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Watershed Institute helps address toxic algae blooms in Rosedale Lake with newly installed wetlands

When Mercer County residents visit Rosedale Lake they will see new wetlands floating on the lake’s surface.

In Pennington, the 20 floating wetlands will help alleviate harmful algae blooms (HABs), which have plagued Rosedale Lake for several years. The wetlands are part of the Watershed Institute’s effort to address the issue of toxic algae blooms.

“The hope is the filter material the islands are made out of will act as a bio-filter, sort of like filter material would for a fish tank. The whole thing is planted with wetland plants,” said Steve Tuorto, director of science at the Watershed. “So the roots and the microbes associated with the plant roots will grow straight down in the water column and filter the water, as it goes past removing any nutrients that enter in those tributaries due to stormwater runoff from the residential communities that are above the lake.”

Part of the Mercer County Park System, Rosedale Lake, has been faced with the issue of toxic HABs since 2019. At the time Mercer County had to shut down all recreation in the lake for public safety.

HABs occur due to several factors and produce toxins that are harmful to people and pets.

“They are certainly happening more frequently due to the warming climate. We are getting more hot days that warms the water. When they get plenty of water and sun they grow,” Tuorto said. “It is also from the nutrients that runoff from the lake primarily through stormwater runoff. With the lake settled down and warm weather the cyanobacteria (organisms that cause the HABs) grow rapidly creating the blooms.”

Through two days the floating wetlands were brought to two separate areas of the lake near flow from tributaries by the Watershed Institutes speedboat and staff. One section of 10 wetlands had been anchored on May 20 and a second section of 10 installed on May 21.

The plants and soil medium will absorb the pollutants and help cleanse the water. According to the Watershed Institute the floating wetlands consist of growing grass, swamp milkweed and cardinal flowers.

“They are really good at removing pollutants including nutrients like nitrogen and phosphorus, which feed the algae blooms, especially the HABs in the lake. After they mature, they sustain themselves and grow really well they will remove enough nutrients that will get less algae blooms,” Tuorto said.

Over the course of the two days, 25 volunteers from the Watershed Institute and Trout Unlimited planted, anchored and deployed the wetlands.

The Watershed Institute will monitor the wetlands for two years.

“We hope the wetlands will last 10 years or more before they need to be hauled out or renovated,” he said. “We will be monitoring the wetlands at Rosedale Lake. The Watershed Institute is partnering with the New Jersey Department of Environmental Protection and also some other organizations throughout the state to begin as big of a statewide program as we can to monitor HABs, to see how frequently and where they are happening.”

A decision on additional wetlands being installed has not been made as the current 20 wetlands will need to be monitored to see if they are successful.

If the wetlands are really successful and are having an impact the Watershed may potentially deploy more wetlands out into the lake, Tuorto added.

Additionally, Mercer County officials will be installing aeration devices and use barley bales to filter stormwater runoff to also combat the forming of HABs.

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