Comparison of carbon-specific growth rates and rates of cellular increase of phytoplankton in large limnetic enclosures

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Abstract
Potential carbon-specific growth rates of phytoplankton were estimated from a series of measurements of photosynthetic radio-carbon uptake over 4- and 24-h exposure periods in the light fields of three large limnetic enclosures (‘Lund Tubes’), each providing different limnological and trophic conditions. Photosynthetic behaviour and short-term, chlorophyll-specific carbon-fixation rates conformed to well-established criteria but, over 24 h, the net retention represented 23–82% of the carbon fixed during the daylight hours. Potential mean growth rates (k'p, of the photo-autotrophic community were calculated as the net exponential rates of daily carbon-accumulation relative to derived, instantaneous estimates of the cell carbon-content. Apparent actual community growth rates (k'D were calculated as the sum of the exponential rates of change of each of the major species present, corrected for probable rates of in situ grazing and sinking, and expressed relative to the fraction of total biomass for which they accounted. The corresponding values were only occasionally similar, k'p generally exceeding K'D by a factor of between 1 and 30 or 1 and 14, depending upon the carbon:chlorophyll ratio used. The ratio, K'p/K'D was found to vary inversely both to k'D and to kn, the net rate of change in phytoplankton biomass, suggesting that measured carbon fixation rates merely represent a capacity for cellular increase which, owing to other likely limitations upon growth, is seldom realized. Apparent rates of loss of whole cells do not account for the loss of carbon; that the ‘unaccounted’ loss rates (K'p–K'D varied in direct proportion to K'p (i.e., losses were least when chlorophyll-specific photosynthetic productivity was itself limited) is best explained by physiological voiding of excess carbon (for instance, by respiration, photorespiration, excretion) prior to the formation of new cells.

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