May 26, 2021

The Watershed Floats Wetlands To Halt Toxic Algae at Rosedale

By Anne Levin

You can boat and fish on Rosedale Lake this summer. Just don’t eat the fish that you catch.

That’s the word from The Watershed Institute, which has begun a project to stop toxic algae blooms and improve water quality at the Mercer County lake. Rosedale is among several New Jersey waterways that have been closed to swimming, fishing, and other activities for the past few summers because of the persistent problem.

Blame pesticides, fertilizers, and other pollutants flowing into the water from tributaries and nearby lands. Climate change, too, is thought to be a culprit. “According to the federal Environmental Protection Agency (EPA), the warmer days caused by climate change may result in more harmful algal blooms (HABs),” reads a press release from The Watershed Institute.

Steve Tuorto, The Watershed’s director of science, said the combination of no rain and the warm temperature of streams are perfect conditions for the HABs. “There’s a lot of algae in the water,” he said. “The harmful blooms are actually a bacteria, and when they grow to high levels they can be toxic.”

Aided by some volunteers from Trout Unlimited, Watershed staff and scientists steered a speedboat into Rosedale Lake over two days last week to drop in the first of 20 floating wetlands. The islands were built on Watershed property using marine foam, canvas, and soil material, before being shipped to Rosedale Lake. The Watershed was contracted by Mercer County to help fight the HABs, with funds from the federal Clean Water Act.

The Watershed’s bacterial action team is monitoring the project, with weekly assessments at about 30 locations including Rosedale Lake, where they will watch the 50-square-foot wetlands for two years to determine their effectiveness. At the same time, officials from Mercer County will add lake aeration devices and barley bales along the shoreline to absorb polluted stormwater runoff coming into the lake.

As part of an earlier pilot project, The Watershed installed seven similar wetlands at an East Windsor senior community. The success of that project helped the organization decide which plants would be best to use on Rosedale Lake.

The harmful blooms have resulted in restrictions not only at Rosedale, but also at Lake Hopatcong in Morris and Sussex counties, and Mountain Lake in Warren County.  In 2017 and 2018, there were a total of 64 HAB advisories, according to the Watershed release. A full list of last year’s advisories is available at

While the algae is still a problem, the situation has cleared up a bit since the New Jersey Department of Environmental Protection issued a warning last year regarding Rosedale Lake. Just when the situation will be resolved is impossible to predict.

“Other than watching, there is no way of telling,” said Tuorto. “It’s basically a monitoring thing. We have to keep an eye on it. If the bloom goes away, restrictions can be eased, but it’s hard to predict. There is every likelihood, though, that the lake will reopen this year. It won’t close for the whole season.”