Greatest axial dimensions (GALD) of phytoplankton cells, colonies and filaments etc, are used to describe the size structure of whole assemblages of species in the shallow eutrophic Loch Leven (S.E. Scotland). Two-weekly samples over the period 1979–1982 have been analysed to determine whether variation in size spectra show seasonal trends. Size frequency distributions are displayed using — for the first time in studies of phytoplankton assemblages — the graphical method based on rankits. The paper describes how individuals to be measured were chosen without bias towards any particular type. Seasonal variation in temperature and nutrient concentrations are discussed in relation to algal size structure.\ud In spite of irregular shifts in species composition and abundance, phytoplankton assemblage size spectra (PASS) exhibit seasonal patterns. Early in the year, when temperatures are low and herbivorous zooplankton sparse, small algae ( 15 µm) predominate. The winter-early spring assemblages often exhibit a normal size frequency distribution. Later in the year larger algae occasionally become relatively more numerous, and skewed or polymodal frequency distributions are recorded. Increases in large algae are usually associated with Daphnia population maxima; the inter-relationship is clearly demonstrated in time-series plots of GALD isopleths and Daphnia numbers.\ud The potential of the PASS method to further knowledge on ecological controls of phytoplankton is discussed. The investigator is compelled to include all species in a sample. The rankit-dimension graphs retain all the data, so the position of each algal measurement and its influence on the size distribution can be observed. This approach may help to identify size ranges of algae removed by a wide variety of grazing zooplankton.\u
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