Fertilizers are a mixture of phosphate, potassium, and nitrogen. These are all naturally-occurring elements. Fertilizers are useful when soil is insufficient in one of these elements to strengthen its productivity. Although fertilizers are useful in many ways, too much can effect the environment negatively.
When runoff of fertilizer occurs, algae and other aquatic plants begin to take over aquatic environments. Nitrogen is normally in short supply in coastal ecosystems. This small supply limits the creation of new organic matter by phytoplankton, tiny plants at the bottom of the marine food chain. When an ecosystem has an increase in nitrogen, phytoplankton populations can increase drastically. This results in an 'algal bloom' and ruins the state of equilibrium between the environment itself and its resources. Oxygen-consuming bacteria consume the plankton that die and sink to the sea floor. When there is a greater amount of plankton sinking to the sea floor, such as following after an algal bloom, bacteria use oxygen more quickly than the water layers above can replace it. This creates low oxygen conditions, which are unsuitable for plant life.

Elizabeth Mastoris

Additional information:
Effects of fertilizer runoff in the Gulf of Mexico-
Fertilizer runoff creates dead zones in streams and rivers-

Agricultural fertilizers polluting coastal ecosystems-

Effects of fertilizer on aquatic plants-