What is the main cause of the algae and weed growth in Rainbow Lake?
Rainbow Lake is an artificial lake that was formed by damming a small stream more than 90 years ago. Initially, the lake was fairly deep and there was little weed growth. However, because the lake was formed by flooding a meadow, the base of the lake was always nutrient rich. Over the years, any weeds that grew in the lake fell to the bottom when they died and decayed, adding nutrients to the lake bottom. The flow and geography of the lake are such that nutrients are not effectively flushed from the lake. Each year, as weeds fell to the bottom, the lake has become shallower little by little; meanwhile, the bottom of the lake has effectively become a nutrient-rich compost pile. Several of the evaluations of the lake that the RLA has commissioned have concluded that this decayed plant matter on the bottom of the lake is the primary source of the nutrients that plant and algae need to grow. As a nutrient source for weed and algae growth, the bottom of the lake is far more significant than any inflows into the lake.
Can the sluice gate at the base of the dam be used to draw off nutrient-rich water from the bottom of the lake?
Consultants have told the RLA that the shallow nature of our lake means that we do not have a stratified lake, i.e., nutrient levels are the same at all depths. Other consultants have suggested that there is a gradient of nutrients in the deeper part of the lake.
Fox Hill Lake used triploid carp to control the weeds, is this an option for Rainbow Lake?
The RLA explored the use of carp with the CT Department of Energy and Environmental Protection several years ago. The State had concerns about our ability to prevent the carp from escaping from the lake. The State would require a large fence to be built within the lake to prevent the fish escaping to Bennett’s Pond. The use of carp would not address our algae problem because they do not usually feed on algae. Additionally, the fish may make the algae problem worse because after feeding on the weeds, the fish excrete bioavailable nutrients into the lake. Finally, having carp in the lake would prevent us from using the copper-based algaecides we rely on for algae control because they are toxic to the fish.
What about deep winter drawdowns?
Several lakes have had success in reducing weed growth by drastically lowering lake levels during the winter months, which causes the dormant weeds to freeze. The RLA Board effectively tried this approach several years ago when the dam needed repair. There was negligible improvement in water quality, meanwhile the community had to look at mudflats for the entire winter. The RLA recently discussed this approach with a marine biologist. His opinion was that it would take many (more than 10) years of drawdowns to have any effect on the water quality. One advantage of drawdowns is that they are not costly.
Would aeration help improve water quality?
Many lakes and ponds have improved water quality by the use of aeration systems. Rainbow Lake does suffer from low levels of oxygenation. However, because of the low flow levels in Rainbow Lake, especially away from the stream entry and exit points, to effectively aerate the lake would require many aeration systems. These systems are expensive and come with the additional requirement that they need a power source. The RLA does not currently own any property with electricity service. Nor could we afford the utility bills associated with running an aeration system with our current voluntary membership system.
Would floating islands help?
Floating islands are platforms of plants that essentially remove nutrients from the lake by using them to grow plants on the platforms. Essentially, the plants on the islands take up the nutrients that would otherwise provide a source for weed and algae growth. Potentially, floating islands could be a solution for Rainbow Lake, although the low flow in the lake would mean that many islands would be required and hence the cost of commercial islands would be high. The RLA Board is looking into whether it would be possible to build our own islands.
Would dredging help?
Dredging would be the best option for the lake because it would both deepen the lake and remove the nutrient-rich layer on the bottom of the lake. The downside of dredging is the cost. The RLA has been quoted costs of around $40,000 for dredging 1 acre; the lake area is 45 acres. As such, dredging on anything other than a small scale would not be feasible. The RLA has been advised that dredging small areas would be beneficial.
Can we skim the algae from surface of the lake?
In principle, it would be possible to remove the ugly-looking filamentous algae from the surface of the lake. The two main challenges, however, are where to dry the wet algae and where to dispose of it once it has dried.