State of the Lake 2021

Executive Summary

Aquatic Ecosystem Research (AER) LLC was engaged by the LakeWononscopomuc Association (LWA) to perform an assessment of waterquality in 2021. The biannual monitoring program is one aspect of the LWA’s lake management strategy with the goal of developing a scientific database todetect changes – positive and/or negative within the lake. The following is an outline of findings from the 2021 waterquality monitoring program at Lake Wononscopomuc. Severalrecommendations are pro- vided 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 startingon June 16th, and strong at both sites from July 22nd to September20th.

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

– The strata of water with <1mg/L of oxygen expanded upwardwith time through October 21st until the bottom 19m of theSite 1 water column and the bottom 7m of the Site 2 watercolumn 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 lowor non-detectable.

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

§ Average hypolimnetic total phosphorus concentrations wereone to two orders of magnitude higher than correspondingepilimnetic lev- els.

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

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

§ Ammonia was not detected in epilimnetic samples. Nitriteand ni- trate were only detected once each and at lowconcentrations.

§ Average hypolimnetic total nitrogen was significantlyhigher (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 threeme- ters 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 highestbio- 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 twodominant genera; it is known for forming dense layers at or belowthe 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 inte- grated sample from the surface and wasdominated by Planktothrix spp.

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

o Epilimnetic levels decreased with time from May 18th to August 26thand re- mained at the relatively low level for the balance of themonitoring season.

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

§ At Site 2, hypolimnetic levels gradually increased to theseason max- imum 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 MarbleValley of Connecticut.

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

§ This is not typical for lakes in the Northeast.

• We hypothesize that it is related to carbonate-basedgeology 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 pe- riods 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 thelake as a whole.
§ Significant negative (decreasing) trends were detected fornitrogen- related variables (e.g., TKN, ammonia)
§ Significant positive (increasing) trends were detected forhypolim- netic total phosphorus.

• Contemporary Lake Wononscopomuc data was compared to thatcollected in the 1990s.
o The most conspicuous change was the increase in specific conductance.
§ Many lakes in snowbelt regions of the country are experiencingthis 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 resultin 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 insolubleminerals in the water column.
This is a feature of lakes with carbonate rich bedrock geology.

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