Benthivorous fish bioturbation reduces methane emissions, but increases total greenhouse gas emissions

Globally, aquatic systems face increasing challenges with respect to increased greenhouse gas (GHG) emissions, eutrophication and strongly altered fish community composition. Although it is known that benthivorous fish can influence sediment and water column biogeochemistry, studies showing causal relationships are largely lacking. Here, we used a mesocosm approach with Common carp (Cyprinus carpio) to unravel the effects of bioturbation on GHG and nutrient dynamics. We hypothesised that fish bioturbation decreases methane (CH4) emissions and increases carbon dioxide (CO2) emissions by increased sediment oxygenation. Additionally, lower phosphorus (P) mobilisation was expected due to increased binding to ferric iron (Fe3+). We found that benthivorous fish increased water turbidity, and reduced CH4 diffusion to the atmosphere by 33%, and ebullition by 67%, probably because of sediment oxygenation. Simultaneously, however, CO2 emissions increased due to higher aerobic decomposition, leading to higher overall GHG emissions. In contrast to our hypothesis, we did not find indications of bioturbation affecting P mobilisation from the sediment, probably because P binding was already high in the control treatment as a result of high porewater Fe:P ratios. We conclude that bioturbation by fish has strong effects on GHG emissions as a result of higher overall decomposition rates offsetting reduced CH4 emissions. Depending on porewater Fe:P ratios, benthivorous fish may additionally reduce P mobilisation.