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May 18, 2008 / Mridul

Global Warming, Eutrophication Creating Death Zones In Oceans

A recent article on the Times Online website looks at the rapidly increasing ‘Dead Zones’ in our seas and oceans. Dead zones are the regions where the marine life cease to survive mainly due to lack of oxygen. Rapid depletion of oxygen in the oceans is leading to great loss of biodiversity which has been deteriorated by 25% during the last three decades.

Global Warming Is Heating The Seas

Solubility of oxygen decreases with increase in temperature so the increasing temperatures deplete the dissolved oxygen leading to death of millions of marine organisms. As stated in the Times article the concentration of DO decreases from 10ml to just 4ml as the temperature increases from 0°C to 25°C. 4ml is the bare minimum required by most marine organisms to survive on while many require higher concentration of DO.

Due to the changing temperatures the directions of the ocean currents have also changed significantly. These currents now carry warm waters to the areas which never received warm waters. The warm and cold waters form layers in different levels of the oceans disturbing the ecosystem of the ocean. This thermal stratification further prevents dissolution of free oxygen in the oceans leading to further loss of marine life.

Excessive Use Of Fertilizers Choking The Oceans

Farmers without the scientific knowledge about the use of these chemicals may use them in excess. During the irrigation these fertilizers mix with the water which then joins the nearby stream/river in the form of surface runoff. The fertilizers are not easily decomposed and require huge amounts of oxygen for there decomposition and they use up all the free oxygen present in the water.

As the DO depletes the marine organisms die whereas algae grow at an exponential rate. These algae owing to the presence of pigmented cells give green, red or brown color to the water. As these algae grow in number they consume more oxygen which results in death of other organisms which now act as food for these algae and hence their growth is further accelerated.

Many areas around the world are suffering from the problem of eutrophication. The Gulf of Mexico, Caspian Sea, Bering Sea and Arabian Sea. The Gulf of Mexico already has a huge Dead Zone which the scientists warn could expand further.

A NASA photo showing phytoplankton growth along the North American coasts

Phytoplankton concentration along the North American Coastline

Efficient Use Of Fertilizers

Most fertilizers contain Phosphorus and Nitrogen on which these algae thrive hence it is that we use fertilizers that a) are biodegradable and b) contain lesser quantities of these elements. Also the farmers need to irrigate their lands in a scientific manner. Each crop requires a definite amount of water to give the best yield hence the farmers shouldn’t over-irrigate their lands since it could lead to more voluminous runoffs.

The rivers and seas do have a self purification or self cleansing mechanism which works to dilute the adverse effects pollutant but we must realize that the rate at which we are dumping pollutants into the oceans and rivers has far exceeded their capacity to self purify themselves. And thus it becomes essential that we help them maintain their ecological balance.


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