Figure 2.14

Plot of sinking rates of killed, stellate eight-celled colonies of Asterionella cells of diminuitive length and volume, compared with the sinking rates calculated for a sphere of the same volume and density (ws calc). Three regressions are fitted to the observed values: (1) applies to all points, (2) to those colonies > 1000 ¡m3 in volume and (3) to those < 1000 ¡m3 in volume. The ratio, 0r = (ws calc)/ws is shown against the horizontal axis with respect to the estimates of ws derived from regression equation (2.1) (curve 4) and those combining equations (2.2) and (2.3) (curve 5). Redrawn with permission, from Jaworski et al. (1988).

measuring about 5.5 ¡m with a basal width of 4.2 ¡m and a height of 2.1 ¡m. Significantly, the cells maintained mutual connections and there remained plenty of eight-armed colonies in the culture. The dry mass of cells and the silica content of the walls diminished with size but overall density increased (from ~1100 to 1200 kg m-3).

The sinking rates (ws) of killed cells measured over the five year period are plotted in Fig. 2.14

against colony volume and compared with ws calc of equivalent spheres having the same densities. As the size of the cells diminished over the first 2 years (during which time the mean colony volume reduced by nearly two-thirds and the mean density difference with the medium almost doubled), sinking rates actually increased slightly. The rising mean of about 7.2 to one of 8.0 ¡m s-1 does not violate the hypothesis that the stellate cell arrangement dominates the sinking rate. Thereafter, with the aggregate volumes of colonies below 1200 ¡m3, sinking rate diminished. The curve fitted to all sinking rate determinations (curve 1 in Fig. 2.14) reveals less of what is happening than do the two separate fitted curves referring to colonies >1200 ¡m3 (curve 2) and those <1200 ¡m3 (curve 3). By taking each of these regression lines and using them to solve form resistance as the calculated sinking rate for a sphere divided by the regression-predicted actual sinking rate, the relationship is further amplified. Curve (4) for all points (corresponding to regression 1) shows the typical form-resistance outline, albeit rather flattened. The right-hand side of curve 5, corresponding to regression 2, quickly takes > 2 and > 3, towards the 'plateau' level shown in Fig. 2.13. To the left of curve 5 (corresponding to regression 3), sinking rate varies more closely with volume (though not so steeply as ws calc); is in the range 1.6-2.4. The change in behaviour has a pivot point when the colony falls below 1200 ¡m3, when individual cell volumes are <150 ¡m3 and the cells are 18-20 ¡m in length. The likely significance of this is that the hydromechanical characteristics of the star shape are eventually overtaken by those of a gyre of ovoids, more reminiscent ofAnabaena than Asterionella.

Some of these measurements are included in the summaries of calculated form resistance owing to shape distortions.

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