Excess nitrogen and phosphorus spur algal growth, deplete oxygen and kill fish

Dense algal growth from excess nutrients blocks sunlight, causing submerged plants to die.

Aquatic plants, like their terrestrial counterparts, require nutrients to grow and reproduce. The growth of algae and other vegetation in water bodies is usually controlled by the nutrient whose supply is most limited. This concept, first described by Justis Lieberg in 1840, is known as "Liebig's Law of the Minimum". Phosphorus is usually the limiting nutrient in brackish or freshwater, while nitrogen is usually the limiting nutrient in saltwater. When excess nutrients applied to the land in the form of manure or commercial fertilizer find their way into the water, blooms or overabundant growth of algae or other aquatic plants can result. Algal blooms at the surface can interfere with photosynthesis of submerged plants by blocking sunlight, causing them to die. When this happens, dissolved oxygen levels near the bottom drop because oxygen demand by decomposing bacteria is great while little or no oxygen is being produced by the dying plants. The problem is compounded when organisms which flourish in oxygen starved environments release hydrogen sulfide and methane. These substances are toxic to fish and other aquatic life.

Excessive algal growth in estuaries can result in the decline of Eelgrass and the loss of shellfish beds. Shellfish die and the beds fail to recognize when thick layers of algae prevent animals such as oysters from pumping water through their bodies to provide adequate food and oxygen. Eelgrass, a submerged grass eaten by many waterfowl, is lost when floating algal mats and phytoplankton in the water reduce light penetration and interfere with photosynthesis.

Some species of fish, as well as other animals lower in the food chain, are very sensitive to low levels of oxygen or food and generally die. The loss of species simplifies the food chain of an ecosystem and makes it more vulnerable to further destruction.