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 withi n the photic zone of pelagic systems and via sedimentation to below th e photic zone and sediments. In order to study the role of nutrient re gimes on the phytoplankton size distribution, nitrogen uptake and grow th response was studied in nitrogen limited cultures of algae. In disc ontinuously diluted nitrogen limited cultures, which were pulsed with nitrate every 3 days, specific nitrate uptake was related to cell volu me. The relation is described by: specific uptake rate = 0.0303 (cel l volume)(0.214). Specific ammonium uptake in ammonium pulsed cultures was not related to cell volume. This result was confirmed by competit ion 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. Ho wever, in the competition experiment, sinking rate of the nitrate grow n population was higher (0.12 m.day(-1)) than that of the ammonium gro wn population (0 m.day(-1)). For natural systems, this might imply tha t in nitrate controlled systems production by larger algae is relative ly high. This may be an additional explanation for the higher sediment ation rates of organic material in coastal or oceanic upwelling areas as compared to ammonium controlled (regenerative) systems.