Eutrophication is the gradual increase in the concentration of biogeochemical gases (Phosphorus and Nitrogen) and other plant nutrients in an aquatic ecosystem such as lake. It is a serious environmental problem since it results in a deterioration of water quality. The productivity or fertility of such an ecosystem naturally increases as the amount of organic material that can be broken down into nutrients increases. This material enters the ecosystem primarily due to runoff from land that carries debris and products of the reproduction and death of terrestrial organisms. Water Blooms, or great concentrations of algae and microscopic organisms, often develop on the surface, preventing the light penetration and oxygen absorption necessary for underwater life. Eutrophic waters are often murky and may support fewer large animals, such as fish and birds, than non-eutrophic waters. Human activities have increased the rate and extent of eutrophication through both point-source discharges and non-point loadings of limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems.

There are basically two different types of Eutrophication. They are: Natural Eutrophication and Cultural Eutrophication.

1 Natural Eutrophication - The process of lake aging caused due to nutrient enrichment and sedimentation is called Natural Eutrophication. During this process, oligotrophic lake gets con­verted into eutrophic lake. It permits the production of phytoplankton, algal blooms and aquatic vegetation including water hyacinth, aquatic weeds, water fern and water lettuce which in turn provide sufficient food for herbivorous zooplankton and fishes.

2.     2 , Cultural Eutrophication-  The cultural eutrophication process consists of a continuous increase in the contribution of biogeochemical nutrients until it exceeds the capacity of the water body, triggering structural changes in the waters. This process is generally speeded up by human activi­ties. It is responsible for the addition of 80% nitrogen and 75% phosphorus to lakes and streams.

Less oxygen dissolved in the water is often referred to as a Dead zone because most marine life either dies, or, if they are mobile such as fish, leave the area. Dead zones are low-oxygen, or hypoxic, areas in the world’s oceans and lakes. Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions. Dead Zones occur due to Eutrophication process. There are around 405 dead zones at present. Arabian Sea Dead Zone is considered largest dead zone of the world. The Chesapeake Bay, on the East Coast of the United States, has one of the first dead zones ever identified, in the 1970s.

References:

https://oceanservice.noaa.gov/facts/eutrophication.html

 https://www.britannica.com/science/eutrophication

 http://www.eniscuola.net/en/2016/11/03/what-is-eutrophication-causes-effects-and-control/

http://www.eniscuola.net/en/2016/11/03/what-is-eutrophication-causes-effects-and-control/

https://oceanservice.noaa.gov/facts/deadzone.html