Invasive invertebrate predator, Bythotrephes longimanus, reverses trophic cascade in a north-temperate lake

Trophic cascades can determine the structure of aquatic food webs, a role often used to manage water quality in lakes. However, trophic cascades are subject to multiple interacting drivers of environmental change. For example, invasive species can dramatically alter food webs, though we know less about invasive species effects on managed trophic cascades. Here, we investigate the changing food web dynamics of Lake Mendota (Wisconsin, U.S.A.) over a 40-yr time period. Piscivore stocking (biomanipulation) beginning in 1988 coupled with a 1987 die-off of cisco (the dominant zooplanktivore) led to decreased zooplanktivory that cascaded to increases in the large-bodied herbivorous zooplankton Daphnia pulicaria and greater water clarity. In 2009, the invasive spiny water flea Bythotrephes longimanus was first detected in Lake Mendota and soon reached record densities. As a result of this invasive invertebrate predator, zooplanktivory increased to 47% of the earlier cisco period. Concomitant with the spiny water flea population explosion were declines in D. pulicaria and water clarity as both exhibited a strong negative relationship to zooplanktivory across the 40-yr time series. We argue that the trophic cascade of 1987-1988 created an environment of high food resources and low fish planktivory that promoted the proliferation of Bythotrephes as it functionally replaced cisco in Lake Mendota's food web. Our results highlight how a manipulated, eutrophic lake was vulnerable to adverse impacts by invasive species, calling attention to the importance of understanding interactions among drivers of environmental change.