When India’s great water bodies like the Dal lake in Kashmir, Lake Loktak in Manipur and Chilka in Orissa were found to contain toxic minerals, the layman first became familiar with eutrophication. These lakes are in thickly populated areas and surrounded by cultivated lands. The fertilizers used for the crop and the minerals from domestic sewage, which wash away into the lakes, and domestic sewage are a major source of eutrophication.
Eutrophication means enrichment of biological productivity and nutrient contents of water-bodies. But this renders the water quality inadequate for domestic, recreational and other uses. It is a natural process, accelerated by human activity. Enrichment of nutritive metals in the water bodies is a result of precipitation, ground water inflow, generally from the contiguous drainage basin. The plants and sediments accumulate gradually, and eventually destroy the water bodies.
Threat to Reservoirs:
Urbanisation, industrialisation, as modern agricultural practices, have threatened the inland fresh water reservoirs with effluents. The indiscriminate dumping of municipal sewage into these ecosystems have added to the eutrophication problem which is now the central concern of water pollution control.
Eutrophic ponds have such high material deposition that a variety of nutrients are released into the water, which support large populations of algae and higher plants. Nitrogen and phosphorus become the main constituents of the water which originate from household detergents and municipal and industrial waste water. The degree of eutrophication is determined by the number of harmful algal blooms: a bloom containing 0.5 to 1 million cells per litre or by the oxygen concentration in the lower depth of the lakes.
The organic waste effects productivity and the composition of aquatic life, increasing the biomass. The increase in nutrients results in more rapid growth and even in the growth of fish. The more serious result is the increase in algal mass and macrophytic vegetation and induces fish kills. Filters of water treatment plants also get clogged and undesirable taste and odours result leading to increased cost of operation.
Much of the nutrients entering the lake are incorporated into algae, which release the nutrients back into water either alive or dead. Some algal blooms release toxic substances that kill fish, domestic animals and birds and the water begins to stink. Blooms also damage the recreational lakes by interfering with fishing, bathing, boating and often reduce the beauty of the lake.
Algae which can indicate eutrophic condition include Oscillatoria, Microcystis, Melosira and Gragilaria etc, while those such as Gonyaulax catenella are involved in the poisoning of shellfish in coastal areas. Cattle have been poisoned by toxins produced by Microcystis. Other deleterious effects are the production of S gas, which is in itself offensive and can also damage white lead paint on houses and cause decreased water transparency.
Reversal of eutrophication is therefore receiving considerable attention. According to Lee, Lake eutrophication can be controlled or its effects minimised by increasing the nutrients and controlling excessive algae growth in lakes. The algal blooms can be removed by harvesting and mechanical removal processes or by setting up natural food webs such as daphnids and certain kind of fish which utilise the phytoplankton for food.
Nutrients such as phosphorus and nitrogen compounds can be removed through precipitations and physic-chemical methods. Chemicals like copper sulphate, sodium arsenite and zirconium oxychloride can also check algal blooming. Nitrogen can be removed by biological nitrification, denitrification and air stripping of ammonia from alkalized waste water.
“Microbial intervention” developed by the Water Research Council of the University of Massachusetts for controlling eutrophication, involves artificial stimulation of bacterial multiplication to disrupt the algal food web because bacteria and algae compete with each other for nutrients from decaying lake sediments.
Water is unique chemical, essential for human survival; its eutrophication is a matter of grave concern. About 80 per cent of India’s diseases are caused by the use of eutrophicated water. Efforts to make important water bodies pollution-free should reduce the amount of waste in effluent water from industries by reutilising or recycling their components. Control of water bodies and of organisms which protect water should be reinforced by all available means including legal enforcement. A beginning should be made with effective sewage disposal in urban areas.