Water is critical to the diverse habitats and working landscapes of the Lake Champlain Basin. The region’s climate provides enough rainfall to feed 14,700 miles of streams and rivers and fill Lake Champlain with 6.8 trillion gallons of water. Vibrant communities, outstanding recreational opportunities, and a strong environmental ethic that rely on this abundance of water attract more people to the Lake Champlain Basin each year. Pressures from human activities, however, threaten to degrade water quality.


Phosphorus is a nutrient that influences lake health and cyanobacteria blooms.

Nutrients are the food that fuels cyanobacteria growth and, when present in excess, nutrients can be a contributing factor to cyanobacteria blooms. Phosphorus is thought to often be a “limiting” nutrient for cyanobacteria growth. This means that cyanobacteria often have enough of other nutrients to grow, so when more phosphorus is available, they grow more readily.

Some forms of phosphorus fuel cyanobacteria more easily than others. For example, phosphorus can be bound to soil particles that erode from streambanks. When in this “particulate” form, the phosphorus is inaccessible to cyanobacteria; it may take months or years for this phosphorus to be released in the Lake in a form that can be used for cyanobacteria growth. In contrast, dissolved forms of phosphorus in fertilizer and wastewater treatment effluent are more readily available for cyanobacteria growth. Targeted efforts to reduce phosphorus loading to Lake Champlain should address all forms of phosphorus.

Many lake segments have phosphorus concentrations that are often near or below targeted limits. However, phosphorus concentrations in Lake Champlain’s shallow bays are often above these limits and generally have not decreased in recent decades.

concentration vs load infographicExcessive phosphorus concentration can have a significant impact on a lake’s ecosystem and is a contributing cause of cyanobacteria blooms. Targeted phosphorus concentration limits for thirteen segments of Lake Champlain (Figure 4) were established in 1991, and the LCBP has supported monitoring efforts for phosphorus concentrations in the Lake since 1990. From 1990 to 2017, most segments did not show long-term trends in phosphorus concentration, though the Northeast Arm showed an increasing trend over this time period. Annual average concentrations often have been near or below targeted limits since 1990 in the Main Lake, Isle La Motte segment, Cumberland Bay, Port Henry, South Lake B segment, Malletts Bay, Burlington Bay, and Shelburne Bay, which make up approximately 82% of the Lake’s volume.

Phosphorus concentrations above targeted limits have been observed in the shallow waters of Missisquoi and St. Albans Bays, the Northeast Arm, and the South Lake A segment. Some of these areas have high phosphorus loads from their contributing sub-watersheds. Also, because there is less water to dilute incoming nutrients, shallow bays are more susceptible to problems associated with excess phosphorus than the deeper bays and Main Lake. Shallow bays are also more affected by phosphorus moving up the water column from sediment at the bottom of the Lake.

phosphorus concentration by lake segment

Figure 4 | Annual mean phosphorus concentration by lake segment, 1990-2017

Phosphorus comes from a variety of sources in the watershed and from lake sediments.

For every square mile of water on Lake Champlain, eighteen square miles of land in the Lake Champlain Basin deliver water to the Lake and contribute sediment, nutrients, and other potential pollutants. For the Great Lakes, this ratio is much lower: there is only 1.5 to 3.4 times as much land as lake surface area in those watersheds. Most nutrients come from sources on the land, so the relatively high land-to-lake area ratio for Lake Champlain poses a significant challenge in limiting nutrient pollution.

FIgure 5 - discharge vs. phosphorus load on the Winooski River

Figure 5 | Mean annual water discharge and phosphorus load for the Winooski River, 1990-2016

Rivers are the pathways for water, sediment, and nutrients to move into the Lake. Each year the Basin’s rivers deliver about 921 metric tons (2 million pounds) of phosphorus. Annual changes in load depend upon the amount of rain and runoff in the watershed (Figure 5). This variability due to precipitation and temperature may confound efforts to reduce phosphorus loading. While management practices may help to reduce inputs, the increasingly intense rainstorms associated with climate change may release more phosphorus, possibly cancelling out some gains made through pollution reduction efforts.

sediment discharge from Otter Creek

Tributaries deliver water, sediment, nutrients, and other pollutants to Lake Champlain tributaries. Photo: LCBP.

Long-term phosphorus loading trends have not improved in most Lake Champlain tributaries. While long-term decreases have been documented in the LaPlatte and Little Ausable Rivers, long-term increases in phosphorus loading have been documented in Lewis and Little Otter Creeks, and in the Poultney River. All other monitored tributaries show no significant long-term trends in phosphorus loading (Figure 6).

Figure 6 - annual tributary phosphorus loading

Figure 6 | Annual tributary phosphorus loading, 1990-2017

Phosphorus from developed land

A variety of land uses contribute phosphorus to Lake Champlain. Addressing each input is critical to achieving long-term reductions in phosphorus load. Developed land can be a substantial source of nutrients and other pollutants to the Lake and can contribute substantially more phosphorus per land area than other land uses (Figure 7). Impervious surfaces, such as parking lots and rooftops, shed rainwater quickly and do not allow stormwater to soak into the ground. High storm flows increase erosion of streambanks, sending more sediment and nutrients downstream to the Lake. Large surges of runoff from developed land can increase the severity of downstream flooding, causing property damage. Green infrastructure designs can reduce these high storm flows by slowing down and storing runoff, thereby reducing the amount of water and nutrients delivered to the Lake.

