Abstract Aquatic communities are one of the most studied systems where alternative states or regime shifts have been detected. We used data spanning a century of time to test whether the zoobenthic community of Lake Mendota, Wisconsin, USA, was relatively stable through time, variable, or whether there was any evidence of alternative community states. We used multivariate statistical analyses to test for community structure similarity and whether detected differences corresponded to major changes in the local environment. Surprisingly, the benthic community in Lake Mendota was not statistically different from the mid 1960s to the present. Similarly, the benthic community was not significantly different from 1914 to the 1950s. However, between the 1950s and mid 1960s there was a dramatic change in the zoobenthic community, including the loss of key taxa and a decrease in the diversity of several major taxa. This dramatic change cannot be attributed to any single environmental factor, and is correlated with multiple factors acting simultaneously, including increased urban development, human population density, intensive agriculture, and the introduction of a major invasive species, Eurasian watermilfoil. The long-term similarity in the benthic community before and after the shift suggests two alternative states that switched with the confluence of multiple stressors.

Invasions of non-native species rank among the leading threats to aquatic ecosystems and biodiversity, and zebra mussels (Dreissena polymorpha) are certainly among those North American invaders with the most dramatic and potentially adverse impacts. Although lakes Mendota and Monona are not yet colonized with the zebra mussel, the establishment of high D. polymorpha densities in the Madison lakes could be rapid, and may happen during the next decade. The goal of this study was three-fold: 1) provide pre-invasion information on the community composition, density, biomass and production of benthic habitats in the Madison lakes; 2) predict the effect of zebra mussel invasion on benthic communities in the Madison lakes through comparisons with data to be obtained in southeastern Wisconsin lakes and an extensive long-term database from Eastern European lakes; 3) estimate the potential effect of zebra mussels on benthic and pelagic communities and associated fisheries in the Madison Lakes.

ACEI-009-06Potential Effects of Zebra Mussels in the Madison Lakes

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