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HS biology (archived)

Course: HS biology (archived) > Unit 25

Lesson 7: Biogeochemical cycles
Science
HS biology (archived)
Ecology
Biogeochemical cycles
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The phosphorus cycle

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The slow cycling of phosphorus through the biosphere. How phosphorus-containing fertilizers can cause aquatic dead zones.

Key points

  • Phosphorus is an essential nutrient found in the macromolecules of humans and other organisms, including DNA.
  • The phosphorus cycle is slow. Most phosphorus in nature exists in the form of phosphate ion—PO43−.
  • Phosphorus is often the limiting nutrient, or nutrient that is most scarce and thus limits growth, in aquatic ecosystems.
  • When nitrogen and phosphorus from fertilizer are carried in runoff to lakes and oceans, they can cause eutrophication, the overgrowth of algae. The algae may deplete oxygen from the water and create a dead zone.

Introduction

Is phosphorus important? That depends—do you like having DNA, cell membranes, or bones in you body? Hint: The answer is probably yes!
Phosphorus is an essential nutrient for living organisms. It’s a key part of nucleic acids, like DNA and of the phospholipids that form our cell membranes. As calcium phosphate, it also makes up the supportive components of our bones.
In nature, phosphorus is often the limiting nutrient—in other words, the nutrient that’s in shortest supply and puts a limit on growth—and this is particularly true for aquatic, freshwater ecosystems.

Natural cycling of phosphorus

The phosphorus cycle is slow compared to other biogeochemical cycles such as the water, carbon, and nitrogen cycles.1
In nature, phosphorus is found mostly in the form of phosphate ions—PO43−. Phosphate compounds are found in sedimentary rocks, and as the rocks weather—wear down over long time periods—the phosphorus they contain slowly leaches into surface water and soils. Volcanic ash, aerosols, and mineral dust can also be significant phosphate sources, though phosphorus has no real gas phase, unlike other elements such as carbon, nitrogen, and sulfur.
Phosphate compounds in the soil can be taken up by plants and, from there, transferred to animals that eat the plants. When plants and animals excrete wastes or die, phosphates may be taken up by detritivores or returned to the soil. Phosphorus-containing compounds may also be carried in surface runoff to rivers, lakes, and oceans, where they are taken up by aquatic organisms.
When phosphorus-containing compounds from the bodies or wastes of marine organisms sink to the floor of the ocean, they form new sedimentary layers. Over long periods of time, phosphorus-containing sedimentary rock may be moved from the ocean to the land by a geological process called uplift. However, this process is very slow, and the average phosphate ion has an oceanic residence time—time in the ocean—of 20,000 to 100,000 years.
This illustration shows the phosphorus cycle. Phosphorus enters the atmosphere from volcanic aerosols. As this aerosol precipitates to earth, it enters terrestrial food webs. Some of the phosphorus from terrestrial food webs dissolves in streams and lakes, and the remainder enters the soil. Another source of phosphorus is fertilizers. Phosphorus enters the ocean via leaching and runoff, where it becomes dissolved in ocean water or enters marine food webs. Some phosphorus falls to the ocean floor where it becomes sediment. If uplifting occurs, this sediment can return to land.
Image credit: Biogeochemical cycles: Figure 5 by OpenStax College, Concepts of Biology, CC BY 4.0; modification of work by John M. Evans and Howard Perlman, USGS

Eutrophication and dead zones

Most fertilizers used in agriculture—and on lawns and gardens—contain both nitrogen and phosphorus, which may be carried to aquatic ecosystems in surface runoff. Fertilizer carried in runoff may cause excessive growth of algae or other microbes that were previously limited by nitrogen or phosphorus. This phenomenon is called eutrophication. At least in some cases, phosphorus, not nitrogen, seems to be the main driver of eutrophication.2
Why is eutrophication harmful? Some algae make water taste or smell bad or produce toxic compounds.2 Also, when all of those algae die and are decomposed by microbes, large amounts of oxygen are used up as their bodies are broken down. This spike in oxygen usage can sharply lower dissolved oxygen levels in the water and may lead to death by anoxia —lack of oxygen—for other aquatic organisms, such as shellfish and finfish.
Regions of lakes and oceans that are depleted of oxygen due to a nutrient influx are called dead zones. The number of dead zones has increased for several years, and more than 400 of these zones existed in 2008. One of the worst dead zones is off the coast of the United States in the Gulf of Mexico. Fertilizer runoff from the Mississippi River Basin created a dead zone of over 8,463 square miles. As you can see in the figure below, dead zones are found in areas of high industrialization and population density around the world.
World map shows areas where dead zones occur. Dead zones are present along the eastern and western shore of the United States, in the North and Mediterranean Seas, and off the east coast of Asia.
Image credit: Biogeochemical cycles: Figure 6 by OpenStax College, Concepts of Biology, CC BY 4.0; original work: Aquatic dead zones by Robert Simmon and Jesse Allen, NASA Earth Observatory
How can eutrophication be reduced or prevented? Fertilizers, phosphorus-containing detergents, and improperly disposed of sewage can all be sources of nitrogen and phosphorus that drive eutrophication. Using less fertilizer, eliminating phosphorus-containing detergents, and ensuring that sewage does not enter waterways—e.g., from a leaky septic system—are all ways that individuals, companies, and governments can help reduce eutrophication.3,4

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