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Nutrients are as critical to a lake ecosystem as they are to terrestrial ecosystems. Aquatic plants and algae require nitrogen, phosphorus, and a suite of micronutrients. When there is too much of a particular nutrient, certain plants or algae can dominate a system. Phosphorus is one of the primary water quality challenges in Lake Champlain. Found in lawn fertilizers, manure, as well as in human and other animal waste, phosphorus causes algal blooms and excessive aquatic plant growth when present in high concentrations. These plants and the water quality problems that occur when they decompose can harm fish and other organisms and limit the use and enjoyment of the Lake.
Phosphorus levels are above established targets in several parts of Lake Champlain, including Missisquoi Bay, St. Albans Bay, and the South Lake, where algal conditions are a nuisance for much of the year. Cyanobacteria (blue-green algae) often pose public health threats during the summer in Missisquoi Bay and other northern parts of the Lake. In an effort to address the problem, Vermont and Québec signed an agreement for reducing phosphorus to Missisquoi Bay that lays out a collaborative approach to the issue.
As a result of targeted efforts and infrastructure improvements in the last 20 years, less than 10% of the phosphorus in the Lake now comes from point sources, such as wastewater treatment facilities and industrial discharges. Runoff from roads and developed areas, and from lawns, farmlands, and other rural areas (called nonpoint sources) contribute more than 90% of the phosphorus. It is also likely that some phosphorus cycles up into the water from the sediments in the lake bottom, although we don’t yet know how much this in-lake cycling contributes to the overall amount of phosphorus in the lake.
A 2007 study conducted for the Lake Champlain Basin Program estimated that 46% of the nonpoint source phosphorus load is from urban land uses and about 38% is from agricultural land (using 2001 satellite imagery and the loading method of analysis). Urban land included all roads, cities, suburbs, lawns, and large-lot buildings. More recently, an LBCP-funded project examining phosphorus sources in the Missisquoi River Basin estimated that about 39% of the phosphorus delivered to the lake is from agricultural use.
Phosphorus Reduction Progress
The phosphorus reduction strategy for Lake Champlain has begun to show promising results. Data gathered between 1990 and 2011 show no significant positive or negative trends in most segments of the Lake (except for Mallet’s Bay, St. Albans Bay, the Northeast Arm, and Port Henry, which are deteriorating). Most segments either consistently meet or exceed the phosphorus targets for that particular lake segment. The most substantial gains in phosphorus pollution reduction have been achieved by targeting point sources, mainly through wastewater treatment plant upgrades.
However, nonpoint source loads still significantly exceed targets in at least four of the five major lake segments. With the exception of Shelburne Bay and a portion of the New York share of the South Lake watershed (South Lake A), all smaller watershed segments exceeded target levels. Conversion of agricultural and forest land to developed land, inadequate implementation of Best Management Practices (BMPs), and streambank erosion continue to slow progress in this area.
More about phosphorus
For the most recent information about phosphorus, please visit the “Where does Phosphorus Come From?” page of our State of the Lake website
To learn about actions taken to address nonpoint phosphorus pollution, please visit the “Reducing Phosphorus Pollution” chapter in The Lake Champlain Management Plan Opportunities for Action.
For additional maps and information, please visit the Lake Champlain Basin Atlas.
Nonpoint Source Pollution Fact Sheet