Ecological health assessment using zooplankton index of biotic integrity (Z-IBI) in urban rivers in Hainan Island, China

The Changwang, Meishe, and Wuyuan are urban rivers in Haikou City, Hainan Island. They are vital water sources, serving various purposes, from farmland irrigation to domestic and industrial use. Unfortunately, anthropogenic activities have influenced these "rivers," negatively impacting water quality and ecological functions; however, there is limited research on their ecological status. Thus, this study assessed the zooplankton community structure and developed the zooplankton index of biotic integrity (Z-IBI) to evaluate the ecological status. Samples were collected and analyzed seasonally in 2019. Generally, 159 zooplankton species belonging to 85 genera, 55 families, and 3 phyla were identified, dominated by Rotifer and Protozoa, followed by Arthropoda (groups of Copepod and Cladocera). The zooplankton density varied from 0.00 to 126.04 ind L-1, with averages of 19.49 ind L-1, 31.40 ind L-1, and 20.38 ind L-1 in Changwang, Meishe, and Wuyuan, respectively. The zooplankton density varied seasonally as summer > spring > winter > autumn. Analysis of similarity (ANOSIM) indicated a significant seasonal difference (R = 0.070, p < 0.001) in zooplankton community but not spatially (R = 0.004, p = 0.413). Thus, SIMPER revealed that Brachionus forficula, Didinium nasutum, Keratella valga, Nauplius, and Polyarthra vulgaris contributed to the seasonal differences. The redundancy analysis (RDA) revealed environmental factors, including total phosphorus (TP), total nitrogen (TN), ammonia nitrogen (NH3-N), chemical oxygen demand (CODMn), dissolved oxygen (DO), temperature, pH, Chl-a, and electrical conductivity (EC) influenced zooplankton species. The Z-IBI was established using Zooplankton abundance, Protozoa abundance, Copepod abundance, Cladoceran biomass %, and Shannon-Wiener diversity index. Based on Z-IBI evaluation, ecological status was "moderate" in Changwang and Wuyuan, while it was "low" in Meishe. Seasonally, the ecological status in autumn was better than in summer and spring. The Z-IBI showed that parameters, including pH, EC, CODMn, TN, TP, and Chl-a, decreased with improving water quality, while water transparency increased with improved water quality. Thus, the "rivers" ecological status could be adequately assessed using the Z-IBI to guide their water quality monitoring and management.