Environmental Impacts

Most aquatic organisms perform optimally within a relatively narrow salinity range. When salinity falls below or rises above this tolerance range, organisms lose their ability to maintain ionic balance through osmoregulation, a fundamental physiological process essential for survival. Salinity stress is often a more significant cause of mortality in aquatic organisms than predation, competition, disease, or parasitic infections. Consequently, changes in salinity regimes and their spatial distribution can alter the distribution and abundance of macrobenthos, seagrass, and other sessile marine organisms.

Natural vertical salinity gradients and the position of isohalines play a critical role in the successful development of fish larvae and juveniles. Salinity also influences the occurrence of pathogenic microorganisms and waterborne diseases within estuarine environments. Certain salinity conditions can promote harmful algal blooms (HABs) and alter microbial processes such as nitrification and denitrification. Overall, large fluctuations in salinity generally reduce species diversity and abundance because relatively few organisms are capable of tolerating wide variations in ionic concentration.

Salinity also influences important geochemical processes. Freshwater sediments typically retain ammonium (NH₄⁺) more effectively than marine sediments because fewer competing cations are present. As salinity increases, calcium and magnesium ions compete for adsorption sites on sediment particles, reducing the adsorption capacity for many dissolved metals and increasing their mobility. Elevated salinity also promotes flocculation, whereby fine suspended particles aggregate into larger particles with higher settling velocities. This process accelerates sediment deposition, reduces suspended sediment concentrations, and improves water clarity within estuarine environments.

The environmental impacts resulting from changes in freshwater inflow to estuaries can be comprehensively evaluated using advanced numerical modeling. Multiple simulation scenarios—including low-, moderate-, and high-risk conditions—can be developed to predict environmental responses under different hydrological and climate conditions. These simulations provide valuable scientific information for environmental planning, infrastructure development, and ecosystem protection.