The Town of Sherborn’s Farm Pond Advisory Committee (FPAC), with the help of the Sherborn Yacht Club and a set of Sherborn resident volunteer citizen scientists, has engaged in water monitoring activities on Farm Pond this summer. These practices have been conducted for the past few decades and include watching for invasive weeds, monitoring for trends related to potentially harmful cyanobacteria (HCB), and collecting data on water quality (water transparency, temperature, depth, dissolved oxygen, and Total Phosphorus).

During the month of August, a month of record low rainfall and high temperatures, Farm Pond’s water clarity showed a rapid and very concerning decline.  FPAC volunteers have been measuring the Secchi disk transparency from a mooring near the center of Farm Pond for the past 24 years. Up until about three years ago this water clarity parameter would typically range over the course of the warmer months from a high of 8 meters to a low of about 5 meters (about 26 to 16 feet). However, starting about three summers ago this range started trending to lower clarity values.  We are now shocked and saddened to report even lower and rapidly dropping water clarity values during the last few months as shown by the table below – with only 1.5 meters of clarity recorded last week on 9/4/22 (about 5 feet).  This low of a Secchi disk measurement has never been observed before at Farm Pond and has been accompanied by a troubling murky green appearance of the water throughout the entire pond.

Secchi Disk Water Transparency Measured in Meters

On Saturday (9/10/22) a transparency measurement was taken at the now closed for the season (no lifeguards) public beach. The level was recorded at only 0.9 meter, or 3 feet, and is now below the 4 feet minimum recommended by the MA Department of Public Health for public bathing beaches.

There have been no observed large surface HCB blooms noticed at the pond as of yet this summer. However, our understanding is that the very low clarity of the water is indicative of a lake-wide significant growth of a major phytoplankton underwater bloom (combination of potentially harmful blue-green cyanobacteria HCB and green algae).  Surface blooms could be imminent.

At this time, we don’t definitively know the exact cause of this recent change at Farm Pond. It has been noted by other experts that climate change may be producing higher temperatures and longer “growing seasons” for algae and HCB in lakes, rivers, and streams both in our local area and worldwide.

In addition to climate change factors (i.e. our significant 2022 drought/sunny days!) this past summer, we have learned from multiple local lake experts that these factors have likely triggered another phenomenon.  Phosphorous normally trapped in the pond sediment (built up over thousands of years) is being released into the upper water column due to a lack of oxygen at depth.   We have evidence of this starting last autumn and over this summer through samples of water collected near the bottom of the pond and testing for Total P.  While Farm Pond’s phosphorous level at the surface (0.5 m) has been measured at close to the method detection limit (8 ug/L), at near bottom depths it has been 10 to 20 times higher (80-160 ug/L).  This release of sediment-bound phosphorous essentially “feeds” algae and HCBs. It could also fuel rapid non-native invasive aquatic weed growth (invasives not yet seen, but we are always worried).

Most of the water clarity degradation happened at the end of the official swimming season (the pond’s waterfront and beach officially closed Aug. 14th) and SYC closed Sept. 11th. However, Sherborn’s Board of Health (BOH) remains involved in this due to the fact that the pond is used nearly year-round by abutters, residents, and others for boating, swimming, and fishing. (Boating is by permit only and is only permitted when boat washing is available April-Nov during specific times).

At this point, swimming should most likely be curtailed at Farm Pond until water clarity conditions improve.  Possible toxicity exposure from HCBs should always be assumed.  The pond is currently posted for the risk posed by this current bloom (See Image).  In addition, a general warning of HCBs being (infrequently) observed in the pond has been posted since the summer of 2020.   (HCBs are native, natural, and have been around for millions of years.  That they exist in Farm Pond is expected, but it is the abundance of them that is the problem).

Going forward, it is now clear that our town will have to deal with the new challenge of trying to manage water quality/transparency in our beloved Farm Pond. There are approaches to be weighed, costs to be considered, and probably much more data that needs to be gathered.  FPAC and other Farm Pond volunteers have attended seminars and reviewed various literature on the subject and have been made well aware of the occurrence of large scale blooms in other water bodies and the potential for these type of blooms in our pond. However, we never expected that it would be happening so quickly at Farm Pond.

More information will be shared as it becomes available. And on behalf of Farm Pond, we need cloudier and colder days ASAP!

Thank you,