at a frequency of 1 min. Discrete sampling was performed along the continuous trails at 16 (in Nov. 2019) to 20 (in Feb. 2020) stations (Fig. 1).

Another sampling strategy used was the temporal sampling at a fixed point in the channel at the mouth of the lagoon (Fig. 1) for 13 hours (one tidal cycle) in Nov. 2019 and for 25 hours (two tidal cycles) in February 2020. During this temporal sampling we used the aforementioned continuous measurements and hourly discrete sampling. Surface water samples were collected at approximately the following five hours. Three rivers including the major contributor Mato Grosso River (Fig. 1) were reached by car the same day as the temporal monitoring at the mouth.

Analytical procedures

Continuous Measurements

For continuous measurements, water was pumped from ~20cm below the water surface at a flow of about 5 L min -1 . About 2 L min -1 to an equilibrator system and the rest to a flow-through customized acrylic chamber containing an YSI 6600 multiparameter probe to measure temperature and salinity.

The salinity probe was calibrated with IAPSO standard seawater and deionized water. The marble equilibrator [START_REF] Frankignoulle | A new design of equilibrator to monitor carbon dioxide in highly dynamic and turbid environments[END_REF][START_REF] Abril | In-situ measurements of dissolved gases (CH4 and CO2) in a wide range of concentrations in a tropical reservoir using an equilibrator[END_REF][START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuárioss tropicais contrastantes: o estuário eutrofizado da Baia de Guanabara (RJ) e o estuário oligotrofico do Rio São Francisco (AL)[END_REF] consisted in an acrylic tube filled with glass marbles where water flows from the top, and a closed air circuit with a flux of about (1 L min -1 ), flowing in the opposite direction from the bottom to the top. The air is dried with drierite and pumped to a non-dispersive infrared (NDIR) gas analyser (LICOR LI-820). Before and after each sampling campaigns, the NDIR was calibrated using gas standards with air pCO2 values of 410, 1007 and 5035 ppmv (White Martins Certified Material, RJ, Brazil). For calibration of the gas analyser, fresh soda lime was used to set the zero and the standard at 1007 ppmv to set the span. The air standards at 410 and 5035 ppmv were used to verify the linearity of the response. The accuracy of the pCO2 measurements is estimated at ±5 ppmv. The partial pressure of atmospheric CO2 was measured in dry air twice a day, at the start and the end of the continuous measurements.

Discrete parameters

Water samples were filtrated on a glass filtration system through pre-combusted GF/F glass fiber filters (porosity 0.7 μm). One filter was weighed prior to filtration and was then dried for suspended particulate matter (SPM) and particulate organic carbon (POC) determination. SPM was determined gravimetrically on the dried pre-weighed filters. POC content were determined after removing carbonates by adding some drops of HCl 2 N on the filters [START_REF] Etcheber | Particulate organic carbon in the estuarine turbidity maxima of the Gironde, Loire and Seine estuaries: origin and lability[END_REF]). The POC analysis was carried on a LECO C-S 125 total carbon analyser based on direct combustion in an induction furnace and infrared absorption determination of the CO2 produced. Another filter was frozen at -20 °C for further chlorophyll a (Chl a) analysis. After extraction in 90% acetone, the Chl a was quantified spectrophotometrically before and after acidification of the samples, with formulations and corrections proposed by [START_REF] Strickland | A practical handbook of seawater analysis[END_REF]. Part of the filtrated water was stored in 50 mL, acidified with 100 µL of ultrapure H3PO4 and stored in the dark until dissolved organic carbon (DOC) analysis. The DOC analysis was performed with a high temperature catalytic oxidation (HTCO) method using a Shimadzu TOC 500 analyser [START_REF] Abril | Behaviour of organic carbon in nine contrasting European estuaries[END_REF]. Another part of the filtrate was stored also in the dark in 150 mL polyethylene flasks for total alkalinity (TA) analysis. TA was titrated with an automated titration system (Metler Toledo model T50) with normalized 0.1 HCl on duplicate 50 mL aliquots and the equivalence point was determined using the linearization of the [START_REF] Gran | Determination of the equivalence point in potentiometric titrations[END_REF] function between pH 4 and pH 3. TA measurements were compared to certified reference material (CRM, provided by A. G. Dickson from Scripps Institution of Oceanography) and the accuracy was of ±5 µmol kg -1 . River samples were conditioned the same way to analyse POC, DOC during both cruises and also Chl a in Feb. 2020.

Carbonate system and CO2 fluxes calculations

We used the CO2sys program [START_REF] Pierrot | MS Excel Program Developed for CO2 System Calculations[END_REF]), version 2.1, to calculate all the chemical species of the carbonate system. The DIC concentrations were calculated from the measured pCO2 and TA, using the carbonic acid constants sets 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], and the CO2 solubility coefficient of [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF]. Considering the analytical accuracies for temperature (0.05°C), TA (±5 µmol kg -1 ) and pCO2 (5ppmv) we estimate the error on DIC calculation at ±5 µmol kg -1 . The water-atmosphere CO2 flux (FCO2) was calculated using the equation, expressed in terms of partial pressure:

FCO2 = kαΔpCO2 ( 1
)
where k is the gas transfer velocity of CO2 (m d -1 or cm h -1 ), α is the solubility coefficient of CO2 (mol m -3 atm -1 ), and ΔpCO2 is the air-water gradient of pCO2 (in atm).

