Extreme Drought Triggers River–Lake Ecosystem Destabilization: Fish Community Collapse Driven by Hydro‐Ecological Rhythm Mismatch

ABSTRACT River–lake systems serve as essential ecosystems supporting fish migration, spawning, feeding, and habitat requirements to complete their life cycles. In recent years, increasingly frequent extreme droughts severely threatened these systems' ecological functions, attracting considerable research attention, yet their ecological impacts remain poorly understood. The Yangtze River‐Poyang Lake system, serving as the primary habitat and germplasm repository for China's economically important fish species, notably the four major Chinese carps, experienced an unprecedented basin‐wide extreme drought in 2022. Four comprehensive field surveys about hydrodynamics, water quality, and fish assemblage were conducted before, during, and after the drought, to systematically capture and elucidate the cascading impacts of this event on fish population dynamics. Such extreme drought induced substantial contraction of hydraulic habitats (water depth decline: > 25 m to 10 m; channel width reduction: 10–3 km), accompanied by intensified sediment scouring and anomalous phytoplankton blooms, leading to a sharp increase in turbidity. These environmental shifts triggered nonlinear biotic responses: local‐scale fish density temporarily surged sixfold due to habitat compression, species diversity plummeted by two‐thirds during the drought, with opportunistic strategists (e.g., Tachysurus fulvidraco ) disproportionately impacted by the shallow, turbid waters. Furthermore, although typical hydrodynamic conditions were restored, fish community recovery from extreme drought remained a challenge. Fish density in post‐drought flood season dropped to as low as 20% of pre‐drought levels, potentially suggesting the system was progressively diverging from its dynamic equilibrium synchronized with fish reproductive cycles, severely impeding natural recruitment of fish populations. This study provides empirical evidence on the ecohydrological tipping points of river–lake systems, offering theoretical support for ecological management under intensifying climate extremes.