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Phosphorus

Phosphorus is an essential nutrient for the plants and animals that make up the aquatic food web. Since phosphorus is the nutrient in shortest supply in most fresh waters, even a modest increase in phosphorus can, under the right conditions, set off a whole chain of undesirable events in a lake or river, including accelerated plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals.

Forms of Phosphorus

Phosphorus has a complicated story. Pure, "elemental" phosphorus (P) is rare. In nature, phosphorus is usually on friendly terms with four oxygen atoms in a relationship known as a phosphate molecule (PO4-3).

Phosphorus in aquatic systems occurs as organic phosphate and inorganic phosphate. Organic phosphate consists of a phosphate molecule associated with a carbon-based molecule, as in plant or animal tissue. Phosphate that is not associated with organic material is inorganic. Inorganic phosphate is the form required by plants. Animals can utilize either organic or inorganic phosphate.

Both organic and inorganic phosphate can either be dissolved in the water or suspended (attached to particles in the water column).

The Phosphorus Cycle

Things would be a little simpler if phosphorus stayed put in one form - but it doesn't. It cycles. Aquatic plants and animals take in dissolved inorganic phosphorus and convert it to organic phosphorus as it becomes part of their tissues.

As plants and animals die or excrete, the organic phosphorus they contain sinks to the bottom, where bacterial decomposition converts it back to inorganic phosphorus. This inorganic phosphorus gets back into the water column when the bottom gets stirred up by animals, chemical interactions, or water currents. Then it's taken up by plants and the cycle begins again.

In lakes that undergo seasonal stratification (layering of the water column by temperature differences from top to bottom), the availability of phosphorus in the water column varies seasonally. In summer and winter, when the lake is stratified, the phosphorus on and near the bottom is trapped by the cooler, more dense bottom waters. During spring and fall overturn, as the surface and bottom water reach the same temperature and mix, this trapped phosphorus becomes available in upper layers of the water column.

Under anoxic conditions (i.e., no detectable dissolved oxygen in the water), phosphorus from lake sediments is released into the water column, via a chemical reaction.

Sources of Phosphorus

There are many sources of phosphorus, both natural and human. These include soil and rocks, wastewater treatment plants, runoff from fertilized lawns and crop land, failing septic systems, runoff from manure storage areas, disturbed land areas, drained wetlands, and commercial cleaning preparations. These sources may be connected to the water body either by a pipe or by the myriad paths stormwater runoff follows from the land to the water. The large number of sources and the variety of routes that phosphorus can take make it difficult to monitor or correct problems with phosphorus over-enrichment.