The atmospheric pCO2 data used was 410 ppmv as measured in air during the campaigns. The gas transfer velocity k was calculated from the gas transfer velocity normalized to a Schmidt number of [START_REF] Jähne | On parameters influencing air-water exchange[END_REF]:

k = k600 (600/Sc) n ( 2
)
where k600 is the normalized gas transfer velocity, Sc is the Schmidt number of a given gas at a given temperature [START_REF] Wanninkhof | Relationship between wind speed and gas exchange[END_REF], and n equals 2/3 as in lakes environments [START_REF] Jähne | On parameters influencing air-water exchange[END_REF]). To calculate k600, in order to account for the uncertainty on this parameter, we used three different parameterizations respectively noted W92, RC01, and A09: the one of [START_REF] Wanninkhof | Relationship between wind speed and gas exchange[END_REF] representative for the global ocean, and the ones of [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],

representative for estuarine systems. The three equations are defined as a function of wind speed at 10 meters (U10), and the A09 equation also considers the estuarine surface area and water current velocity. To determine k600 we used U10 by the Instituto Nacional de Meteorologia (INMET) at the Sampaio Correia automatic weather station, located ~10 km from Saquarema lagoon (Fig. 1). In the A09 model, we used an average water current set to 20 cm s -1 for the Saquarema lagoon and the contribution on k600 was less than 8%. The fluxes were computed with the three parameterizations of k600. However, none of these parameterizations adapt perfectly to hydrographic and morphological features of the Saquarema lagoon; the W92 equation is valid for the open ocean and probably underestimates the k600 in the lagoon, whereas the RC01 and A09 equations are valid for funnelshaped macrotidal estuaries, and probably overestimates the k600. Therefore, we present here the CO2 fluxes calculated with the three different parameterizations, and the fluxes calculated as the average of the three.

Statistical analysis

A Kolmogorov-Smirnov test was performed to check the normality of the data and the Levene's test the homoscedasticity of the variances. All statistical analyses were based on α = 0.05. When the data showed a normal distribution with equal variances, we used the analysis of variance, ANOVA to examine the seasonal and spatial variation in the parameters. When the data did not follow a normal distribution, we used the Kruskall-Wallis test and, to examine the daytime variation in the parameters, we used the Mann-Whitney test. Correlation analysis between variables was performed with the Spearman rank coefficient. We used the PAST 4.0 program to perform statistical tests.

Results

Spatial and temporal variations of carbon species and ancillary parameters

The two sampling periods were quite contrasted in terms of meteorological and hydrological conditions. The data were compiled according to spatial and temporal samplings and the two sampling periods are presented separately. Table 1 summarizes the average concentrations, and respective standard deviations and ranges, of relevant parameters for this study (T, S, pCO2, SPM, DOC, POC, Chl a, TA and DIC). The most outstanding feature of the two campaigns was the presence of higher salinity and colder waters entering the lagoon during the high tide in Feb. 2020 (Figs. 3 and4).

Upwelling waters with temperatures down to 16 °C and salinities up to 34.47 were observed at the mouth of the Saquarema lagoon in Feb. 2020 while in Nov. 2019 maximum salinity was 30.2 and minimum temperature was 23.5 ºC (Figs. 3 and5; Table 1). A second important difference between seasons and particularly the spatial surveys was the warmer temperature inside the lagoon in summer in Feb. 2020 (29.56 ± 1.14 º C) compared to spring in Nov. 2019 (24.17 ± 0.77 ºC) (Figs. 2 and3).

In the Saquarema lagoon the salinity ranged from 14.3 to 34.6 and was strongly correlated with carbonate chemistry parameters, including TA (r = 0.960; Fig. 4 I and 4 J) and DIC (r = 0.925; Fig.

the lowest. In salinity 14.3, the TA and DIC concentrations were, for the two cruises, 1466 and 1091 μmol kg -1 , respectively. For marine sampled in Feb. 2019 (salinity 34.6) the TA and DIC concentration were respectively 2376 and 2211 μmol kg -1 (Fig. 6). Furthermore, both TA and DIC averages were significantly different between the two sampling periods (Mann-Whitney, p<0.0001).

During both campaigns, pCO2 showed undersaturated conditions inside the lagoon with a mean of 156 ± 82 ppmv during the spatial survey (Figs. 2 and3). Surface water pCO2 decreased with salinity inside the lagoon (Fig. 3). However, in Feb. 2020, the cold marine waters sampled during flood tides at the outlet of the lagoon presented oversaturated pCO2 values with a mean value of 497 ± 40 ppmv (salinities between 33 and 35). For both spatial and temporal surveys during the two campaigns, DOC, POC and Chl a increased inside the lagoon, contrarily to salinity, DIC, TA and pCO2 (Fig. 4). The minimum concentration of Chl a was 2.7 μg L -1 at the mouth of the lagoon at high tide and salinity 35 (Fig. 4G and4H). In the most inner part of the lagoon, in salinities below 15, the concentration of the Chl a reached 217 μg L -1 (Fig. 4G and4H). DOC concentrations showed a minimum of 1.0 mg L -1 at high salinity (~35) and a maximum of the 19.8 mg L -1 values at salinities of ~17 (Fig. 4A and4B). POC varied between 0.4 mg L -1 in seawater entering the lagoon and 12.3 mg L -1 in the Mombaça compartment inside the lagoon (Fig. 4C and4D).

Spatial variation of pCO2 in surface waters and CO2 exchanges at the air-water interface

Inside the lagoon, during both spatial sampling in Nov. 2019 and Feb. 2020, water was undersaturated in CO2 with respect to the atmosphere and strong spatial gradients occurred between the inner lagoon and its mouth. The lagoon was a sink of CO2 during the measurements at daytime. For the whole lagoon, an average pCO2 of 177 ± 73 ppmv was observed in Nov. 2019 and 126 ± 52 ppmv in Feb.

