Comparing the performance of the water quality index and phytoplankton index of biotic integrity in assessing the ecological status of three urban rivers in Haikou City, China

The urban rivers, Changwang, Meishe, and Wuyuan, are vital water resources in Haikou City, China. However, pollution from various anthropogenic activities negatively impacts their ecological status. This study aimed to develop and compare the performance of a minimum water quality index (WQImin) and phytoplankton index of biotic integrity (P-IBI) in assessing the rivers' ecological status for protection purposes. Samples were collected and analyzed seasonally from March to December 2019. The results revealed that TP, DO, COD, TN, and turbidity were critical for developing the WQImin. The WQImin values showed no significant spatial and seasonal differences. Thus, it rated the rivers “moderate,” with the highest and lowest WQImin values in winter and summer. The P-IBI was developed using phytoplankton density, Chl-a, biomass, and Margalef richness. Notably, the P-IBI values differed significantly at seasonal but not spatial scales. It rated Changwang and Wuyuan “moderate,” whereas Meishe was “low.” Moreover, Autumn and summer had the highest and lowest P-IBI values. Interestingly, the P-IBI demonstrated that parameters such as COD, TN, NH4+-N, EC, pH, turbidity, and temperature decreased with improving water quality, while water transparency increased with improved water quality. The WQImin and P-IBI assessment results were consistent. The indices showed that summer had poor water quality than other seasons. In addition, the indices negatively correlated with the trophic level index (TLI). In contrast, the P-IBI ratings were significantly lower than WQImin at spatial and seasonal scales. Hence, this study suggests that WQImin and P-IBI can effectively assess the ecological conditions of the rivers and that the critical parameters and metrics for their development might vary in different regions. In addition, P-IBI may provide stricter standards than WQImin for water quality assessment.