Phosphorous Information Sheet

Background

Phosphorus is an essential element for the growth and metabolism of plants and animals. It is a major component of nucleic acids and cell membranes. Despite its importance, in most streams phosphorus is found in low concentrations, therefore becoming the limiting factor for aquatic plant growth. The amount in water is generally no more than .1 mg/L.

Phosphorus occurs in two forms: organic and inorganic phosphates (PO4-). Organic phosphates are derived from living plants, animals, their byproducts, and remains. Inorganic phosphate (also known as orthophosphates, free phosphates, or reactive phosphates) naturally occur and are bonded to soil particles. In streams, plants and algae readily absorb inorganic phosphates, which causes them to grow rapidly. Our test kits determine the concentration of inorganic phosphate only.

Inputs of phosphorus into streams are derived from natural and unnatural sources. Phosphorus tends to bind to soil and sediment particles and can be washed into streams when soil erodes. Soil erosion can be natural process during rain events, especially in regions where the geologic conditions include erodible sediment or bedrock and fast flowing waters. These areas can have high phosphorus concentrations. When humans alter the land in ways that increase soil erosion (deforestation, removal of riparian zones, construction sites, poor agricultural tilling practices), an overabundance of phosphorus can enter a waterway.

Some other unnatural sources of phosphorus inputs into streams include human wastes, industrial wastes, inputs from fertilizer runoff, and the drainage of wetlands. Old, outdated sewage treatment plants usually contribute to increased phosphorus levels in streams, especially in heavy precipitation events. This is because many storm drains are connected to sewage lines causing the sewage treatment plants to overflow and have to release untreated sewage to a stream. Rural households with faulty septic tanks can also add phosphorus. Human waste is not the only culprit; animal waste from farms contributes, especially from livestock that graze near or even in a stream.

Industries can add phosphorus when they add any organic wastes (food waste from processing plants), cleaning detergents that contain phosphates, and phosphoric acid industrial strength cleaners. Fertilizers that are placed onto lawns and crops usually contain phosphorus. If applied in excess, the phosphorus from the fertilizers is washed into streams from rain events.

Phosphorus is often locked up at the bottom of lakes and wetlands, bound to the bottom sediments or metal ions that have sunk to the bottom (see the Ferrous Iron Information Sheet). The phosphorus is naturally reintroduced to a lake's water column during fall and spring overturns, caused by temperature inversions in the water. Any human disturbance of a wetland or lake bottom can also release this locked up phosphorus. Completing draining wetlands for development or farmland is a disturbance that can add phosphorus to streams. By bringing the organic matter and soil/sediment bound phosphorus that were locked up in wetland bottoms to a land surface, storms erode this new soil into local waterways.

Human Impact

Alteration of the concentrations of phosphorus in natural systems by humans' influence usually results in drastic changes in the ecology of the stream. Because levels of phosphates in nature are usually low and are the limiting factor in the growth of plants in most naturally controlled systems, too much phosphorus can unbalance the system.

Excessive phosphorus causes rapid growth in photosynthetic aquatic life such as phytoplankton (algae, some protists, and cyanobacteria), and macrophytes (flowering, leafy plants and mosses). As little as .03 mg/L phosphates can stimulate excessive plant growth. Resulting algal blooms, or an explosive growth in algae, can turn water into a soupy green appearance. The extra plant life adds plenty of oxygen to the water, which is a good thing, but eventually those extra plants die, and aerobic (oxygen demanding) bacteria decompose them, consuming oxygen in the process. This process is known as eutrophication. The resulting reduction in the available amount of dissolved oxygen in the water harm aquatic life, disrupting their ability to breathe, reproduce, and remain active. For instance, streams with high amounts of dissolved oxygen are able to support pollution sensitive fish such as trout, while lower dissolved oxygen streams are only able to support more tolerant fish such as carp. If oxygen depletion is extreme, aquatic life must move or perish. See the dissolved oxygen information sheet for more information.

One major problem with phosphate pollution of streams is that there is no biological process that removes dissolved phosphates from streams. The phosphates are simply cycled into plants then back into the water when the plants die - to be reused by other plants later.

Water Quality Criteria

According to the Environmental Protection Agency (EPA) the concentration of phosphorus in sewage waste should be less than 1 mg/L. Currently there is no established value of phosphorus for drinking water.

Example Data for Phosphorus - French Creek

The Department of Environmental Protection (DEP) has systematically collected water quality data from French Creek, at Meadville, since 1973. A summary of these data is below to provide an understanding of past values of phosphorus and demonstrate trends or relationships of past total phosphorus concentrations.

Average: 0.082 mg/L

Maximum: 0.430 mg/L

Minimum: 0.010 mg/L

Graph 1 shows, in general, a trend of higher phosphorus concentrations at low discharges and high discharges in French Creek. At low flow, increased phosphorus is probably caused by man-made additions. At high flow, the increased phosphorus is probably from soil erosion caused by storm events. The concentrations of phosphorus are lowest between discharges of 500 ft3/s to 2000 ft3/s. Graph 2 shows the annual patterns of concentrations of phosphorus as well as the long-term trend for 1975-1994. Some of this variation in phosphorus is probably explained naturally by faster flows, associated with higher discharges, causing more soil and sediment erosion throughout the watershed. One unnatural source of phosphates that could contribute to phosphates into the French Creek watershed include runoff from fertilizers placed upon crops and lawns, industrial wastes, sewage effluent, and disturbances of land by humans.