Phosphorus in water

Why is phosphorus important?

Both phosphorus and nitrogen are essential nutrients for the plants and animals that make up the aquatic food web. Since phosphorus is the nutrient in short 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 stream including accelerated plant growth, algae blooms, low dissolved oxygen, and the death of certain fish, invertebrates, and other aquatic animals.

There are many sources of phosphorus, both natural and human. These include soil and rocks, wastewater treatment plants, runoff from fertilised lawns and cropland, failing septic systems, runoff from animal manure storage areas, disturbed land areas, drained wetlands, water treatment, and commercial cleaning preparations.

Phosphates enter waterways from human and animal waste, phosphorus rich bedrock, laundry, cleaning, industrial effluents, and fertiliser runoff. These phosphates become detrimental when they over fertilise aquatic plants and cause stepped up eutrophication. This process results from the increase of nutrients within the body of water which, in turn, create plant growth. The plants die more quickly than they can be decomposed. This dead plant matter builds up and together with sediment entering the water, fills in the river bed, bed of the bay, or lake making it more shallow. Normally this process takes thousands of years. Cultural eutrophication is an unnatural speeding up of this process because of man's addition of phosphates, nitrogen, and sediment to the water. Bodies of water are being aged at a much faster rate than geological forces can create new ones.In testing for cultural eutrophication, one would expect to find an algal bloom or scum on the water accompanied by a fishy smell to the water and a low dissolved oxygen content. Do not expect to find a high phosphate reading if the algae are already blooming, as the phosphates will already be in the algae, not in the water. The algae bloom should start where running water enters the lake or bay, so test the water before the area where the bloom begins for high phosphate and nitrate levels.

Healthy Parameters

0.01 - 0.03 mg/L - the level in uncontaminated waters

0.025 - 0.1 mg/L - level at which plant growth is stimulated

0.1 mg/L - maximum acceptable to avoid accelerated eutrophication

> 0.1 mg/L - accelerated growth and consequent problems

If too much phosphate is present in the water the algae and weeds will grow rapidly, may choke the waterway, and use up large amounts of precious oxygen (in the absence of photosynthesis and as the algae and plants die and are consumed by aerobic bacteria.) The result may be the death of many fish and aquatic organisms.

Phosphates are not toxic to people or animals unless they are present in very high levels. Digestive problems could occur from extremely high levels of phosphate.

Criteria: The following criteria for total phosphorus were recommended by US EPA (1986):

1. no more than 0.1 mg/L for streams which do not empty into reservoirs,

2. no more than 0.05 mg/L for streams discharging into reservoirs, and

3. no more than 0.025 mg/L for reservoirs.

Forms of phosphorus

Phosphorus has a complicated story. Pure, "elemental" phosphorus (P) is rare. In nature, phosphorus usually exists as part of a phosphate molecule (PO₄). 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 phosphorus is the form required by plants. Animals can use either organic or inorganic phosphate.

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