Colonization dynamics of the invasive predatory cladoceran, Bythotrephes longimanus, inferred from sediment records

Despite the central role of colonization to invasion biology, little is known about the early dynamics of founder populations because this time period is often unobserved and short-lived. This study documents with high resolution the early population ecology of an invading freshwater zooplankton, Bythotrephes longimanus (Cladocera : Cercopagidae), by measuring accumulation rates of its caudal spine in sediments. Using dated (²¹⁰Pb and ¹³⁷Cs) sediment cores from four spatially distinct sites in Island Lake Reservoir (Minnesota, USA), we describe its first presence, early distribution, growth trajectory, and early impacts on prey. The sediment record shows that B. longimanus first appeared and was widely distributed in the lake in 1982 (± 2 yr, standard deviation), 8 yr before its first detection in the water, making it one of the earliest documented invasions in North America and suggesting that ecosystems may serve as dispersal hubs for years prior to detection. Logistic growth models describing spine accumulation rates show that B. longimanus required about two decades to achieve an annual equilibrium (K). Prolonged buildup to K may owe to several factors, including accumulation of a sufficiently large bank of resting eggs, the obligate overwintering life stage. Early exponential growth was incongruent with the presence of a lag phase. Post invasion, Daphnia mendotae became proportionally the most abundant daphniid in the lake, but the timing of the switch in prey species composition coincided more with the proliferation of B. longimanus density and its attainment of K than with its arrival to the lake.