2020. Undersaturated pCO2 values occurred in the interior of the lagoon with the lowest value down to ~60 ppmv at low salinities. In Fora lagoon close to the outlet to the sea, we observed higher, but still undersaturated pCO2 values (301 ppmv in Nov. 2019 and 250 ppmv in Feb. 2020). There was significant spatial differences between the measured pCO2 in the Fora and Mombaça/Jardim compartments (Kruskall-Wallis, p<0.001; Table S1). However, we found no significant difference between the Mombaça and Jardim compartments (Kruskall-Wallis, p>0.05). The salinity was significantly different between the three compartments in the lagoon (ANOVA, p<0,001). In contrast, temperature was not significantly different between compartments (Kruskall-Wallis, p>0.05). There was also significant seasonal variability in salinity and temperature (Table S2): inside the lagoon water was significantly warmer in Feb. 2020 compared to Nov. 2019 (ANOVA, p>0,05) at the outlet of the lagoon, the seawater at high tide was cooler and more saline in Feb. 2020 compared to Nov.

2019 (ANOVA, p>0,05).

Wind speed was in general low during the samplings, on average between 2.03 ± 0.9 m s -1 and 2.83 ± 0.6 m s -1 in Nov. of 2019. In Feb. of 2020, the wind speed average was 2.8 ± 1.0 m s -1 in at day time (Table 2). As expected, the k600 values calculated with the equation of [START_REF] Abril | Turbidity limits gas exchange in a large macrotidal estuary[END_REF] representative for macro-tidal estuaries were higher than those calculated with the relationships of [START_REF] Raymond | Gas Exchange in Rivers and Estuaries: Choosing a Gas Transfer Velocity[END_REF] representative for a large variety of estuaries, and of [START_REF] Wanninkhof | Relationship between wind speed and gas exchange[END_REF] representative for the open ocean. Inside the Saquarema lagoon was a sink of atmospheric CO2 during the two sampled period. Averaged calculated CO2 fluxes with the three k600 parameterizations varied between -2.6 ± 4.8 mmol m -2 h -1 in Nov. 2019 and -5.6 ± 2.4 mmol m -2 h -1 in Feb. 2020.

Tidal variation of pCO2 in surface waters and CO2 exchanges at the air-water interface

During the temporal monitoring experiments at the mouth of the lagoon significant variation of salinity, temperature and pCO2 occurred in summer (Feb. 2020) (Fig. 5; Table 1) but not in spring in Nov. 2019 (Table 1). Maximum water pCO2 of 524 ppmv was observed at high tide (Table 1, Fig. 3), with corresponding temperature of 16.3 °C and a salinity of 34.4. At the same sampling point but at low tide, water pCO2 was 194 ppmv for temperature of 28.16 °C and a salinity of 28.36, showing simultaneous warming and mixing with brackish waters, and biological uptake of CO2. The pCO2

was not significantly different between day and night (p >0.05 Mann-Whitney test), but it was between ebb and flood tides. Calculated CO2 fluxes were -2.7 ± 0.5 mmol m -2 h -1 in Nov. 2019. In Feb. 2020 the CO2 fluxes average varied between -4.6 ± 1.8 mmol m -2 h -1 in the ebb tide and +0.8 ± 2.1 mmol m -2 h -1 at the mouth of the lagoon during the intrusion of upwelling waters with flood tides.

Discussion

Mixing between lagoon and ocean waters as drivers of air-sea CO2 fluxes

The contrasting climatic conditions between the two surveys were reflected by the distribution of the salinity gradient in the lagoon, as well as the carbonate species, particularly pCO2. In Nov. 2019, we could not observe an entrance of pure oceanic water during the 12h measurements. Highest salinity at the mouth of the lagoon at high tide was only 30.2 (Table 1) at that season, when, in addition, the Cabo Frio Upwelling is not active [START_REF] Coelho-Souza | Biophysical interactions in the Cabo Frio upwelling system, Southeastern Brazil[END_REF]. The monitoring in Nov. 2019 occurred during particularly bad meteorological conditions, with strong S-SW winds the day before sampling.

Such conditions generally promote a large entrance of shelf water in the lagoon [START_REF] Oliveira | Análise da hidrodinâmica do sistema lagunar de Saquarema frente às projeções de aumento do nível médio do mar. Programa de Pós-graduação e Pesquisa de Engenharia[END_REF]); however, this was not clear during our monitoring, even at high tide (see Table 1). Because salinity remained lower than pure seawater salinity, this water contained a significant fraction of CO2-depleted water from the lagoon and pCO2 values remained undersaturated as those inside the lagoon, at about 300 ppmv and salinities up to 31.9 (Fig. 3; Table 1). In contrast, in Feb. 2020, the marine end-member was very marked, with colder and more saline waters, supersaturated in CO2. At that season the Cabo

Frio upwelling was at maximum intensity [START_REF] Coelho-Souza | Biophysical interactions in the Cabo Frio upwelling system, Southeastern Brazil[END_REF], and throughout the entire flood tides, the cold SACW with a pCO2 of 520 ppmv entered the lagoon (Fig. 5). The well-defined temperature and pCO2 gradients between the lagoon and the ocean in Feb. 2020 allow a detailed quantitative analysis of carbonate thermodynamics during the mixing of two end-members: the first one is located inside the lagoon at salinity 10 and the second one is the SACW sampled during flood tides at the mouth of the lagoon at salinity ~ 35.

