A ‘red tide’ of single-celled organisms stretches 20 miles off La Jolla, Calif., in 2005. Some blooms are toxic. (Peter Franks, Scripps Institution of Oceanography, University of California, San Diego)

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A 'red tide' of single-celled organisms stretches 20 miles off La Jolla, Calif., in 2005. Some blooms are toxic. (Peter Franks, Scripps Institution of Oceanography, University of California, San Diego)