The primary cause of eutrophication in waterways is the excessive input of nutrients, particularly nitrogen and phosphorus, often resulting from agricultural runoff, sewage disposal, and urban runoff. These nutrients stimulate the overgrowth of aquatic plants and algae, leading to hypoxia or oxygen depletion as the decomposing algae consume oxygen. This process adversely affects aquatic life, leading to fish kills and loss of biodiversity. Ultimately, eutrophication disrupts ecosystem function and limits the recreational and economic value of affected waterways, necessitating management strategies to mitigate nutrient loading.