Spatiotemporal Dynamics of Water Quality and Plankton Communities in Hydrologically Connected Reservoirs

This study investigates the spatiotemporal dynamics of water quality, phytoplankton, and zooplankton communities in two hydrologically connected reservoirs, Lantan and Renyitan, where water is transferred from Renyitan to Lantan. Seasonal and spatial analyses were conducted on key water quality parameters, including ammonia nitrogen (NH₄-N), total phosphorus (TP), dissolved oxygen (DO), and chemical oxygen demand (COD), and their influence on plankton abundance and diversity was assessed. Zooplankton abundances averaged 1500 ± 200 individuals/mL during the wet season, significantly higher than the dry season’s 1200 ± 150 individuals/mL (p = 0.0096, d = 1.58). Phytoplankton abundances also increased significantly during the wet season (p = 0.013), driven by nutrient enrichment from surface runoff and hydrological mixing. Diversity indices, such as the Shannon-Wiener Index (H') and Margalef’s Richness Index (DMg), displayed notable seasonal variations (p < 0.001), suggesting greater diversity and community balance in the wet season. Depth-related variations were more pronounced for zooplankton, with higher species richness (50 ± 8 species) and diversity (H' = 2.5 ± 0.2) in shallow zones, compared to opportunistic dominance in deeper waters (Dominance Index D = 0.7 ± 0.1). Phytoplankton depth-related differences were minor, with shallow samples averaging 5200 cells/mL compared to 4900 cells/mL in deep waters (p = 0.537). Inter-reservoir comparisons revealed higher biodiversity and community balance in Renyitan, whereas Lantan exhibited localized nutrient imbalances, promoting dominance of specific taxa.  Non-Metric Multidimensional Scaling (NMDS) analysis highlighted significant seasonal shifts in plankton communities, with broader dispersion in the wet season due to dynamic environmental conditions. Deep-water habitats exhibited greater ecological stability, clustering tightly around NMDS centroids. Canonical correlation analysis (CCA) identified TN, TP, and DO as critical environmental drivers (p < 0.01). These findings emphasize the influence of seasonal and depth-related dynamics on plankton communities within connected reservoirs. Insights derived from this study provide valuable foundations for nutrient management, bloom mitigation strategies, and sustainable reservoir ecosystem management. Future research incorporating molecular tools and long-term monitoring is recommended to enhance understanding of community resilience in the face of climate-driven changes.