EFFECT OF NITROGEN-SOURCE ON THE SIZE DISTRIBUTION WITHIN MARINE-PHYTOPLANKTON POPULATIONS

The size distribution within phytoplankton populations is important in determining direction and magnitude of carbon and energy fluxes within the photic zone of pelagic systems and via sedimentation to below the photic zone and sediments. In order to study the role of nutrient regimes on the phytoplankton size distribution, nitrogen uptake and growth response was studied in nitrogen limited cultures of algae. In discontinuously diluted nitrogen limited cultures, which were pulsed with nitrate every 3 days, specific nitrate uptake was related to cell volume. The relation is described by: specific uptake rate = 0.0303 * (cell volume)(0.214). Specific ammonium uptake in ammonium pulsed cultures was not related to cell volume. This result was confirmed by competition experiments in continuous cultures. Large diatoms became dominant when nitrate was supplied as the only nitrogen source once in 3 days. In ammonium pulsed cultures a large (Ditylum brightwellii) and a small (Skeletonema costatum) species both dominated. Sinking rates of algae in monoculture were not dependent on cell size or nitrogen source. However, in the competition experiment, sinking rate of the nitrate grown population was higher (0.12 m.day(-1)) than that of the ammonium grown population (0 m.day(-1)). For natural systems, this might imply that in nitrate controlled systems production by larger algae is relatively high. This may be an additional explanation for the higher sedimentation rates of organic material in coastal or oceanic upwelling areas as compared to ammonium controlled (regenerative) systems.