The natural history of the Hardangerfjord

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Abstract
On the basis of a 13 months phytoplankton survey of Hardangerfjorden, south-west Norway, and the coastal waters Outside (Braarud et al. 1974), the discrepancy between the quantitative phytoplankton cycle whithin the fjord and that of the offshore area from autumn to early spring is discussed. Low light supply, small Zooplankton populations, and low phosphate concentrations within the upper 30 m layer characterize the period October–January for the fjord area. A pronounced Skeletonema costatum maximum in October–November in the fjord had no counterpart offshore where an extreme diatom scarcity prevailed. The degree of exposure to wind, through its effect upon vertical transport of the plankton algae, was found to be a main cause of this difference. This applies also to the time difference of 3–4 weeks between the advanced spring diatom maximum in the fjord and that offshore. Low grazing activity appears to be an essential prerequisite for the occurrence of large diatom populations in the fjord in autumn as well as in early spring. In both cases there is low light intensity. An example is given of a great increase in the diatom population being accompanied by an increase in phosphate concentration of the fjord water.