Inside the lagoon, our continuous measurements in Feb. 2020 went down to salinity 10 (Fig. 3), whereas the less saline discrete sample for TA analysis had a salinity of 14.3 (Fig. 4). We could extrapolate the strong linear correlation between TA and salinity (Fig. 4L) to estimate TA at salinity 10. In the salinity range 0 to 10 that occurs at the outlet of several small rivers, changes were too abrupt to allow precise surface water sampling, in addition to being very shallow, making it impossible to travel with the boat. According to previous works on the hydrology of the Saquarema lagoon (Alves 2013), the mixing between lagoon and seawater during the field campaigns occurred Where TAcons (S), DICconc (S) and Tcons (T) are respectively the TA, DIC and T at salinity S resulting from the conservative mixing of the two end-members. Then, from these three parameters, we calculated the theoretical pCO2(S) resulting from conservative mixing of the two end-members by solving the carbonate system for each 0.5 salinity increments between 10 and 35, [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][START_REF] Cotovicz | Sources and sinks of CO2 in the delta of the Paraíba do Sul River (Southeastern Brazil) modulated by carbonate thermodynamics, gas exchange and ecosystem metabolism during estuarine mixing[END_REF][START_REF] Abril | Thermodynamic uptake of atmospheric CO2 in the oligotrophic and semiarid São Francisco estuary (NE Brazil)[END_REF]. This led to a first theoretical conservative pCO2 mixing curve (Fig. 6). We also evaluate the theoretical impact of water heating on pCO2. To do this, we performed the same pCO2 calculation for each 0.5 salinity interval, using, this time, the in situ measured temperature, TAcons(S) and DICcons (S). Finally, we have binned our measured data for 0.5 salinity intervals between 10 and 35 to allow a quantitative comparison between the observed pCO2 and the two modelled pCO2.

Results of this analysis are shown in Fig. 6 and reveal that in Feb. 2020, when the cold SACW supersaturated in CO2 starts mixing with the lagoon waters at the tidal time scale in the Fora compartment, it simultaneously undergoes important heating that should potentially increase the pCO2 until a maximum value of 580ppmv at salinity 29. At salinities lower than 29, the mixing with undersaturated lagoon water would make the pCO2 decrease until salinity 10. The pCO2 field measurements do not follow the trend predicted by the mixing model, revealing the occurrence of strong biological uptake of CO2 within the lagoon. At the tidal timescale, from salinity 35 to 29, although heating should in theory increase pCO2 by about 60ppmv, the observed pCO2 rapidly drops from 520 to 190 ppmv. Between salinities 29 and 10, on a timescale longer than a month, the pCO2 continues to decrease remaining below the theoretical conservative mixing line (Fig. 6), within the range 80-200ppmv, and similar to the situation in Nov. 2019 (Fig. 3C). The mixing model in Feb.

2020 indicates that in the Fora compartment, the biological uptake of CO2 by the planktonic autotrophic community was efficient enough to absorb the excess CO2 carried by upwelling waters, plus the one resulting from the heating of the oceanic cold waters, making the lagoon to act as a net CO2 sink. This situation in Saquarema lagoon during the intrusion of upwelling waters, contrasts with the one in a Baja California coastal lagoon in Mexico, where CO2 outgassing prevails particularly during the periods of upwelling [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] Ávila-López | Air-Water CO2 Fluxes and Net Ecosystem Production Changes in a Baja California Coastal Lagoon During the Anomalous North Pacific Warm Condition[END_REF]). In the Aby and Tendo lagoons in Ivory Coast, a CO2 sink has also been reported, although it was related to a thermohaline stratification of the water column [START_REF] Koné | Seasonal variability of carbon dioxide in the Rivers and lagoons of Ivory Coast (West Africa)[END_REF]. Despite its specific hydrological settings, the Saquarema Lagoon acts as a CO2 sink similarly to the neighbouring Guanabara Bay and Araruama hypersaline lagoon [START_REF] Cotovicz | Comparações entre medições em tempo real da pCO2 aquática com estimativas indiretas em dois estuárioss tropicais contrastantes: o estuário eutrofizado da Baia de Guanabara (RJ) e o estuário oligotrofico do Rio São Francisco (AL)[END_REF]2021), with phytoplankton productivity acting as the major driver on CO2 air-sea fluxes.

Another interesting fact revealed by our study is the relative stability between the two surveys of the low pCO2 values in the most confined waters of the lagoon, in the Jardim and Mombaça compartments where water residence time is longer (Table 1; Fig. 2). Seasonally, the average daytime pCO2 inside the lagoon was significantly lower (Mann-Whitney, p>0,001; Table S2) in Feb. 2020 (126 ± 52 ppmv) than in Nov. 2019 (177 ± 73 ppmv) (Fig. 2; Table 1). During the spatial surveys, waters were about 5°C cooler in Nov. 2019 compared to Feb. 2020, which corresponds to a theoretical thermal pCO2 increase of about 7 ppmv, instead of the observed decrease of about 51 ppmv. In Feb. 2020 the Chl a concentrations were significantly (Mann-Whitney, p<0,001) higher than in Nov. 2019 (Fig. 4), revealing that the modest seasonal pCO2 changes inside the lagoon was associated to enhanced phytoplankton production in Feb. 2020. Feb. 2020 was characterized by denser phytoplankton blooms, apparently adapted to rapidly absorb large quantities of CO2 diffusing from the atmosphere as well as all the dissolved CO2 brought with the SACW at that season. In contrast to the average values, the lowest pCO2 values were identical at both seasons at 60ppmv (Fig. 3; Table 1). These pCO2 minima were observed in the 15-25 salinity range around midday, before thermal winds start to blow. This apparent constant threshold value for CO2 consumption in the lagoon waters may be due to a combination of various factors including surface heating and transient stratification, nutrient and/or CO2 limitation in the photic layer, photo-inhibition of phytoplankton species, including eukaryotes and cyanobacteria [START_REF] Caraco | Direct and indirect effects of CO2 on competition between cyanobacteria and eukaryotic phytoplankton[END_REF][START_REF] Phlips | Factors Affecting the Abundance of Phytoplankton in a Restricted Subtropical Lagoon, the Indian River Lagoon, Florida, USA[END_REF], as well as higher reaeration rates at very low pCO2.

