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

Ecosystems 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.