Successional conditions of Dolichospermum and Microcystis in Taihu Lake, China

Dolichospermum spp. and Microcystis spp. are two common cyanobacteria that form blooms in the Changjiang (Yangtze) River basin, but the environmental conditions for their succession in large lakes are still unclear. Based on daily monitoring data from Meiliang Bay in Taihu Lake from March to June, 2016–2018, we studied the environmental conditions necessary for the succession of these two cyanobacteria. Results show that from March to June, the dominant genera of cyanobacteria experienced succession and co-dominated with Microcystis. The succession process included three stages. In Stage I, the biomass of Dolichospermum and Microcystis was similar (March), but Dolichospermum was dominant for most of the period. In Stage II, dominance alternated between Dolichospermum and Microcystis (April to mid-May). In Stage III, the biomass of Microcystis dominated (mid-May to June). In addition, temperature and nutrients across the three stages varied significantly. The average temperature increased continuously from 10.9 to 18.4, and to 24.2 °C. The total nitrogen content decreased from 2.87 to 2.40, and to 1.86 mg/L. The total phosphorus content increased from 0.08 to 0.09, and to 0.12 mg/L. Correlation analysis revealed that Microcystis biomass was positively correlated with temperature and total phosphorus. Dolichospermum biomass was positively correlated with total nitrogen. Classification and regression tree displays that when the temperature was below 18.1 °C, Dolichospermum dominated; above 18.1 °C, Microcystis took over. Further analysis revealed that when temperature reached 18 °C, the biomass of Microcystis increased exponentially, and the biomass of Dolichospermum exhibited a Gaussian distribution trend. This finding indicated that temperature was the key factor in the succession of Dolichospermum and Microcystis in nutrient-rich shallow lakes. As nitrogen and phosphorus concentrations decrease, the dominant species of cyanobacteria will diversify its development. The results of this study provide a foundation for risk prediction and control strategies for cyanobacterial blooms in lakes and reservoirs.