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

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.

Key words:

• ¹⁵N assimilation
• size fractionation
• picoplankton
• nanoplankton
• microplankton
• chlorophyll a
• paniculate nitrogen
• paniculate carbon
• particulate phosphorus
• upwelling
• freshwater runoff
• f‐ratio