Links between inorganic and organic carbon

In addition to the clear spatial and temporal negative correlations between pCO2 and phytoplanktonic biomass (Fig. 7), the links between inorganic and organic carbon is a crucial point in order to explain the drivers of the CO2 sinks in the Saquarema lagoon. During the 25h cycle at the mouth of the lagoon in Feb. 2020, the POC and DOC expressed as mg L -1 , the POC% and the SPM concentrations varied all together (Fig. 5). Lagoon waters flowing to the ocean during the ebb tides had much greater concentrations of DOC, POC and SPM and their SPM were enriched in POC, compared to the SACW entering during the flood tides. Consequently, the lagoon behaved as a source of POC and DOC for the offshore marine waters. When considering the entire dataset (spatial and temporal monitoring), a negative correlation is observed between DIC and Total Organic Carbon (TOC) (Fig. 9). The SACW have the highest pCO2, highest DIC and lower TOC concentrations, whereas the less saline samples inside the Monbaça and Jardim compartments have lower pCO2 and DIC and higher TOC. However, the slope -2.3±0.1 of the correlation in fig. 9 reveals that the landward DIC decrease is only about half of the landward TOC increase. At the outlet of the lagoon in Feb. 2020, the TOC concentrations during ebb tides were 11.8±4.6 mgC L -1 higher than those during flood tides, whereas the DIC concentrations during ebb tide were only 5.1 ± 2.7 mgC L -1 lower than those during flood tide. The exchanges between the lagoon and the ocean can be estimated thanks to the average tidal prim in the lagoon, estimated from a modelling work to 1.4 10 6 m 3 on average [START_REF] Oliveira | Análise da hidrodinâmica do sistema lagunar de Saquarema frente às projeções de aumento do nível médio do mar. Programa de Pós-graduação e Pesquisa de Engenharia[END_REF]. Using this tidal prism and the differences in carbon concentration between ebb and flood in Feb. 2020 (Fig. 5), we calculate exports from the lagoon of 21 tC d -1 of DOC and 14 tC d -1 of POC and an import of only 15 tC d -1 of DIC. The penetration of atmospheric CO2 through the air-lagoon interface (Table 2) provided an additional 1.4 tC d -1 of DIC in Feb. 2020 (0.8 tC d -1 of DIC in Nov. 2019). Although they are very rough estimates, these fluxes suggest first, that the carbon flux at the air-lagoon interface is one order of magnitude lower than the tidal exchange of DIC with the ocean, and second, that the lagoon exports more than twice as much TOC than it imports DIC. Consequently, if phytoplankton converts DIC to TOC with a 1:1 ratio, an additional source of TOC would be necessary to balance the carbon budget of the lagoon.

Several processes can potentially contribute to this disequilibrium between inorganic and organic carbon along the salinity gradient: (1) dilution of DIC and import of TOC with freshwater; [START_REF] Carreira | Changes in the sedimentary organic carbon pool of a fertilized tropical estuary, Guanabara Bay, Brazil: an elemental, isotopic and molecular marker approach[END_REF] phytoplankton production that consumes DIC and produces POC and eventually DOC; (3) gas exchange and invasion of atmospheric CO2 that will contribute to DIC; (4) other sources of POC and DOC occurring in the most confined waters of the lagoon, where waters are CO2-depleted. Estuarine DOC and POC can have three origins: terrestrial, marine and estuarine primary production [START_REF] Abril | Behaviour of organic carbon in nine contrasting European estuaries[END_REF][START_REF] Bauer | The changing carbon cycle of the coastal ocean[END_REF]). In the case of the Saquarema lagoon, wastewater carried by small rivers and flowing from multiple point sources all around the lagoon, is also an additional potential source of POC and DOC [START_REF] Carmouze | Metabolism of Saquarema Lagoon, Brazil. Biogeochemistry[END_REF].

The impact of mixing of the lagoon waters with freshwater is more difficult to evaluate quantitatively because our river data are partial due to logistical constrains, because sampling in many more small rivers as well as hydrological data would be necessary, and because the distribution of the carbon species in the 0-10 salinity range was inaccessible. The significant seasonal increase in POC and DOC concentrations in the rivers and inside the lagoon (Fig. 4) as well as the lower salinity in the lagoon (Table 1) in Feb. 2020, suggest that rivers could deliver more POC and DOC during the rainy summer.

However, this season is also warmer and more productive for phytoplankton as attested by the higher Chl a concentration and the lower pCO2. At the river stations, DOC and POC concentrations were higher than in seawater, but lower than in the Mombaça inner compartment of the lagoon at salinity between 15 and 20 where Chl a concentrations are maximal at both seasons (Fig. 4), suggesting also the presence of phytoplankton-derived POC and DOC. POC:Chl a ratios indicate very different contributions of phytoplankton biomass to the POC in the three sampled rivers, in the inner lagoon and at the mouth, and in the SACW (Fig. 8). In all the Feb. 2020 samples, and half of the Nov. 2019 samples in the Mombaça and Jardim compartments, as well as in the SACW samples, this ratio was within the range of 15 and 140 which corresponds to a predominant contribution of living phytoplankton to the POC [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] Abril | Behaviour of organic carbon in nine contrasting European estuaries[END_REF][START_REF] Sathyendranath | Carbon to chlorophyll ratio and growth rate of phytoplankton in the sea[END_REF][START_REF] Cloern | Phytoplankton primary production in the world's estuarine-coastal ecosystems[END_REF]. In river samples, the ratio was much higher (Fig 8), which confirms a terrestrial and/or domestic origin of POC [START_REF] Abril | Behaviour of organic carbon in nine contrasting European estuaries[END_REF].

