Lab 7 Identifying Limiting Nutrient - Sayre School

Lab 7: The Ecosystem Drawing
Phosphorus is an important element for making DNA and RNA, and is also a component of
ATP which serves as an energy source. The phosphorus cycle differs from the other major
cycles because it does not include a gas phase.
When it rains, phosphates are removed from the rocks (via weathering) and are distributed
throughout both soils and water. Plants take up the phosphate from the soil. The phosphates
then move from plants to animals when herbivores eat plants and carnivores herbivores.
The phosphates in the animal tissue eventually return to the soil through excretion, and
decomposition of plants and animals. The same process occurs in aquatic ecosystem.
algae bloom off
coast of Florida
While obviously beneficial for many biological processes, an excessive amount of phosphorus
is considered a pollutant in water. Phosphate stimulates the growth of plankton and plants.
When there are no longer enough nutrients, the plants die. Excess growth of decomposers
tends to consume large amounts of dissolved oxygen, potentially suffocating fish and other
marine animals. While the excess dead plants block available sunlight to bottom dwelling
species. This is known as eutrophication.
Humans can alter the phosphorus cycle in many ways, including deforesting the tropical rain
forests and the using agricultural fertilizers. As the forest is cut, heavy rains quickly wash away
nutrients originally stored in plants. Agricultural runoff provides much of the phosphate
found in waterways. Crops often cannot absorb all of the fertilizer in the soils, causing excess
fertilizer runoff and increasing phosphate levels in rivers and other bodies of water.
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glossary 7.6