Water pollution is a large set of adverse effects uponwater bodies such as lakes, rivers, oceans, and groundwater caused by humanactivities. Although natural phenomena such as volcanoes, algae blooms, storms,and earthquakes also cause major changes in water quality and the ecologicalstatus of water, these are not deemed to be pollution. Water pollution has manycauses and characteristics. Increases in nutrient loading may lead toeutrophication. Organic wastes such as sewage impose high oxygen demands on thereceiving water leading to oxygen depletion with potentially severe impacts onthe whole eco-system. Industries discharge a variety of pollutants in theirwastewater including heavy metals, organic toxins, oils, nutrients, and solids.Discharges can also have thermal effects, especially those from power stations,and these too reduce the available oxygen. Silt-bearing runoff from manyactivities including construction sites, deforestation and agriculture caninhibit the penetration of sunlight through the water column, restrictingphotosynthesis and causing blanketing of the lake or river bed, inturn damaging ecological systems.