QO 0 10 15 20 DO 0 3 10 15 Spfci Fii phowuywvtfietro raw rrfl Cmachlj h mn C mc oflll Cc h1 101

Fig. 12.13 Variation with depth of specific photosynthetic rates of phyto-plankton originating from different depths, in Lake Tahoe (CaliforniaNevada, USA) (by permission, from Tilzer and Goldman (1978), Ecology, 59, 810-21. Copyright © 1978, the Ecological Society of America). The phytoplankton were collected from the surface ( ), 50 m (-----) and

105 m (-), and incubated with 14CO2 at a series of depths between the surface and 105 m. (a) Specific photosynthetic rate per unit chlorophyll. (b) Specific rate per unit cellular biomass, expressed as carbon. The lower maximum photosynthetic rate per unit chlorophyll of the deeper water cells is due to their higher pigment content.

circulation. This suggests that even within a mixing layer, photosynthetic differences between the cells at the top and the bottom might develop as turbulence diminishes. Lewis et al. (1984) obtained evidence that this can indeed happen. In Bedford Basin (Nova Scotia, Canada) they measured DPm, the difference between Pm (per unit chlorophyll) at the surface and at the base of the mixing region, and also measured the rate of turbulent kinetic energy (TKE) dissipation, e (m2s~3), on 19 occasions over the period of a month. While on many occasions, especially at high rates of TKE dissipation, DPm did not differ significantly from zero, on other occasions, as e diminished to low values, DPm increased markedly, indicating that photoadaptation had in fact occurred.

In McMurdo Sound, Antarctica, in the austral spring Robinson et al. (1995a) found the light saturation onset parameter, Ek, of the diatom-dominated algal community in the platelet layer immediately underneath the 2.2 m thick ice layer was higher (5.9-12.6 mmol photons m~2s_1) than that (1.3-4.5 mmol photons m-2s-1) for the diatom community at the benthic surface (26 m depth). The initial slope, a, of the P versus Ed curve was much higher for the benthic algae than for the platelet layer algae - 0.07-0.10 and 0.003-0.011 (mg C mg chl a-1 hr-1 [mmol photons m-2 s-1]-1), respectively - indicating a higher photosynthetic efficiency at low light levels. As pointed out earlier, strictly speaking, since the two diatom communities are not taxonomically identical, this should be regarded as phylogenetic, rather than ontogenetic, adaptation.

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