The conservation of river ecosystems is fundamental to advancing ecological civilization. As key indicators of aquatic ecosystem health, fish populations face dual pressures from dam construction and emerging pollutants. Traditional ecological risk assessments often focus on individual stressors, overlooking the critical combined effects arising from dam-induced alterations in hydrological, thermal, and ionic (hydro-thermal-salinity) regimes and concurrent exposure to emerging pollutants exposure. This review aims to synthesize the impacts of damming on key environmental factors and elucidate the mechanisms of their combined effects with emerging pollutants on fish, thereby providing a scientific basis for comprehensive ecological risk assessment and new insights for risk management in dammed rivers.

This paper employs a systematic review methodology. It begins by summarizing the documented alterations in key environmental factors caused by dam construction, including hydrodynamics, water temperature, and nutrient regimes. The analysis reveals that dam construction profoundly transforms river ecosystems through multiple interconnected pathways. Hydrologically, such projects significantly alter natural flow regimes, manifesting in reduced velocities, homogenized flow patterns, and diminished extreme hydrological events. Thermally, reservoir operations induce stratification, leading to downstream temperature inversion phenomena that can extend tens to hundreds of kilometers, accompanied by delayed thermal responses and flattened annual fluctuations. In terms of nutrient cycling, reservoirs prolong hydraulic residence time, functioning as “biogeochemical reactors” that not only retain nitrogen and phosphorus but also promote their transformation into more bioavailable forms, with particularly pronounced effects in cascade systems. Subsequently, it separately analyzes the impacts of these altered environmental factors on fish physiology, growth, and reproduction. These environmental changes directly affect fish at multiple biological levels. Specific flow velocities trigger spawning in drifting-egg species, while suitable vorticity ranges are crucial for juvenile behavior and adult reproduction. Water temperature regulates metabolism, growth, and reproduction, with deviations from optimal ranges inducing oxidative stress and immune suppression. Regarding nutrients, while ammonia nitrogen may promote growth at low concentrations, elevated levels cause tissue damage and impair reproductive capacity. Next, it summarizes the processes of uptake, bioaccumulation, metabolism, and toxicity of various emerging pollutants in fish. The results show that emerging pollutants accumulate in fish through dietary uptake, and, following metabolic processing, some are transformed into more toxic compounds that disrupt endocrine function and inhibit development and reproduction. Finally, building on this foundation, the review constructs a comprehensive analytical framework to explore how environmental factors modulate the ecological risks of emerging pollutants through three potential pathways: 1) modifying the environmental behavior of emerging pollutants; 2) influencing the toxicokinetics of emerging pollutants within fish bodies; and 3) directly altering the sensitivity of fish to emerging pollutants, leading to synergistic or antagonistic combined toxicity. Specifically, environmental changes induced by damming interact with emerging pollutant exposure through multiple mechanisms. Altered hydro-thermal conditions play a decisive role in pollutant bioavailability. They achieve this by altering how chemicals partition, transport, and transform between water and sediments. Beyond the physical environment, such variations directly affect fish physiology. Changes in metabolic rates and foraging behavior reshape the toxicokinetics of emerging pollutants, determining how they are absorbed, distributed, and ultimately excreted. It is also important to consider these environmental variations as independent physiological stressors. Because they often suppress immune function and increase oxidative stress, the natural tolerance thresholds of fish may shift markedly. This dynamic ultimately creates conditions for complex synergistic, or, in some cases, antagonistic, combined toxicity.

Future research should prioritize: 1) mechanistic investigations into the combined effects of emerging pollutants under the influence of environmental influences, grounded in toxicological mechanisms; 2) studies conducted at environmentally relevant concentrations and focusing on endangered species, employing long-term, low-dose exposure scenarios to enhance ecological relevance; and 3) exploration of the feasibility of optimizing reservoir ecological operation strategies to regulate environmental factors, thereby enhancing the self-purification capacity of water bodies and the tolerance of fish to pollutants. This review provides a critical theoretical basis and a forward-looking perspective for developing an integrated ecological risk management framework for dammed rivers.