Farm Pond Water Quality Monitoring - 2022 Summary

Declining Conditions, Low Lake Water Clarity & Elevated Nutrient Levels

The past 2022 season at Sherborn's Farm Pond was concerning for those of us who regularly monitor this treasured lake for several important water quality parameters. Along with the two preceding 2021 and 2020 seasons where cyanobacteria issues first became of concern, the water clarity continued to decline, reaching dramatic lows in the late summer of 2022. Accompanying the drop in water clarity were visual observations of opaque green water with poor visibility at points along the lake's entire shoreline and from various docks. Pond visitors who were normally accustomed to "seeing the bottom" when wading out into waist deep water, boating, or standing on docks noticed this was no longer possible at times. 2022 also brought along another drought season with low lake water levels.

Evidence of this poor water transparency trend include:

The median Secchi disk transparency depth (SD) for the 2022 season was at an all-time low of 4.0 meters/13 feet (12 sampling events, April through November). For 24 years of FPAC field work, including 2022, this historical median is now at 6.2 meters (20.3 feet, 1 meter = 3.28 feet).

Four of these readings of SD for 2022 were the lowest ever observed at Farm Pond in 24 years (1.0 to 2.7 meters, 3.3 to 8.9 feet) and occurred from mid-July to early October.

The last four seasons, 2019-2022, have all exhibited median SD's below the longer-term 6.2 meter level (5.1, 5.8, 5.8, and 4.0 meters, for 2019-2022).

SD levels measured right at the Town's public beach in early September were at 0.9 meters (3 feet). MA DPH guidelines for public swimming areas are set at a SD minimum of 4 feet, or 1.2 meters for swimmer safety.

This decline in water clarity is attributed to much higher concentrations of natural populations of blue-green cyanobacteria and green algae (two distinct classifications of organisms). The growth of these organisms is favored with increasing water temperatures and water nutrients (phosphorus, nitrogen) concentrations.

This monitoring program routinely samples levels of Total Phosphorus (TP) at the lake surface and the 24-year average (39 samples) over that period is now at 8.7 ug/L, or ppb. This general level of phosphorus is indicative of a low-nutrient (oligotrophic) lake that would be expected to exhibit high water clarity. However, TP sampling in late summer 2022 through early fall revealed TP concentrations at surface at 13 to 21 ug/L, which represent much higher nutrient levels that would be predicted to fuel cyanobacteria and algal blooms. Moreover, additional new volunteer TP sampling at mid- and bottom- depths had even higher TP levels in late season - with mid-depth levels at 11 to 24.6 ug/L and bottom-depths at 66 to 73 ug/L. Any mixing of these lower stratified layers with the surface layer due to high wind weather events/storms are known to be capable of quickly introducing nutrients to the upper water regions where cyanobacteria and green algae may thrive.

Our observations are consistent with a growing understanding of the affects of climate change on northern hemisphere oligotrophic lakes world-wide, where there is today a general trend of increasing phosphorus concentrations and water clarity deterioration in formally pristine lakes. Phosphorus first enters Farm Pond continually from multiple sources, primarily: watershed runoff from surrounding land areas (properties and streets, impervious surfaces, fertilizer applications, etc), groundwater (including contributions from nearby septic systems), and atmospheric deposition via rain/snow. Over time, the phosphorus cycles throughout the flora and fauna within the lake, eventually building up in the bottom sediments. Re-introduction of P into the water column from the bottom sediments is known to occur and can be accelerated by periods of drought and increasing ambient temperatures.

The Town has recently obtained state funding for a Farm Pond Watershed Based Plan (WBP) to start this important project in early 2023 with a lakes management consultant, with the goal of further understanding the recent changes at Farm Pond by a more extensive two years of more comprehensive field work and study of current lake and watershed conditions. A final report, expected in early 2025, will provide the Town with management recommendations going forward.

For a more detailed review of all the 24 years of FPAC volunteer data collection and sampling at Farm Pond please review two accompanying documents: