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a Original citations quote content of SiO2, except where stated otherwise.

a Original citations quote content of SiO2, except where stated otherwise.

Figure 1.9

The silicon content of selected diatoms from the freshwater (•) or marine (■) phytoplankton or other aquatic habitat (A), plotted on log/log scales against (a) cell volume and (b) surface area. Ast refers to Asterionella, Fra to Fragilaria and Ste to species of Stephanodiscus; Bac refers to Bacillaria, Dit to Ditylum, Nit to Nitzschia, Ske to Skeletonema and Tha to Thalassosira. The equations of the least-squares regression fitted to the data in (a) is log [Si] = 0.707 log v -0.263 (r = 0.85); that for (b) is log [Si] = 1.197 log s - 1.634 (r = 0.83). Redrawn, with permission, from Reynolds (1986a).

Figure 1.9

The silicon content of selected diatoms from the freshwater (•) or marine (■) phytoplankton or other aquatic habitat (A), plotted on log/log scales against (a) cell volume and (b) surface area. Ast refers to Asterionella, Fra to Fragilaria and Ste to species of Stephanodiscus; Bac refers to Bacillaria, Dit to Ditylum, Nit to Nitzschia, Ske to Skeletonema and Tha to Thalassosira. The equations of the least-squares regression fitted to the data in (a) is log [Si] = 0.707 log v -0.263 (r = 0.85); that for (b) is log [Si] = 1.197 log s - 1.634 (r = 0.83). Redrawn, with permission, from Reynolds (1986a).

collectively that the quantities of the components vary within generally consistent limits and, though they do fluctuate, the ratios with other constituents do not vary by more than can be reasonably explained in these terms.

For instance, carbon generally makes up about half the dry organic mass of organic cells. The normal content of phytoplankton strains cultured under ideal laboratory conditions of constant saturating illumination, constant temperature and an adequate supply of all nutrients, was found to be 51-56% of the ash-free dry weight (Ketchum and Redfield, 1949). A slightly lower range (45-51%) was derived by Round (1965) and Fogg (1975) from measurements on freshwater phytoplankton. However, the same sources of data showed extremes of about 35%, in cells deprived of light or a supply of inorganic carbon, and 70%, if deficiencies of other elements impeded the opportunities for growth.

The importance of carbon assimilation by photoautotrophs to system dynamics has encouraged interest in being able to make direct estimates of organismic carbon content as a function of biovolume. It will be obvious, from the recognition of the variability in the absolute contents of carbon, its proportion of wet or dry biomass, and the relative fractions of ash and vacuolar space, that any general relationship must be subject to a generous margin of error. For instance, Mullin et al. (1966) derived an order-of-magnitude range of 0.012-0.26 pg C Mm-3 for a selection of 14 marine phytoplankters that included large and small diatoms. Reynolds' (1984a) analysis of data, pertaining exclusively to freshwater forms, adopted simultaneous approaches to diatoms and non-diatoms. The relatively low ash content and absence of large vacuoles among the latter permitted a much narrower relationship between carbon and biovolume (averaging 0.21-0.24 pg C Mm-3). Supposing carbon makes up a little under half of the ash-free dry mass and that dry mass averages 0.47 pg Mm-3 (Fig. 1.8), this figure is highly plausible. For diatoms, there seemed little alternative but to calculate carbon as a function of the silica-free dry mass.

This approach does not satisfy the quest for a volume-to-carbon conversion for mixed diatom-dominated assemblages, which continues to tax ecosystem ecologists. A recent re-exploration by Gosselain et al. (2000) confirms the wisdom of separating diatoms from other plankters. It provides an evaluation of several of the available formulaic methods for estimating the carbon contents of various diatoms.

Of the other elements comprising biomass, nitrogen accounts for some 4-9% of the ash-free dry mass of freshwater phytoplankters, depending on growth conditions (Ketchum and Redfield,

Table 1.5 The silicon content of some planktic diatoms relative to cell volume and surface area

Species

Si

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