Cyanos Activity May 21, 2020

Reconsider activities and limit exposure to the water.

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 Thursday May 21, 2020, AER visited Bantam Lake to conduct monthly water quality monitoring and biweekly Cyanobacteria monitoring as part of the ongoing lake management program. Water column profile data and Secchi transparencies were collected from 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. The concentrated plankton sample was transferred into a 25mL glass amber bottle and stored at 3°C. Approximately 500mL of the top three meters of lake water were integrated and collected for algae counts at both 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. Addition preparation and methods were undertaken as described in earlier memos. 

Nineteen algal genera were identified in samples collected on May 21st . Five of those were from the Cyanobacteria; four genera were observed from each of the Chlorophyta (Green Algae), Chrysophyta (Golden Algae), and Bacillariophyta (Diatoms) taxa. The Cyanobacteria genus, Aphanizomenon spp. (Fig. 1), continued to be most abundant genus observed accounting for 95 and 82% of all cells counted at Site 1 and Site 2, respectively. Other Cyanobacteria genera observed included Aphanothece spp., Coelosphaerium spp., Dolichospermum spp., and Woronichinia spp. 

 The Cyanobacteria cell concentration at the North Bay Site was 27,556 cells/mL. The concentration at the Center Lake Site was 60,004 cells/mL. Both concentrations were above the intervention threshold of 20,000 cells/mL, and indicative of Visual Rank Category 2 (CT DPH & CT DEEP 2019). Additional information on Cyanobacteria and recommended public interventions are available at https://portal.ct.gov/DPH/Environmental-Health/Environmental-Health-Section/Blue-Green-Algae-Blooms. 

Despite the higher concentrations of Cyanobacteria, water clarity was relatively good. Secchi transparencies were measured between 2.57 and 2.98 meters at the four sites (Table 1). One reason for the contrasting water quality characteristics (i.e. high cell concentration but good Secchi transparencies) is the nature of Aphanizomenon spp. In the lake in its vegetative state, it forms rafts of filaments. The filaments are comprised of small cells that are often <3 to 5µm in length. The small cells in a raft do not always result in a more turbid condition since they are tightly grouped together. Once samples are treated, the filaments constituting the rafts separate (Fig. 1). 

Surface water temperatures increased by approximately 4°C since May 5th but temperatures at the bottom did not. The subsequent temperature/density gradient resulted in stratification below 4 meters of depth at the North Bay, Center Lake and Folly Point sites. That stratification and oxygen demand at the sediment-water interface resulted in oxygen concentrations at the bottom of those sites being lower than corresponding concentrations at the surface. At the thermally mixed South Bay site, oxygen concentrations were consistent throughout the water column (Table 1). 

Literature Cited Connecticut Department of Public Health and Connecticut Department of Energy and Environmental Protection. 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 

Data is collected and analyzed by Aquatic Ecosystem Research who is contracted by Bantam Lake Protective Association.