Trophic And Non-Trophic Effects Of Fish And Macroinvertebrates On Carbon Emissions

Shallow aquatic systems exchange large amounts of carbon dioxide (CO2) and methane (CH4) with the atmosphere. The production and consumption of both gases is determined by the interplay between abiotic (such as oxygen availability) and biotic (such as community structure and trophic interactions) factors. Fish communities play a key role in driving carbon fluxes in benthic and pelagic habitats. Previous studies indicate that trophic interactions in the water column, as well as in the benthic zone can strongly affect aquatic CO2 and CH4 net emissions. However, the overall effect of fish on both pelagic and benthic processes remains largely unresolved, representing the main focus of our experimental study. We evaluated the effects of benthic and pelagic fish on zooplankton and macroinvertebrates; on CO2 and CH4 diffusion and ebullition, as well as on CH4 production and oxidation, using a full-factorial aquarium experiment. We compared five treatments: absence of fish (control); permanent presence of benthivorous fish (common carps, benthic) or zooplanktivorous fish (sticklebacks, pelagic); and intermittent presence of carps or sticklebacks. We found trophic and non-trophic effects of fish on CO2 and CH4 emissions. Intermittent presence of benthivorous fish promoted a short-term increase in CH4 ebullition, probably due to the physical disturbance of the sediment. As CH4 ebullition was the major contributor to the total greenhouse gas (GHG) emissions, incidental bioturbation by benthivorous fish was a key factor triggering total carbon emissions from our aquariums. Trophic effects impacted GHG dynamics in different ways in the water column and the sediment. Fish predation on zooplankton led to a top-down trophic cascade effect on methane-oxidising bacteria. This effect was, however, not strong enough as to substantially alter CH4 diffusion rates. Top-down trophic effects of zooplanktivorous and benthivorous fish on benthic macroinvertebrates, however, were more pronounced. Continuous fish predation reduced benthic macroinvertebrates biomass decreasing the oxygen penetration depth, which in turn strongly reduced water-atmosphere CO2 fluxes while it increased CH4 emission. Our work shows that fish can strongly impact GHG production and consumption processes as well as emission pathways, through trophic and non-trophic effects. Furthermore, our findings suggest their impact on benthic organisms is an important factor regulating carbon (CO2 and CH4) emissions.