Cyanos Activity July 17, 2018

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 Monday July 16, 2018, AER visited Bantam Lake to collect profile data and algae samples as part of the ongoing lake monitoring initiative. Water column profile data 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 Point Folly (N 41.70773 W -73.22638), and at a site in the South Bay region of the lake (N 41.69015 W -73.22728). Algae samples were collected at the North Bay and Center Lake as originally planned. Additional samples were collected for algal analyses from the South Bay Site and from the shoreline at the public beach in the North Bay area. Algae samples were analyzed by methods described in prior memos. 

Cyanobacteria cell concentrations continued to increase at the sites monitored since the start of the season (Fig. 1). The concentrations of blue green cells at the North Bay and Center Lake Sites were 16,495 and 22,781 cells/mL and represented 98 and 97% of all algal cells counted in those samples, respectively. Cyanobacteria cell concentrations at the South Bay Site and at the public beach in the North Bay of Bantam Lake were 44,008 and 29,340 cells/mL, respectively. Cyanobacteria cell concentrations of >20,000 and <100,000 are characteristic of Visual Rank Category 2 based on the public health guidance provided by the CT DEEP / CT DPH. Recommended actions under this scenario are to notify both State agencies, increase regular visual surveillance until conditions change, and consider cautionary postings at public access points (Ref. 1). 

Bantam Lake waters were visually more turbid than observed in past site visits. Secchi transparencies ranged from a low of 2.23m at the South Bay Site to 2.77m at the North Bay Site. The July 16th lake average was 2.60m, which was a decrease from the 3.17m average on July 2nd. The South Bay Site, which is the shallowest site of the four sites,regularly has the lowest Secchi transparency of the four sites. The lower transparency is in concordance with it containing the highest cell concentrations of the four sites sampled. 

As observed in the July 2nd samples, Aphanizomenon spp., Dolichospermum spp., and Woronichinia spp. are the most abundant cyanobacteria genera at all sites. As noted earlier, the Cyanophyta accounted for the vast majority of algal cells counted. Relative abundances of cyanobacteria from the four samples analyzed ranged between 97 and 99%. All three genera can form blooms and produce toxins. No other observed genera constituted ≥1.5% of the total cells counted. 

It is worth noting that heterocysts were observed in many of the two filamentous genera, Aphanizomenon spp. and Dolichospermum spp. Heterocysts are specialized cells where atmospheric nitrogen in the water is transformed into a form of nitrogen that cells can utilize and provide an adaptive advantage to cyanobacteria over other taxa, particularly when nitrogen is limiting. However, nutrient data from samples collected on June 18 th suggest that Bantam was phosphorus limited. 

Surface and bottom water temperatures did not change appreciably at the deeper sites since July 2nd. At the surface temperatures ranged from 27.3 to 27.9°C. Bottom water temperatures at the North Bay Site and the Point Folly Site where maximum depth was measured between 6.1 and 6.5m were 20.4 and 19.6°C, respectively. At the Center Lake Site, where maximum depth is a little greater at 8.13m, bottom temperature was several degrees lower at 17.4°C. 

The one notable exception occurred with the bottom temperature at Site 4, where maximum depth is <5m. At that site, temperature increased from 22.1 to 25.8°C between July 2nd and July 16th . The entire water column at the South Bay Site only differed in temperature by <2°C and no thermocline was detected. At the other sites a thermocline was situated between 4 and 5m at the North Bay and Point Folly Sites and between 5 and 6m at the Center Lake Site. The thermoclines acted as the upper boundary of oxygen concentrations of <1mg/L. At the South Bay Site, where a thermocline was not detected, oxygen concentrations at the bottom were >5mg/L. 

Also worth noting from the profile data collected is that from the surface to 3m depth pH ranged from 8.4 to 8.65. When pH is greater than 8.3, bicarbonate is the dominant form of carbon available to the pelagic algal community. Cyanobacteria have another adaptive advantage over other algal groups because they can utilize bicarbonate to fulfill their carbon requirements while other groups are dependent upon carbon dioxide, which is limited at elevated pH. Consequently, higher pH promotes the dominance of blue-green algae because carbon resources are limited for other algal genera. 

Ref. 1 See Guidance to Local Health Departments For Blue–Green Algae Blooms in Recreational Freshwaters. June 2017. CT DPH & CT DEEP. http://www.ct.gov/deep/lib/deep/water/water_quality_management/monitoringpubs/bluegreenalgaeblooms_guidanceforlhds_2017version.pdf

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

Figure 1: Bantam Lake cyanobacteria cell count seasonal trend.

Figure 2: Bantam Lake Secchi depth seasonal trend.