THE FATE OF AN EARLY DIATOM SPRING BLOOM IN THE KATTEGAT

Sedimentation rates were determined over a 9 week period covering the rise and fall of a phytoplankton spring bloom in the southern Kattegat (1991). The bloom which was totally dominated by the diatom Skeletonema costatum occurred early compared to previous years and reached its maximum already mid February coinciding with the establishing of a pycnocline in the beginning of February. The sedimentation of PON during the period corresponded with the amount of available nitrate at the onset of the spring bloom, indicating that most of the new spring production settled to the bottom. While a substantial part of the POC of the water mass consisted of detritus, the sedimenting matter originated almost entirely from intact phytoplankton. Many of the sinking diatom cells formed aggregates during the decline of the bloom peak, coinciding with an increased whole-community sinking. The decrease of the below-pycnocline oxygen concentration ended three weeks after the spring bloom peak and corresponded to 73% of the oxygen demand of the sedimenting matter.