Dynamics of the invasive spiny water flea, Bythotrephes longimanus, in Lake Simcoe, Ontario, Canada

The invasion of the nonindigenous zooplankter Bythotrephes longimanus in Lake Simcoe has the potential to impact overall ecosystem functioning, yet there is currently no information regarding the status and dynamics of its population. We used a combination of uni- and multivariate analyses to examine variation in Bythotrephes interannual and seasonal abundance, and life-history characteristics in Lake Simcoe from 1999 to 2007, and we explored the biotic (predators, prey) and/or abiotic (water quality, temperature) factors that may have regulated its population over this period. The mean annual abundance of Bythotrephes varied by an order of magnitude over the period of study and was significantly associated with epilimnetic temperature and prey and predator abundance. Bythotrephes typically appeared in early summer and had 2 seasonal abundance peaks, which varied in magnitude and timing within and among years. August and September Secchi depth and total phosphorus concentration predicted groups of years with similar Bythotrephes phenologies, lending further support to the role of water clarity as a significant component of this visual predator's habitat in its invaded range. Bythotrephes body size and reproductive patterns also varied seasonally, possibly reflecting the seasonal availability of cladoceran prey, predation pressure from planktivorous fish, or adaptations to avoid unfavourable conditions. Overall, a complex interaction of fish predation, food availability, temperature, and water clarity controlled the dynamics of Bythotrephes in Lake Simcoe. Changes to the Lake Simcoe ecosystem due to Bythotrephes invasion may have important consequences for the ecosystem services provided by the lake and merit further investigation.