Nitrogen nutrition of the phytoplankton in the Oslofjord

The uptake of ammonium and nitrate by phytoplankton in the inner Oslo-fjord Norway, was investigated by means of 15N. An optical emission spectrometer was used to determine 15N: 14N ratios. NH 4 was the more important N source after May. Absolute as well as relative (specific) uptake rates of NH 4 and NO 3 were higher in the summer (June–August) than at other times. The maximum NH 4 uptake rate was shown to be exponentially related to temperature. Uptake of NO 3 was strongly inhibited by NH 4 concentrations >1–2 μM that were found near the shore in spring and autumn. Short (3–15 h) turnover times of the dissolved NH 4 and NO 3 pools in the summer were due to the combined effects of large algal standing stocks, low ambient substrate concentrations, high water temperatures and near absence of a suppression of NO 2 uptake by ambient NH 4.