The Institute of Geo-Lake Sciences of the Chinese Academy of Sciences found that extreme weather events exacerbate the rebound of cyanobacterial blooms in lakes

Reporter Cai Shuwen Intern Yan Qinghe

Why, after extensive and high-intensity pollution control and ecological restoration, the cyanobacterial blooms in Taihu Lake have rebounded? In view of this "rebound confusion," an international research team led by Qin Boqiang, a researcher at the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, has made the latest progress in carrying out multidisciplinary interdisciplinary research such as meteorology, hydrology, ecology, and biogeochemistry, and proposed a positive feedback mechanism of "regional extreme climatic conditions exacerbating bloom outbreaks." The relevant results were recently published in the top international journal in the field of water resources and water environment, "Water Resources Research".

"In the context of global warming, regional extreme weather events will further worsen lake water quality, exacerbate lake eutrophication and bloom outbreaks." Qin Boqiang said that taking the rebound of cyanobacterial blooms in Taihu Lake in 2017 as an example, it was mainly driven by the high external load caused by the floods in the Taihu Lake Basin in 2016 and the significant warm winters in 2016 and 2017.

Studies have shown that the super-strong El Niño events of 2015 and 2016, coupled with warm phases such as the North Atlantic decadal oscillation (AMO) and the Pacific Decade Oscillation (PDO), together induced heavy rainfall in the Taihu Lake Basin in 2016 and the subsequent warm winter.

Qin Boqiang analyzed that the abnormally high rainfall caused a large number of pollutants containing nitrogen and phosphorus in the basin to flow into Taihu Lake with runoff through erosion; and the end of 2016 to the beginning of 2017 was the warmest winter in the entire Taihu Lake Basin since 1960, resulting in a high biomass of overwintering cyanobacteria, when the area of cyanobacterial blooms in Taihu Lake even exceeded the average level of previous summers. "The growth of cyanobacteria in the Taihu Lake basin in winter is mainly affected by temperature." Qin Boqiang said that compared with the 1970s and 1980s, the current trend of climate warming is particularly obvious in winter, and the winter temperature in the Taihu Lake area is generally 5-6 °C, just reaching the lower limit of cyanobacteria growth. The high temperature in winter makes cyanobacterial blooms occur in winter, which is a phenomenon that was not seen before.

After the occurrence of blooms, the photosynthesis and microbial degradation of cyanobacteria led to an increase in the pH of the water body and a decrease in the concentration of dissolved oxygen at the bottom of the lake, which promoted the mineralization and degradation of the nutrient salts of the sediment. Coupled with the shallow average depth of Taihu Lake, the sediment is more likely to resuspend and resuspend to release a large amount of nitrogen and phosphorus to the water column, further exacerbating the bloom outbreak, thus forming a vicious circle of "nutrient salt accumulation - bloom outbreak - endogenous release - eutrophication intensification", providing a continuous source of nutrients for the continuous occurrence of blooms. This is also the main reason why large-scale cyanobacterial blooms lasted from May to November of that year in 2017.

How to cut off this positive feedback mechanism of cyanobacterial bloom rebound? What implications does the study find for the next step of Taihu Lake governance and conservation policies and ecological restoration programs? Qin Boqiang believes that the treatment of sediment must be paid attention to: first, we must continue to carry out the endogenous pollution control work caused by sediment; second, through the gradual restoration of grass-based ecosystems to curb the suspension of sediment and the release of nutrient salts. In addition, Qin Boqiang mentioned that the construction of rainwater and sewage diversion can be strengthened, the discharge standards of sewage treatment can be improved, the prevention and control of non-point source pollution can be carried out, and the load of surface runoff on Taihu Lake can be reduced. "In fact, extreme weather, as a stimulus to the cycle of cyanobacterial blooms, is more pronounced in large, shallow and eutrophication lakes, so different water bodies should also be treated by lakes."

Qin Boqiang said that under the trend of global warming, eutrophication and global expansion of blooms, therefore, in-depth and sustained promotion of returning fishing to lakes, restoring natural wetlands to regulate the climate, energy conservation and emission reduction is China's due meaning to achieve the "double carbon" commitment, and it is also more urgent in the rebound of cyanobacterial bloom control in Taihu Lake. At the same time, climate change affects the governance of water bodies, but also reminds the public of the importance of environmental protection, all sectors of society should pay more attention to climate change, enhance environmental awareness, and join hands to protect the ecological environment.