Abstract
The uptake rates of nitrate and ammonium at six depths at one station in Oslofjorden were measured on eleven dates by means of the 15N-isotope method. Other measured variables were nitrate and ammonium concentration, light extinction and irradiance, tempearture, salinity, phytoplankton composition and biomass, mean cell volume, chlorophyll a, and particulate nitrogen. Ammonium was the more important N-nutrient in the summer at 0, 2, and 4 m depths, while nitrate was the dominant N-nutrient at 6, 8, and 16 m depths and in the winter. There was no clear indication of N-limitation on any of the sampling dates. Statistical analysis showed that physical and chemical variables were more important than biological ones in explaining N-uptake. Temperature was the most significant covariate. Depth was a better indicator of vertical variation than light extinction. Day-length was an important factor when N-uptake was normalized to chlorophyll a.