Abstract

We investigated the effects of dreissenid mussel (Dreissena polymorpha and D. rostriformis bugensis) invasions on the concentrations of chlorophyll a (Chl) and total phosphorus(TP), and the Chl:TP ratio within 27 north-temperate lakes that spanned large gradients in lake size and trophic status, using two approaches: (i) regression analysis and (ii) hierarchical modeling. Overall, Chl declined by 40%–45% after dreissenid invasion and the magnitude of effect was structured by epilimnetic volume within stratified, but not nonstratified (mixed) lakes. Declines in TP over the invasion period were significant only for stratified systems and were smaller (∼16% reduction from pre-invasion values) than for Chl. Across the complete trophic gradient (i.e., among lakes) the regression approach (analysis of covariance) indicated a significant decline in the intercept of the Chl:TP, but not the slope. In contrast, hierarchical modeling indicated a decline in the slope of Chl:TP among lakes between pre- and post-invasion periods. As with Chl, the effect of dreissenids on Chl:TP declined with increases in epilimnetic volume in stratified, but not nonstratified lakes. Dreissenid effects on Chl were prolonged, with no indication of diminishing within 7–10 years of invasion.