Early changes in the benthic community of a eutrophic lake following zebra mussel (Dreissena polymorpha) invasion

In the context-dependent world of biological invasions, biologists understand few general patterns of spread and impact. One possible exception is the zebra mussel (Dreissena polymorpha), an invader that routinely restructures food webs through an ecosystem engineering process termed “benthification.” By efficiently consuming phytoplankton, zebra mussels can increase light penetration and nutrient concentrations in the benthos of a lake, thereby stimulating growth of benthic periphyton (phytobenthos) and macroinvertebrates (zoobenthos). Few studies monitor the response of these benthic communities to invasion. We documented early changes in phytobenthos and zoobenthos as zebra mussels invaded eutrophic Lake Mendota (Wisconsin, USA). From 2015 to 2018, the number of zebra mussel individuals reached densities >30 000 m−2 on hard substrates and 3000 m−2 in macrophyte beds. Community data showed classic signs of benthification, including 300% increases in (non-zebra mussel) zoobenthos and phytobenthos abundance on average across a depth gradient, including significant increases at depths where zebra mussels did colonize. Deep macrophyte biomass increased 900%, but water clarity showed no significant rapid increase. We speculate that nutrient enrichment may be more strongly responsible than increased light penetration for the benthic response of Lake Mendota. Continued integration of benthic production and processes into our study of lake ecosystems will be critical to understanding whole ecosystem function, especially as zebra mussels continue to “benthify” lakes within their invaded range.