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

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Abstract
Uptake of 15N-labeled NH4+, NO3- and urea by picoplankton (< 1 .mu.m), nanoplankton (< 10 .mu.m) and the whole phytoplankton community was investigated in oceanic, shelf and inshore waters in the southern Benguela upwelling system during Dec. 1983. Picoplankton and nanoplankton chlorophyll a accounted for 2-49% and 13-99%, respectively, of the whole community chlorophyll a. Picoplankton were relatively more active in oceanic as opposed to coastal waters. N uptake by picoplankton was 27% of that by the intact phytoplankton community in oceanic waters compared to .apprx. 10% for the coastal regions. Similarly, turnover rates of particulate N in the < 1 .mu.m size class were faster in oceanic than shelf or inshore waters. Relative preference indices showed a consistent preference for NH4+, to a lesser extent for urea, compared to NO3- by all 3 size classes. Nevertheless, NO3- was quantitatively the most important N source for the nutrition of the intact community over the shelf, supplying 71% of the total N assimilated. The proportion of NO3- assimilated was directly related to algal size class at the shelf and inshore stations with picoplankton and nanoplankton production being based largely on regenerated N (NH4+ and urea). In oceanic waters, production in all size classes was supported principally by regenerated N. Results are discussed with reference to the effect of qualitative and quantitative aspects of N availability on phytoplankton community structure.