Assessing ecological status using phytoplankton functional groups in three urban rivers in Hainan Island, China

The urban rivers, including Changwang, Meishe, and Wuyuan in Haikou City, Hainan Island, are vital water sources for agricultural production and support industrial and domestic activities. Despite the rivers experiencing anthropogenic impacts, limited studies have assessed their water quality. Accordingly, this study assessed the phytoplankton community structure, utilized the river phytoplankton assemblage index (Q index) to evaluate the ecological status, and compared its performance with the comprehensive trophic level index (TLI). Sample collection and microscopy analysis was conducted seasonally in 2019. Two hundred ninety-eight phytoplankton species belonging to 8 phyla were identified, predominated by Chlorophyta, Bacillariophyta, and Cyanophyta. The phytoplankton biomass ranged from 0.04 to 34.98 mg L, with averages of 3.06 ± 0.71, 5.16 ± 1.92, and 2.70 ± 0.76 mg L in Changwang, Meishe, and Wuyuan, respectively. The phytoplankton biomass varied seasonally, recording the highest and lowest values in summer and autumn, respectively. The phytoplankton species were classified into 26 functional groups, which exhibited spatial and seasonal differences in their biomass and composition. The redundancy analysis (RDA) revealed that NH-N, TP, COD, Chl-a, salinity, and temperature were the main environmental factors influencing phytoplankton functional groups. The average Q index values in Changwang, Meishe, and Wuyuan were 3.39 ± 0.61, 3.44 ± 0.51, and 3.22 ± 0.67, and all the rivers were rated “good” in status. Seasonally, the Q index and TLI revealed that the rivers’ ecological condition was better in autumn and winter compared to spring and summer. Generally, the Q index performed better, indicating that parameters such as NH-N, COD, TP, and Chl-a decreased with improving ecological conditions from “poor” to “excellent” status. In addition, the Q index exhibited a significant negative relationship with TLI, suggesting that low Q index values may indicate increased eutrophication or deteriorated water quality. Thus, the ecological condition of the urban rivers could be adequately assessed using the Q index to guide their water quality monitoring and management.