Synergistic Effects of Warming and Internal Nutrient Loading Interfere with the Long-Term Stability of Lake Restoration and Induce Sudden Re-eutrophication

Phosphorus (P) precipitation is among the most effective
treatments
to mitigate lake eutrophication. However, after a period of high effectiveness,
studies have shown possible re-eutrophication and the return of harmful
algal blooms. While such abrupt ecological changes were attributed
to the internal P loading, the role of lake warming and its potential
synergistic effects with internal loading, thus far, has been understudied.
Here, in a eutrophic lake in central Germany, we quantified the driving
mechanisms of the abrupt re-eutrophication and cyanobacterial blooms
in 2016 (30 years after the first P precipitation). A process-based
lake ecosystem model (GOTM-WET) was established using a high-frequency
monitoring data set covering contrasting trophic states. Model analyses
suggested that the internal P release accounted for 68% of the cyanobacterial
biomass proliferation, while lake warming contributed to 32%, including
direct effects via promoting growth (18%) and synergistic effects
via intensifying internal P loading (14%). The model further showed
that the synergy was attributed to prolonged lake hypolimnion warming
and oxygen depletion. Our study unravels the substantial role of lake
warming in promoting cyanobacterial blooms in re-eutrophicated lakes.
The warming effects on cyanobacteria via promoting internal loading
need more attention in lake management, particularly for urban lakes.