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Particulate organic 14C accumulated linearly for periods of 10 to 12 h when natural assemblages of phytoplankton from the Celtic Sea were incubated with '"C-bicarbonate at a constant illumination of 145 pm01 photons m-' S-'. Biomass-specific photosynthesis rates (P/B) estimated for the > 1.0 klm slze fraction ranged from 0.05 to 0.11 h-' (0.6 to 1.3 d-' for a 12 h photoperiod) at a temperature of 11 "C. Llnear particulate 14C accumulation, and other independent measurements, suggest that the photosynthetically active phytoplankton biomass was not increasing during the incubations (i.e. the growth rate was negligible). A rapid turnover of phytoplankton carbon and/or accumulation of energy storage reserves are possible mechanisms which could account for the absence of growth when photosynthesis rates were high. There was no evidence for enhanced phytoplankton growth rates in samples d~luted with filtered seawater, indicating that microzooplankton grazing did not greatly influence productivity estimates in these sanlples. Estimates of phytoplankton respiration rates from the dark loss of organic 14C ranged from 20 to 80 O/O of the rate of photosynthetic accumulation of I4C in the light, and cannot fully account for the apparent discrepancy between photosynthesis and growth. This leaves the accumulation of non-photosynthetic biomass under the experimental culture conditions as a likely, but untested possibility. Indirect evidence is cited to support this hypothesis.

Estimating the growth and loss rates of phytoplankton from time-series observations of 14C-bicarbonate uptake