State of the Lake 2021

Executive Summary

Aquatic Ecosystem Research (AER) LLC was engaged by the Lake Wononscopomuc Association (LWA) to perform an assessment of water quality in 2021. 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 2021 water quality monitoring program at Lake Wononscopomuc. Several recommendations are provided at the end of the report.

• The lake was thermally stratified by the May 18th sampling date and remained stratified throughout the season, including on October 21st.

– Resistance to mixing at the thermocline was strong at Site 1 starting on June 16th, and strong at both sites from July 22nd to September 20th.

– Oxygen concentrations of <1mg/L were first observed at the strata near the bottom of the water column starting on June 16th at both sites.

– The strata of water with <1mg/L of oxygen expanded upward with time through October 21st until the bottom 19m of the Site 1 water column and the bottom 7m of the Site 2 water column were anoxic.

• The lake exhibited early mesotrophic to mesotrophic characteristics.

– Season average and summer average Secchi disk transparency were indicative of early mesotrophic productivity.

– Monthly epilimnetic total phosphorus measurements spanned oligotrophic to eutrophic concentrations.

§ The highest epilimnetic phosphorus concentrations were measured between May 18th and July 22nd; however, fromAugust 26th through October 21st, concentrations were lower non-detectable.

§ Season averages were indicative of early-mesotrophic tome to-trophic trophic conditions.

§ Average hypolimnetic total phosphorus concentrations were one to two orders of magnitude higher than corresponding epilimnetic levels.

• Hypolimnetic concentrations increased throughAugust 26th then decreased by September 20th; but, were again elevated by October 21st.

– Average epilimnetic total nitrogen concentrations were indicative of meso-trophic conditions.

– Ammonia was not detected in epilimnetic samples. Nitrite and nitrate were only detected once each and at low concentrations.

– Average hypolimnetic total nitrogen was significantly higher (p<0.05) and comprised largely of ammonia.

• Concentrations increased over time at both sites.

• Algal cell concentrations were low in the integrated samples of the top three meters of the water column.

o Relative abundances of cyanobacteria cells were high onOctober 21st at Site 1; and, on September 20th through October21st at Site 2.

o Relative concentrations of phycocyanin suggested that the highest bio- mass of cyanobacteria existed between the 12 and 15m stratum at Site 1 and the 11 and 14m strata at Site 2 on all sampling dates.

o The filamentous cyanobacteria, Planktothrix spp., was one of two dominant genera; it is known for forming dense layers at or below the thermocline.

§ A sample was collected from Site 1 between the depths of 13and 14m on June 16th; it had ~30X more cells than the corresponding integrated sample from the surface and was dominated by Planktothrix spp.

• Specific conductance was high as it is for most lakes in the Marble Valley ofConnecticut.

o Epilimnetic levels decreased with time from May 18th to August 26th and remained at the relatively low level for the balance of the monitoring season.

o Hypolimnetic levels increased through August 26th at Site 1 and were slightly lower for the balance of the season

§ At Site 2, hypolimnetic levels gradually increased to the season maximum on October 21st.

• Alkalinity was high as it is for most lakes in the Marble Valley of Connecticut.

o Epilimnetic levels were higher earlier in the season and decreased to lowest levels by August 26th before modestly increasing throughOctober 21st.

o Hypolimnetic levels were lowest on May 18th and increased for most of the season. The maxima were reached on August 26th at Site 1 and on October 21st at Site 2.

• The lake water pH was high and not uncommon for lakes in the Marble Valley of Connecticut.

o In most instances, other than on May 18th, hypolimnetic pH was greater than epilimnetic pH.

§ This is not typical for lakes in the Northeast.

• We hypothesize that it is related to carbonate-based geology of the Marble Valley of Connecticut.

• Oxidation-reduction potentials (ORP) exhibited different characteristics at the two sampling sites.

o Site 1 exhibited a pattern more typical to lakes in Connecticut with high ORP in surface waters and low ORP in strata near the bottom after protracted periods of anoxia.

o Site 2 ORP levels were only high on May 18th and at the bottom strata. ORP was atypically low with highest levels measured in mid-depth regions of the water column.

• Statistical analyses of water quality trends from data collected in 2015,2017, 2019, and 2021 revealed that the lake has changed during that time.
o Changes were detected in the epilimnion, hypolimnion, and in the lake as a whole.
§ Significant negative (decreasing) trends were detected for nitrogen- related variables (e.g., TKN, ammonia)
§ Significant positive (increasing) trends were detected for hypolimnetic total phosphorus.

• Contemporary Lake Wononscopomuc data was compared to that collected in the 1990s.
o The most conspicuous change was the increase in specific conductance.
§ Many lakes in snow belt regions of the country are experiencing this type of change and it has been shown to be a result of increasing use of deicing road salts.

• The high pH and high calcium concentrations in the lake water may result in the coprecipitation of phosphorus.
o This can result in algal productivity that is lower than what the lake could support if phosphorus was not constantly forming insoluble minerals in the water column.
This is a feature of lakes with carbonate rich bedrock geology.

Read/Download Complete Report (PDF)