Eutrophication in lakes of Nepal

Increasing inputs of inorganic fertilizers and pesticides and inefficient use of organic wastes in agriculture has adversely affected the quality of soil and water resources. The excessive enrichment of surface water with nutrients corresponded by high production of autotrophs has caused eutrophication in several lakes of the world. According to the Survey of the State of the World’s Lakes, a project promoted by the International Lake Environment Committee, eutrophication has affected 54% of Asian lakes. In Nepal about 720,000 ha area is occupied by water bodies and some 3.2% is occupied by lakes including ghole, ponds and reservoirs. Lakes and ponds play important role as it is the habitat for a wide variety of aquatic plants and animals. Lakes have vital role in national economic activities, since Pokhara valley has been a tourist destination as it consists three most beautiful lakes like Rupa, Fewa and Begnas.

 Eutrophication is a natural process that typically occurs as lakes ages. However, human accelerated eutrophication (called "cultural eutrophication") occurs more rapidly, and causes problems in the affected water bodies. Nutrients enter the watersheds via precipitation, fertilizers, nitrogen fixation, irrigation, and weathering. The nutrient losses are also derived from runoff, leaching, floods, subsurface flow, deep seepage from agriculture fields and animal farms. Agriculture activities and livestock are the two main nutrient sources responsible for the eutrophication, besides human–urban and industrial wastewater discharges accelerated eutrophication of surface water by high phosphate and nitrate losses from domestic waste. Nutrient enrichment is the starting point of eutrophication in any water body and is followed by uncontrolled growth of primary producers which depletes oxygen owing to decomposition of algal organic matter. The undesirable overgrowth of phytoplankton and their subsequent death forms a greenish slime layer over the surface of water body, which restricts the light penetration. When the abundant aquatic plants decompose, much oxygen is consumed by those decomposers causing deficiency of dissolved oxygen. Lowered oxygen results in the death of fish that need high levels of dissolved oxygen.

The application of herbicides and algaecides for control of eutrophication can be ineffective as it can be quite expensive and impractical for a large ecosystem. The totally mechanical process of lake clearing can be unfeasible for the country like ours. Besides, control prevention techniques would be better option to reduce the concentrations of the nutrients (nitrogen and phosphorus) in lakes. The possible activities to be undertaken to prevent the introduction of nutrients in lakes can be reduction of fertilizer runoff from fields by using agricultural techniques like proper planning of fertilization and use of slow release fertilizers. To limit Nitrogen and phosphorus loads from households’ reduction in use of phosphorous in detergent and proper management of waste water from animal husbandry is important.