Besides being a distinguishing constituent of phytoplankton and having a universal distribution among all the photoautotrophic algae and cyanobacteria, the photosynthetic pigment chlorophyll a is also widely used as a convenient index of phytoplankton biomass. This makes its contribution to cellular composition extremely important to extrapolating to phyto-plankton abundance and to its use as a base for estimating phytoplankton productivity. Although there have been many values published alluding to the absolute chlorophyll a contents of freshwater phytoplankton (some reviewed in Reynolds, 1984a), these quantities are now known to be so variable that they have little value by themselves. The variability is most obviously linked to the cell's requirement for carbon and the light energy available to drive its fixation. In broad
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