Urbanization significantly impacts river ecosystems, altering fish community structure and dynamics and posing challenges to the sustainable management of these vital resources. In the heavily urbanized Haihe River in China, understanding the spatial and seasonal variations of fish communities and the environmental factors influencing them is crucial for effective conservation and sustainable management. This study investigated fish communities and environmental variables at ten sites along an urbanization gradient in the upstream reach of the Haihe River over four seasons in 2023. A total of 6710 individual fishes representing 30 species were collected. The results showed that the most urbanized section exhibited higher species diversity but was dominated by tolerant, omnivorous species with similar functional traits, indicating functional homogenization. In contrast, less urbanized sections displayed greater seasonal fluctuations and supported species with specialized traits. Key environmental factors influencing fish community dynamics included nitrogen levels, water temperature, dissolved oxygen, and the abundance of the submerged macrophyte Potamogeton crispus. These factors varied spatially and seasonally, mediating the effects of urbanization on fish communities. The findings highlight the importance of environmental factors in shaping fish community dynamics in urban rivers and underscore the need for integrated management strategies that consider both anthropogenic impacts and natural influences to conserve fish diversity, river maintain ecosystem health and ensure long-term sustainability. Sustainable management practices that balance development with environmental protection are vital for preserving ecological integrity and enhancing the resilience of urban river systems to challenges such as urbanization.

The rising challenge of eutrophication in aquatic systems globally necessitates an understanding of phytoplankton community dynamics under diverse biomanipulation approaches. This study, conducted from June 2022 to July 2023 in the Yuqiao Reservoir’s ponds in China, explored phytoplankton dynamics across ponds under different biomanipulation strategies. The study included a pond (BL) without fish stocking, a pond (CH) stocked with carnivorous and herbivorous fish, and another pond (CFD) incorporating a mix of carnivorous, filter-feeding, and detritus-feeding fish. Substantial seasonal variations in phytoplankton density and biomass were observed. In the BL pond, phytoplankton density ranged from 0.23 × 10⁷ to 3.21 × 10⁷ ind/L and biomass from 0.71 to 7.10 mg/L, with cyanobacteria predominantly in warmer seasons and a shift to cryptophytes and chrysophytes in winter. The CH pond exhibited a density range from 0.61 × 10⁷ to 8.04 × 10⁷ ind/L and biomass of 1.11 to 7.58 mg/L. Remarkably, the CFD pond demonstrated a significant reduction in both density (0.11 × 10⁷ to 2.36 × 10⁷ ind/L) and biomass (0.27 to 5.95 mg/L), indicating the effective implementation of its biomanipulation strategy. Key environmental factors including total nitrogen, water temperature, pH, chlorophyll-a, and total phosphorus played a significant role in shaping phytoplankton communities. The study highlights the importance of tailored biomanipulation strategies in aquatic ecosystem management, emphasizing long-term monitoring for sustainable management of eutrophication.