SEASONAL CHANGES IN BENTHIC DIATOM COMMUNITIES COLONIZING GLASS SLIDES IN ABURATSUBO BAY, JAPAN

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Abstract
The seasonal changes in communities of benthic diatoms colonizing glass slides were observed in Aburatsubo Bay, Kanagawa Prefecture, Japan. Once every month during the study periods June 1984 to July 1985 and February to October 1987, glass slides were immersed at 1 m depth for 10 days. Thirty diatom species belonging to 14 genera were identified. Each of these species constituted more than 1% of the total cells in at least one sample. Direct sunlight duration, which is the total time exposed to direct sunlight during the immersion period, and grazing pressure seem to be major factors controlling the seasonal changes in diatom density. Generally, the total cell density of diatoms increased as the direct sunlight duration increased. However, diatom densities were comparatively low under heavy grazing pressure of amphipods in spite of long direct sunlight durations, especially in summer. Physical factors of the seawater, such as water temperature, salinity and nutrients were measured and determined to be minor factors affecting seasonal changes in diatom density. The diatom species were classified into 7 types of growth form, and species in the same group were found to have similar patterns of seasonal variation. Seasonal variation in the total diatom density was largely attributed to sharp fluctuations of species in two groups, Types A and G, under the influence of changes in direct sunlight duration and grazing by amphipods. Type A is “solitary cell with prostrate habit and swift gliding movement” and Type G is “colony enclosed in a filamentous or a branched, arborescent, mucous tube”, namely tube-dwelling diatoms.