Toxic ‘Rust Tide’ Makes An Unwelcome Return To The Peconic Estuary, Local Bays

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Blooms of a toxic red algae that is known to have killed fish and shellfish have once again blossomed in the waters of the East End.

The algae, a dinoflagelate organism known as Cochlodinium, appeared in northeastern Shinnecock Bay and western Peconic Bay last week and has begun spreading. It is the 10th year in a row that the algae blooms have appeared in late summer, usually lasting until late September or early October.

Last year, the algae spread into East Hampton waters for the first time, blooming in Three Mile Harbor until mid-October, and is thought to be linked to a massive die-off of scallops in Peconic and Gardiners bays and along the North Fork.

Scientists have dubbed these blooms a “rust tide” for the dark reddish-brown color the algae blooms give to the water, to contrast it with the infamous “brown tide” that devastated shellfish stocks in the 1980s and 1990s, and the “red tide” algae that has bloomed in recent years in western Peconic Bay and has been shown to be toxic, even fatal, to humans.

The Cochlodinium species is not harmful to humans, even if ingested, but can kill fish and shellfish in large numbers if they are exposed to it for as long as an hour. Last year, for the first time, scientists said they saw direct evidence of the algae having killed large numbers of fish in the wild: a die-off of hundreds of menhaden and other fish species in a small enclosed harbor off Flanders Bay. Then, in late summer, enormous numbers of bay scallops, which had been poised to produce their most robust harvests in the Peconics and Gardiners Bay since the brown tides last bloomed in 1995, died suddenly.

“I saw some data from East Hampton waters last year where the densities were stable until you got to August,” said Christopher Gobler, Ph.D, a professor at Stony Brook University’s School of Atmospheric and Marine Sciences. “In August, September and October, they took a nosedive, from 20 scallops per meter down to two scallops per meter, at exactly the time the [Cochlodinium] bloom was going on.”

Cochlodinium presents a particular issue for shellfish, since the tiny individual organisms can swim through the water column and spend most of the dark hours near the bottom of the bays before rising again to the surface during the day, where they are visible to scientists.

Densities of the rust tide blooms are still relatively low, though some signs of very dense blooms are already starting to pop up in parts of Flanders Bay. Densities of 500 per milliliter are shown to be sufficient to kill fish and shellfish, and some of the blooms have already exceeded 10,000 per milliliter.

The degree of damage will depend on how long the blooms last, which will be determined by temperatures in the coming weeks. Hot weather in August and September will prolong the blooms, cooler or stormy weather will likely put an end to them. The blooms quickly dissipate once water temperatures touch 60 degrees, which typically happens by early October. This year’s blooms started a little bit early than the 10-year average, but several weeks later than they did in 2010 and 2012, when prolonged hot spells in July warmed bay waters quickly.

Dr. Gobler noted that the blooms have been proven to feed on nitrogen, and that high nitrogen levels in local waters, the result of discharge from residential septic systems, are believed to be the main culprit for the numerous and increasingly destructive harmful algal blooms that have spread across East End waters in recent decades.

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