The declining cyanobacterial blooms in Lake Taihu (China) in 2021: The interplay of nutrients and meteorological determinants

Abstract

The cyanobacterial blooms are global environmental perils and have been extensively compounded and changeable in Lake Taihu with emerging significantly higher intensities during 2016–2019 than those in 2010–2015. However, the cyanobacterial density and chlorophyll-a (chl-a) contents of Lake Taihu in 2021 have been downturned in recent years, and the underlying mechanisms have become a heated subject in multiple aspects. Based on the monthly occurrence of cyanobacterial blooms at 31 monitoring stations in Lake Taihu from 2010 to 2021, combined with the data of environmental conditions such as water quality, pollutants load into the lake, and hydrometeorological factors, this study seeks to unravel the characteristics of cyanobacterial blooms in Lake Taihu in 2021, the shifting kinetics of the environmental factors, and discusses the intrinsic causes of the lower cyanobacterial bloom intensity in 2021. The results revealed that the wind speed in Lake Taihu remained stable in 2021, with an average rise of 0.5 °C in the ambient temperature. Similarly, the rainfall incidence and the total rainy days from July to August were 1.6 and 1.2 times of the average value from 2010 to 2020 respectively. Among the critical nutrients, a respective decline of 49 %, 56 %, and 78 % was recorded for the total nitrogen (TN), ammonia nitrogen (NH4+-N), and nitrate nitrogen (NO3–-N). Moreover, the random forest results evidenced that the NO3–-N, and NH4+-N were the key factors to affect the cyanobacterial density. This reduction in the nutrient levels caused by the abated nutrient loading into the lake coupled with the disproportionate rainfall and rainy days were the central causes for the obvious drop in the intensity of cyanobacterial blooms. Henceforth, the present work advocates to accelerate the robust technological application and management measures such as dual control of pollutant concentration and total amount to further reduce the nitrogen and phosphorus load to Lake Taihu.