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1 Log/log plot of cell chlorophyll-a content against cell volume of various freshwater phytoplankters (data in Table 1.3): •, Cyanobacteria; O, diatoms; □, chlorophytes; ■. others). Regression equations are fitted to the data for Cyanobacteria (1, log chl = 1.00 log v - 2.26), diatoms (2, log chl = 1.45 log v - 3.77) and chlorophytes (3, log chl = 0.88 log v - 1.51) and to all points (4, log chl = 0.98 log v -2.07). Redrawn from Reynolds (1984a).

terms, the weaker is the photon flux, and the greater is the probability of limitation of growth rate by light, then the greater is the need for the light-harvesting centres of which the chlorophyll is an essential component. Where appropriate, this behaviour may be accompanied by the production of additional quotas of accessory pigments (for a fuller discussion, see Chapter 3). Synthesis of chlorophyll a is also sensitive to nutrient supply and deployment, directly or as a consequence of altered internal resource allocation. The measurements of biomass-specific estimates of chlorophyll a presented in Reynolds (1984a) range over an order of magnitude, between 0.0015 and 0.0197 pg Mm-3 of live cell volume, which corresponds to 3 to 39 mg g-1 of dry mass (0.3% to 3.9%). The true range is probably wider: later data showed that the chlorophyll-a content of just one species of cyanobacterium, Planktothrix cf. mougeotii, may vary nearly ninefold (0.45-3.9%

dry weight) between its deep-stratified and free-mixed phases (Reynolds 1997a).

On the other hand and analogously with Redfield stoichiometry, the probabilistic relationships derived from mixed populations over periods of time point to 0.003-0.007 pg Mm-3, or roughly 0.013-0.031 pg chl (pg cell C)-1, or 0.7-1.6% of the ash-free dry mass as each being typical. For many purposes, the common approximations that chlorophyll accounts for 1% of the dry mass and about 2% of the value of the cell carbon quota are not at all unreasonable average estimates. Indeed, field chlorophyll measurements are commonly converted to approximate producer biomass, expressed as active cell carbon by the application of a ratio 50: 1 by weight. A margin of variation, from 30 : 1 to 70 : 1, should nevertheless be allowed. It should be borne in mind too that the amount of chlorophyll a is proportionate to cell volume and not cell number, bigger cells carrying proportionately more chlorophyll than small ones, and that there may be systematic interphyletic differences in the typical cell-specific contents. The plot of data taken from Reynolds (1984a) emphasises both points (Fig. 1.10).

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