Introduction
What are Dead Zones?
Dead zones are areas of low oxygen in the world's oceans and lakes and occur when dissolved oxygen (DO) concentration falls to or below 2 mg of O₂/liter. This low level of oxygen is also referred to as Hypoxia and makes these areas unfavourable for sustaining life and that is why these areas are referred to as dead zones. Once the dissolved oxygen levels decline below 0.5 ml O₂/liter in a body of water, mass mortality occurs.
Do you know that the Baltic Sea is home to seven of the world’s 10 largest marine dead zones and such zones are increasing at an alarming pace around the world?
This acceleration in these dead zones has been attributed to increased runoff from agricultural fertilizers and sewage which increases the amount of nutrients available for algae, leading to the eutrophication process.
But not all dead zones are caused by pollution. The largest dead zone in the world which is the lower portion of the Black Sea occurs naturally. Natural causes of dead zones include variations in wind and ocean circulation patterns, as well as coastal upwelling. Long water residence times, high temperatures, and high levels of sunlight penetration through the water column are all environmental factors that influence the formation or intensity of a dead zone.
Lately, the size of the dead zones or Oxygen Minimum Zones (OMZ) in the Arabian Sea has increased. It is the densest and most intense OMZ among the world's oceans and spans approximately two million square kilometres.
According to a paper published by Nature in 2014, the oxygen deficiency in coastal waters is caused by excessive sewage from densely populated cities such as Mumbai and Karachi.
Causes
These Dead zones can occur naturally, but scientists are concerned about the areas that have been created or enhanced by human activity. Physical, chemical, and biological factors all contribute to the formation of dead zones, but nutrient pollution is considered to be the primary cause.
The surface runoff from agricultural fields transports large amounts of nutrients, which eventually end up in lakes and rivers. Because of the abundance of nutrients, excessive algal blooms and phytoplankton growth consume available oxygen and emit carbon dioxide. This leads to fish and plants competing for oxygen, causing aquatic ecosystems to deoxygenate.
Decomposers also exacerbate the problem by continuing aerobic decomposition, which adds to the excess carbon dioxide and creates hypoxic conditions.
Eutrophication also causes the formation of a layer on the surface that restricts sunlight penetration inhibiting the ability to perform photosynthesis by underwater plants. This leads to decreased concentration of oxygen available to organisms which die as a result.
These algal blooms caused due to Eutrophication are sometimes referred to as "red tides" or "brown tides", but these are not related to tides in any way. This nomenclature is dependent on the colour of the algae and the fact that these algal blooms impact the entire aquatic ecosystem. Additionally, when there is excess cyanobacteria concentration in waterbodies, it is referred to as cyanobacterial bloom.