Long-term empirical evidence, early warning signals, and multiple drivers of regime shifts in a lake ecosystem

1. Catastrophic regime shifts in various ecosystems are increasing with
the intensification of anthropogenic pressures. Understanding and
predicting critical transitions are thus a key challenge in ecology.
Previous studies have mainly focused on single environmental drivers
(e.g., eutrophication) and early warning signals (EWSs) prior to
population collapse. However, how multiple environmental stressors
interact to shape ecological behaviour and whether EWSs were detectable
prior to the recovery process in lake ecosystems are largely unknown. 2.
We present long-term empirical evidence of the critical transition and
hysteresis with the combined pressures of climate warming, eutrophication
and trophic cascade effects by fish stocking in a subtropical Chinese lake
in the Yangtze floodplain. The catastrophic regime shifts are
cross-validated by 64-year multi-trophic level monitoring data and
paleo-diatom records. 3. We show that EWSs are detectable in both the
collapse and recovery trajectories and that including body size
information in composite EWSs requires shorter time series data and can
improve the predictive ability of regime shifts. Although full recovery
has not yet been observed, EWSs prior to recovery provide us with the
opportunity to take measures for a clear-water regime. 4. Climate warming
and top-down cascade effects have a negative influence on water clarity by
altering lower trophic level abundance and body size, which in turn have a
negative effect on macrophyte abundance. Furthermore, we identify a shift
in the dominant driving forces from bottom-up to top-down after regime
shifts, decoupling the relationships between nutrients and biological
components and thus decreasing the efficiency of nutrient reduction. 5.
Synthesis This study provides new insights into ecological hysteresis
under multiple external stressors and improves our understanding of
trait-based EWSs in both the collapse and recovery processes in natural
freshwater ecosystems. For management practice, our work suggests that
slowing down climate warming and weakening the fish predation pressure on
food webs are necessary to increase the effectiveness of nutrient
reduction in the restoration of lakes.