National Geographic : 1974 Mar
Stain of pollution, nurtured by ill-treated sewage, flowers in an algal bloom where Mattawoman Creek meets the Potomac River near Washington, D. C. Seen as red tendrils against green water in this infrared aerial photograph, such recurrent blooms serve as an alarm, warning that serious problems lie upstream. Thin white line of dead fish draws a boundary between benign water near shore and the poison ous algal bloom of Gymnodinium breve, at times so rust-colored that it is called a red tide. The beach fringes Florida's Sanibel Island, a paradise for shell collectors. An estimated 50 million pounds of fish died in one such outbreak. Fifth columns of Gonyaulax cells, here dividing into a chain, may build up rapidly. Because of a natural toxicity, high concentrations can form harmful red tides. One Gonyaulax species poisons the sea life that feeds on it but has no ill effects on man. But other species can build up in shellfish in quantities lethal to human beings. In 1972 a red tide closed many New England clam beds.