Algal blooms, oxygen-depleted zones affected Long Island waters in 2024, report says

Last summer, dozens of Long Island’s ponds, lakes, bays and estuaries were affected by oxygen-depleted zones and harmful algal blooms, researchers from Stony Brook University found, which were triggered by excessive levels of nitrogen and exacerbated by record-high temperatures.

The researchers, led by Christopher Gobler, a coastal ecologist at Stony Brook’s marine sciences department, recorded 36 different bodies of water where dissolved oxygen fell below 3 milligrams per liter, a level that can be fatal to marine wildlife. Gobler said that was a record number of dead zones for Long Island; last year there were 30. 

"That’s a remarkable amount of low-oxygen water," said James McClelland, a senior scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts, who was not involved in the research but reviewed a summary of Gobler’s results. "Far more than I’d expect under more natural conditions."

Nitrogen pollution and the resulting algal blooms can trigger a cascade of damage to coastal ecosystems: slow growth and die-offs of shellfish and other marine animals and depleted sea grass beds and salt marsh grasses, which are critical to healthy coastal ecosystems. "Once you’ve lost them, it’s very hard to get them back," McClelland said.

Gobler presented some of his findings at a lecture Friday sponsored by Save the Great South Bay. The report is being released Wednesday.

More than 25 marine waterways were afflicted with harmful algal blooms (HABs), Gobler said, including Dinophysis, which produce toxins that can cause gastrointestinal illness in people who eat contaminated shellfish.

"We’ve yet to find a location without it," Gobler said at the lecture. "This organism is everywhere," though often at concentrations too low to cause concern. But when environmental conditions are right, the unicellular plankton can quickly multiply, making shellfish in those waters unsafe to eat.

Gobler emphasized that there have been improvements in Long Island’s waterways. In past years, a summertime dead zone spread roughly two-thirds the length of Long Island Sound. But extensive improvements in the wastewater treatment systems in New York City significantly curbed the nitrogen flowing toward the Sound; the hypoxic region this year extended only as far as Bayville.

Last year, five shellfish beds were closed temporarily because of algal blooms; this year no closures were necessary.

A bloom of Dinophysis in Reeves Bay in Riverhead last summer reached densities of 60 million cells per liter and remained at more than a million per liter for more than a month. That’s an improvement over readings of 100 million cells per liter in Seatuck Cove in 2023. Before that, however, the highest concentration known to Gobler and other experts in the field was 2 million cells per liter.

More than two dozen freshwater ponds and lakes experienced outbreaks of blue-green algae, 17 of them on the South Fork. Last year, Gobler reported 13 on the South Fork. These algae, or cyanobacteria, can produce toxins such as microcystin, a liver toxin and possible carcinogen. Microcystin can kill a dog that swims in an affected pond or lake.

Six fish kills appeared in several creeks and ponds on the South Shore, including in Watchogue Creek, Mill Pond in Water Mill and Mecox Bay. In some spots, Gobler said, the water was "slick with fish," floating belly-up on the surface. In each case, the dead fish appeared in waters where dissolved oxygen had dropped well below 3 milligrams per liter.

Low oxygen levels or hypoxia frequently develop where excess nitrogen and other nutrients stimulate aquatic plant growth, including macroalgae (seaweeds) and microalgae. The algae release oxygen into the water as they photosynthesize but when they die and decompose they use up oxygen.

The link between excess nitrogen and the growth of algae, including harmful algae, has been well understood for decades, McClelland said. "There are lots of details about how that plays out in different systems," he said; for example, how much water gets flushed out with the tides.

In some coastal waters the nitrogen pollution comes primarily from crop fertilizer and waste from industrial dairy or hog farms. On Long Island, nitrogen pollution sources include sewage outfalls and lawn and agricultural fertilizer.

But in 2011, researchers at the Marine Biological Laboratory released a landmark study that found "wastewater is the dominant source of nitrogen to Great South Bay, particularly wastewater from septic systems." Authors Ivan Valiela and Erin Kinney calculated that septic systems "account for 67% of the total land derived nitrogen load to Great South Bay." Their research was later published in the peer-reviewed Journal of Coastal Research.

Since then, multiple scientific studies have confirmed the link of nitrogen pollution in marine ecosystems with wastewater, from Tampa Bay to Cape Cod.

Algal blooms and hypoxic conditions occur in bays and inlets close to the shore, where the excess nutrients flow out with groundwater. Shallow water bodies with less tidal flushing are especially at risk; farther into the ocean, the nutrients are highly diluted.

Warm air and water encourages runaway plant growth, so experts warn that algal blooms will increase as temperatures climb.

Overgrowth of algae, even when it doesn’t create biotoxins, can disrupt marine and freshwater ecosystems. Nitrogen pollution "affects our marshes, fisheries and wildlife in general," Maureen Dunn, a water quality scientist at Seatuck Environmental Association, said. "The best thing we can do to improve Long Island’s coastal ecosystems is to "reduce the overburden of nitrogen in the environment."

Suffolk County residents this election will be voting on Proposition 2, which would fund expanded sewers and grants to homeowners for nitrogen-removing septic systems by raising the sales tax rate in Suffolk from 8.625% to 8.75%. The fund is expected to raise nearly $3 billion over 50 years.