Dry weight

Characteristically, the major constituent of the live plankter is water. If the organism is air-dried to remove all uncombined water, the residue will comprise both organic (mainly protoplasm and storage condensates) and inorganic (such as the carbonate or silica impregnated into the cell walls) fractions. Oxidation of the organic fraction, by further heating in air to ~500 °C, yields an ash approximating to the original inorganic constituents. The relative masses of the ash and ash-free (i.e. organic) fractions of the original material may then be back-calculated.

The dry weights and the ash contents of a selection of freshwater phytoplankton are presented in Table 1.3. Generally, cell dry mass (Wc) increases with increasing cell volume (v), as shown in Fig. 1.8. The regression, fitted to all data points, has the equation Wc = 0.47 v0 99, with a high coefficient of correlation (0.97). At first sight, the relationship yields the useful general prediction that the dry weights of live

Table 1.3 Air-dry weights, ash-free free dry weights, chlorophyll content and volume of individual cells3 from natural populations (all values are means of collected data having considerable ranges of variability)


Dry weight (Pg cell-1)

Ash-free dry weight (pg cell 1)

(pg cell-1)

Volume (^m3 cell


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