Note4Students

From UPSC perspective, the following things are important :

Prelims level : Dead Zones

Mains level : Impact of excessive water pollution

• Scientists say this year’s oceanic ‘dead zone’ in the Gulf of Mexico will be one of the largest in recorded history.
• It’s expected to grow to over 8,000 sq. miles, and scientists predict severe harm to marine habitat, impacting fish harvests.

Dead Zone

• Dead zones are hypoxic (low-oxygen) areas in the world’s oceans and large lakes.
• They are caused by “excessive nutrient pollution from human activities coupled with other factors that deplete the oxygen required to support most marine life in bottom and near-bottom water.
• Historically, many of these sites were naturally occurring.
• However, in the 1970s, oceanographers began noting increased instances and expanses of dead zones.
• These occur near inhabited coastlines, where aquatic life is most concentrated.
• The vast middle portions of the oceans, which naturally have little life, are not considered “dead zones”.

Why do they occur?

• Dead zones can be caused by natural and by anthropogenic factors.
• Natural causes include coastal upwelling and changes in wind and water circulation patterns.
• Use of chemical fertilizers is considered the major human-related cause of dead zones around the world.
• Runoff from sewage, urban land use, and fertilizers can also contribute to eutrophication
• They can be caused by an increase in nutrients (particularly nitrogen and phosphorus) in the water, known as eutrophication.
• These chemicals are the fundamental building blocks of single-celled, plant-like organisms that live in the water column, and whose growth is limited in part by the availability of these materials.
• Eutrophication can lead to rapid increases in the density of certain types of these phytoplankton, a phenomenon known as an algal bloom.

How is hypoxia created?

• The major groups of algae are Cyanobacteria, green algae, Dinoflagellates, Coccolithophores and Diatom algae.
• Cyanobacteria are not good food for zooplankton and fish and hence accumulate in water, die, and then decompose.
• The bacterial degradation of their biomass consumes the oxygen in the water, thereby creating the state of hypoxia.

With inputs from:

National Geographic