Figure 7 - annual phosphorus loading and land cover

Figure 7 | Annual phosphorus loading and land cover in the Lake Champlain Basin

Phosphorus from developed lands accounts for approximately 16% (147 metric tons or 323,600 pounds) of the total phosphorus load to Lake Champlain each year. Most of this phosphorus comes from nonpoint sources in the built environment, such as impervious pavement, lawns, and rooftops. In contrast, wastewater treatment facilities, an example of point source phosphorus, contribute 6% of the annual phosphorus load to the Lake. Regulations banning phosphorus from laundry detergents in 1976 greatly reduced the amount of phosphorus entering wastewater treatment facilities, and technology upgrades since then have increased the amount of phosphorus removal before discharge to the Lake (Figure 8).

Figure 8 - phosphorus load from wastewater treatment facilities

Figure 8 – Phosphorus load from wastewater treatment facilities, 1990-2016

Phosphorus from agricultural land

Farming has long been a way of life for many in the Lake Champlain region, but agriculture can have a significant impact on water quality. Runoff and erosion from barnyards, laneways to pastures, and animal congregation areas can carry excessive pollution to nearby waterways and eventually to Lake Champlain. Most conventional farms rely in part on commercial fertilizers and feed additives, and organic agricultural practices also can involve the application of fertilizer to increase crop productivity. Cattle farms and other animal-based operations generate large amounts of manure that must be disposed of or used carefully; farmers frequently spread manure on their fields to recycle nutrients back into crops and pastures. A portion of the nutrients within fertilizers, additives, and manure is washed off the land and into a waterway before it can soak into the ground.

Approximately 38% of the phosphorus load to the Lake comes from agriculture (352 metric tons or 775,000 pounds each year). One key challenge in addressing this long-term problem is that part of the load comes from legacy phosphorus—that is, phosphorus that has been added to soils over decades of farming. Such legacy phosphorus will continue to be released into the Basin’s waterways even as efforts to reduce future loading are underway.

Phosphorus from streambank sediments

The erosion of streambanks accounts for approximately 18% (165 metric tons or 365,000 pounds) of the annual phosphorus load to the Lake. Streambanks that lack significant woody plant growth can be susceptible to erosion during floods. Streambank erosion often occurs when lands near a river are altered to accommodate land use activity such as construction of a road, cultivation of farmlands, or any activity that removes or prevents a vegetated woody buffer. When streambanks erode and collapse into a stream, the change in flow patterns can cause increased erosion and the release of phosphorus in the future. Collapsed streambanks at the edge of agricultural fields may be especially high in legacy phosphorus.

Phosphorus from lake sediments

Stores of phosphorus are locked up in lake-bottom sediments as materials delivered from the watershed have settled to the bottom over many decades. Chemical reactions associated with low oxygen levels can cause this phosphorus to be released into the water. This phosphorus can fuel cyanobacteria growth in the shallow bays where sunlight penetrates closer to the bottom sediments. Hot, calm weather not only creates the conditions for this release of phosphorus, it is the type of weather that cyanobacteria prefer for growth.

Many efforts are underway to reduce phosphorus loading and ultimately reduce phosphorus concentrations in Lake Champlain.

Lake Champlain has been the focus of renewed investments in watershed management practices by the U.S. federal government, state and provincial agencies, and municipalities. In 2015, the Vermont legislature passed the Clean Water Act (Act 64), which created the Clean Water Fund to reduce the amount of phosphorus and other pollution entering the state’s waterways. In 2016, the U.S. Environmental Protection Agency produced an updated Vermont Total Maximum Daily Load (TMDL) for phosphorus loading into twelve Vermont segments of Lake Champlain, while New York continues to work toward the TMDL set in 2002 for New York segments of the watershed. Vermont and Québec adopted an agreement concerning phosphorus reduction in Missisquoi Bay in 2002. The agreement reaffirmed the phosphorus concentration limit for the bay and established a phosphorus loading limit for the bay’s watershed. The two jurisdictions currently are working toward a renewed agreement and shared common goals for the restoration of Missisquoi Bay.

Farmers, resource management agencies, and local watershed organizations have long recognized that farms in the Basin play a major role in the nutrient pollution problem. Several initiatives are underway to help the agricultural sector do its part in meeting targeted phosphorus loading limits and ultimately reducing in-lake phosphorus concentrations. Ongoing grant programs, wastewater treatment upgrades, agricultural support to implement best management practices, and outreach programs all work together to meet the critical need to reduce phosphorus loading.

water quality science in the field

A variety of research studies are examining ways to reduce phosphorus loading. Photo: Stone Environmental, Inc.

Dive In: What You Can Do

Test your turf. Before fertilizing, test lawns and gardens to determine the actual need. It may be possible to use less fertilizer than anticipated or even none at all.

Healthy soil, healthy lawn. Foster soil health in your lawn and garden rather than relying on lawn care products that import more nutrients into the Basin.

Let it grow. Set your lawn mower blades to 3” and leave grass clippings. Tall grass is healthier and has deeper roots that hold more water and reduce runoff.

Rein in the rain. Redirect your gutter downspouts to a lawn, plant a rain garden, or install a rain barrel.

Wash vehicles on grass. Wash your vehicle on a lawn instead of a driveway to prevent detergents from running into the Lake. Alternatively, use a carwash where the water is treated after use.

Shore up the water’s edge. Plant native vegetation along shorelines and riverbanks to hold soil in place and reduce erosion.