Concerning DOC, the facts that concentrations inside the lagoon were highest in summer and were higher than in the river and ocean end-members at both seasons (Fig. 4) indicate an internal source that can be multiple sewage point sources and phytoplankton exudation or lysis. On one occasion, high DOC was also observed in the most polluted river with a maximum value in February, when runoff was apparently higher than in November. The distributions of DOC along the salinity gradient indicate a net production inside or input into the lagoon at both seasons, as well as a close to conservative dilution in the 15-35 salinity range during the mixing of lagoon waters with seawater (Fig. 4). One interesting observation comes from the positive correlation between DOC and Chl a and the negative correlation between DOC and pCO2 (Fig. 7). The same observation was made in the most eutrophic waters of the Guanabara Bay [START_REF] Cotovicz | Predominance of phytoplankton-derived dissolved and particulate organic carbon in a highly eutrophic tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF] suggests that part of DOC comes from the exudation or lysis of microalgae, a common process in coastal waters [START_REF] Myklestad | Dissolved organic carbon from phytoplankton[END_REF][START_REF] Augusti | Phytoplankton lysis predicts dissolved organic carbon release in marine plankton communities[END_REF]. It is interesting to note that both in the Guanabara Bay and in the Saquarema lagoon, DOC concentrations consistently increased to values above 10 mg L -1 , when pCO2 was below 200 ppmv (Fig. 7; [START_REF] Cotovicz | Predominance of phytoplankton-derived dissolved and particulate organic carbon in a highly eutrophic tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF], thus, when CO2 limitation of phytoplankton can occur, together with a strong depletion of inorganic nutrients in the surface water (Lui et al. 2017). This could be due to changes in the composition and physiological state of the planktonic microbial community induced by a combination between physicochemical, biological, climatic and hydrographic factors, in these two urbanized coastal environments.

Impact of eutrophication on the carbon cycle of the Saquarema Lagoon

Cultural eutrophication of coastal ecosystems is a secular trend in many developing countries, and particularly in the densely populated state of Rio de Janeiro in Brazil [START_REF] Knoppers | Coastal lagoons of southeastern Brazil: physical and biogeochemical characteristics[END_REF].

Eutrophication is due to massive anthropogenic inputs of organic (mainly domestic) and, to a lesser extent, inorganic (agricultural or industrial) nutrients that enrich estuarine waters and sediments with bioavailable nitrogen and phosphorus [START_REF] Rabalais | Global change and eutrophication of coastal waters[END_REF]). In addition, in the case of shallow lagoons, the frequent resuspension of sediments caused by tidal exchange and winds, becomes an important internal source of recycled nutrients, making the lagoons and bays in the region extremely productive [START_REF] Carmouze | Metabolism of Saquarema Lagoon, Brazil. Biogeochemistry[END_REF]. Because of the transparency of their waters, tropical lagoons are recognized as naturally autotrophic ecosystems and eutrophication will enhance this ecosystem property, making them more productive [START_REF] Carmouze | Metabolism of Saquarema Lagoon, Brazil. Biogeochemistry[END_REF][START_REF] Knoppers | Coastal lagoons of southeastern Brazil: physical and biogeochemical characteristics[END_REF]. Nutrient accumulation in coastal bays and lagoons leads to massive algal blooms, to a general increase of water turbidity, a reduction of light at the bottom, and the progressive loss of native submerged aquatic vegetation and biofilms [START_REF] Carmouze | The eutrophication of the Lagoon of Saquarema, Brazil[END_REF]. Our study reveals a CO2 sink that can be in part explained by eutrophication that enhances autotrophy more than heterotrophy, as reported in diverse coastal ecosystems (e.g. [START_REF] Gypens | Effect of eutrophication on air-sea CO2 fluxes in the coastal Southern North Sea: a model study of the past 50 years[END_REF], and as attested by the very high Chl a concentrations (Fig. 4GH; Table 1).

In the Saquarema lagoon, inorganic nutrients concentrations remain relatively stable since the 90's with dissolved inorganic nitrogen (DIN) around 5-20 µmol L -1 and dissolved inorganic phosphorus around 0,5-1 µmol L -1 [START_REF] Carmouze | The eutrophication of the Lagoon of Saquarema, Brazil[END_REF][START_REF] Carloni | Mudanças na qualidade da água de uma laguna do leste fluminense, geradas pela abertura permanente de conexão com o mar[END_REF]). However, inorganic nutrients concentrations are not relevant indices of eutrophication because most of the nitrogen and phosphorus are present in the organic form, which is rarely analysed. Indeed, if the POC concentration is 8 mg L -1 (Table 1), and if we assume a C/N Redfield ratio of 10, then the particulate organic nitrogen concentration would be 70 mg L-1, that is 2.5-10 times the DIN concentration. Some previous works in Saquarema have documented Chl a, POC and DOC concentrations in the 80's and 90's in the Jardim and Fora compartments [START_REF] Pereira | Estudo Experimental da Influência do Sedimento sobre as Atividades Metabólicas na Laguna de Saquarema -RJ[END_REF]; Table 3). Although these data are occasional and not fully representative of spatial and seasonal variations in the lagoon, they suggest a decadal increase of these three parameters, as well as a decrease of the POC:Chl a ratio, and thus, an increase of the stock of phytoplankton-derived organic carbon with time, particularly in the Jardim compartment (Table 3). Indeed, the most inner compartments of the lagoon have longer water residence times, shallower depths, making them more adapted to the long-term accumulation of nutrients in waters and sediments and enhanced planktonic primary production. In present days, the conditions that create the CO2 sink in the inner compartments of the Saquarema lagoon appear relatively stable between the two campaigns (see 4.1).

