Sewage treatment plants, agricultural activities and industries produce effluents and runoff rich in nutrients. The most important nutrients include carbon, nitrogen and phosphorous compounds, phosphates, nitrates and ammonia. Nutrient assimilation within lakes and rivers results in an increase in the biomass of aquatic plants and animals. Excessive additions of plant nutrients, silt and organic matter can substantially deteriorate water quality. Studies of lake eutrophication and assimilative capacity of rivers examine the capacity of the receiving waters to assimilate additional loadings and ensure that increases in discharges will not substantially degrade water quality.