Question

The most immediate (final) cause of dead zones in the Gulf of Mexico is...

a) excess photosynthesis in coastal waters

b) decomposition of algae

c) transport of nutrients downstream

d) fertilizers applied to agricultural fields

e) leaching of fertilizers into streams

Answer 1

The most immediate (final) cause of dead zones in the Gulf of Mexico is - c) transport of nutrients downstream.

Explanation :

• All the other options except c are other associated phenomenon that are responsible for dead zones. However, the root cause is the third option.
• When nutrients, especially like oxygen and phosphorus, run off into the gulf, they lead to a boom of algal growth. These nutrients come from the fertilisers that are applied to the agricultural fields. The excess fertilisers leach off into the streams, especially during monsoon. Apart from fertilisers, the source of these nutrients can be traced to a variety of other human activities.
• The increase in algal bloom as a result of these nutrients in the gulf water is called eutrophication. These algal blooms, especially the cyanobacteria are not consumed by the aquatic animals and hence they accumulate in water and then subsequently die and decompose.
• It is this bacterial decomposition of these algae that exhaust the oxygen in the water resulting in the formation of a state of hypoxia.
• Thus it can be seen that out of all the options, the ultimate or the final cause is the transport of nutrients downstream.