Nutrient Enrichment and Eutrophication Processes in Freshwater Ecosystems Implications for AquaticBiodiversity Conservation

Nutrient loading caused by agriculture, effluent discharge, and urbanization is currently considered to be one of the major stressors affecting freshwater ecosystems through the process of eutrophication and subsequent loss of aquatic biodiversity. In the present study, the effects of nutrient pollution on water quality, degree of eutrophication, and aquatic biodiversity along a chain of a river and a reservoir affected by different types of land use, including an upstream reference site, agricultural zone, urban-effluent site, and a downstream reservoir, were investigated. Samples of nutrients, dissolved oxygen, water clarity, and algal biomass, as well as surveys of algae, aquatic invertebrates, fish, and aquatic macrophytes, were conducted at 12 sites during three seasons. Concentrations of nutrients increased dramatically from the reference site to the urban-effluent site, with total nitrogen and total phosphorus concentrations being six and ten times higher, respectively. The peak of algal biomass was observed prior to the monsoon season, and cyanobacteria dominated in the enriched areas. The number of invertebrate taxa decreased from 34 to 12; pollution-sensitive taxa disappeared almost completely from the urban-effluent site; and fish diversity was reduced by more than half. Phosphorus pollution and low dissolved oxygen explained more than 50% of the biodiversity reduction. The ecosystem status score was found to have fallen from 84 in the reference area to 27 in the most enriched area. These results indicate that nutrient enrichment functions as the main cause of decline in biodiversity by influencing algal domination, oxygen depletion, and habitat transformation. Nutrient reduction in the entire catchment area, wastewater treatment, and preservation of remaining reference habitats are key in protecting freshwater biodiversity.