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.
Eutrophication also causes a chain reaction in the ecosystem when excess algae and plant matter decompose and emit large amounts of carbon dioxide. This causes ocean acidification by lowering the pH of seawater which slows fish and shellfish growth and can prevent bivalve mollusk shell formation. As a result, commercial and recreational fisheries catch less, resulting in smaller harvests and more expensive seafood.
Although eutrophication also happens naturally over time as lakes age and fill with sediment., it is human activities that have accelerated the rate and extent of eutrophication by introducing limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems.
Threats to Human Health and Biodiversity
Algal blooms also bring on larger-scale issues like human illness. When filter feeders such as oysters and other shellfish consume algal bloom-related microbes, they become poisonous and their consumption as seafood can cause illness or even death in humans.
Algal blooms can also kill marine mammals and shorebirds that rely on the marine ecosystem for sustenance. Scavenger birds like herons and marine mammals like sea lions rely on fish for survival and with fewer fish below algal blooms, these creatures lose a vital food source.
Conclusion
It is no secret that mindless use of fertilisers has brought us here. People in general and farmers, in particular, must be made aware of this issue. The Indian government has taken some steps such as introducing soil health card scheme that enables the judicious and appropriate use of fertilisers based on the soil's requirements; this is a step forward as it will reduce waste and runoff.
But a multidisciplinary, integrated strategy that takes into account the full range of climate variables is required to track and potentially reverse the spread of these dead zones. In order to reverse this trend, terrace farming and maintaining riverbank vegetation as nutrient sinks are viable options and they will require a systemic effort in forms of appropriate infrastructural development and progressive policy changes.