Cyanos Activity May 25, 2021
Disclaimer: The information presented below reflects conditions throughout the lake and may differ from conditions on specific shorelines. For information regarding beach closings please contact the Torrington Area Health District or local town officials.
On Tuesday, May 25th , AER visited Bantam Lake to conduct biweekly Cyanobacteria monitoring and monthly water quality monitoring as part of the ongoing lake management program. Data collected in the field included measurements of temperature, dissolved oxygen, specific conductance, and pH at one-meter intervals from the top to the bottom of the water column, total depth, and Secchi disk transparency. These data were collected at four sites: the North Bay Site (N 41.71087° W -73.21155°), the Center Lake Site (N 41.70056° W -73.22102°), a site west of Folly Point (N 41.70773 W - 73.22638), and at a site in the South Bay region of the lake (N 41.69015 W -73.22728).
A plankton tow sample using a 10µm mesh plankton net was collected at the Center Lake site. Approximately 500 mL from the top three meters of lake water were integrated and collected for algae counts at the North Bay and Center Lake sites using a three-meter-long sampling tube. These samples were preserved with Lugol’s solution shortly after collection and stored at 3C. Methods for analyses of the phytoplankton net sample and the integrated samples discussed in our April 30th memo were followed.
The lake appeared clearer than normal and Secchi disk transparency measurements corroborated those observations. Secchi disk transparency ranged from a low of 3.50m at the South Bay Site, to a high of 3.94m at the Folly Point Site (Table 1). These measurements were greater than any recorded earlier this year or any from all of last year.
Thirty-three algal genera were observed between the net tow and integrated whole water samples. The number of Chlorophyta (aka Green Algae) genera increased since the start of the season from three to eleven genera. The numbers of Cyanobacteria (aka Blue-green Algae) genera observed since the start of the season increased from 4 to 9. Numbers of genera of Bacillariophyta (aka Diatoms) and Chrysophyta (aka Golden Algae) were five and four, respectively. Other taxonomic group were represented by two or less genera.
Cyanobacteria cell concentrations were 8,962 and 7,676 cells/mL at the North Bay and Center Lake Sites, respectively. These were below the intervention threshold of 20,000 cells/mL, and indicative of Visual Rank Category 1 (CT DPH & CT DEEP 2019). It is worth noting that the Cyanobacteria cell concentrations represented 84 and 91% of all cells in those respective samples and a significant shift in community structure. In prior weeks, Golden Algae, and to a lesser degree Diatoms, were the most numerous taxa.
The most numerous Cyanobacteria were Aphanizomenon spp., Dolichospermum spp., and Woronichinia spp. (Fig. 1). Aphanizomenon spp. was the genus responsible for the bloom conditions experienced early in the season last year. These species are considered toxigenic (CT DPH & CT DEEP 2019, EPA 2020)
Concurrent with the shift in the pelagic algae community structure was the shift in the temperature / density dynamics within the water column. For the first time this season, the water column was stratified and at all four sites. At three of the four sites a thermocline was detected between 4 and 5 meters of depth; at the South Bay Site, the thermocline was detected between 3 and 4 meters of depth. This was exemplified in the greater differences between surface temperature and temperatures at the bottom of the water column at each site (Table 1). Stratification also prevented mixing of sufficient oxygen concentrations to lower depths to keep pace with the consumption of oxygen via cellular respiration at the bottom, resulting in reduced oxygen levels at the bottom (Table 1).
Literature Cited
Connecticut Department of Public Health and Connecticut Department of Energy and Environmental Protection [CT DPH & CT DEEP]. 2019. Guidance to Local Health Departments for Blue–Green Algae Blooms in Recreational Freshwaters. See https://portal.ct.gov/-/media/Departments-and-Agencies/DPH/dph/environmental_health/BEACH/Blue-Green-AlgaeBlooms_June2019_FINAL.pdf?la=en
United States Environmental Protection Agency [US EPA]. 2020. Health Effects from Cyanotoxins. https://www.epa.gov/cyanohabs/health-effects-cyanotoxins
Data is collected and analyzed by Aquatic Ecosystem Research who is contracted by Bantam Lake Protective Association.
