Increased variability and sudden ecosystem state change in Lake Winnipeg, Canada, caused by 20^thcentury agriculture

Abstract

Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self-reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously-elevated variability characteristic of alternate stable states or a "paradox of enrichment," respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N 2 -fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (15-20 lg P L 21

Seeking alternative stable states in a deep lake

“…The mean total phosphorous (TP) concentrations were reconstructed using diatom and pigment-based inference models. On the other hand, pigments are known to be sensitive to early changes in pressure (Bunting et al, 2016;McGowan et al, 2005), but they do not reflect TP concentrations when P is no longer a limiting factor, which is the case at very high TP levels. On the one hand, the diatom responses to perturbation may be delayed (Davidson et al, 2011) and could give seasonally biased results, as diatoms are often most abundant in the spring and autumn (Reynolds, 1984).…”

“…Instead, because deep lakes are an ideal context for measuring sequential sedimentation, they provide an opportunity for high-resolution palaeo-ecological reconstruction allowing to test for early nutrient-driven regime shifts triggering alternative stable states (Bunting et al, 2016;Randsalu-Wendrup, Conley, Carstensen, & Fritz, 2016;Wang et al, 2012). Instead, because deep lakes are an ideal context for measuring sequential sedimentation, they provide an opportunity for high-resolution palaeo-ecological reconstruction allowing to test for early nutrient-driven regime shifts triggering alternative stable states (Bunting et al, 2016;Randsalu-Wendrup, Conley, Carstensen, & Fritz, 2016;Wang et al, 2012).…”

“…Indeed, an ecosystem state variable responding through a smooth and continuous pressure-status relationship to a strong stepwise change of this driver over time (as, e.g. Therefore, we used a multistep process aimed at testing all three necessary properties of a regime shift (sudden change, threshold-response and bistability, Andersen et al, 2009;Bunting et al, 2016). Therefore, we used a multistep process aimed at testing all three necessary properties of a regime shift (sudden change, threshold-response and bistability, Andersen et al, 2009;Bunting et al, 2016).…”

Variance and Rate‐of‐Change as Early Warning Signals for a Critical Transition in an Aquatic Ecosystem State: A Test Case From Tasmania, Australia

“…Environmental pressures or perturbations can erode the resilience of an ecosystem such that, in cases where multiple stable states exist, a threshold is crossed and the system shifts into a new stable state (Bunting et al, ; Carpenter & Brock, ; Wang et al, ). These critical transitions are a nonlinear ecosystem response to a change in conditions, such as nutrient influx, climate, and/or land use change (Lenton, ; Scheffer et al, , ; Scheffer & Carpenter, ; Wang et al, ).…”

“…Thus, it is important to select appropriate metrics when using palaeoecological data to measure resilience and EWS. Variance, for example, has been a successfully employed measure for palaeoecological data without interpolation, allowing an effective test for EWS and critical transitions (Bunting et al, ; Dakos et al, ). In addition, ROC can be used to measure EWS, without interpolation, as an erosion of resilience before a critical transition (Scheffer et al, ; Siteur et al, ).…”