Abstract
An investigation of the first spring phytoplankton bloom in Trondheimsfjorden (63° N) was carried out at one station about 2 km off Trondheim harbour in the period 28 February–11 April 1975. Hydrography, chlorophyll a, ATP and particulate organic phosphorus, carbon, and nitrogen (the two last-named corrected for detrital interference) were measured, and phytoplankton was sampled for counting and determination.
The phytoplankton, mainly diatoms (important species: Skeletonema, Thalassiosira spp., Chaetoceros socialis) and Phaeocystis developed in a nearly exponential fasion. Estimates of growth rates are given for some prominent species; the majority were in the range of 0.4–0.7 div. day-1 (temperature range 3–6° C).
The chl/N and chl/C ratios increased at the start of the bloom, as did the ATP/N and ATP/C ratios. Chl/N reached particularly high values below 10m (chl/N ∼ 0.1). Very high values (∼0.15) were observed in communities dominated by resting spores of C. socialis.
Towards the end of the bloom nutrient-deficient communities were observed (lowered levels of cellular N and P). N and P are presumably close to equally limiting the phytoplankton growth.