SummaryThe species composition and year‐month frequency together with the abundance of the four periphytic communities of Plover Cove Reservoir (the epiphyton, epilithon, epipelon and epipsammon) were compared both in terms of all species present and in terms of major (dominant/frequent) species using the ‘circle’ method. All four communities were similar in terms of overall species composition but the richest variety of species occurred in the epiphyton. The diatom floras on wood and rock surfaces closely resembled each other as did the two sand grain floras. The former floras were, however, much more diversified than the latter floras and stood distinctly apart from them. In terms of their dominant/frequent species, the epipelic/epipsammic communities and the epiphytic/epilithic communities had the same number of common taxa (three), a further indication of the closeness between the two communities in each combination. On the other hand, the fact that the three common taxa were different for each combination again reveals the distinct differences between the two combinations.Investigations concerning the effect of the nature of the substrate on diatom growth revealed that it had little effect in qualitative terms. Quantitatively there would appear to be some effect, since differences in abundance and frequency and thus in the seasonal cycle of diatoms growing on different substrates were observed. The conditions in the microhabitats of each substrate undoubtedly determined the dominance and abundance of the flora. However, the effect of the nature of the various substrates on diatom growth varied from species to species and in general, the substrates showed little qualitative selectivity. Thus, no truly differential species were found to any great extent on any of the four substrates.The relationships between the growth of these four periphytic diatom communities and the physical and chemical conditions have also been investigated in terms of cell number and chlorophyll content. Both graphical and statistical analyses of the results are discussed, and it is suggested that fluctuations in water level, temperature and nutrients (especially silica, nitrate and phosphate) are the major factors governing the seasonal patterns of variation in periphyton standing crop. Thus, diatom growth is maximum when nutrient concentrations and water level are high and when water temperature is low, and as temperature increases and nutrients decrease so diatom growth falls. It is probable that an interplay of these factors, together with the detrimental effects of heavy rainfall and typhoons during summer, is responsible for the seasonal changes in periphyton in Plover Cove Reservoir exhibiting a winter peak as opposed to the commonly occurring spring and autumn growth peaks.