Synergistic control of CO ₂ emissions by fish and nutrients in a humic tropical lake

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 CO ₂ partial pressure (pCO ₂) in the surface waters of a tropical humic lake (Lake Cabiúnas, Brazil). The experiment included the addition of nutrients and fish to the mesocosms in a factorial design. Overall, persistent CO ₂ emissions to the atmosphere, supported by an intense net heterotrophy, were observed in all treatments and replicates over the 6-week study period. The CO ₂ 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 CO ₂ 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 CO ₂ emissions by 35% (by 193 ± 7 mmol CO ₂ m ^-2 day ^-1). These results indicate that high CO ₂ 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 CO ₂ emissions from tropical humic lakes.