The invasion of North American and Western European freshwaters by dreissenid (zebra and quagga) mussels has resulted in profound changes to the biogeochemistry, flora, and fauna of these ecosystems.  With the exception of a few studies, most investigations on the impact of dreissenid mussels have focused on the biogeochemistry or a small number of taxa in a single ecosystem.  In this study we have reviewed >500 peer reviewed articles, grey literature, and long-term monitoring data. Our dataset currently contains >50 lakes, 9 rivers, and 18 microcosm/mesocosm experiments.  We have used a meta analysis approach, with an unweighted log response ratio metric (LR), to determine the significance of dreissenid induced changes in biogeochemistry and primary and secondary producers. Using this approach, each lake, river, and experiment, are given equal weighting.  Where data are sufficient, results are presented by sub-ecosystem (littoral, pelagic, profundal, river, experimental).

Results/Conclusions

In lakes, dreissenid mussel invasion has been accompanied by, on average, a 50% decline in phytoplankton biomass, a 30% increase in water transparency, and 30-40% increases in soluble phosphorus and silica concentrations.  In large rivers, phytoplankton biomass has declined, on average, by 80%, accompanied by a 90% increase in water transparency, and a 680% increase in soluble phosphorus concentrations.   Post dreissenid invasion, littoral sediments have 90% higher organic content, the depth of the littoral zone has increased by 30%, and macrophyte coverage has increased by 180%.  Across all systems, zooplankton abundance has declined by 70%, while biomass has declined by 40%.  Cladoceran and copepod abundance declined by 40-50%, while rotifer abundance declined by 80%.   The response of zoobenthic communities to dreissenid invasion showed significant increases in biomass within rivers (80%) and the littoral zones of lakes (160%), and significant decreases in profundal zones (50%).  The most responsive zoobenthic taxa were littoral amphipods (>1000% increase) and gastropods (>200% increase), and unionid mussels (85-95% decline).  Our results indicate the role of dreissenids in dramatically altering the flow of energy and nutrients from pelagic to benthic zones, with significant effects on the biogeochemistry, flora and fauna of lake and river ecosystems.