Rise of a native apex predator and an invasive zooplankton cause successive ecological regime shifts in a North Temperate Lake

Abstract

AbstractEcosystems can undergo abrupt regime shifts as a result of many factors. Shifts between turbid and clearwater states are well understood for human‐impacted shallow lakes, but are not well understood in other types of lakes. Here we use long‐term data to describe abrupt shifts in water clarity in Trout Lake, an oligotrophic lake with a largely undeveloped watershed. For several decades mean summer water clarity averaged 4.5 m, but then around 2007 water clarity sharply increased and the “clear water regime” persisted for nearly a decade. Nutrient availability did not explain these changes, but rather they were explained by a classic top‐down trophic cascade. Around 2007, the population of the apex pelagic predator, Lake Trout, substantially increased. This was accompanied by a sharp decline in the lake's major pelagic prey fish, the zooplanktivorous Cisco. In turn, there was an increase in large‐bodied zooplankton taxa (calanoids, Daphnia), which reduced algal biomass. This clear water regime was then disrupted in 2014 by the invasion of a predatory zooplankton, Bythotrephes cederstroemi. This invasion corresponded to strong impacts on lower trophic levels (decrease in large‐bodied zooplankton and decreased water clarity), but more minor impacts on higher trophic levels (increased Cisco, decreased Lake Trout abundances)—in effect reversing the trophic cascade and shifting Trout Lake to a novel ecosystem state. Our study provides a long‐term, empirically based example of successive ecological regime shifts that occurred from the rise of an apex predator and a mid‐trophic level invasion in an undeveloped, oligotrophic lake.