COLD SPRING HARBOR - Marine scientists are voicing concerns as potentially harmful algae blooms continue to spread into local waterways. Officials with the Department of Environmental Conservation say the blooms, known as brown and red tides, can affect shellfish. They say the problem is caused when excess nitrogen seeps into the ground from waste water, septic systems, cesspools and fertilizers. Mussels and clams filter the algae toxins into their tissues, then transfer the materials to humans when they're eaten. Experts say shellfish poisoning is a risk, but that monitoring the algae blooms has kept the spread under control. Activists say the more troublesome areas include Huntington and Northport harbors, as well as Shinnecock and Peconic bays. Brown tide: Trouble brewing for Long Island's baysBrown Tide: Blight on the Bays - A special report