The New York State Department of Environmental Conservation has warn?ed that cyanobacteria, which is informally known as blue-green algae, has been found in Big Reed Pond off East Lake Drive in Montauk and could be hazardous to people and animals, especially dogs that take a dip in the pond or even walk too close to its edge. Big Reed Pond, as well as Lake Agawam in Southampton and Marratooka Lake in Mattituck, has been declared off-limits for people and dogs by the D.E.C.