An assessment of the long-term phenology and impact of Bythotrephes longimanus in Island Lake Reservoir, Minnesota, using sediment records.

Abstract

Bythotrephes longimanus, a predacious zooplankter, has invaded >130 inland lakes in North America, yet few studies have documented the long-term phenology and impact of an invasion. This study used Bythotrephes spines and Daphnia ephippia in 210Pb-dated sediment cores from four sites in Island Lake Reservoir (Minnesota, United States) to document the establishment and growth of the Bythotrephes population and to examine corresponding changes in daphnid species composition and density during a 40-year period spanning the invasion. Based on sediment evidence, Bythotrephes invaded the reservoir in 1982.2, or about 8 years prior to what was its first detection in the water column in 1990. If accurate, this makes Island Lake Reservoir the first documented inland lake invaded in North America. Year to year population density of Bythotrephes followed a logistic growth model, and generally took about 10 years to reach annual carrying capacity. A notable impact of Bythotrephes on the daphnid community occurred within the first few years but did not intensify with increased Bythotrephes density over time. Daphnid species composition shifted in the western basin toward D. mendotae dominance, but did not shift in the eastern basin. The reservoir maintained a population of its largest-bodied daphnid, D. pulex, despite Bythotrephes presence. Coincident with Bythotrephes invasion, mean daphnid ephippium size decreased both within species and for the whole 3-species daphnid assemblage. This research suggests that lakes could harbor Bythotrephes for years before its first detection in the water column, that annual population build-up of Bythotrephes may require years before reaching peak annual densities, that the impact of Bythotrephes on food webs occurs relatively quickly, and that long-term impacts of Bythotrephes in a reservoir may be less severe than in natural lake ecosystems.