On a sticky July afternoon, as the sun beat down and the temperature climbed into the high 80s, several dozen people gathered on the banks of a murky pond in Morningside Park in Manhattan to talk about a slimy green problem.

The pond, built in 1989, is a highlight of the leafy park, which runs for 13 blocks through Harlem and Morningside Heights. But in recent years, it has turned a sickly shade of green as algae has overtaken its surface. And on this Saturday, scientists from Columbia University and the city’s Parks Department began a new research effort at the site into the spread of harmful algae blooms worldwide.

For the university, the project represents a new chapter in its complicated and sometimes tense relationship with the surrounding community over this section of the park. The pond itself was built on the site of a proposed Columbia gymnasium, which was abandoned after students and Harlem residents objected and the issue set off the bitter Columbia student protests of 1968.

The pond’s small size, and the amount of its water that has been taken over by algae, makes it a perfect case study, said Joaquim Goes, the project’s lead researcher and a biology professor at Columbia’s Lamont-Doherty Earth Observatory. For years, Dr. Goes has studied toxic algal blooms around the world, even monitoring a bloom that grows to “three times the size of Texas” every year off the coast of Oman, in the Arabian Peninsula.

The city approached the university about the algae problem a month and a half ago, Dr. Goes said. By taking samples from the pond and trying different remedies, his team hopes to figure out the best way to mitigate the spread of harmful algae and to create an “early warning system” for future blooms, he said.

“If we can control it from the source, we can prevent it from spreading,” he said.

Most algae are harmless to humans and animals, but some, called cyanobacteria or blue-green algae, can be toxic.

Harmful algae blooms thrive when there are excess nutrients — like phosphorus or nitrogen — in a waterway, along with lots of sunlight and calm water. The blooms, which typically appear during the summer, have spread steadily throughout the city’s freshwater ponds and lakes. The problem is exacerbated by climate change, which can cause blooms to “occur more often, in more water bodies and to be more intense,” according to the Environmental Protection Agency.

The city has been monitoring the blooms since 2016, said Rebecca Swadek, the Parks Department’s director of wetlands management. “They’re definitely spread out throughout the city,” she said.

In 2020, the Parks Department published a news release with safety tips for avoiding the toxic blooms, primarily advising parkgoers and their pets to stay out of the water and to rinse off immediately if they came in contact with algae. Exposure can cause eye or throat irritation, breathing difficulties and dizziness, according to the department.

The New York State Department of Environmental Conservation maintains an online map of places where harmful algal blooms have been confirmed throughout the state. The state has also issued $371 million in grants to reduce pollution that contributes to the blooms, along with $14 million for research and monitoring projects.

When students and residents near the park began protesting Columbia’s construction of the gym in the late ’60s, a main criticism was of the building’s design: While students could enter it on the western side of the park, the public would have had access through a basement-level entrance on the eastern side, and only to part of the building. The university later built the gymnasium elsewhere. In 1989, the crater left behind by the abandoned project was converted into an ornamental pond and waterfall.

Columbia University’s new president, Minouche Shafik, standing near the pond on Saturday, praised the university and surrounding community for working together now to improve the park “rather than fighting over a neighborhood asset.”

As scientists and officials work to control the algae, some residents have another goal: getting the waterfall flowing again. It was rehabilitated in 2018, but currently doesn’t work.

Part of the Columbia University project will include work by the engineering school to repair broken water pumps and restore the waterfall (which has not been affected by the algae).

“I would say that the pond and the waterfall are, in people’s minds, the most impressive feature of the park,” said Brad W. Taylor, an architect and president of the volunteer organization Friends of Morningside Park, who created a petition in June to push for restoration of the waterfall.

“That’s often the first thing they mention,” he said.

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