Larger lakes are hit harder and longer by Dreissena invasion

Invasive quagga mussels are swiftly spreading across Europe and North America, causing major ecological and economic impacts even in lakes already invaded by zebra mussels. A key challenge is to understand the magnitude and timing of mussel impacts across a wide range of freshwater ecosystems, to predict potential impacts on uninvaded systems. We assembled high-resolution data sets spanning 18 North American and European invaded lakes and five key ecosystem variables (Secchi depth, phosphorus and chlorophyll concentration, phytoplankton and zooplankton biomass). We then address the questions: How have the pace and magnitude of dreissenid impacts varied with lake morphometry (area, depth, volume)? How have impacts differed between polymictic and stratified lakes? Which lakes have exhibited recovery following zebra and quagga mussel invasions? We found that impacts manifest earlier in polymictic than in stratified lakes. After the initial impacts, lakes invaded by zebra mussels usually exhibited signs of partial ecosystem recovery, which was interrupted in lakes that experienced a secondary invasion by quagga mussels. In the Laurentian Great Lakes, mussel populations reached maximum biomass later, and ecosystem impacts manifested more slowly but culminated at a higher overall impact level. Furthermore, no ecosystem recovery in the Great Lakes has been observed to date, even 30 years after mussel invasions, though recovery may be delayed in large lakes. Our results indicated that lake size is more important than other parameters in determining Dreissena impact, and findings from smaller inland lakes cannot be directly applied to larger systems (e.g., the Great Lakes) and vice versa.