The foul odor being smelt by some near the Great South Bay could actually be toxic in large concentrations, experts say.

Scientists say large amounts of decaying seaweed originally from Japan are being reported along the South Shore and in Heckscher State Park.

Some people say it smells like rotten eggs or spoiled meat, but many agree it can be overwhelming and makes walking around the shoreline and other parts of the bay unpleasant.

Stony Brook University professor Dr. Christopher Gobler says the outbreak was not expected and that it’s a major warning sign of nitrogen water pollution in the bay.

He says the seaweed called dasysiphonica japonica thrives on high nitrogen and carbon dioxide in the water from sewer runoff, and conditions in Suffolk County are perfect for its fast growth.

“And it’s just yet another symptom of the series of insults we have to the bay—water quality impairments, mahogany giant brown tide and this bloom—all of which are harmful to marine life and all of which are promoted by excessive nitrogen,” Gobler says.

He says the smell is from hydrogen sulfide gas, which is coming off of the seaweed.

Gobler says in high enough concentrations, the gas can be harmful to both people and pets so it’s best not to spend a lot of time at parks if the odor is smelt.

The seaweed and its associated red tide can also reduce oxygen in the water, threatening sea life.

Oyster farmer Keith Powell says it’s a growing concern.

“And it seems each year the times that these blooms are out of control does seem to be increasing,” Powell says.

Scientists say this type of invasive seaweed has been discovered up the shoreline of the Great South Bay all the way east to the Peconic Bay.