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

Abstract

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 (δ13C-DIC up to 5.55‰), and highest pH (locally counter-acting the process of ocean acidification). 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 day−1), a moderate sink during summer (− 5.50 to − 4.67 mmolC m−2 day−1), with a net community production (NCP) of 93.7 mmolC m−2 day−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.

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Acknowledgments

The authors thank Renan Cardoso, Marcelo Muniz and Roberto Meigikos (Laboratory of Radioecology and Environmental Change-Federal Fluminense University) provided access to IRMS equipment and high valuable 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).

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Correspondence to Luiz C. Cotovicz Jr..

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Cotovicz, L.C., Knoppers, B.A., Régis, C.R. et al. Eutrophication overcoming carbonate precipitation in a tropical hypersaline coastal lagoon acting as a CO2 sink (Araruama Lagoon, SE Brazil). Biogeochemistry 156, 231–254 (2021). https://doi.org/10.1007/s10533-021-00842-3

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  • DOI: https://doi.org/10.1007/s10533-021-00842-3

Keywords

  • Carbonate chemistry
  • Coastal eutrophication
  • Hypersaline waters
  • Climate change
  • Atmospheric CO2 sink