Abstract. In contrast to its small surface area, the coastal zone plays a disproportionate role in the global carbon cycle. Carbon production, transformation, emission and burial rates at the land-ocean interface are significant at the global scale but still poorly known, especially in tropical regions. Surface water pCO 2 and ancillary parameters were monitored during nine field campaigns between April 2013 and April 2014 in Guanabara Bay, a tropical eutrophic to hypertrophic semienclosed estuarine embayment surrounded by the city of Rio de Janeiro, southeast Brazil. Water pCO 2 varied between 22 and 3715 ppmv in the bay, showing spatial, diurnal and seasonal trends that mirrored those of dissolved oxygen (DO) and chlorophyll a (Chl a). Marked pCO 2 undersaturation was prevalent in the shallow, confined and thermally stratified waters of the upper bay, whereas pCO 2 oversaturation was restricted to sites close to the small river mouths and small sewage channels, which covered only 10 % of the bay's area. Substantial daily variations in pCO 2 (up to 395 ppmv between dawn and dusk) were also registered and could be integrated temporally and spatially for the establishment of net diurnal, seasonal and annual CO 2 fluxes. In contrast to other estuaries worldwide, Guanabara Bay behaved as a net sink of atmospheric CO 2 , a property enhanced by the concomitant effects of strong radiation intensity, thermal stratification, and high availability of nutrients, which promotes phytoplankton development and net autotrophy. The calculated CO 2 fluxes for Guanabara Bay ranged between −9.6 and −18.3 mol C m −2 yr −1 , of the same order of magnitude as the organic carbon burial and organic carbon inputs from the watershed. The positive and high net community production (52.1 mol C m −2 yr −1 ) confirms the high carbon production in the bay. This autotrophic metabolism is apparently enhanced by eutrophication. Our results show that global CO 2 budgetary assertions still lack information on tropical, marine-dominated estuarine systems, which are affected by thermal stratification and eutrophication and behave specifically with respect to atmospheric CO 2 .

The increasing concentrations of methane (CH 4 ) in the atmosphere stress the importance of monitoring and quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014, we measured the spatial CH 4 concentrations, identified major sources and calculated the fluxes at the airwater interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presented high spatial variability of CH 4 concentrations, without a significant trend with salinity, but observed the influence of the urban areas at its watershed. Although the more polluted sector of the bay accounts for about 10% of the sampled surface area, it contributed to one half of the bay's total CH 4 emissions. In most cases, high CH 4 concentrations seemed be sustained by allochtonous sources such as the sewage network and polluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at the inner and centre of the Bay, CH 4 concentrations were not significantly higher in bottom hypoxic waters than in surface waters, suggesting that CH 4 diffusion from these sediments was modest, due to the prevalence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565-980 lmol m 22 d 21 ) are well above those of most estuaries worldwide, showing that urban pollution can be an important source of CH 4 to the coastal waters and even more significant than the presence of organic-rich environments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO 2 -equivalent, CH 4 emissions reduced the net CO 2 sink in Guanabara Bay by 16%.

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Dissolved inorganic carbon (DIC), its stable isotope composition (δ 13 C-DIC) and ancillary parameters of the water column were investigated in a eutrophic tropical marine-dominated estuary surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil). Most negative δ 13 C-DIC signatures (down to −6.1‰) were found in polluted regions affected by direct sewage discharges where net heterotrophy induces high partial pressure of CO 2 (pCO 2) and DIC concentrations. Keeling plot was applied to this polluted region and determined the δ 13 C-DIC sewage signature source of −12.2‰, which is very consistent with isotopic signature found in wastewater treatment plans. These negative δ 13 C-DIC signatures (i.e., DIC depleted in 13 C) were restricted to the vicinity of urban outlets, whereas in the largest area of the bay δ 13 C-DIC signatures were more positive (i.e., DIC enriched in 13 C). The most positive δ 13 C-DIC signatures (up to 4.6‰) were found in surface waters dominated by large phytoplankton blooms, with positive correlation with chlorophyll a (Chl a). In the largest area of the bay, the preferential uptake of the lighter stable carbon isotope (12 C) during photosynthesis followed the Rayleigh distillation, and appeared as the most important driver of δ 13 C-DIC variations. This reveals an important isotopic fractionation (ε) by phytoplankton due to successive algal blooms that has turned the remaining DIC pool enriched with the heavier stable carbon isotope (13 C). The calculated diel apparent ε showed higher values in the morning (18.7‰-21.6‰) and decreasing in the afternoon (6.8‰-11.1‰). ε was positively correlated to the pCO 2 (R 2 = 0.88, p = 0.005) and DIC concentrations (R 2 = 0.73, p = 0.02), suggesting a decline in carbon assimilation efficiency and decreasing uptake of the lighter carbon under CO 2 limiting conditions. The eutrophic coastal waters of Guanabara Bay have δ 13 C-DIC signatures well above that found in estuaries, shelf and ocean waters worldwide.

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HighlightsOxygen-depleted river waters were associated with high concentrations of TA and DIC TA and DIC decreased in the mixing region due to re-oxidation processes Extreme high concentrations and emissions of CO2 and CH4 were found in hypoxic/anoxic polluted river waters Concentrations and emissions of GHG decreased seaward direction as results of mixing, degassing and biological uptakeThe diffusive CH4 emissions were more important than CO2 emissions in terms of global warming potential

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Aim: There are few studies dealing with the biogeochemical processes occurring in small estuaries receiving high sewage loading in tropical regions. The aim of this investigation was to characterize the biogeochemical behavior of nutrients in superficial waters collected at the Iguaçu estuarine system, during specific conditions (neap tide), located at the inner sector of a heavily eutrophicated embayment (Guanabara Bay, SE Brazil). Methods Physical and chemical variables were measured in situ (pH, temperature, conductivity, salinity, total dissolved solids, transparency, dissolved oxygen), whereas suspended particulate matter, chlorophyll a, phaepigments and nutrients (carbon, nitrogen and phosphorus forms) were measured in laboratory across the mesohaline estuarine gradient. Results The Iguaçu River mouth is in a high stage of eutrophication, considering nutrient concentrations, chlorophyll a and transparency of water column. Results indicate a transition from heterotrophic conditions to autotrophic conditions, since the nutrients concentrations showed a decreasing pattern along the saline gradient, while the chlorophyll an increased over the transects. The pH values and chlorophyll : phaeopigments ratios are significantly related to the amount and quality of organic matter contents, especially at transects under strong marine influence. More than 95% of the dissolved and total nitrogen concentrations are represented by NH4+ contributions, which are related to the ammonification of organic matter contents in this region, indicating the existence of untreated sewage loads in this area. Conclusion In this study, the Iguaçu River seemed to contribute with high inputs of nutrients that support important phytoplankton production at the inner regions of the bay related to the CO2 sink and autotrophic metabolism, showing the importance of verifying the biogeochemical behaviors of nutrients in estuarine areas, even in small scale.