ABSTRACT
The rates of dark(14)CO2 fixation by natural phytoplankton communities growing in eutrophic and oligotrophic waters were studied with short-term in situ experiments. Three aspects were investigated: (1) the time course incorporation of(14)CO2 in darkness, (2) the depth variability in dark(14)CO2 fixation, and (3) the variability in(14)CO2 fixation within a year. The highest dark(14)CO2 incorporation rates were observed during the first interval of incubation (20 min) after which they approached a constant rate with time. The observed differences in dark(14)CO2 fixation rates between populations from different depths were associated with differences in species composition as well as with physiological differences caused by exposure to different illumination conditions prior to their exposure to darkness. Autocorrelation coefficients were computed for the analysis of variability of dark(14)CO2 fixation rates within a year. It was suggested that dark(14)CO2 incorporation might be a periodic phenomenon depending mainly on the productive capacity of the phytoplankton community.
