Explanation:

Hypoxic areas in the world's oceans and large lakes.

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.