An ecosystem can be as small as a puddle of water, or as vast as a desert. It can be defined as a specific area comprised of living organisms -- e.g., flora and fauna -- and the non-living factors that make up their habitat. Within that ecosystem, a limiting nutrient is a relatively scarce naturally occurring element. Growth only occurs as long as the nutrient is available.
Lakes and rivers are freshwater systems that depend on phosphorous and nitrogen to maintain the balance of plant and animal life in them. Generally speaking, phosphorous is the limiting nutrient in freshwater systems, meaning less phosphorous occurs naturally in rivers and lakes than nitrogen; this limits the amount of plant life that can grow in a body of water. When phosphorous quantities rise, plants grow to nuisance levels, choking rivers and making navigation difficult. In lakes, excess phosphorous fuels algal blooms that deplete water of oxygen and can lead to fish kills; this phenomena is known as eutrophication. Excess phosphorous enters bodies of water from fertilizer runoff on lawns and sewage treatment plants.
Nitrogen and phosphorous both occur naturally in the ocean, where they support the growth of aquatic plants that shellfish and other marine organisms feed on. Nitrogen is usually the limiting nutrient that keeps ocean ecosystems in balance. When it increases in quantity, phytoplankton blooms can result. The microscopic plant grows at an accelerated rate, forming a green scum on the water’s surface near land. Excess nitrogen enters ocean ecosystems through storm water runoff and burning fossil fuels.
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Plants that live in land-based ecosystems, such as a forest, require thirteen different minerals to live. When one of these nutrients is missing or in short supply, it is considered a limiting nutrient. Phosphorous and nitrogen are usually limiting nutrients because plants require large amounts of them on a daily basis. However, micronutrients like iron and boron can be limiting nutrients if they are scarce whereas adequate amounts of nitrogen and phosphorous exist. A soil nutrient in limited supply results in stunted growth or a lower number of plants in an ecosystem.
Impact On Ecosystems
Limiting nutrients are important because they determine the quantity of plants available for animals to feed on. This influences how many animals can live in a certain habitat. When a limiting nutrient is too scarce, animal populations decline; when it increases, animal populations swell. The limiting nutrient in an ecosystem should not be confused with limiting factors such as food, shelter, temperature and space, all of which affect rise and decline of animal populations. The term "limiting nutrient" refers to an element used in the production of food, but not the food itself.