Spring bloom nutrient dynamics in the Oslofjord

Primary production and uptake rates of ammonium, nitrate and silicic acid were measured during the 1996 spring bloom in the Oslofjord, southern Norway, using isotope methodology (14C, 15N and 32Si). Chlorophyll a (chl a) standing stock peaked at 38 mg chl a m–3. Diatoms, especially Thalassiosira nordenskioeldii, dominated the plankton community. The diatom bloom followed a sigmoid growth pattern until it abruptly declined. Both nitrate and silicic acid were depleted from the surface layer during the bloom. Nitrate was the major nitrogen-source for phytoplankton growth, the f-ratio was >0.9 until the bloom declined. Virtually all the silicic acid was converted into diatom frustules (biogenic silica), and silicon controlled the primary production during the bloom.