Exploring the impacts of natural or artificial shading on water quality, hydrobionts, and evaporation in freshwater environments

Water surface shading sourced from natural plants or artificial covers have been increasingly applied in freshwater systems; however, their effects on water quality, aquatic organisms and water loss remain insufficiently understood. We conducted a systematic review of published studies to provide a comprehensive synthesis of current knowledge on the influence of shading on freshwater environments. A total of 653 records were identified, of which 122 studies were retained after screening and eligibility assessment. The findings suggest that shading alters abiotic parameters such as light and wind, as well as processes such as gas exchange and water mixing. These changes can influence the physical, chemical, and biological characteristics of freshwater ecosystems. Shading generally reduces water temperature and dissolved oxygen concentrations while increasing water transparency and phosphorus availability. However, its effects on nitrogen transformation are less consistent and appear to be strongly influenced by site-specific environmental conditions. The reduction of sunlight suppresses primary production, which slows down the nutrient uptake by phytoplankton or aquatic plants. While shaded areas can serve as habitats for freshwater species, the concomitant decreases in primary production and dissolved oxygen pose critical constraints on the growth, behavior, and species composition of zooplankton and fish. Moreover, shading reduce water surface evaporation, with biological cover generally demonstrating lower effectiveness than physical covers in limiting evaporative losses due to plant transpiration. These findings provide a scientific basis for selecting appropriate shading strategies in freshwater systems, enabling managers to balance water quality improvement and ecosystem protection while pursuing evaporation control and economic benefits.