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

Abstract

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.

Ecosystem size filters life‐history strategies to shape community assembly in lakes

“…The management status of the study lakes is complicated, but still relevant to discuss in relation to fish assembly dynamics on this landscape. On the one hand, Trout Lake has received substantial management attention in the form of fish stocking of native lake trout (Martin et al, 2022). Yet most other study lakes have not been greatly, if ever stocked.…”

Nonlinear dynamics, resilience, and regime shifts in aquatic communities and ecosystems: an overview

“…Some of the oldest and most convincing examples of regime shifts come from the study of shallow lakes. In this volume, long‐term studies of larger and deeper lakes reveal regimes of water clarity and phytoplankton related to food web shifts in a north temperate lake (Trout Lake, Wisconsin, USA) (Martin et al 2022) and to climate‐driven changes in the inputs of colored dissolved organic carbon in a subtropical lake (Lake Annie, Florida, USA) (Sullivan et al 2022). Collectively these studies of regime shifts, as well as others presented in this volume (e.g., Benito et al 2022), indicate that ecosystems under the influence of changing internal and external forces can move quickly to a different and long‐lasting state, affecting ecosystem services.…”

Long‐term analysis of body condition reveals species coupling and the impacts of an invasion

“…During the years 2001–2006, 2014, 2016, and 2018, the captured sample size for lake trout was too low for a population estimate to be calculated. Figure adapted from Martin et al (2022).…”

“…In this study, we examined long‐term changes in body condition of a coupled predator and prey: lake trout ( Salvelinus namaycush ; predator) and cisco ( Coregonus artedi ; prey) respectively, from Trout Lake, Vilas County, Wisconsin, USA. In the mid‐2000s, the lake trout population increased substantially, which caused a trophic cascade around 2007 (Figure 1) (Martin et al, 2022; Parks & Rypel, 2018). This shifted the lake from having low lake trout–high cisco densities to high lake trout–low cisco density (Figure 1).…”