Hydrological connectivity and nutrient thresholds drive seasonal dynamics of fish community in the Changjiang River-Shijiu Lake connected system

Floodplains exhibit high fish diversity and have supported regional fisheries for centuries. However, understanding of how the synergistic effects of hydrological connectivity and nutrient thresholds collectively drive fish community succession remains fragmented, particularly in large systems like the Changjiang River-Shijiu Lake continuum. Here, this study employed comprehensive eDNA metabarcoding (180 samples across 15 sites) for the first seasonal monitoring in this system, overcoming traditional survey limitations in dynamic habitats and enabling non-invasive monitoring in "Yangtze Protection" no-fishing zones. Key findings reveal: (1) Significant seasonal distribution patterns, with peak richness in summer (37 taxa) and lowest in autumn (22 taxa); 13 core taxa persisted year-round, while 35 were season-specific. (2) Diversity indices (Shannon: 2.41 ± 0.43; Simpson: 0.83 ± 0.11) were highest in summer (P < 0.01). (3) Hydrological connectivity drove spatial heterogeneity; a 3.75-m winter water-level decline significantly altered communities (Adonis, R² = 0.21, P < 0.05). (4) COD was the key annual driver (9.64-15.36 %), while TDN (15.41 %) and NH₃-N (15.38 %) were primary summer-limiting factors. The results elucidate synergistic regulation of fish communities by hydrological rhythms and nutrient thresholds, advocating for seasonal connectivity maintenance (winter habitat preservation) and nutrient control targets (summer TDN < 0.8 mg/L) to conserve biodiversity in connected floodplain systems. These findings elucidate synergistic hydrological-nutrient regulation of fish communities, advocating seasonal connectivity maintenance (e.g., winter habitat preservation) and targeted nutrient control (e.g., summer TDN reduction) to safeguard biodiversity in connected river-lake ecosystems.