Heatwave: Algae are disturbing lakes in New Jersey

Heatwave: Algae are disturbing lakes in New Jersey


The ongoing heat wave in North Jersey is not only causing discomfort for many residents, it could also cause problems for the region’s lakes, which have already suffered environmental problems in recent summers.

Lake Hopatcong and Greenwood Lake have been temporarily closed several times in recent years due to harmful algal blooms (HABs). Bloom concentrations have not risen to levels that would prompt closures this year, but authorities fear that could soon change if the heat doesn’t ease.

“We’re holding our breath,” said Marty Kane, chairman of the Lake Hopatcong Foundation, in an interview with on Wednesday afternoon. “Mother Nature isn’t helping us right now.”

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What are harmful algal blooms?

HABs are caused by cyanobacteria that occur naturally in freshwater and form dense mats that resemble pea soup or spilled paint, according to the New Jersey Department of Environmental Protection. The bacteria can cause numerous symptoms in humans when ingested, including headaches, sore throats, nausea and diarrhea, while direct contact with the algae can cause a rash.

The flowers tend to thrive in standing, warm, nutrient-rich water—all characteristics that are enhanced by prolonged periods of hot, sunny weather.

Kane said he’s keeping an eye on New York, where several beaches in Westchester and Putnam counties were closed last week because of the flake bloom. Recent measurements have measured water samples from Lake Hopatcong at 30 degrees, he said, and “I’m guessing we’re probably going to set records for water temperatures this summer.”

An interactive map on the DEP website shows more than a dozen samples taken from different parts of Lake Hopatcong over the past month that indicate a HAB warning, which is triggered when cyanobacteria levels are between 20,000 and 80,000 cells per milliliter. If they exceed the 80,000-cell mark, an algae warning goes into effect. During this time, the DEP recommends closing public beaches and warns the public to avoid contact with the water.

The highest reading this year at Lake Hopatcong was 67,125 cells per milliliter, measured on Skytop Road on May 31. The most recent samples released, a series of measurements on July 2, found a maximum concentration of 50,750 cells per milliliter on the beach at Hopatcong State Park.

Previous closures at lakes in New Jersey

Repeated high HAB concentrations forced beaches at Lake Hopatcong and Greenwood Lake to close for much of the summer in 2019. Bacterial blooms partially returned in 2020 and 2022, and although they were not as severe, authorities began consulting environmental experts to prevent the bacteria from forming.

Lake Hopatcong undergoes an alum treatment in the fall, a short-term method that reduces phosphorus levels in the water and causes algal blooms. The longer-term solution, Kane said, is a large-scale aeration system that diffuses oxygen into the lake bottom to dissolve algae.

Kane and his colleagues at the Lake Hopatcong Foundation and similar organizations across the state have implemented various treatment plans designed to prevent HABs from occurring, but what they could really use right now is a few days of cooler weather.

“We hope we have done enough,” Kane said, “but these are the worst conditions for an algal bloom.”

Anyone who observes a suspected HAB bacteria species should not touch the water and protect pets, as contact with cyanobacteria can make them lethargic and lose their appetite, according to the Lake Hopatcong Foundation. Residents can report the bloom using an online form or by calling the NJDEP hotline at 1-877-WARNDEP (927-6337).