Synergistic effects of warming and internal nutrient loading interfere with the long-term stability of lake restoration and induce sudden re-eutrophication
Urban lakes are globally ubiquitous and are usually highly eutrophic, pointing to an increase in frequency, duration and magnitude of harmful algal blooms as wide-spread threats to ecological and human health.
Over half a century, phosphate (P) precipitation is among the most effective treatments to mitigate eutrophication in these lakes. However, after a period of high effectiveness, re-eutrophication would possibly occur leading to the return of harmful algal blooms. While such abrupt ecological changes were presumably attributed to internal P loading, the role of lake warming and its potential synergistic effects with the internal loading, thus far, has been largely understudied.
Researchers led by Dr. KONG Xiangzhen and Prof. Dr. XUE Bin from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, along with their international collaborators, have addressed the question by quantifying the contributions of lake warming and the potential synergistic effects with internal P loading in an urban lake located in central Germany, which suffered from the abrupt re-eutrophication and cyanobacterial blooms in 2016 (30 years after the first P precipitation).
Their findings were published in Environmental Science & Technology on Feb. 20.
In this study, a process-based lake ecosystem model (GOTM-WET) was established using a high-frequency monitoring dataset covering eutro-/oligo-trophic states over 30 years.
Model analyses suggested that, for the abrupt occurrence of cyanobacterial blooms, internal P release accounts for 68% of the biomass proliferation, while lake warming contributed to 32%, including both direct effects via promoting growth (18%) and synergistic effects via intensifying internal P loading (14%). The model further revealed that the synergy was attributed to prolonged lake hypolimnion warming and oxygen depletion.
“Our study exemplifies how process-based mechanistic modeling could help to tease apart important drivers of abrupt shifts and cyanobacterial blooms in lakes, particularly in an era of rapid global changes including climate change and human activities.” said Dr. Kong.
This study unravels the substantial role of lake warming in promoting cyanobacterial blooms in re-eutrophicated lakes. The indirect effects of warming on cyanobacteria via promoting internal loading need more attention in future lake research and management.
“Our findings will have far-reaching consequences for lake restoration and management as the nutrient targets we applied so far to reach or maintain a certain trophic state will not work in a far warmer future and need to be adjusted, i.e. stronger nutrient level reduction and higher efforts in restoration are demanded.” said Dr. Kong.
Nanjing Institute of Geography and Limnology