Abstract
Using experimental mesocosms, we tested the strength of bottom-up controls by nutrients and top-down controls by an omnivorous fish (Hyphessobrycon bifasciatus; family Characidae), and the interaction between them on the CO2 partial pressure (pCO(2)) in the surface waters of a tropical humic lake (Lake Cabiunas, Brazil). The experiment included the addition of nutrients and fish to the mesocosms in a factorial design. Overall, persistent CO2 emissions to the atmosphere, supported by an intense net heterotrophy, were observed in all treatments and replicates over the 6-week study period. The CO2 efflux (average +/- standard error) integrated over the experiment was similar among the control mesocosms and those receiving only fish or only nutrients (309 +/- 2, 303 +/- 16, and 297 +/- 17 mmol CO2 m(-2) day(-1), respectively). However, the addition of nutrients in the presence of fish resulted in a high algal biomass and daytime net autotrophy, reducing the CO2 emissions by 35% (by 193 +/- 7 mmol CO2 m(-2) day(-1)). These results indicate that high CO2 emissions persist following the eutrophication of humic waters, but that the magnitude of these emissions might depend on the structure of the food web. In conclusion, fish and nutrients may act in a synergistic manner to modulate persistent CO2 emissions from tropical humic lakes.