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Abstract:
Grain size analysis was carried out on dried sediments whose epipelic diatom floras had been previously investigated. Changes in the percentages of different particle fractions over a two year period were plotted and compared with changes in percentage abundance of different diatom genera, or groups of genera. Fluctuations in stream discharge were reflected in the relative proportions of coarser and finer sediment, and the relative stability of discharge in a spring region by a more constant, contrasting grain size distribution. Epipsammic and epilithic species were more abundant when the coarser sediment fraction was larger; epipelic Navicula and Nitzschia species increased on finer sediments. Different Navicula species showed contrasting occurence patterns. The role of sediments as nutrient traps is discussed and offered as an explanation of the presence of more eutrophic species of finer sediments. The activity of epipelic algae themselves also modifies the nature of the sediment, fostering more rapid accumulation of fine particles and contributing organic matter.
