Nutrient limitation

Ecologists are often interested in the primary productivity of an ecosystem, which is the rate at which organic matter is created by producers. One factor that controls the primary productivity of an ecosystem is the amount of available nutrients. If a nutrient is in short supply, it will limit an organism’s growth. When an ecosystem is limited bt a single nutrient that is scarce or cycles very slowly, this substance is called a limiting nutrient.

Farmers, who are well aware of this phenomenon, apply fertilizers to their crops to boost their productivity. Fertilizers usually contain three nutrients help plants grow larger and more quickly than they would in unfertilized soil.

The open oceans of the world can e considered nutrient-poor environments compared to the land. Seawater contains at most only 0.00005 percent nitrogen, or 1/10000 of the amount typically found in soil. In the ocean and other saltwater environments, nitrogen is often the limiting nutrient. In some areas of the ocean, however, silica or even iron can be the limiting nutrient. In streams, lakes, and freshwater environments, phosphorus is typically the limiting nutrient.

When an aquatic ecosystem receives a large input of a limiting nutrient---for example, runoff from heavily fertilized fields---the result is often an immediate increase in the amount of algae and other producers. This result is called an algal bloom. Why do algal blooms occur? There are more nutrients available, so the producers can grow and reproduce more quickly. If there are not enough consumers to eat the excess algae, conditions can become so favorable for growth that algae cover the surface of the water. Algal blooms can sometimes upset the health of an ecosystem.