3) What are the two most likely causes of the dead zones? (please don't use humans as a cause, list specific actions or byproducts of humans)

4) Describe how fertilizers cause hypoxia and consequently dead zones in coastal ecosystems.

The coastal upwelling and changes in wind and water circulation patterns is the main natural cause of dead zones.

Fertilizers cause hypoxia due to the over growth of algae on the surface of water bodies.

Explanation:

Coastal upwelling is the cause of dead zones in which water that is present in depth comes to the surface of ocean due to the movement of water because of wind. Fertilizers cause hypoxia due to the over growth of algae on the surface of water bodies. The fertilizers increases the algal growth by providing nutrients necessary for its growth. This over growth decreases oxygen concentration in water and affect marine organisms.