The phytoplankton of the Gulf of Nicoya (Costa Rica), a tropical estuary on the Pacific coast of Central America, is dominated by diatoms, of which species of Chaetoceros and Rhizosolenia are most numerous, and by dinoflagellates which form red tides. Similarity coefficients
showed the greatest resemblance between diatom floras of upper and lower Gulf when compared with other eastern tropical Pacific coastal waters. Total diatom species increased dramatically during the rainy season. Although most of the Chaetoceros species are known to form resting spores,
few spores are seen in the Gulf. The red tide dinoflagellates appear not to be toxic, and the dominant diatoms are mostly cosmopolitan in distribution although some characteristically tropical species are occasionally abundant.
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