Phytoplankton communities in water bodies at Dungeness,U.K.: analysis of seasonal changes in response to environmental factors

The species richness and the dynamics of the ciliatecommunitieswere studied at three stations along a streamlet formed bywaterdischarged from a geothermal sulphur spring. Altogether, 73specieswere identified. Ciliate coenoses were composed primarily offreshwater forms, but they were numerically dominated byeuryhalinespecies. The great fluctuation in both number of species andindividuals observed along the streamlet were due to vigorouschemical changes in the physico-chemical characteristics ofthewater. These changes were caused mainly by the sulphide andsaltgradient observed along the water course. The maximum value ofsulphide was observed in August with 391 µM l^–1, andthehighest conductivity value was observed in April with 18 mScm^–1. The most characteristic and abundant species weresmall-sized bacterivorous scuticociliates which have a rapidecological adaptability and whose tolerance limits toenvironmentalchanges are wide. Bacterivorous scuticociliates such as Cyclidium citrullus, C. glaucoma, Cinetochilummargaritaceum, and Uronema nigricans were numericallydominant when the thermal water contained the highestconcentrationof sulphide. The diversity index values changed sharply intimeboth inside and among sampling stations, indicating unstablecommunities which depended on highly fluctuatingconditions.