What causes the size and shape of the dead zone to vary?

Question

Regarding hypoxia and dead zones

Answer 1

The depletion of oxygen

The formation of oxygen-depleted subsurface waters are associated with nutrient-rich discharge (eutrophication). Bio-available nutrients in the discharge can stimulate algal blooms, which die and are eaten by bacteria, depleting the oxygen in the subsurface water.

The size and shape of the algal bloom determines the size and shape of the dead zone.

Nutrient-rich discharge is mostly due to a combination of natural processes, as well as possible human activities such as land clearing which result in the development of widespread algal blooms and associated low-oxygen bottom water.

An example of a natural process involved is enclosed bodies of water such as fjords or the Black Sea that have shallow sills at their entrances causing water to be stagnant there for a long time. The eastern tropical Pacific Ocean and Northern Indian Ocean have lowered oxygen concentrations which are thought to be in regions where there is minimal circulation to replace the oxygen that is consumed.

Off the coast of Cape Perpetua, Oregon, there is also a dead zone with a 2006 reported size of 300 square miles. This dead zone is unique in that it only exists during the summer, perhaps due to wind patterns.