Editor’s Note: This post is the second of a series that began last week with the post titled “Cleaning up Carmi: Land and Water.”
At a meeting of the Lake Carmi Implementation Team in Franklin on Thursday, April 19th, Vermont Lake Assessment Coordinator Mark Mitchell used lake monitoring data from 2016 and 2017 to tell the story of how unusual environmental conditions during 2017 led to extreme cyanobacteria blooms. Given the possibility that the same perfect storm could occur again in the future, the Watershed Management Division recommends management of legacy phosphorus in the lake, while work continues to reduce nutrient runoff from the watershed.
Biweekly monitoring conducted in Lake Carmi by the division during 2016 and 2017 showed normal lake stratification during early summer. Lakes stratify when warm air above heats the upper portion of the water column, with the cooler, denser water below. Typically, oxygen is low near the lake bottom, a condition called “hypoxia,” as algae, plants, and other organic matter decompose.
During the period of hypoxia, legacy phosphorus is released from the sediment (internal loading) ready to be mixed into the water column when surface waters cool and the lake mixes. In late August 2017, unusually cool weather caused early destratification, allowing phosphorus that had accumulated in the lowest layer to mix into the water column, supercharging the cyanobacteria. Subsequently, many warm, calm days caused the lake to stratify again. More internal phosphorus loading occurred before the lake cooled and mixed again in early October. This allowed the cyanobacteria bloom to continue through October and into November. For a detailed account of this series of events, see Mark Mitchell’s presentation and the accompanying fact sheet.
With much work to reduce runoff of nutrients from the watershed into Lake Carmi already accomplished, the extreme cyanobacteria blooms of 2017 spurred the division to recommend an in-lake treatment for Lake Carmi. Initially, a pilot aeration project was proposed for one lobe of the lake; however, a team of consultants hired this past winter provided convincing arguments for a whole lake treatment.
The final report from the team of consultants evaluating alternative in-lake treatments is available on the division’s “Restoring Lake Carmi” web page. The team considered treatments ranging from artificial circulation to hypolimnetic aeration/oxygenation to phosphorus binding with aluminum (an alum treatment). The final recommendation was artificial circulation, with two different whole-lake approaches possible: diffused air circulation and downdraft pumping. The estimated cost of a whole-lake artificial circulation project is $1.2 – $1.6 million.
The same team of consultants has been selected to model and design the project, with design to be completed in June. The division anticipates installation in 2018, with the system ready to prevent hypoxia during summer 2019.
The Watershed Management Division continues to advocate for accelerated work on land to prevent additional nutrient loading from the watershed, in conjunction with the in-lake treatment, as both approaches are necessary to restore Lake Carmi.