Cyanos Activity September 23, 2019
Do not expose yourself or your pets 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 September 23rd AER visited Bantam Lake to perform biweekly cyanobacteria monitoring as part of the ongoing lake management program. Samples collected for algal identifications and enumerations were from the North Bay site
(N 41.71087° W -73.21155°) and Center Lake site (N 41.70056° W -73.22102°). A
plankton net tow sample, using a 10µm mesh plankton net, was also collected
at the Center Lake site. Samples were treated, stored, and analyzed following
methods discussed in a previous memo.
Water column profile and Secchi transparency data were also collected at the
North Bay Site, the Center Lake Site, as well as at 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).
- Cyanobacteria concentrations in samples collected on September 23rd were 162,438 and 190,387 cells/mL at the North Bay and Center Lake sites, respectively (Table 1). These were 2X to 3X higher than concentrations observed in September 9th . In the guidance provided to municipalities (CT DPH & CT DEEP 2019) cyanobacteria concentration >100,000 cells/mL are equated with Visual Rank Category 3 with the following interventions recommended: Update/inform CT DPH & CT DEEP and expand risk communication efforts.
- Collect samples for analysis and/or increase frequency of visual assessment.
- POSTED BEACH CLOSURE: If public has beach access, alert water users that a blue-green algae bloom is present. (See Table 6 of CT DPH & CT DEEP 2019 for more detail.)
Aphanizomenon spp. followed by Dolichospermum spp. were the dominant cyanobacteria genera observed at both sites. Both are toxigenic genera (CT DPH &
CT DEEP 2019).
The season high cyanobacteria cell concentrations occurred concurrently with
season low Secchi transparencies. The greatest Secchi transparency on September 23rd was 1.42m and measured at the South Bay site. Secchi transparencies at the other sites were all approximately 1m (Table 1).
Differences in temperatures between surface and bottom waters on September
9
th were <1°C and reflective of a mixed water column. On September 23rd differences between those depths at the North Bay, Center Lake and Folly Point
sites were modestly higher, e.g. at the North Bay site the difference was 2.3°C
(Table 1). This implies that between September 9th and the 23rd the water column had begun to stratify at these sites. Highest resistance to mixing occurred
between 3 and 4 meters at the three sites. The water column at the shallower
South Bay site was mixed.
Oxygen concentrations throughout the water column were patterned after the
stratification patterns. At the North Bay, Center Lake and Folly Point sites, oxygen levels were between 10 and 11.5 mg/L above 4 meters of depth. From 4 meter to the bottom, oxygen concentrations decreased to ≤1.3 mg/L. At the mixed
South Bay site, oxygen concentrations ranged from 8.5 to 4.3 mg/L from the
top to the bottom of the water column.
Literature Cited
Data is collected and analyzed by Aquatic Ecosystem Research, who is contracted by Bantam Lake Protective Association.
