Variation and Driving Factors of Black Water Event Intensity in Lake Taihu During 2009 to 2018

Abstract

Using daily survey and monitoring "black water event" (BWE) results in the sensitive area of Lake Taihu from April to October, 2009-2018, as well as the BWE analyzed data for relative meteorological, hydrological, chemical, and algal bloom conditions, the characteristics and yearly differences of BWEs were summarized. A BWE control strategy was suggested. There were 75 BWE occurrences detected in the past 10 years. The average area of a BWE was 1.35 km2, with a maximum area of 9.20 km2. The BWEs lasted for an average of three days, while the longest lasted 16 days. The BWEs significantly increased organic matter, total nitrogen, total phosphorus, ammonia, and sulfate, among others. All the BWEs occurred at water temperatures over 20℃. All the BWE occurrences started between May and September. The yearly BWE intensity (BWEI) varied significantly among years, with the strongest intensity in 2017 and the second strongest in 2018. The BWEI was significantly positively related to yearly algal bloom intensity (ABI) and average daily water temperature from May to September, while there was no significant relationship with major nutrient indicators. This suggests that climatic variation among years will significantly influence the risk of a BWE in Lake Taihu. The occurrence of a BWE was significantly influenced by a polluted river mouth. Almost all the BWEs occurred near river mouths, except for five macrophyte-related BWEs. This suggests that sediment pollution and its resuspension could be strongly related to the occurrence of a BWE. When considering degradation factors, a BWE could be classified as an algal-related BWE and a macrophyte-related BWE. The algal-related BWE could be further classified into three types:river-related BWE, bloom transport BWE, and local origin BWE. This research suggests that algal bloom control will be the fundamental countermeasure to decrease the risk of a BWE. Bloom abatement treatments, including mechanical bloom cleaning, water current adjustment engineering, coast-away bloom cleaning engineering, mechanical aeration treatment, and sediment dredging near river mouths would be effective methods to abate the risk of a BWE.