Influence of ecosystem engineers on ecosystem processes is mediated by lake sediment properties

Invasive species may impact biotic community structure, ecosystem processes, and associated goods and services. Their impact may be especially strong when they also serve as ecosystem engineers (i.e. organisms affecting the physical habitat and resources for other species). Dreissenid mussels fill both these roles, having invaded the Laurentian Great Lakes in the late 1980s, and also serve as ecosystem engineers by altering nutrient fluxes and influencing the microbial food web through direct nutrient release and excretion of feces and pseudo-feces at the water-sediment interface. We conducted laboratory experiments to investigate how the different functional traits of invasive quagga mussels (filtering activity and direct nutrient release) and native chironomid larvae (tube building and bioirrigation) interact with lake sediment of differing organic matter content to influence biogeochemical fluxes and water quality. Our results showed that sediment reworking and ventilation activities by chironomid larvae increased oxygen penetration in the sediment, affecting primarily pore water chemistry, whereas invasive mussels enhanced nutrient releases in the surface water. However, sediment organic matter modulated the effects of ecosystem engineers on system-level processes; chironomids had a greater influence on sediment reworking and microbial-mediated processes in organic-rich sediments, whereas quagga mussels enhanced nutrient concentrations in the overlying water of organic-poor sediments. These results have management implications, as the effects of invasive mussels on the biogeochemical functioning in the Great Lakes region and elsewhere can alter system bioenergetics and promote harmful algal blooms.