AME 51:153-168 (2008)  -  DOI: https://doi.org/10.3354/ame01178

ABSTRACT: A mesocosm experiment was conducted during the spring of 2003 in Raunefjord, west Norway. Inorganic nutrients (16 µmol l^–1 nitrate, 1 µmol l^–1 phosphate) were added to 2 enclosures with 10% of those concentrations (1.6 µmol l^–1 nitrate, 0.1 µmol l^–1 phosphate) added daily thereafter; a third unamended mesocosm was used as a control. Nitrogen (N) (ammonium, nitrate, urea and amino acid) uptake rates for >0.8 µm (largely composed of phytoplankton) and 0.2 to 0.8 µm (largely composed of bacteria) size classes were measured, as well as nutrient, chlorophyll, phytoplankton and microzooplankton concentrations. The nutrient additions initiated a process resulting in a large bloom of Phaeocystis pouchetii colonies. There was a 2.5 wk lag, during which a smaller bloom of phototrophic flagellates, followed by diatoms, formed in all mesocosms; diatoms increased until silicic acid was depleted. After the flagellate and diatom bloom dissipated, the mesocosms were depleted of inorganic N. Dissolved organic N (DON) remained constant in all mesocosms during the flagellate and diatom bloom but increased with the onset of the large P. pouchetii bloom, largely due to release of urea and amino acids, which made up >93% of the DON pool at the end of the experiment. Uptake rates of urea often accounted for the largest percentage of total N uptake in all mesocosms for both the >0.8 µm and 0.2 to 0.8 µm size classes, generally providing the largest percentage (up to 88%) of bacterial N demand of any substrate. This was particularly true during the Phaeocystis bloom when urea uptake rates were up to 65 times greater than the other N substrates. We observed a switch from new production, when nitrate uptake dominated during the diatom bloom, to regenerated production during the Phaeocystis bloom. The present study adds to the growing body of evidence that organic N is a substantial source of N for both phytoplankton and bacteria.

KEY WORDS: Nitrogen uptake · Mesocosm experiments · Phaeocystis · Diatoms · Bacteria