If we compare with other densely populated tropical and sub-tropical regions, the inner part of the Saquarema lagoon was a sink of atmospheric CO2 ranging from -5.6 to -2.6 mmol C m -2 h -1 (Table 2), similar to those reported the Guanabara Bay (-7.7 to -3.53 mmol C m -2 h -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 higher than those reported in the Araruama hypersaline lagoon (-0.2 to -0.6 mmol C m -2 h -1 ; [START_REF] Cotovicz | Eutrophication overcoming carbonate precipitation in a tropical hypersaline coastal lagoon acting as a CO2 sink (Araruama Lagoon, SE Brazil)[END_REF], in the Aby Lagoon in Ivory Coast (-0.3 mmol C m -2 h -1 ; Koné et al. 2009) and in Tokyo Bay (-0.4 mmol C m -2 h -1 ; [START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF]. The mechanism of carbon cycling in coastal environments impacted by urban pollution has been discussed by [START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF] as the balance between the more active respiration due to the presence of sewage derived organic matter and the more active photosynthesis driven by nutrient accumulation. This leads to a border between nearshore waters close to the sewage outlet that act as CO2 sources because degradation of organic matter locally predominates and offshore waters that act as CO2 sink because phytoplankton production overwhelms the CO2 budget. Cultural eutrophication apparently tends to shift this border toward the landside [START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF]. In Tokyo Bay, implementation of sewage treatments since the 70's have reduced the loads of biodegradable organic carbon but not the loads of nutrients; as a result, the respiration was reduced but the phytoplanktonic production was favoured, and the Tokyo Bay acts as a strong CO2 sink [START_REF] Kubo | Coastal urbanization alters carbon cycling in Tokyo Bay[END_REF]. In Guanabara and Tokyo bays, and in Araruama lagoon, the sewage loads are concentrated to well identified regions that emit CO2 to the atmosphere but represent a limited surface area [START_REF] Cotovicz | A strong CO2 sink enhanced by eutrophication in a tropical coastal embayment (Guanabara Bay, Rio de Janeiro, Brazil)[END_REF]2021;[START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF]). In the Saquarema lagoon, the CO2 supersaturation of water was linked to the intrusion of SACW in Feb. 2020 that acted as a weak source of CO2, of 0.2±0.3mmol C m -2 h -1 , for kW92, well within the range reported in the continental shelf of the subtropical Atlantic Ocean (ranging from 0.01 to 0.4 mmol C m -2 h -1 , [START_REF] Ito | Net sea-air CO2 fluxes and modelled pCO2 in the southwestern subtropical Atlantic continental shelf during spring 2010 and summer 2011[END_REF]).

CO2 supersaturation did not occur in the inner compartments where domestic carbon and nutrient are carried by rivers but also from multiple point sources all around the lagoon [START_REF] Carloni | Mudanças na qualidade da água de uma laguna do leste fluminense, geradas pela abertura permanente de conexão com o mar[END_REF]). This suggests that, even if degradation of organic matter derived from sewage is probably occurring, in the environmental conditions of the lagoon, it cannot outcompete with the high phytoplanktonic productivity that creates the CO2 sink. In addition, the TOC export to the ocean was larger than the DIC import (see 4.2) which suggests that this flux included organic carbon originating from sewage.

Despite the large differences in water depth and residence time, eutrophication has turned the Saquarema lagoon to a CO2 sink, similarly to the 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], the Tokyo Bay [START_REF] Kubo | A significant net sink for CO2 in Tokyo Bay[END_REF], and the hypersaline Arararuama Lagoon. If the CO2 fluxes were very similar in all these systems, the fluxes of POC and DOC are probably very different. One important issue to be investigated in the next future is the fate of the organic C present in these environments: the POC and DOC that is not remineralized is partially exported to the ocean, and partially stored in the sediments. In 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] have emphasized that the air-water CO2 flux was quantitatively similar to the recent organic C burial in the sediments [START_REF] Carreira | Changes in the sedimentary organic carbon pool of a fertilized tropical estuary, Guanabara Bay, Brazil: an elemental, isotopic and molecular marker approach[END_REF]. No carbon burial data are available in the Saquarema lagoon. However, recent works in the neighbouring Itaipu lagoon indicate an important increase of organic matter storage in the sediment linked to eutrophication [START_REF] Cerda | Coupling fallout 210Pb and stables isotopes (δ13C, δ15N) for catchment urbanization reconstruction in southeastern coastal zone of Brazil[END_REF]. More hydrological and sedimentological data would be necessary in order to quantitatively compare the CO2 sink with carbon burial and export. In the sea, this organic material may undergo partial degradation and only more refractory compounds would persist longer times in oceanic water masses.

Conclusions

The Saquarema lagoon behaved as CO2 sink during the two sampling surveys. Although the sampling periods are probably not fully representative of the temporal variation, the similar values of pCO2, POC, DOC and Chl a reveal a relative stability of the autotrophic character of the inner compartments of the lagoon. Important contrasts were observed at the mouth of the lagoon between seasons, with the entrance of SACW in Feb. 2020. The response of the lagoon carbonate system to the intrusion of the cold SACW supersaturated in CO2 was dominated by biological uptake, pCO2 showing a strong non-conservative behaviour during the mixing of seawater with lagoon waters. The negative correlation for both surveys between Chl a, POC, DOC on the one hand and pCO2 and DIC on the other hand reveal that phytoplankton inside the lagoon was very efficient at converting inorganic carbon to both particulate and dissolved organic carbon. Our analysis also suggests the TOC exported to the ocean apparently come partly from sewage sources and partly from phytoplankton production inside the lagoon. Our study confirms that intense eutrophication of tropical semi-enclosed littoral ecosystems, including bays and lagoons, turns these surface waters to act as CO2 sinks and accumulators and exporters of organic carbon. 1.

  at two different timescale: the mixing between salinity 35 and 29 occurs in the Fora compartment on a short timescale, related to the tide; the mixing between salinity 29 and 10 occurs all the compartments until the Mombaça compartment (Fig.1) on much a longer time scale (56-58 days; Alves 2013). From the TA and pCO2 values in these two end-members, we calculate the DIC values and we established the equations of the conservative mixing lines for temperature, TA and DIC as a function of salinity as follows:𝑇𝐴 𝑐𝑜𝑛𝑠 (𝑆) = 𝑇𝐴(10) + (𝑆 -10).𝑇𝐴(35) -𝑇𝐴(10)

Figure 1 .

 1 Figure 1. Map of land use and occupation of the watershed of the Saquarema Lagoon. The red diamonds represent the sampling stations during the first campaign (Nov 2019), the orange triangles represent the sampling stations during the second campaign (Fev 2020), the blue square represent the temporal monitoring , black point represents the Sampaio Correa meteorological station (22.87° S, 42.61°W) where wind speed was measured and the grey points indicate the sampled river stations.

