Chapter 20, Problem 1CR

To determine

The nature and causes of the annual dead zone in the Gulf of Mexico.

Answer to Problem 1CR

The wastewater from cities, factories, and agricultural fields, and runoff of the fertilizers used in agricultural fields into the Gulf of Mexico are the major causes for the anuual dead zone in the Gulf of Mexico. The nitrates and phosphates reaching the water through the fertilizers and wastes causes algal bloom that depletes orxygen content of that particular region of the water body, leading to the formation of a dead zone.

Explanation of Solution

The Mississippi River is a vast basin that ends up in the Gulf of Mexico. It is a significant supplier of the country’s fish and shellfish. The river basin lies within 31 states and contains about two-thirds of the continental U.S. land area.

The nature of the dead zone contains very less amount of oxygen. The condition in the area is suffocating and polluted. The cause of this nature is the water draining from the farms, cities, factories, and sewage plants directly into the Mississippi River. A large amount of crop fertilizers containing nitrates and phosphates flow into the river. The huge quatity of sediment, generally from soil eroded from the Mississippi River basin, reaching the Gulf of Mexico can also lead to the death of the benthic forms of aquatic life.

The excess of plant nutrients that over-fertilize becomes the reason for the excessive growth of algae (phytoplanktons). This is known as algal bloom or eutrophication. They die and sink to the bottoms, which are further decomposed by the hordes of bacteria. During this process, a lot of oxygen may be consumed which results in the depletion of the dissolved oxygen. This massive volume of water with a very less amount of dissolved oxygen is known as a dead zone. It is so because it contains very little or no marine life.