Brawley Consulting Group, LLC (BCG) was engaged by the Lake Wononscopomuc Association (LWA) to perform an assessment of water quality in 2023. The biannual monitoring program is one aspect of the LWA’s lake management strategy with the goal of developing a scientific database to detect changes – positive and/or negative – within the lake. The following is an outline of findings from the 2023 water quality monitoring program at Lake Wononscopomuc. Several recommendations are provided at the end of the report.

• The water columns at two sampling sites were highly stable, stratified, and exhibited strong resistance to mixing at the thermocline throughout the season. This resulted in anoxic conditions at the bottom which expanded upward in the water column with time. By mid-October at Site 1, the bottom 15 meters of the 30-meter water column, had oxygen concentrations of <1 mg/L.
• The lake exhibited mostly early mesotrophic to mesotrophic characteristics in 2023. Epilimnetic total phosphorus levels were within the early mesotrophic range, while epilimnetic total nitrogen levels were characteristic of mesotrophic conditions. Average summer Secchi disk transparency was just above the early mesotrophic range and within the mesotrophic range. Algae and cyanobacteria cell concentrations indicated even less productivity, i.e., were more characteristic of oligotrophic conditions. Relative phycocyanin was also indicative of low cyanobacteria growth.
• Nutrient and other chemical characteristics of the hypolimnion differed from those in the epilimnion and were an outcome of the anoxic / highly reduced environment at the lower depths. On average, total phosphorus, total nitrogen, and ammonia levels in the hypolimnetic were higher than those in the epilimnion at least partly due to internal loading processes. Hypolimnetic alkalinity was also on average higher than epilimnetic alkalinity, but not as high as might be anticipated given the protracted period of anoxia at the bottom. The pH at the bottom was often higher than corresponding epilimnetic pH which is uncommon. The hypolimnetic pH, alkalinity and phosphorus levels were likely impacted by the co-precipitation process (see below) that we believe occurs in the lake.
• The two most abundant cyanobacteria genera observed in the top three meters of the water column were genera capable of regulating buoyancy and were likely responsible for the lake’s highest cyanobacteria biomass observed in the upper levels of the hypolimnion at 10 to 13 meters of depth. They are also morst likely the genera that occasionally form shoreline blooms after becoming positively buoyant, reach the surface, and moved to the shore by light winds. Diatoms and golden algae were dominant or co-dominant up through July before cyanobacteria became the dominant taxon.
• Statistical analyses using 2015 to 2023 data indicate that the lake had significantly changed, but much of statistical models were driven by measurements of nitrogen-related variables, which were reported in 2015 and 2017 as much higher, and much lower since then. Total phosphorus in the epilimnion appears to be decreasing in a statistically significant fashion, but another trophic-related variable – Secchi disk transparency – has not exhibited significant change over the last 8 years. Specific conductance continued to trend up and was the variable showing the most change since the early 1990s.
• Co-precipitation is a phenomenon that likely occurs at Lake Wononscopomuc due to high pH and calcium levels. The process precipitates soluble reactive phosphorus out of the water column, making it unusable by algae and cyanobacteria. We hypothesize that once the precipitate reaches lower, anoxic depths, the compound coverts back into the soluble form and increases carbonate and bicarbonate thus increasing pH. The process would also increase soluble reactive phosphorus in the hypolimnion. This could provide a supplemental source of hypolimnetic phosphorus in addition to that released from sediments as part of the internal loading process. These higher hypolimnetic levels may contribute to the cyanobacteria productivity in the upper levels of the hypolimnion.