Nitrogen uptake by size-fractionated phytoplankton populations in the southern Benguela upwelling system

Uptake of 15N-labelled ammonium, nitrate and urea by picoplankton (< 1 pm), nanoplankton (< 10 pm) and the whole phytoplankton community was investigated in oceanic, shelf and inshore waters in the southern Benguela upwelling system during December 1983.Picoplankton and nanoplankton chlorophyll a accounted for 2 to 49 % and 13 to 99 % respectively, of the whole community chlorophyll a. Picoplankton were relatively more active in oceanic as opposed to coastal waters.Nitrogen uptake by picoplankton was 27 % of that by the intact phytoplankton community in oceanic waters compared to ca 10 % for the coastal regions.Similarly, turnover rates of particulate nitrogen in the < 1 pm size class were faster in oceanic than shelf or inshore waters.Relative preference indices showed a consistent preference for ammonium, to a lesser extent for urea, compared to nitrate by all 3 size classes.Nevertheless, nitrate was quantitatively the most important nitrogen source for the nutrition of the intact community over the shelf, supplying 71 % of the total nitrogen assimilated.The proportion of nitrate assimilated was directly related to algal size class at the shelf and inshore stations with picoplankton and nanoplankton production being based largely on regenerated nitrogen (ammonium and urea).In oceanic waters, production in all size classes was supported principally be regenerated nitrogen.Results are discussed with reference to the effect of qualitative and quantitative aspects of nitrogen availability on phytoplankton community structure.