The trophic status of a lake segment is a measure of how much plant growth is supported by phosphorus in the water. Excessive amounts of phosphorus can lead to reduced water quality, including reduced clarity and excessive growth of aquatic plants that impact both the organisms living in the Lake and human uses of the Lake. Eutrophic areas have excessive levels of phosphorus and are highly impacted, whereas mesotrophic and oligotrophic areas have moderate and low levels of phosphorus and associated impacts.

Unlike many lakes, Lake Champlain is divided into several distinct lake segments. Each segment has a different level of phosphorus, which determines that segment's trophic level. Lake Champlain's segments range from the oligotrophic Mallett's Bay to the highly eutrophic South Lake.

The Phosphorus Levels in Lake Champlain Map [37 KB] shows the trophic status of Lake Champlain's 13 segments, based on the average concentration of phosphorus in the water between 1990 and 2003. The charts associated with each segment show the average phosphorus concentrations measured by the LCBP’s Long Term Water Quality and Biological Monitoring Program from 1990 through 2003. The in-lake criteria lines are the phosphorus concentrations that the LCBP partners have established as phosphorus concentration goals.

The LCBP partners have agreed to reduce phosphorus inputs from point sources, such as wastewater (sewage) treatment plants, and nonpoint sources, such as runoff from urban and agricultural areas, with the ultimate goal of lowering in-lake phosphorus concentrations to levels that are acceptable for the health and human use of the Lake. Although phosphorus discharges from some tributaries and wastewater treatment plants have declined since the mid-1990s agreement, monitoring programs have yet to detect significant reductions in phosphorus concentrations in the Lake.

Scientists believe this delay between the implementation of phosphorus controls and a measurable response in Lake Champlain's water quality is normal. This delay or "lag time" may in part be caused by accumulation and storage of phosphorus in sediments. For example, research conducted on St. Albans Bay determined that a "lag time" of several decades may occur before the bay will fully respond to wastewater treatment plant upgrades completed in the 1980’s. This "lag time" is all the more reason to reduce phosphorus loading to the Lake as soon as possible.

• Long-Term Water Quality and Biological Monitoring Project for Lake Champlain Webpages
• Internal Phosphorus Loading Studies of St. Albans Bay. VT Department of Environmental Conservation. 1994.
• Long-Term Water Quality and Biological Monitoring Project for Lake Champlain. Cumulative Report for Project Years 1992- 1996. LCBP Technical Report #26, 1998.