Primary production and consumption of organic matter in the southern North Sea during the spring bloom of 1975

Abstract

From February to June, 1975, primary production (of particulate plus dissolved organic matter) in the eastern part of the southern North Sea was 90 to 120 g C·m −2, except for the area close to the Rhine outflow (70 g C·m −2). The production potential of surface samples was closely related to the chlorophyll a concentration; below an incident light intensity of 2.5·10 6 cal·m −2·day −1, light and chlorophyll a explain practically all of the variance in primary production at the surface. Consumption of organic matter was estimated by way of budget calculations of oxygen concentrations: 70 to 90 g C·m −2 were consumed by heterotrophs in the period February to June. Consumption fell farthest behind production during short periods of vigorous Phaeocystis blooms. In the area close to the Rhine outflow the situation was exceptional because more organic matter was consumed by heterotrophs than was produced by algae in the first months of the growing season. Turnover of organic matter other than that contributed by photosynthesis is apparently slow in the Southern Bight, except in the region where the Rhine discharges its waters into the North Sea. In this area there are two important sources of organic matter for organisms at the end of the food chain: the phytoplankton production, and allochthonous organic matter flushed in by the rivers.