Long Islanders spending time along the waters of the South Shore this holiday weekend may spot an unwelcome algae presence.

Dr. Chris Gobler, of Stony Brook University's School of Marine and Atmospheric Sciences, says the so-called mahogany tide algae bloom has been spotted along the South Shore. It has turned up in parts of the Great South Bay and the Shinnecock Bay, and in places like East Hampton and Quogue.

"We haven't really seen this organism spread out to the bays," says Gobler. "If we have this on top of what we normally have, it could be one of the worst seasons."

The algae discolors the water to a rusty hue and causes low oxygen levels in the water, which can harm sea grasses. Some studies have shown that the tide can be lethal to oysters and scallops, Gobler says. Mahogany tide contributed to fish die-offs in the Peconic River last year, according to scientists.

Global analysis has shown that the mahogany tide is the direct result of high levels of nutrients coming from the land into the sea. Environmentalists blame Long Island's aging private cesspool systems and excess nitrogen from fertilizers seeping into the water.