The Nutrient:
Phosphorus is a naturally occurring nutrient that is necessary for plant growth in lake ecosystems. Phosphorus concentrations control how much productivity, or life, there is a lake. Introduce large amounts of phosphorus and plant/plankton populations boom; phosphorus is the limiting nutrient.
Phosphorus also occurs in many man-made products, including human waste, soaps/detergents and fertilizers. These phosphorus sources can greatly affect the water quality of our lakes and ponds. Old, outdated septic systems can leak phosphorus from human waste and laundry/dish soaps into surface waters. While rain run-off flowing over freshly fertilized lawns can pick up fertilizer and carry it into our streams and lakes. We see especially high concentrations of phosphorus in water bodies and soils close to agricultural operations that use animal waste or chemicals to fertilize their lands.
The Impact:
Increased phosphorus concentrations leads to increased plant and plankton life, or “productivity.” In many ecosystems, the term productivity is a positive one, indicating a large amount and variety of plant and animal life. Over-productive lake ecosystems, however, are not necessarily good, and can lead to an imbalance of nutrients in the water body and “eutrophication.”
Lakes can be classified based on their productivity as “oligotrophic” (having low nutrient concentrations, little productivity and very clear waters), “mesotrophic” (having intermediate nutrient concentrations and productivity and fairly clear waters) and “eutrophic” (having high nutrient concentrations and productivity and very murky waters). Lakes naturally progress through these three different stages (over hundreds of years), but the rate with which they progress is driven by the availability of nutrients, namely phosphorus. If human impacts introduce excessive amounts of phosphorus, a lake will progress through the trophic levels quickly (over tens of years).
Eutrophic lakes are heavily populated by plant life, when these plants die they decompose, a process that removes large amounts oxygen from the water column and creates “anoxic” conditions. Fish and invertebrates, which use this oxygen to breathe, suffocate and die in anoxic waters, further contributing to decomposition and anoxia.
The presence of surplus phosphorus in lakes also contributes to the algal life in lakes and ponds. Some algae known as cyanobacteria or blue-green algae have been found to be neurotoxins, and have the potential to pose a threat to humans and animals who swim in or drink waters with cyanobacteria blooms.
Eutrophic conditions also pose a risk to our enjoyment of a lake or pond. The thick infestation of plants makes swimming unpleasant, if not impossible, chokes up boat propellers and snags fishing lines. Extra productivity and decomposition could also be accompanied by unpleasant odors.
The Management
The easiest way to limit the amount of phosphorus in a lake ecosystem is to limit/eliminate the human sources:
* Be sure your septic system is up-to-date and functioning properly.
* Eliminate grass lawns leading up to the edge of the water, or plant a buffer strip of native, water-loving plants between your lawn and the water’s edge (this will also discourage nuisance waterfowl from accessing your property)
* Never fertilize lake-side land, not even with organic fertilizers.
* Use phosphorus-free detergents and soaps.
* Limit the amount of impervious surfaces on your property.
* For more on limiting your impact, please read the SLA’s 50 Ways to Save the Squam Lakes brochure
Water Quality Monitoring Sites
Click on a site below to download a chart with phosphorus levels. You will need Adobe Reader (free software) to open the files.
