INTERANNUAL MONITORING OF WATER ENVIRONMENT AND SPATIOTEMPORAL DYNAMICS OF PHYTOPLANKTON COMMUNITY AND BLOOMING RISK IN A TROPICAL WATER SOURCE RESERVOIR, CHINA

Shiyan Reservoir is situated in a densely populated urban area in Shenzhen, China, where its water quality and ecological health are crucial for the safety and security of the city's drinking water supply. To assess these factors, 13 water quality and ecological indicators were monitored over a 14month period. Chlorophyll-a concentrations in Shiyan Reservoir ranged from 21.25 to 88.77 mu g/L, with pronounced temporal heterogeneity and substantial variation. The highest chlorophyll-a levels, averaging around 85 mu g/L, were recorded from April to July, indicating a high risk of algal blooms during this period. The reservoir's annual average water temperature (WT) was 25.31 degrees C, with temperatures rising to approximately 30 degrees C from May to September, further increasing bloom risk. Chemical oxygen demand (COD) ranged from 1.71 to 3.08 mg/L, and total organic carbon (TOC) varied from 2.22 to 5.13 mg/L, indicating minimal organic pollution that meets Class I surface water standards. However, total nitrogen (TN) pollution remains severe, with concentrations between 1.66 and 2.49 mg/L, aligning with Class V surface water standards. Although recent reductions in TN levels have been observed, it continues to be the primary pollutant in the reservoir. Total phosphorus (TP) concentrations, ranging from 0.01 to 0.043 mg/L, are sufficiently low to meet Class I-II surface water standards. The reservoir's monthly average integrated trophic state index fluctuated between 43.13 and 53.81, with an annual average of 50.49, indicating a state of light eutrophication from April to May and moderate eutrophication in other months. Correlation analyses revealed WT as the primary factor influencing phytoplankton dynamics during the flood season, with a correlation coefficient of 0.69. In the flood season, WT, TOC, nitrate nitrogen, and transparency were the main factors affecting phytoplankton growth, while in the dry season, TOC, dissolved oxygen (DO), and WT were the primary drivers. Two-way ANOVA results indicated that temporal factors and sampling location, along with their interactions, explained the variations in environmental factors, with temporal heterogeneity significantly exceeding spatial heterogeneity. Cyanobacteria dominated from May to October, while diatoms were prevalent from November to February. Correlation and canonical correspondence analyses (CCA) highlighted significant differences in the environmental drivers of cyanobacteria, diatoms, and green algae. WT was the primary influencing factor for all three algae types, while organic matter and nutrients from runoff during the pre-flood season acted as critical triggers for cyanobacterial blooms. Additionally, a negative correlation was found between the nitrogen-to-phosphorus ratio and the abundance of cyanobacteria, green algae, and diatoms, suggesting that phosphorus may be a limiting factor for algal growth in Shiyan Reservoir.