Carbon Dioxide Concentration and Emissions along a Trophic Gradient in Tropical Karst Lakes

Inland aquatic ecosystems are valuable sentinels of anthropic-associated changes (e.g., agriculture and tourism). Eutrophication has become of primary importance in altering aquatic ecosystem functioning. Quantifying the CO2 emissions by inland aquatic ecosystems of different trophic statuses may provide helpful information about the role of eutrophication on greenhouse gas emissions. This study investigated diel and seasonal carbon dioxide (CO2) concentrations and emissions in three tropical karst lakes with different trophic statuses. We measured CO2 emissions using static floating chambers twice daily during the rainy/warm and dry/cold seasons while the lakes were thermally stratified and mixed, respectively. The CO2 concentration was estimated by gas chromatography and photoacoustic spectroscopy. The results showed a significant seasonal variation in the dissolved CO2 concentration (C-CO2) and the CO2 flux (F-CO2), with the largest values in the rainy/warm season but not along the diel cycle. The C-CO2 values ranged from 13.3 to 168.6 mu mol L-1 averaging 41.9 +/- 35.3 mu mol L-1 over the rainy/warm season and from 12.9 to 38.0 mu mol L-1 with an average of 21.0 +/- 7.2 mu mol L-1 over the dry/cold season. The F-CO2 values ranged from 0.2 to 12.1 g CO2 m(-2) d(-1) averaging 4.9 +/- 4.0 g CO2 m(-2) d(-1) over the rainy/warm season and from 0.1 to 1.7 g CO2 m(-2) d(-1) with an average of 0.8 +/- 0.5 g CO2 m(-2) d(-1) over the dry/cold season. During the rainy/warm season the emission was higher in the eutrophic lake San Lorenzo (9.1 +/- 1.2 g CO2 m(-2) d(-1)), and during the dry/cold the highest emission was recorded in the mesotrophic lake San Jose (1.42 +/- 0.2 g CO2 m(-2) d(-1)). Our results indicated that eutrophication in tropical karst lakes increased CO2 evasion rates to the atmosphere mainly due to the persistence of anoxia in most of the lake's water column, which maintained high rates of anaerobic respiration coupled with the anaerobic oxidation of methane. Contrarily, groundwater inflows that provide rich-dissolved inorganic carbon waters sustain emissions in meso and oligotrophic karstic tropical lakes.