Changes in spring phytoplankton communities and nutrient dynamics in the eastern basin of Lake Erie since the invasion of Dreissena spp.

Distinct changes have occurred in the size and composition of the spring phytoplankton community in the eastern basin of Lake Erie following the introduction of Dreissena. Since 1996, total phytoplankton biovolume has decreased to approximately 20% of pre-Dreissena levels, whereas postinvasion concentrations of spring soluble nutrients, particularly silica, have been substantially elevated compared with earlier years. Spring dominance has shifted from a mix of pennate and large centric diatoms and pyrrophytes to three centric diatoms with high silica requirements: Aulacoseira islandica, Stephanodiscus hantzschii, and Stephanodiscus parvus, and the overall diversity and species richness of the spring phytoplankton community has declined significantly. In addition, current April silica concentrations are approximately twice as high as historical (i.e., 1960s-1980s) winter maxima, indicating that the silica content of the lake has increased since the dreissenid invasion. These results suggest that the severe silica depletion caused by increased anthropogenic inputs of nutrients during the last century has been mitigated through a decrease in diatom production, most likely brought about by dreissenid grazing.