Ecological studies on the phytoplankton of Boknafjorden, western Norway. 1. The effect of water exchange processes and environmental factors on temporal and vertical variability of biomass

Temporal and vertical variability of phytoplankton biomass in terms of in situ fluorescence (ISF), chlorophyll a (chl a), particulate carbon (C), nitrogen (N), and phosphorus (P) were studied throughout 1981 in Boknafjorden, a deep-silled fjord of southwestern Norway. Local blooms of autochthonous populations of the diatom Skeletonema costatum were found in stratified waters of sheltered areas in late February. The first widespread diatom bloom occurred in late March and was probably transported in the brackish ‘upper layer’ from the Norwegian Coastal Current. A second diatom bloom was observed during an upwelling period in early May. After this event, a two-layer system could be recognized. Below the pycnocline shade-adapted cells formed a distinct maximum layer of chl a. Most likely they were introduced to the ‘intermediate layer’ by inflow of coastal water which compensated for the outflow of ‘fjord water’ above. A similar situation appeared in late June, during which autochthonous populations of the coccolithophorid Emiliania huxleyi made up the major part of the biomass of the ‘upper layer’. On 25 June more than 50 % of the chl a content of the upper 40 m was found deeper than the 1 % light depth. Grazing by copepods and very low nutrient concentrations kept the phytoplankton biomass low throughout summer. Mass occurrence of the dinoflagellate Gyrodinium aureolum was encountered in the uppermost part of the ‘upper layer’ in early October during an inflow of warm and brackish coastal water deprived of nutrients. Very high chl a /C ratios (0.046–0.095) indicate that this species was extremely well adapted for growth under low irradiances.