Identifying major contributors to algal blooms in Lake Dianchi by analyzing river-lake water quality correlations in the watershed

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Globally intensified hazardous algal blooms (HABs) can and have already threatened public health and ecosystem services. For the hypereutrophic plateau Lake Dianchi in China, although comprehensive measures have been made regarding nutrients reduction and HABs biomass mitigation in the past decades, massive HABs are still frequently observable. HABs management strategies could hardly be successful unless relationships between the surrounding rivers’ water quality and HABs inside the lake are taken into serious consideration. This study characterized the river-lake water quality differences during 2010–2015, explored the relationships of riverine pollutants loading to lake water quality, and investigated the correlations of single and joint riverway water quality parameters to lacustrine HABs respectively via Spearman correlation analysis and multiple stepwise linear regression. Results indicated that surrounding inflow rivers displayed greater fluctuations than the lake in terms of water quality, and pollutants especially CODCr and nutrients were generally of higher concentrations in rivers that flow into Caohai and northern Waihai, than at the nearby state controlling sampling sites in the lake. Comparing to the rest of the lake, HABs in northern Waihai was much more influenced by the water quality of inflow rivers. Water temperature (WT) and SS were uniformly recognized as significant positive correlation factors between riverine water quality and lacustrine HABs, while DO as negative factor. When multiple factors were considered, WT was the predominant positive contributor to lacustrine HABs among most of the inflow rivers. Impact of riverine nutrients, however, was equivocal, due to the fact that nutrients concentrations in both rivers and the lake were already way beyond the hyper-eutrophication criteria. Taking into account multiple factors that may influence the threshold of nutrients to trigger HABs, the target nutrients limits for inflow rivers as well as Lake Dianchi itself was suggested to be much lower than previously recommended, and years of nutrients reduction efforts together with other remediation projects are required to ultimately mitigate HABs inside the lake.