What does it tell us about a lake?

Phosphorus levels in a lake can tell us how productive a lake is in plant growth, and can tell us how human activities on the land surrounding the lake are affecting the lake system. Again, as phosphorus increases, so does plant growth such as algae.

The problem with too much algal growth in a lake is that the blooms can form green scums on the top of the water which may have a foul odor, and may turn off people that would otherwise recreate on the lake. More importantly, too much algal growth can disrupt the balance between natural systems in a lake. For example, one consequence of increased algal production is that as the algae die, they fall to the bottom of the lake as dead organic matter. The process of decomposition by bacteria of this organic matter at the bottom of the lake uses valuable oxygen in the deeper, colder waters of lakes. Many lakes with phosphorus and algal population problems become anoxic in the summer, meaning that there is no oxygen in the deep waters of the lake. This can put many cold-water species at risk of death, or extinction as conditions may not be adequate for reproduction or survival to adult stage.

On the other hand, in places where the sediments receive sunlight, algae are found living on the sediments. The algae produce oxygen, which will reduce the flow of phosphorus from the sediment to the water. Beyond that, these algae may consume large quantities of phosphorus, further reducing the phosphorus entering the system from sediments.

Other lake chemistry may affect phosphorus conditions in a lake. For example, the pH of water affects the transport of phosphorus between the sediments and water in a lake. As the pH of water rises, it promotes the release of phosphorus from sediments. Primary production from plants such as algae increases the pH of water, therefore increasing the amount of phosphorus released from sediments. The increase of phosphorus increases the amount of algal growth, making the situation worse!

Phosphorus pollution can be considered a point source pollution if there is a direct source that can be identified as contributing large amounts of phosphorus to the lake. An example of a point source might be a factory that spills its wastes into the lake with one drain pipe. Most phosphorus pollution, though, is the result of non-point source pollution, meaning that there are many small sources of phosphorus that add up to equal large amounts when they all enter the lake. Examples of non-point source pollution of phosphorus include camp roads, driveways and paths on steep slopes, fertilizers washed into the lake from lawns on the shore, failed septic systems, and pet wastes.

If a lake is found to have phosphorus problems, a state agency or a local lake association will usually perform a watershed survey which identifies point and non-point sources of pollution to the lake. In many cases funding is found to address the problems affecting the lake.

Even if the input of phosphorus from the land surrounding the lake is reduced, the lake may still face many tens or hundreds of years of problems with high phosphorus and algal blooms. Because lakes are pretty much closed systems, the phosphorus that was deposited in the past is still there in the sediments. And remember that increased pH of the water (because of algal blooms) can trigger the release of phosphorus from the sediments of the lake. This is called internal loading.

TP varies from 1 ppb to 110 ppb with the average being 14 ppb. It is measured in parts per billion (ppb) or parts per million (ppm).
Note: 1 ppb = 0.001 ppm.

Total phosphorous levels in a lake or pond can be related to lake classification:

Care to Compare?

The data used to create this graph were provided to PEARL by the Maine Department of Environmental Protection (DEP). DEP summary data were used, which can be viewed by clicking "Browse Data Sets" then the "Water Quality" button from PEARL 's homepage. The distribution of mean (or average) total phosphorus levels recorded at 867 sampling sites are shown. Need data? You can search for a lake or pond from the PEARL homepage.

Out of 867 sampling stations:

• 52, or 6.0%, had total phosphorous levels of 0 - 4.9 ppb
• 391, or 45.1%, had total phosphorous levels of 5 - 9.9 ppb
• 227, or 26.2%, had total phosphorous levels of 10 - 14.9 ppb
• 104, or 12.0%, had total phosphorous levels of 15 - 19.9 ppb
• 40, or 4.6%, had total phosphorous levels of 20 - 24.9 ppb
• 19, or 2.2%, had total phosphorous levels of 25 - 29.9 ppb
• 12, or 1.4%, had total phosphorous levels of 30 - 34.9 ppb

22, or 2.5%, had total phosphorous levels ≥35 ppb

Resources:
Bronmark, Christer and Lars-Anders Hansson. 1998. The Biology of Lakes and Ponds. New York. Oxford University Press.