Thaís Laque1, Rachel B Freita1, Bias M. Faria2, Fábio Roland3, and Vinicius F Farjalla1. (1) Federal University of Rio de Janeiro, (2) CENPES-PETROBRAS, (3) Federal University of Juiz de Fora
Background/Question/Methods Recent studies have shown that lakes are CO2 supersaturated ecosystems, i.e. total respiration exceeds photosynthetic production. Allochthonous dissolved organic carbon (DOC) plays a major role in CO2 saturation as it may sustain bacterial respiration in low productive ecosystems. Therefore, some of the main causes of CO2 supersaturation in lakes are the bacterial respiration of allochthonous DOC in oxic sediments and in the water column and the photo-degradation of allochthonous DOC in surface waters. Tropical coastal lagoons of Rio de Janeiro State comprise a gradient of humic-DOC concentrations, reaching values up to 160 mg C L-1. In this study, we measured the partial pressure of CO2 (pCO2) in 19 tropical coastal lagoons of Rio de Janeiro State, Brazil. Our main goals were to analyze the magnitude of pCO2 values in these lagoons and to assess the pCO2 in relation to various environmental characteristics. Samplings were monthly performed in a central point in each lagoon, during one year. Partial pressure of CO2 was measured in triplicates using an Infrared Gas Analyzer (IRGA, EGM-4, PP-Systems). The following environmental variables were also determined in each lagoon: depth, light penetration in the water column (Secchi Disk), water temperature, salinity, oxygen concentration, pH, DOC concentration (TOC-Analyzer, Shimadzu), water color (430 nm), phosphate concentration and chlorophyll-a concentration. Linear regressions were used to identify dependence between parameters. To access the relative importance of each parameter on the pCO2, stepwise multiple regression analysis was performed.
Results/Conclusions In general, all lagoons were CO2 supersaturated, but pCO2 varies among lagoons, reaching values up to 3000 µatm in the sub-surface in some highly-humic ecosystems. These values are among the greatest observed in the surface of freshwater ecosystems. Multiple regression using stepwise method revealed that pCO2 was positively related to DOC concentration, although the total variance explained was low (R2 = 0.162, p < 0.05). Therefore, we concluded that most lagoons were supersaturated with CO2 along the entire DOC gradient. Furthermore, pCO2 was positively correlated to DOC content, reflecting dependence between allochthonous DOC and heterotrophy in these humic costal lagoons. Finally, some of these lagoons showed the greatest pCO2 values in surface waters of freshwater ecosystems, probably due to extremely high humic-DOC concentrations.
