Two nutrients that aquatic plants and animals need to grow are Nitrogen and Phosphorus.

Nitrogen

Aquatic organisms need nitrogen to live and can find it in different sources throughout nature. There are two ways a living organism can get the nitrogen it needs:

1. Eat aquatic plants, such as blue-green algae, which take nitrogen from the water and convert it to ammonia or nitrate.
2. Eat other organisms that have already eaten the aquatic plants.

Phosphorus

Phosphorus, just like nitrogen, hydrogen or oxygen, is an element found in nature. Phosphorus is usually found in the form of phosphate but is used up quickly in nature because plants and animals need it to grow.

Algae doesn't need much nitrogen or phosphorus to live, so when many nutrients are in the environment, the algae can grow at a very fast rate. The large amount of algal growth is called an algal bloom. When the algae dies and decomposes, the dissolved oxygen is used up, some times to the point where animals cannot survive.

Increased nutrients can cause increased algae growth.

But how does the nitrogen get into nature in the first place?

When humans release large amounts of nutrients into the environment, it is called cultural eutrophication. Different sources of nutrients are fertilizers that are in the runoff from a farm, human wastes and animal wastes.