What is phosphorus and why is it a concern?

“Total phosphorus,” also referred to as “phosphorus,” is the measure of the phosphorus available for plant and algae growth. “Phosphate” is one form of phosphorus.

While phosphorus is a nutrient that is essential for life, too much phosphorus can result in excessive plant growth and algal blooms. Lakes with high levels of phosphorus are considered “eutrophic.”

Phosphorus is naturally occurring in soil, so erosion of shorelines contributes phosphorus to the lake. Since fertilizers and some herbicides contain phosphorus, phosphorus often enters lakes in run-off from lawns and farms. Leaking septic tanks also contribute phosphorus.

Lake Score Cards

Lake Bomoseen currently has good water quality, especially compared to many other lakes in Vermont. The DEC’s Vermont Lakes and Ponds Management and Protection Program (VLPP) publishes “Lake Score Cards” for 851 lakes and ponds in Vermont. Lake Bomoseen’s score card is shown below; the full version is available from the DEC here. To see the complete list of lakes and ponds with score cards, click here.

• The Lake Score cards show the annual mean values for spring phosphorus, summer phosphorus, summer Secchi, and summer chlorophyll-a. Where there are enough sample points, a trendline is used to show the overall direction that the values have been trending over the years available.

• The background color behind the points indicates the trophic state that the values correspond to—hypereutrophic, eutrophic, mesotrophic, or oligotropic.

• According to the VLPP, “Total phosphorus (TP) concentrations at spring turnover represent the potential available phosphorus that will feed primary producers like aquatic plants and planktonic algae during the following summer.” These values have been collected by scientists in the VLPP’s Spring Phosphorus program.

  • Bomoseen’s values have been trending downward since 1980; the average value is 13.7 ug/L (micrograms per liter). This corresponds to the mesotrophic state.

  • For comparison, Lake St. Catherine’s average value is slightly higher at 14.6 ug/L (stable trend), and Lake Iroquois’s average value is 27.9 ug/L. Many of Lake Iroquois’s annual mean spring phosphorus values correspond to the eutrophic state.

• Summer phosphorus (and Secchi disk and summer chlorophyll-a values) are recorded as part of the Vermont Lay Monitoring Program, in which volunteers take measurements throughout the summer.

  • The trend for summer phosphorus in Lake Bomoseen is in an upward direction, which warrants further investigation. It’s important to note that there are no values since 2019. What have the summer phosphorus levels been most recently?

  • Lake Bomoseen’s average summer phosphorus value is 13.3 ug/L, well below the eutrophic threshold of 30 ug/L.

  • For comparison, Lake St. Catherine’s average summer phosphorus value is 15.1 ug/L and Lake Iroquois’s is 24.8 ug/L.

• A Secchi disk is an 8” metal disk with black and white quadrants. It is lowered into the water, and the depth at which it disappears from human sight is recorded. Secchi disk values are measures of water clarity or transparency. Values are affected by suspended solids in the water and the presence of planktonic algae. (The word “turbidity”—cloudiness caused by suspended solids—is often used in association with Secchi disk measurements.) In general, higher values, which indicate more transparency—are more desirable.

  • Lake Bomoseen’s Secchi disk values have been trending toward higher values (desirable) since 1990. The average summer Secchi disk reading is 7.4 m, which is well in the oligotrophic state.

  • For comparison, Lake St. Catherine’s Secchi disk readings have been decreasing over the years. Its average summer Secchi disk reading is 5 m, with most of its readings corresponding to the mesotrophic state. Lake Iroquois’s readings have been stable, but the average summer Secchi disk reading is only 3.9 m.

  • Note that the Lake Score Cards display the Secchi disk values with 0 at the the top of the plot, as if you were looking through the water column. The values for Lake Bomoseen are Highly Significantly Increasing, but the trendline points in a downward direction on the plot.

• Chlorophyll-a is a photosynthetic pigment found in plants and phytoplankton (including cyanobacteria). The amount of chlorophyll-a in the water column serves as a good approximation of the amount of algae present. High values and increasing trends are cause for concern.

  • Lake Bomoseen’s average chlorophyll-a value is 2.0 ug/L, corresponding to the oligotrophic state, indicating that Lake Bomoseen does not have high levels of planktonic algae (including cyanobacteria). The trend has been stable.

  • For comparison, Lake St. Catherine’s average chlorophyll-a value is 4.2 ug/L (mesotrophic), and Lake Iroquois’s is 10.6 ug/L (eutrophic).

All of Lake Bomoseen’s recorded values fall into the mesotrophic or oligotrophic states. This suggests that Bomoseen is NOT a eutrophic lake, nor is it trending overall toward being a eutrophic lake.

We do not know what effect application of ProcellaCOR EC will have on these water quality parameters in Lake Bomoseen.

For more information on the VLPP’s scoring methodology, please read their document “The New Vermont Inland Score Card.”

The Lake Bomoseen score card shown above is a screenshot from the Vermont Department of Environmental Conservation’s Lake Score Card page. Accessed September 4, 2022.

Note 1: Above each of the graphs there is a p-value. A p-value is a measurement that is determined through statistics based on the data set. When the p-value is calculated to be 0.05 or more, the trend in the data is considered “stable.” When the p-value is calculated to be less than 0.05, the trend is “significantly increasing” or “significantly decreasing.” In this case, “significant” refers to the statistical meaning of the word.

Note 2: For phosphorus and chlorophyll-a, trends that are stable or decreasing are desirable for lake health. Too much phosphorus and chlorophyll-a can be problematic for the reasons explained above. For Secchi disk transparency, values refer to depth. Larger values mean that light can penetrate deeper due to less suspended solids. Larger values and increasing trends are thus more desirable for lake health.