Non-Point Source - Runoff
All water flows to the ocean. Whether a farmer in the Midwest or homeowner on the coast, the fertilizers and pesticides used for lawn care and farming practices eventually end up in the oceans. When it rains, the water carries loose soil from the land, into local bodies of water. Fertilizers and pesticides used within local watersheds find their way to the ocean. This is considered a nonpoint source pollution because it is not possible to determine the origin of the fertilizer.
Fertilizers contain nitrogen and phosphorus, which are important nutrients for crops. When these fertilizers reach bodies of water, the result is a rapid growth in phytoplankton, or algae. Many people may describe this rapid growth, or bloom, as explosive because the algae and plants seem to appear to grow overnight The algae bloom occurs because algae reproduce very fast when there are plenty of nutrients available. Once the bloom has happened, the algae compete for sunlight and nutrients. When the nutrients run out, there is a mass die-off of algae. The first organisms to happily respond to the increase in dead organisms are the bacteria that live off the dead plants and algae. Since the bacteria find that they have an endless food supply, they too begin to experience a rapid growth. Bacteria are dependent upon the available oxygen in the ocean in order to decompose the dead algae and plants.
As the bacteria continue to reproduce, they remove large amounts of oxygen in the water. The lack of oxygen begins to kill fish and other animals, often called a fish kill. With the addition of more dead organisms, bacteria keep reproducing, continuing to decrease the amount of oxygen in the ocean.
Just as with the oil deposit, excess amounts of algae can block the suns rays from entering the ocean. When this occurs, phytoplankton and plants can no longer perform photosynthesis. This disruption in the food web will negatively affect all animals involved by decreasing the amount of food available to them.
Another issue with nonpoint source pollution is that pollutants can get into the food chain. Sometimes pollutants including heavy metals like mercury and other contaminants like pesticides run off of the land and are taken up by phytoplankton. When zooplankton eat the phytoplankton, they get the pollutants inside of them. Then the zooplankton are eaten, and so forth. Animals such as fish, seals, and polar bears that are near the top of the food chain often get a lot of pollutants building up inside their bodies.
Nonpoint source pollutants are also known to cause disease in marine animals. Some sea turtles, for example, get infections from bacteria that run off the land.