The assessment of size variation in Loch Leven phytoplankton: methodology and some of its uses in the study of factors influencing size

The dimensions of individuals of many phytoplankton species vary considerably during the population cycle and during long-term succession, as well as at any one point in time. Moreover, the type of size-frequency distribution also fluctuates by being, for example, normal on one occasion and variously departing from this condition on another. As a consequence, parametric and many non-parametric methods are of limited value in the statistical analysis of size data. In this paper the graphical method of Sokal and Rohlf (1969), based on rankits, has been used to assess size spectra of algal plankton. Brief consideration is given to the effects of sample preservation on algal size, the need for a random choice of individuals to be measured, and the size of measuring unit to employ. Proformas are described which enable the microscopist without computer facilities to record measurements in a manner facilitating easy tabulation with rankits in readiness for graphing. Seasonal information on size-frequency distributions for a number of algae are illustrated with rankit-dimension plots. A distinction is drawn in these results between what are usually normally, or near-normally distributed values, and those showing marked departures from the normal distribution. The tendency to follow one or other of these 2 main types of distribution appears to be largely a function of the range of values (ratio of maximum to minimum value) in the sample, with the wider-ranging populations generally departing the most from the normal. The orientation of the measured axis with respect to the plane of cell division may also affect the form of the plots, and results must be interpreted with especial care where the mono-specificity of the samples is in doubt. Some experimental results are described in order to illustrate some uses of the rankit graph method in investigating environmental controls of size. These results indicate also the potential ability that dense swarms of zooplankton (Daphnia and Cyclops) have for altering the length spectra of Oscillatoria and Melosira threads