Water Quality Indicators: Total Phosphorous

 

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How to use this page: This is an indicator page. Examine this page for detail on the indicator and use that information to establish a metric for the indication of water quality. This is the final step in examining a sustainable community for its environmental attributes, water subattributes, and finally a water indicator. After completing this page, please go back and review other indicators and see discern metrics and weights for the AHD process.

 

Why is Total Phosphorous a Water Quality indicator?


Phosphate will stimulate the growth of plankton and aquatic plants which provide food for larger organisms, including: zooplankton, fish, humans, and other mammals. Plankton represent the base of the food chain. Initially, this increased productivity will cause an increase in the fish population and overall biological diversity of the system. But as the phosphate loading continues and there is a build-up of phosphate in the lake or surfacewater ecosystem, the aging process of lake or surface water ecosystem will be accelerated. The overproduction of lake or water body can lead to an imbalance in the nutrient and material cycling process (Ricklefs, 1993). Eutrophication (from the Greek - meaning "well nourished") is enhanced production of primary producers resulting in reduced stability of the ecosystem. Excessive nutrient inputs, usually nitrogen and phosphate, have been shown to be the main cause of eutrophication over the past 30 years. This aging process can result in large fluctuations in the lake water quality and trophic status and in some cases periodic blooms of cyanobacteria.


Phosphorus is one of the key elements necessary for growth of plants and animals. Phosphates ( PO4) are formed from this element. Phosphates exist in three forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical formula. Ortho forms are produced by natural processes and are found in sewage. Poly forms are used for treating boiler waters and in detergents. In water, they change into the ortho form. Organic phosphates are important in nature. Their occurrence may result from the breakdown of organic pesticides which contain phosphates. They may exist in solution, as particles, loose fragments or in the bodies of aquatic organisms.

How is the level of Total Phosphorous influenced?


Rainfall can cause varying amounts of phosphates to wash from farm soils into nearby waterways. Phosphate will stimulate the growth of plankton and aquatic plants which provide food for fish. This may cause an increase in the fish population and improve the overall water quality. However, if an excess of phosphate enters the waterway, algae and aquatic plants will grow wildly, choke up the waterway and use up large amounts of oxygen. This condition is known as eutrophication or over-fertilization of receiving waters. This rapid growth of aquatic vegetation eventually dies and as it decays it uses up oxygen. This process in turn causes the death of aquatic life because of the lowering of dissolved oxygen levels.


In situations where eutrophication occurs, the natural cycles become overwhelmed by an excess of one or more of the following: nutrients such as nitrate, phosphate, or organic waste. The excessive inputs, usually a result of human activity and development, appear to cause an imbalance in the "production versus consumption" of living material (biomass) in an ecosystem. The system then reacts by producing more phytoplankton/vegetation than can be consumed by ecosystem. This overproduction can lead to a variety of problems ranging from anoxic waters (through decomposition) to toxic algal blooms and decrease in diversity, food supply and habitat destruction. Eutrophication as a water quality issue has had a high profile since the late 1980s, following the widespread occurrence of blue-green algal blooms in some fresh waters. Some blue-green algae can at times produce toxins, which are harmful to humans, pets and farm animals.


Under aerobic conditions (presence of oxygen), the natural cycles may be more or less in balance until an excess of nitrate (nitrogen) and/or phosphate enters the system. At this time the water plants and algae begin to grow more rapidly than normal. As this happens there is also an excess die off of the plants and algae as sunlight is blocked at lower levels. Bacteria try to decompose the organic waste, consuming the oxygen, and releasing more phosphate which is known as "recycling or internal cycling". Some of the phosphate may be precipitated as iron phosphate and stored in the sediment where it can then be released if anoxic conditions develop.
In anaerobic conditions (absence of oxygen), as conditions worsen as more phosphates and nitrates may be added to the water, all of the oxygen may be used up by bacteria in trying to decompose all of the waste. Different bacteria continue to carry on decomposition reactions, however the products are drastically different. The carbon is converted to methane gas instead of carbon dioxide, sulfur is converted to hydrogen sulfide gas. Some of the sulfide may be precipitated as iron sulfide. Under anaerobic conditions the iron phosphate precipitates in the sediments may be released from the sediments making the phosphate bioavailable. This is a key component of the growth and decay cycle. The pond, stream, or lake may gradually fill with decaying and partially decomposed plant materials to make a swamp, which is the natural aging process. The problem is that this process has been significantly accelerated.
           

What level of Total Phosphorous a is desirable for a sustainable community?


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. 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.

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Where to go now that you have reviewed an indicator:

Now that you have reviewed an indicator you should create a metric (see Step 4 of the Analytical Hierarchy Processes) that indicates the importance of this indicator in your decision process. Keep this step in mind as you go through one of the other indicators below. Once you have chosen a metric for each of your indicators, you should decide how they collectively measure the sustainability of water by weighting each indicator (see Step 5 of the Analytical Hierarchy Process). These will be applied in an algorithm (see Step 6 of the Analytical Hierarchy Process) to give you the final measurement for Water and Sustainability.

 

 

Author: Shawn Dayson Shifflett