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

Potential carbon-specific growth rates of phytoplankton wereestimated from a series of measurements of photosynthetic radio-carbonuptake over 4- and 24-h exposure periods in the light fieldsof three large limnetic enclosures (‘Lund Tubes’),each providing different limnological and trophic conditions.Photosynthetic behaviour and short-term, chlorophyll-specificcarbon-fixation rates conformed to well-established criteriabut, over 24 h, the net retention represented 23–82% ofthe carbon fixed during the daylight hours. Potential mean growthrates (k'_p, of the photo-autotrophic community were calculatedas the net exponential rates of daily carbon-accumulation relativeto derived, instantaneous estimates of the cell carbon-content.Apparent actual community growth rates (k'_D were calculatedas the sum of the exponential rates of change of each of themajor species present, corrected for probable rates of in situgrazing and sinking, and expressed relative to the fractionof total biomass for which they accounted. The correspondingvalues were only occasionally similar, k'_p generally exceedingK'_D by a factor of between 1 and 30 or 1 and 14, depending uponthe carbon:chlorophyll ratio used. The ratio, K'_p/K'_D was foundto vary inversely both to k'_D and to k_n, the net rate of changein phytoplankton biomass, suggesting that measured carbon fixationrates 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 theloss of carbon; that the ‘unaccounted’ loss rates(K'_p–K'_D varied in direct proportion to K'_p (i.e., losseswere least when chlorophyll-specific photosynthetic productivitywas itself limited) is best explained by physiological voidingof excess carbon (for instance, by respiration, photorespiration,excretion) prior to the formation of new cells.