by Jenn Rogers

Last summer Rosedale Lake in Mercer Meadows was affected by a toxic cyanobacteria harmful algal bloom (HAB). Most will remember that this bloom closed the lake to public use from early July through mid-November. This HAB was considered a high-risk closure by state standards due to the toxicity level and cell counts in the lake. At 56,300,000 cells/milliliter, Rosedale Lake had the HAB with the highest cell count in New Jersey for 2019.

The Mercer County Park Commission, in partnership with the Watershed Institute, was awarded a $185,000 grant from the New Jersey Department of Environmental Protection earlier this spring to prevent freshwater HABs at Rosedale Lake.

The County’s Open Space Trust Fund is also financing this project by providing $52,000 in matching dollars to perform the project. To alleviate HAB pressures, we will take a combined approach of reducing the nutrient load entering the lake and creating conditions in the waterbody that are not favorable to HABs.

What might these efforts entail?

1. Installing barley straw bales at the two tributaries entering Rosedale Lake. Barley straw acts as a sponge, absorbing excess nutrients from the surrounding water. Because of the lake’s history with HAB we will be placing 150 bales between the two inlets to the lake.

2. In addition to barley bales to reduce the nutrient load on the waterbody, the Watershed Institute will be designing, constructing and deploying 1,000 square feet of constructed floating wetlands (CFW) in the lake. CFWs are man-made ecosystems that mimic natural wetlands and support plant growth hydroponically. The root systems of the plants hosted in the floating wetland filter, consume and break down nutrients. It is estimated that one 250-square-foot CFW has the root and microbial surface area equivalent to nearly 175 times that of a natural wetland, and has the potential to mitigate approximately 11,000 pounds of algal biomass growth per year.

3. Since cyanobacteria species prefer calm waters, the project team will install an aeration system in the lake. Enhancing water movement can improve the water chemistry and make dissolved nutrients less available to algae. This system will be installed at the north end of the lake, where water levels are deeper.

This combined approach will control the oversaturation of nutrients in the waterbody, thereby decreasing the risk of HABs at Rosedale Lake.

Our 2020 Roots for Rivers Reforestation Grant will also assist in enhancing water quality at the lake by planting approximately 1,500 trees and shrubs in the five-acre lawn and meadow matrix adjacent to one of the lake’s tributaries.

Jenn Rogers is director of stewardship for the Mercer County Park Commission. For more information, visit www.mercercounty­parks.org

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