Mechanisms of freshwater zoogeochemistry: A review and meta‐analysis of animal impacts on ecosystem function

Freshwater ecosystems are under immense pressure from global environmental change. These stressors have led to biogeochemical alterations, rapid rates of defaunation and losses of associated animal-driven ecosystem functions. Zoogeochemistry provides a unique perspective on the dual challenges of global change and defaunation in freshwater ecosystems, but zoogeochemical frameworks that explicitly link conservation biology with biogeochemistry have not yet been fully embraced by freshwater ecologists. We conducted a systematic review and meta-analysis of the mechanistic roles that animals play in freshwater biogeochemistry, with the aim of advancing the application of zoogeochemistry in freshwaters. We reviewed 14 major mechanisms of freshwater zoogeochemistry. Our review shows that a diversity of taxa impact ecosystem-level elemental pools and fluxes, within and across freshwater ecosystem boundaries. However, these effects are not always placed in context at the ecosystem level, which limits the scope of our meta-analysis. Zoogeochemical impacts tended to be stronger in wetlands and lakes than in streams and estuaries. Consumption, excretion, and respiration had the strongest direct impacts on elemental pools and fluxes; bioturbation, consumption, and egestion had the strongest indirect impacts. These mechanisms often interacted, generating complex influences on elemental pools and fluxes. However, creative experimental designs can be used to isolate individual mechanisms, allowing complex effects with multiple mechanisms to be incorporated into ecosystem models. Quantifying and modelling zoogeochemical effects in freshwater ecosystems will allow ecologists and managers to make informed decisions that balance biodiversity conservation with healthy ecosystem function.