Figure 2 .

 2 Figure 2. Maps showing the boats tracks and the spatial continuous measurements of salinity, temperature and pCO2 in surface waters of Saquarema lagoon for both sampling campaigns in Nov 2019 -and Feb 2020.

Figure 3 :

 3 Figure 3: Distribution of pCO2 and temperature as a function of salinity during the spatial and temporal monitoring for the two campaigns. "FP" refers to fixed point monitoring at the lagoon outlet, "Spatial" to spatial monitoring during the day inside the lagoon.

Figure 4 :

 4 Figure 4: Distributions of DOC, POC, POC %, Chl a, DIC, TA as a function of salinity for the two

Figure 5 :

 5 Figure 5: Diurnal variation of the concentrations of the main water parameters at the mouth of the lagoon during Feb. 2020. Note that a temporal monitoring of 12h was also performed in Nov. 2019, but the parameters were relatively constant, and the average values are reported in Table1.

Figure 6 :

 6 Figure 6: Comparison of observed water pCO2 (red) with the results of the mixing model of South Atlantic Central Water (Salinity 34.6) with lagoon waters (Salinity 10). The "conservative mixing" curve (black) is calculated by considering TA, DIC and temperature conservativity between the two end-members; The "conservative mixing corrected for temperature" curve (blue) is calculated by considering TA, DIC conservativity between the two end-members and the observed temperature along the salinity gradient.

Figure 7 :

 7 Figure 7: Relationships between: (A) DOC versus Chl a; (B) POC versus Chl a; (C) TOC versus Chl a; (D) DOC and pCO2; (E) POC and pCO2; (F) TOC and pCO2. The dashed lines at 400 ppmv represent the global average atmospheric CO2 concentration. "FP" refers to fixed point monitoring at the lagoon outlet, "Spatial" to spatial monitoring during the day inside the lagoon.

Figure 8 :Figure 9 :

 89 Figure 8: plots of (A) POC% (percent of SPM) versus SPM; (B) POC:Chl a vs Chl a, of the two samples campaigns for the spring and summer. The shaded area represents POC:Chl a range for living phytoplankton. "FP" refers to the monitoring at fixed point at the lagoon outlet, "Spatial" to spatial monitoring during the day inside the lagoon.

Table 1 .

 1 Mean (± standard deviation), minimum and maximum of the principal physicochemical parameters of the waters of Saquarema Lagoon for the sampling periods.

	November 2019	February 2020
	Spatial sampling	Temporal sampling (12h)	Spatial sampling	Temporal sampling (24h)
	Temperature
	(°C)

Table 2 .

 2 Mean values measured in situ of ΔpCO2 (pCO2water -pCO2air) and wind speed at 10 m height (U10). Calculated mean values for gas exchange coefficient (k600) and CO2 fluxes at the air-water interface in each sampling campaign. W92 is the data calculated according to k600 of Wanninkhof

	(1992), RC01 is the data calculated according to k600 of Raymond and Cole (2001), and A09 is data
	calculated according to k600 of Abril et al. (2009).
			Nov. 2019		Feb. 2020
			Spatial sampling	Temporal sampling	Spatial sampling	Temporal sampling (ebb tide)	Temporal sampling (flood tide)
	∆pCO2 (ppmv)		-233±73	-157±19	-283±52	-192±40	94±68
	U10 (m s-1 )		2.03±0.89 2.83±0.64	2.8±1.02	1.46±0.51	1.69±1.02
		W92	1.4±0.3	2.5±0.2	2.4±0.5	0.8±0.1	1.1±0.5
	k600 (cm h -1 )	RC01	3.9±2.6	5.1±2.4	5.1±2.7	3.2±2.3	3.4±2.7
		A09	7.0±4.1	9.0±3.4	9.0±4.4	5.5±3.1	6.1±4.4
		W92	-0.9±0.9	-1.2±0.1	-2.5±0.5	-0.4±0.2	0.2±0.3
	FCO2 (mmol C m -2 h -1 )	RC01 A09	-2.5±2.3 -4.5±4.2	-2.6±0.3 -4.5±0.5	-5.2±1.1 -9.2±2.1	-1.5±0.9 -2.7±1.6	0.8±1.0 1.4±1.8
		Average	-2.6±4.8	-2.7±0.5	-5.6±2.4	-4.6±1.8	0.8±2.1

Table 3 .

 3 Comparison of the values of DOC, POC, Chl a observed during the study with values in the late 80's and 90's.

		Pereira (1991)		This study
		Jardim*	Fora#	Jardim	Fora
	DOC	12.6±3.5	8.6±2.6 13.8±5.6	7.5±2.5
	POC	7.1±1.4	1.7±0.3 8.7±2.9	4.1±0.9
	Chl-a	16.7±10.3	13.3±3.3 116.8±85.2	45.2±23.4
	*from Mar. 89 to Jul.89; # from Oct.89 to Jan.90

Table S1 :

 S1 Statistical analysis comparing the spatial variation of parameters measured between

Acknowledgments

This research was funded by the Fundo Brasileiro para a Biodiversidade (FUNBIO) project: "Mecanismos Reguladores da Produção Pesqueira nos Sistemas Lagunares do Leste Fluminense: estado atual e cenários futuros" and the France-Brazil International Research Project VELITROP (Vulnérabilité des Ecosystèmes Littoraux Tropicaux face à l'Eutrophisation) funded by the French National Centre for Scientific Research (CNRS-INEE). We thank Leandro Alves, Mauricio Lorenzo Cerda Lema, Francisco Fernando Lamego Simões Filho, and Tailan Moretti Mattos from the Aquatic Biogeochemitry Lab. at the Universidade Federal Fluminense for their valuable help in the field.