Structural Characteristics and Influencing Factors of Zooplankton Communities in the Duliujian River under Flood Discharge Disturbance

This study, based on the 2023 "23·7" extreme flood event in the Haihe River Basin, analyzes the zooplankton community structure and its correlation with environmental factors during the flood discharge process in the Duliujian River, revealing the phased impacts of this flood disturbance on the aquatic ecosystem. The study selected three monitoring points-Jinhai Highway Bridge, Yangchengzhuang Bridge and Tuanpo Bridge-for analysis. The results indicate a total of 38 zooplankton species belonging to 4 phyla were identified, with rotifers and protozoans being the most abundant. The top three dominant species were Brachionus calyciflorus, Asplanchna priodonta and Cyclops sp. Rotifers exhibited the highest abundance, accounting for 50% of the total zooplankton, while copepods contributed the highest biomass, accounting for 57%. Permutational Multivariate Analysis of Variance (PERMANOVA) found no significant difference (P>0.05) in zooplankton community structure during versus after the flood discharge period. Rotifers and protozoans were the dominant groups during and after discharge, respectively. Water quality, evaluated using physicochemical indicators and diversity indices, was classified as moderately polluted. Principal Component Analysis (PCA) showed that instantaneous flow rate was the primary influencing factor during discharge, while total suspended solids (TSS) was the primary influencing factor after discharge. Spearman correlation analysis revealed that during discharge, protozoans showed a significant positive correlation with ammonium nitrogen (NH₄⁺-N) (P < 0.01), while cladocerans and copepods were negatively correlated with nutrients and pH (P < 0.01). After discharge, rotifers were positively correlated with total phosphorus (TP), nitrate nitrogen (NO₃^--N) and water temperature (T) (P < 0.05). Redundancy analysis (RDA) indicated that pH, DO, TN, and TP were the main drivers of zooplankton community structure during discharge, while, TN, TP, NO₃^--N, TSS and T were the main drivers after discharge. This study provides a scientific basis for research on the adaptive mechanisms of estuarine ecosystems under flood disturbance and offers reference value for flood discharge management decisions and subsequent aquatic ecological restoration.