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

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 (pCO(2)) 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 pCO(2) undersaturation was prevalent at the innermost region with shallow, confined, and phytoplankton-dominated waters, with a strong enrichment of heavier carbon isotope (delta C-13-DIC up to 5.55 parts per thousand), 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 O-2 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.

Automated summary

This study investigated the carbonate chemistry in the semiarid eutrophic Araruama Lagoon in Brazil. The high salinity waters had higher concentrations of total alkalinity and dissolved inorganic carbon compared to seawater, with calcium carbonate precipitation partially explaining these deficits. The lagoon acted as a significant CO2 sink due to the uptake by photosynthesis, with simultaneous and antagonistic actions of CaCO3 precipitation and autotrophic metabolism on CO2 fluxes.