Eutrophication:

Materials such as sewage or organic wastes from milk plants, canneries, slaughter houses, paper mills, starch factories and fish processing plants, and runoff from agricultural lands greatly increase the productivity of waters and cause algae to grow in abundance, so that sometimes water surface gets entirely covered by  algae. This is called 'algal bloom'.  In general, the entire water body becomes a green nourishing soup. Eventually, the death of this algae promotes the growth of a very large  'decomposer'  population.

The decomposers break down the dead algae using a lot of oxygen present  in  the water.  In addition, the living algae also consume oxygen from water at night for  their respiration. The decomposers, and the algae cause decreased amounts of oxygen available to fishes, ultimately causing them to die. The problem of eutrophication or excessive nourishment leading to loss of life in water bodies mainly occurs in ponds and lakes and not in flowing water. Now the question arises what is the solution to  this problem? One solution  is to process the sewage thoroughly, to remove nutrients to prevent 'algal bloom'.  Since such a treatment is expensive, further  ways are being explored. One suggestion is to use this water to  irrigate crops and/ or to grow algae and aquatic plants  in a man made shallow pond which can be used for making biogas.