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.