Nitrogen assimilation by three size fractions of the winter phytoplankton off Westland, New Zealand

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Abstract
The assimilation of ammonium, nitrate, and urea by the phytoplankton community off the Westland coast was measured in a detailed series of 15N‐assays in mid winter, 1987. The average nitrogen (N) uptake rates for the three substrates for three size classes of phytoplankton (< 2, < 20, < 200 μm) followed the distribution of biomass as measured by chlorophyll a, with higher values inshore. Nitrate accounted for 56% of the total N utilised by the entire community (< 200 μm), whereas ammonium and urea made up c. 19 and 25%, respectively. Picoplankton (< 2 μm) made up 33–44% of the total chlorophyll a and contributed about one‐third of the N uptake by the entire community. The relative preference index (RPI) for all three size classes consistently indicated a strong preference for regenerated N (NH4 +). NH4 + and urea became relatively more important sources of N for picoplankton in offshore oceanic waters, and to a lesser extent for nanoplankton (2–20 μm). However, the ratio of NO3 ‐ to total N assimilated (i.e., the proportion of new production) for the three size classes was high in all waters sampled (f‐ratio = 0.5–0.7). These data imply that new N provided a greater nutritional source than regenerated N for the growth of the dominant size components (< 2, 2–20 μm) of the midwinter phytoplankton community.