The relatively small size of planktic algae has been alluded to in Chapter 1. For instance, the diameters of the spheres of equivalent volumes to species named in Table 1.2 cover a range from 1 to 450 ¡m. It has been suggested that this is itself an adaptive feature for living in fine-grained turbulence. Many species may present rather greater maximal dimensions if (presumably) the rate of turbulent energy dissipation allows. The effect of size on the settling velocities of centric diatoms was demonstrated empirically by Smayda (1970). A striking feature of his regression of the logarithm of velocity ws on the average cell diameter (d) is that its slope lies closer to 1 than 2, as would have been expected from the Stokes equation (2.14). The regression fitted to the plot of sinking rates of killed Stephanodiscus cells against the diameter (in Fig. 2.8) also has a slope of ~1.1. The observations suggest that the larger size (and, hence, the larger internal space) is compensated by a lower overall unit density (cf. Section 1.5.2 and Fig. 1.9). The implication is that more of the overall density of the small diatom is explained by a relatively greater silicon content. However, it is probable that the effect is enhanced by the fact that a relatively greater part of the internal space of larger diatoms is occupied by cell sap rather than cytoplasm (Walsby and Reynolds, 1980) and that many marine diatoms, at least, are known to be able to vary the sap density relative to that of sea water (Gross and Zeuthen, 1948; Anderson and Sweeney, 1978). This mechanism of density regulation is not available to freshwater algae (see Section 2.5.2) but, either way, density effects may be vital to the suspension of larger diatoms in the sea, if the reduction of sinking rates is the ultimate adaptive aim. According to Smayda's (1970) data, cells of Coscinodiscus wailesii should settle at a rate of 40 m d-1, had they the same net density as the much smaller Cyclotella nana, rather than the observed 8-9 m d-1.

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