Weihai nationally controlled rivers health assessment based on Periphytic algae and Phytoplankton index of biotic integrity

River health assessment is not only a "diagnostic tool" for ecological conservation, but also a core approach to achieving sustainable water resource utilization and addressing climate change challenges. However, there is currently a lack of a unified health assessment index system. This study selected Periphytic algae and Phytoplankton as indicator organisms to assess four nationally controlled rivers in Weihai City which including Muzhu River, Rushan River, Huanglei River, and Gu River. The index of biological integrity (IBI) and comprehensive pollution index (CPI) were employed for monitoring and evaluation. Additionally, correlation analyses were conducted with 10 water quality parameters: pH, dissolved oxygen (DO), total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH₃-N), potassium permanganate index (COD_Mn), chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD₅), Cd and Pb. The aim was to establish a robust river health assessment methodology. The results showed that the algal community structures in the four rivers exhibited the following characteristics: Bacillariophyta were the most abundant taxa among periphytic algae, while Chlorophyta dominated the Phytoplankton communities. The CPI values ranged from 0.63 to 0.90, all corresponding to a "moderate" water quality rating. Across all sampling sites, Periphytic algae IBI (Pe-IBI) scores fell between 40 and 43, also indicating a "good" ecological status. The ranking of rivers based on Periphytic algae integrity was: Rushan River, Huanglei River, Gu River, Muzhu River. Phytoplankton IBI (Ph-IBI) values across all sampling sites ranged from 39 to 43, consistently corresponding to a "good" ecological status rating. The integrity ranking based on Phytoplankton communities was: Muzhu River, Huanglei River, Rushan River, Gu River. The Pe-IBI index showed significant positive correlations with CPI values, pH, TN and Pb (r=0.848, 0.843, 0.868 and 0.590, P<0.05), while exhibiting a significant negative correlation with TP (r=-0.515, P<0.05). The Ph-IBI index demonstrated significant positive correlations with NH₃-N and BOD₅(r=0.600 and 0.513, P<0.05), but significant negative correlations with pH and DO (r=-0.542 and -0.733, P <0.05). No significant correlation was observed between Ph-IBI and CPI values (P>0.05). The results of this study demonstrate that Periphytic algae serve as more reliable bioindicators of aquatic environmental conditions than Phytoplankton in the nationally controlled rivers of Weihai City. This research provides fundamental data and technical support for accurately assessing the health status of aquatic ecosystems in Weihai's nationally controlled rivers, thereby facilitating water ecological monitoring and evaluation.