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Abstract

Three protist strains were grown under suspended solid stress conditions (0, 31.25, 62.5, 125, 250, 500, 1000, and 2000 mg SS L-1). Quadruplicate cultures in 500 ml-PC bottles were subsampled to measure chlorophyll fluorescence every 24 hrs. A total of 32 culture bottles (quadruplicate bottles for 8 SS concentrations) were prepared and incubated at 20癈, 30 psu under 50 μE m-2s-1 using rotating wheels (4 rpm) for each of the 3 test strains, a cryptomonad, a diatom Skeletonema sp. and a dinoflagellate Prorocentrum minimum. In all the three test strains, population size steadily increased at least during the first 3 days regardless of the SS concentrations in the culture media, followed by retarded growth (a cryptomonad and Prorocentrum minimum) or population decline (Skeletonema sp.). Mean daily growth rates during the first 3 d along the SS concentration gradient were compared within each strain as well as among the strains. Skeletonema sp. and a cryptomonad exhibited positively related growth rate with SS concentrations while P. minimum did negatively. Present result implies that the increased SS concentration may sometimes stimulate population growth of non-flagellated or nano-flagellate protists in contrasts to the case of dinoflagellates. Extended speculation might open an early discussion on the potentially negative effect of the increased SS concentrations on red-tide dinoflagellate species.

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