Size-fractionated measurements of nitrogen uptake in aged upwelled waters: Implications for pelagic food webs

Uptake of NO3−, NH4+, and urea was measured for the 200–20-, 20–2-, and <2-µm size classes within the chlorophyll maximum layer in the Benguela upwelling system. Picoplankton and nanoplankton dominated in terms of biomass and activity both inshore and offshore. Net plankton N uptake declined drastically at night, whereas the smaller size classes maintained rates close to daytime levels. There was some evidence for N resource partitioning. Reduced N generally made up a higher proportion of the total N uptake by nanoplankton and picoplankton (for both size classes, mean = 94%) than by net plankton (mean = 63%). The contribution of nanoplankton and picoplankton NH4+ uptake always exceeded 50% of the total primary N production. In a regeneration-based model with small cells dominating primary production it is conservatively estimated that carnivory on microzooplankton can contribute 14% toward the production of an omnivorous mesozooplankton assemblage.