A study of the diatom flora living on Thalassia testudinum König in Biscayne Bay was carried out from November 1964 to July 1965. This paper records 42 species of pennate diatoms, representing 20 genera. One new species has been described. The ranges of salinity and temperature
within which the studied species thrive are established. Salinity and temperature do not appear to have a direct effect on the distribution and succession of the diatoms living on Thalassia. The characteristics of the sediments and/or the presence of substances produced by other organisms
appear to have more influence on the occurrence of the diatoms living on Thalassia than do salinity and temperature.
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