What Causes Dead Zones in Oceans?

Hurricanes may help limit the expansion of dead zones by dispersing algae and aerating the sea water.

Dead zones are the areas in oceans and large bodies of freshwater like lakes, where the level of dissolved oxygen is too low to sustain marine life. In other words, these are hypoxic zones, where the amount of oxygen dissolved in water depletes to such extent, that fish and other aquatic life cannot survive.

In such zones, most marine life either dies or migrates to other areas, thus turning these hypoxic zones into biological deserts.

Dead or hypoxic zones can occur naturally, but there has been a staggering increase in their numbers, owing to human activities that pollute ocean water. It has been observed that the numbers of dead zones are almost doubling in every decade since 1960.

So far, NASA has identified 415 dead zones, and the numbers are expected to increase significantly in the coming years.

Dead zones are formed in the oceans and large freshwater bodies due to eutrophication, which refers to an increase in chemical nutrients, particularly nitrogen and phosphorous, in the water.

These two nutrients are known to stimulate the growth of plants and phytoplanktons like algae. This, in turn, causes an explosion of algae population in water, known as an algal bloom. As a result of algal bloom, the dissolved oxygen in water reduces.

Though algae can produce oxygen in the daytime with the help of photosynthesis, they use the dissolved oxygen in the night.

Moreover, an algal bloom is often followed by the death of large quantities of algae, as these phytoplanktons are short-lived. Some algae can also produce toxins, which can cause death of marine life, including fish.

The dead algae, fish, and other organic matter are then decomposed by bacteria using oxygen. This depletes the level of dissolved oxygen in water, and eventually a dead zone is created when the level of oxygen becomes too low to support marine life.

Several biological, chemical, and physical factors can be associated with the formation and expansion of oceanic dead zones. Basically, dead zones are created in the oceans due to an increase in chemical nutrients in water, that can be caused by both natural factors, as well as human activities, which are discussed below.

The most important natural cause of dead zones is coastal upwelling. It refers to a phenomenon wherein the deep water of the ocean is pushed upwards to replace the wind-driven surface water. The deep ocean water is rich in nutrients, and therefore, promotes algal growth. It is believed that climate change and global warming may have an association with the increased frequency of the wind patterns that cause coastal upwelling.

Water stratification refers to a process where the least dense water forms a layer above the denser water, and the difference in density prevents the mixing of these layers. It occurs when freshwater from rivers sits above the denser saline water of the ocean. As a result, the oxygen added to the top layer via the air cannot travel down to the bottom layer. Climate change can increase river water runoff, and thus may have a role to play in the creation of such dead zones.

Apart from the aforementioned causes, dead zones can have an association with changes in wind and water circulation pattern. Enclosed bodies of water, where water remains stagnant, are more likely to have low levels of dissolved oxygen. A dead zone can develop in such water bodies when the circulation of water is not enough to replace the oxygen that has been used.

Certain human activities are considered as the main reasons behind the expansion of dead zones. Increased use of chemical fertilizers, mainly nitrogenous fertilizers, is one of the most important factors that contributes to the process of dead zone formation. Chemical fertilizers, as well as pesticides and insecticides used in intensive farming, escape into water bodies like rivers and eventually end up in the ocean.

Dead zones can have a great impact on benthic biomass and biodiversity. As marine life cannot survive in dead zones, their increasing numbers can have an adverse effect on seafood industry. This can eventually affect the economies of coastal states.

Apart from this, dead zones can contribute to climate change due to an increased production of nitrous oxide in these areas. Nitrous oxide is a greenhouse gas, that is produced in the low-oxygen water of dead zones by microbial respiration and denitrification (the process of reducing nitrogen by bacteria).

It is possible to reverse dead zones, with the Black Sea being the best example in this regard.

The Black Sea was the largest dead zone in the late 80s. But between 1991 and 2001, the process was reversed when the prices of fertilizers increased significantly following the collapse of the Soviet Union, and the centrally planned economies of Eastern and Central Europe.

As a result, the use of fertilizers dropped drastically, which reduced the amount of nitrogen entering the sea water. It is true that this reversal was unintentional, but it proved that dead zones can be reversed with certain simple measures given below.

» Reducing the use of fertilizers, and adjusting the time of fertilizer application, so as to limit runoff to the coastal waters.

» Conservation of wetlands and floodplains, which can help control and filter runoff by taking up the nutrients.

» Proper monitoring of septic system and sewage treatment facilities, and prevention of sewage leak. This can help reduce waste and nutrients discharged into the surface and ground water. Even recycling of sewage can prove beneficial in this regard.

» Preventing the entry of animal waste into water bodies.

» Reducing the industrial discharge of chemicals, nutrients, and organic matter.

So, like many other ecological problems, expanding dead zones can also be reversed by controlling certain human activities. However, the actual process of reversal can take decades, and therefore, our first priority should be a reduction in water pollution. By closely monitoring the things we put into the oceans, we can help maintain the required oxygen level in ocean water. Otherwise, the fragile ecosystem of the oceans will soon reach a point where it won't be able to sustain life.