Nutrients - Phosphorus & Nitrogen


Nutrients are elements that are needed by plants and animals for growth.


Why does it cause pollution?

When people discuss water quality and nutrients that cause pollution, they are usually talking about nitrogen and phosphorus. Most other nutrients, like calcium, magnesium, sodium, and potassium are naturally in high abundance in nature. Nitrogen and phosphorus are frequently called “limiting nutrients.” This means that at any given time, one or the other of these nutrients may be the hardest nutrient for a plant to acquire and therefore be the only nutrient that is limiting the plant's growth. So, the plant's growth is often directly related to the amount of whichever limiting nutrient (nitrogen or phosphorus) that it is are able to get.

Generally speaking, phosphorus is a limiting nutrient in freshwater systems and nitrogen is a limiting nutrient in saltwater systems.

So, back to our fundamental question, “Why do nutrients cause pollution?” Limiting nutrients cause pollution when they enter a waterbody in relatively high amounts. If the conditions are right, where there is a lot of sunlight, the water is clear, and the temperature is high, nutrients cause algae to grow. When this happens, the algae use the sunlight to make food for themselves and increase their numbers. This is called an algae bloom. Algae blooms can result in large floating “mats” of algae. When the algae die, however, they sink as dead organic matter. Organic matter on the bottom of the water column is used by bacteria for food. In order for the bacteria to break down the organic matter they use dissolved oxygen. This can lead to hypoxia and fish kills.


How does it get into water?

Farmers and homeowners use nutrients in the form of inorganic fertilizers to make plants grow. Farmers use fertilizers to produce food and fiber crops; whereas homeowners use fertilizers to make their grass greener. However, when it rains, excess fertilizers can dissolve in the rainwater and run off of the land and into the nearest bayou or stream.

Untreated sewage runoff is also a large source of nutrients. Before modern, man-made inorganic fertilizers people in agriculture used animal waste or manure to fertilize their fields. Sometimes they even used human waste. It is high in nutrients, including phosphorus and nitrogen. Municipal (city) sewage systems and individual home sewage systems contribute large amounts of nutrients to waterways each year.


What land uses are the source of this type of pollution in the B-T Basin?

This type of pollutant does not cause documented problems in the Barataria and Terrebonne Basins. In some other places, nutrient pollution might be caused by urban or agricultural runoff, or by untreated sewage.

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