Planktonic nitrogen uptake in the south-eastern Kattegat

Abstract

The relationships between the uptake rates of nitrate and of the regenerated nutrients, ammonium, and urea have been studied in the surface waters of the south-eastern Kattegat. During the spring bloom, the plankton nitrogen nutrition was mainly based on nitrate, which had been accumulated during the nonproductive winter period. The nitrate uptake during this bloom, which was followed with frequent samplings in the early part of March in 1984, dominated the sum of ammonium, nitrate, and urea uptake and constituted 80% of the total daily uptake rate of 2 μmol N · 1 −1 · day −1 in the upper (0–6 m) water mass. During summer and early fall when the surface water was nutrient depleted, the production was based on recycled nitrogen, reflected in the dominance of ammonium and urea uptake (≈ 50 and 30% of the total uptake, respectively). In September, the daily nitrogen uptake rate was half as compared with the spring bloom. The size fraction < 10 μm dominated the nitrogen uptake in early summer, while in spring and fall this size fraction accounted for 30–35% of the total nitrogen uptake. The nitrogen uptake over the whole productive season was estimated (as an average for all the data from 1982–1986) to 2.26 mol N · m −2 · yr −1, which, converted to carbon uptake by assuming Redfield stoichiometry, would correspond to 180 g C · m −2 · yr −1. The spring bloom represented 16% of this, i.e., converted to carbon uptake 29 g C · m −2 during approximately a 2-wk period. On an annual basis, the new production, as estimated by the fraction of nitrate uptake of the total uptake represented 25%, and the regenerated production was separated into about two-thirds ammonium uptake and one third urea uptake. The conversion of values of uptake rates of ammonium, nitrate and urea, achieved from 15N experiments, to carbon productivity indicated a good correlation with the yearly 14C estimate of productivity.