New records of cell carbon and cell volume are given for five species of freshwater algae cultured optimally for use in physiological studies on Daphnia spp. These are listed with other values for freshwater algae from the literature, giving a total of 41 determinations for 25 species, together with details of culture media employed, formulae used for calculation of volume and techniques for carbon determination. The following significant regression of cell carbon on cell volume was fitted to 37 points: C = 0.1204V 1031 (pg C./im1) and the regression coefficient was found not to differ significantly from 1.0. Significant simple linear and polynomial regressions could also be fitted to the untransformed data. There was a significant reduction of residual mean square in the polynomial regressions when tested against the linear regressions and this demonstrated the curvilinearity of the relationship. The polynomial regressions predicted a more or less constant carbon-volume ratio of —0.2 pg C /im~J over the size range of freshwater algae available but the double logarithmic regression was better for comparison. The double logarithmic regression is com- pared with other regressions for freshwater and marine algal species and it appears that freshwater and marine species differ in the way carbon content per unit volume changes with cell size, as this relationship is inverse in marine algae. It is suggested that the relationship described here might be applied to estimate the availability of edible algae in nature to herbivorous zooplankton.
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