Figure 5.5

The initial slopes, ar, of growth rate on light intensity from Fig. 5.4, plotted against the corresponding values of msv-1, the product of maximum dimension and the surface-to-volume ratio, of each of the same selection of species of algae: Ana, Anabaena flos-aquae; Aphan, Aphanizomenon flos-aquae; Coel, Coelastrum microporum; Diet, Dictyosphaerium pulchellum; Fra b, Fragilaria bidens; Lim red, Limnothrix redekei; Mic, Microcystis aeruginosa; Monor, Monoraphidium sp.; Ped b, Pediastrum boryanum; Pla ag, Planktothrix agardhii; Scen q, Scenedesmus quadricauda. The least-squares regression fitted to the points is ar = 0.257 (msv-1)0'236. Redrawn with permission from Reynolds (1997a).

flux density, which does, eventually, force a dependence of r on I. The steeper is the slope of light-dependent growth (ar), the more efficient is the dedication of harvested light energy. Values of ar (expressed in units of specific replication rate (r) d-1 (mol photon)-1 m-2 per d, which simplifies to (mol photon)-1 m2), are derived from the initial, light-dependent slopes in Fig. 5.4. Following Reynolds (1989b), they are plotted in Fig. 5.5 against the dimensionless product of the surface-to-volume ratio (sv-1) and the maximum dimension of the alga (m). This had been found to provide the most satisfactory morphological descriptor of the interspecific variability in ar. It also corresponded with the interpretation that the greatest flexibility of algae to enhance their light-dependent growth efficiency evidently resided with those having the greatest morphological attenuation of form. Ostensibly, slender and flattened shapes make the best light antennae, at least when oriented correctly in the photon-flux field. The filamentous arrangement cells in the Oscillatoriales seems to be supremely efficient in this context, as both the numerous studies referred to above and the affinity the Planktothrix and Limnothrix species for turbid, well-mixed lakes (see Section 7.2.3) would indicate. Recalculating from the data of Post et al. (1985), acclimated P. agardhii may maintain a maximum growth rate of 9.8 x 10-6 mol C (mol cell C)-1 s-1 at 20 °C, to as low as 18 |imol photons m-2 s-1; ar ~ 0.54 mol C (mol cell C)-1 (mol photon)-1 m2. The analogous experimental data for the diatom Asterionella have not been located but, piecing together information from field populations, Reynolds (1994a) showed it to rival the reputation of Planktothrix as a low-light adapting organism. From a specific growth rate, r20 = 20.6 x 10-6 mol C (mol cell C)-1 s-1 and a chlorophyll content of 2.3 pg cell-1, that is ~0.324 g chla (mol cell C)-1, the sustaining chlorophyll-specific yield is 63.6 x 10-6 mol C (g chla)-1 s-1. In turn, this requires a photon flux of, theoretically, not less than 509 x 10 6 mol photons (g chla)

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