Cyanos Activity August 27, 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, August 27, 2018, AER visited Bantam Lake to conduct biweekly cyanobacteria monitoring as part of the ongoing lake management program. Water column profile data and Secchi transparency 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 predetermined North Bay and Center Lake sites. An additional sample was also collected at the South Bay site since Secchi transparencies there have been regularly lower than transparencies at the other three sites, suggesting that there could be greater algal productivity. Algae samples were analyzed by methods described in an earlier memo. 

The cyanobacteria community across the lake had fully recovered from the copper sulfate treatment of July 26th. Cyanobacteria cell concentrations measured during the 20 days following the treatment (samples collected on July 30th and August 16th) ranged between 588 and 2,702 cells/mL at the North Bay and Center Lake sites (Fig. 1). Cyanobacteria cell concentrations at the same two sites on August 27th were 18,875 and 28,978 cells/mL, respectively, one to two orders of magnitude higher than that observed on August 16th, and the highest cell concentrations for both sites for the season. Cyanobacteria were the dominant phytoplankton taxa at 84 to 86% of the total flora at the two sites. The Chlorophyta or green algae were the second most important group at approximately 13 to 14% of the planktonic algal community. 

The cyanobacteria cell concentrations at the three sites neared or were within the 20,000 – 100,000 cells/mL range for Visual Rank Category 2 designation. During these conditions the CT DEEP and CT DPH recommend the following: Increase regular visual surveillance until conditions change and consider cautionary postings at public access points (See Appendix C, Example B in https://www.ct.gov/deep/lib/deep/water/water_quality_management/monitoringpubs/bluegreenalgaeblooms_guidanceforlhds_2017version.pdf). 

Secchi transparencies in recent sampling events (both AER and White Memorial sampling events) reflect the increased cyanobacteria cell concentrations. Average Secchi transparency based on the four sites on August 24th was 2.02 m, and on August 27th the average was 2.42 m. This is in contrast to the average of 2.77 m on August 30th, just days after the treatment. 

At the South Bay site, Secchi transparency was only slightly lower than that observed at the Center Lake and Folly Point sites. These three sites were notably lower than clarity measured at the North Bay site on August 27th (Table 1). Cyanobacteria cell concentrations at the South Bay site were similar to those observed at the North Bay and Center Lake sites (Fig. 1). The cyanobacteria genus Aphanizomenon spp. continued to dominate the algal community at 61 to 69% of the total among the three sites sampled. 

The cyanobacteria genus Dolichospermum spp. (Anabaena spp.) was the second most abundant, ranging from 10 to 15% of the total. Both species can regulate buoyancy, are nitrogen fixers, and some are capable of synthesizing toxins. Specimen filaments of both genus did contain heterocyst which are the specialized cells capable of nitrogen fixation. 

It is worth noting that bloom conditions were visible upon arrival and early on during our time on the lake on August 27th visit. At our second site – the Center Lake site – for example, streaks of cyanobacteria parallel with the prevailing wind direction (approximately out of the south). Toward the end of the sampling event and especially at the third and fourth sites, Point Folly and South Bay, respectively streaks had dissipated. It is not uncommon and has been observed by the White Memorial staff, that blooms are more visible in the morning and less visible later in the day. This is especially true when mornings are preceded by warm, very calm nights with light winds start up in the late morning as occurred on August 27th. 

Surface water temperatures were about 1°C lower than those on August 16th and within 0.5°C of each other at the North Bay, Folly Point and Center Lake sites (Table 1). Surface water temperatures at the South Bay site were 1.3°C, or more, warmer than those measured at the other sites, likely due to the shallowness of the site. Bottom water temperatures had not changed appreciably since August 16th with the exception of the South Bay site where the bottom temperature was approximately 1°C lower. Anoxic conditions were observed at and below 6m depth at the North Bay and Point Folly sites; at and below 7m depth at the Center Lake site; and were not encountered at the South Bay site where bottom oxygen levels were 4.1mg/L. 

The thermocline was observed near the bottom of three sites: between 5 and 6m depth at the North Bay and Point Folly sites, and between 6 and 7m depth at the Center Lake site. At the South Bay site a thermocline was not detected based on water temperature/density gradients within the water column. 

Nutrient data for August 16th and earlier dates in surface waters (1m depth) have been included in Table 2. Total phosphorus levels at the North Bay site were slightly elevated. Of particular note was the low chlorophyll-a concentrations, indicative of lower algae levels and the efficacy of the treatment up to that point in time.

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