Even to get accurate measurements of sinking rate (ws) is problematic. Here, the main issue is to be able to keep the water static, when almost any conventional observation system involving uninsulated light sources is beset by generated convection. Techniques have been developed or applied, using some combination of strict ther-mostatic control, thin observation cells (Wiseman and Reynolds, 1981) or non-heat-generating measuring systems based on fluorometry (see, for instance, Eppley et al., 1967; Tilman and Kilham, 1976; Jaworski et al., 1981). Another approach has been to make measurements in solutions of high viscosity: Davey and Walsby (1985) used glycerol. Alternatively, to measure the rate of loss across a boundary layer from an initially mixed suspension works with convection and yields acceptable results on field-collected material when sophisticated techniques may not be readily available (Reynolds et al., 1986). This actually imitates, in part, the way that plankton settles from natural water columns (see Section 2.6).

Once confidence was gained in the measurement of sinking rates, another, more tantalising source of variability was detected. Several, quite independent investigations of the species-specific sinking rates of diatoms each yielded order-of-magnitude variability. For any given species, the sinking rates seemed least when the cells were healthy and physiologiclly active but were as much as three to seven times faster in similar cells that were naturally moribund (Eppley et al., 1967; Smayda, 1970; Reynolds, 1973a), or whose photosynthesis was experimentally inhibited or carbon-limited (Jaworski et al., 1981), or which had been exposed to sublethal doses of algi-cide (Margalef, 1957; Smayda, 1974), or had been otherwise freshly killed (Wiseman and Reynolds, 1981). However, comparing the fastest rates from each of the studies that had made measurements on comparable material (eight-celled stellate coenobia of the freshwater diatom Asterionella formosa), some conformity among the various results became apparent (Jaworski et al., 1988).

Wiseman et al. (1983) had previously established that the one sure way to get the consistent, inter-experimental results necessary to be able to investigate the morphological form resistance of

Figure 2.8

Log/log plot of the instantaneous intrinsic settling rates (ws) of Stephanodiscus rotula cells, collected from the field and plotted against mean cell diameter, ds (o). There is apparently no correlation. However, when corresponding samples are killed by heat prior to determination (•), a strong positive correlation is found. Redrawn from Reynolds (1984a).

Log/log plot of the instantaneous intrinsic settling rates (ws) of Stephanodiscus rotula cells, collected from the field and plotted against mean cell diameter, ds (o). There is apparently no correlation. However, when corresponding samples are killed by heat prior to determination (•), a strong positive correlation is found. Redrawn from Reynolds (1984a).

phytoplankton was to first kill the diatoms under test. A case in point is shown in Fig. 2.8, where the mean sinking rates of Stephanodiscus rotula cells sampled during the increase and decrease phases of a natural lake population seem to vary randomly. However, the corresponding rates of cells killed by dipping in boiling water just before measurement were demonstrably correlated to size. It is now quite generally accepted that live, healthy diatoms have the capacity to lower their sinking rates below that of dead or moribund ones. The mechanism of change is not obviously contributed by variability in size or shape or even density. Sinking of live diatoms and, possibly, other algae is plainly influenced by the intervention of further, vital components that must be taken into account in any judgement on how phy-toplankton regulate their sinking rates.

To go on now to review some of the analytical investigations into the sinking rates of phytoplankton and the role of form resistance provides the simultaneous opportunity to observe the cumulative influences of the biotic components of the modified Stokes equation (2.16).

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