Studies on the phytoplankton ecology of the trondheimsfjord. III. Dynamics of phytoplankton blooms in relation to environmental factors, bioassay experiments and parameters for the physiological state of the populations

Abstract

Quantitative phytoplankton sampling was carried out at weekly intervals at one station in the central part of the Trondheimsfjord and at irregular intervals at one station near Trondheim Harbour during March–October, 1970 and 1971. Various developmental stages of diatom blooms have been observed, which have been related to variations in freshwater discharge, hydrography, nutrients (nitrate, orthophosphate, and reactive silicate in sea water and river water), light, the results of bioassay experiments, parameters for the physiological state of natural phytoplankton populations, and to data on phytoplankton and hydrography collected during 1963–1969. Two spring blooms of diatoms are persistent from year to year in the area. The first one starts in March, triggered by an increase in the incident radiation and culminates in early April. It develops analogously to a batch culture and is nourished mainly by nutrients accumulated during the winter. The second takes place in brackish waters during May–June concomitant with floods in rivers. The magnitude of its populations corresponds to discharge maxima unless disturbed by hydrographical irregularities and heavy grazing by Calanus finmarchicus (Gunnerus). This bloom is analogous to a continuous culture and is nourished by nutrients in entrainment water and to a lesser extent by those in the river water. Furthermore, the unpredictable development of diatom blooms in the autumn seems to follow peaks in the discharge unless prevented by too low salinity and poor incident light. In autumns of little discharge and with turbulence in the upper 5–10 m dinoflagellates predominate. In high salinity waters nitrogen seems generally more limiting than phosphorus for phytoplankton growth The N/P atomic ratio of such waters with no phytoplankton growth was 10–12 in contrast to 13–18 in the phytoplankton. Due to the high N/P ratio of 40–50 in river water, phosphorus was more limiting than nitrogen in some brackish waters. On two occasions trace metals seemed to be the most limiting.