Impacts of omnivorous invaders on community structure and biomass of multiple trophic levels are often complex and poorly understood. Responses to omnivory may vary by trophic level and depend upon regional connectivity in the invaded landscape. Here, we tested for differences in multi-trophic planktonic species diversity and ecosystem attributes among reservoirs with reproducing populations of omnivorous zebra mussels (Dreissena polymorpha) relative to uninvaded reservoirs at the southern invasion front in North America. Reservoirs were selected in two partially invaded riverine metaecosystem networks under contrasting levels of hydrologic connectivity to encompass community and ecosystem patterns at the invasion front and to evaluate differences in regional (basin connectivity) versus local (reservoir) structuring processes. Additionally, multi-year trends in phytoplankton biomass, water column transparency, and total phosphorus were compared pre- and post-invasion. The results highlight greater differences in local ecosystem variables than community properties in reservoirs of contrasting invasion status. Following predictions, reductions in zooplankton and phytoplankton biomass, increases in water column transparency, and decreases in total phosphorus were observed in invaded reservoirs. Reductions in total phosphorus persisted for up to five years following invasion. These directional patterns suggest that omnivorous zebra mussels are influencing ecosystems of hydrologically connected reservoirs at the southern invasion front.