Activity of heterotrophic bacteria and its coupling to primary production during the spring phytoplankton bloom in the southern bight of the North Sea

Parallel measurements were made of particulate and dissolved products of primary production, utilization rate of amino acids, monosaccharides, and glycollate, thymidine incorporation into DNA, and exoproteolytic activity before and during the spring bloom at different stations in the southern bight of the North Sea and in the English Channel. High correlations were found between the three methods used for estimating the activities of heterotrophic bacteria. A reasonable quantitative agreement was found between the estimate of bacterial production based on thymidine incorporation into DNA and the estimate of total carbon utilization based on the sum of the utilization rates of amino acids, monosaccharides, and glycollate. A close coupling between microheterotrophic activity and primary production was demonstrated. Examination of the nitrogen balance during the spring bloom shows that microheterotrophic activity could play an important role in food‐web dynamics by partly satisfying the nitrogen needs of phytoplankton, which represents about twice the mineral nitrogen stock initially present in the water column.