Measurements of primary production, dark fixation and vertical distribution of the microbenthic algae in the Øresund

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Abstract
Estimations of primary production of the micro benthic algae have been carried out in 0resund in shallow water off Niva and in deeper water off Helsingør, using a He technique. At Niva the highest actual production occurred in May in the Ruppia-flat; in the following months it was highest just off the beach. The Ruppia is assumed to have a beneficial influence on the primary production in early summer since its roots bind the sand and this produces a sheltered habitat. Later in the year the opposite will be the case as the growing Ruppia will shut out some of the light and cause a competition for nutrient salts. At Niva the vertical distribution of diatoms is fairly even down to 4 cm depth while off Helsingør at a depth of 8 m the entire standing crop is found between 0 and 2 mm. At the end of the year the algae concentrate in the uppermost 2 mm. This concentration is strongest for the pseudobenthic algae and commences for these algae about a week before a corresponding concentration of the psamrnophytic algae. This could be explained in terms of reduced grazing effeCt and migration of the algae. In Niv\.a the percentual dark fixation increases from the beach and seawards. It was highest for the psammophytic algae. It increased with increasing depth in the sediment. – Strong fluctuations in the dark fixation from 4-5 cm depth may be explained by displacements of a redoxdiscontinuity layer at this depth.