Interaction between pelagial and benthal during autumn in Kiel Bight. I. Development and sedimentation of phytoplankton blooms

A detailed investigation of production, sedimentation and plankton biomass and composition in relation to the physico-chemical environment was conducted during autumn and early winter in western Kiel Bight. Two major phytoplankton blooms occurred: a Ceratium bloom in October and a diatom (Thalassiosira decipiens, Chaetoceros spp.) bloom in November. The Ceratium bloom was initiated by admixture of nutrient-rich bottom water to the surface layers. Mass mortality of ceratia in the water column, which preceded sedimentation of the bulk of their biomass, could not be related to environmental changes. It is therefore conjectured that mass mortality of the Ceratium bloom — a recurrent phenomenon in Kiel Bight—is triggered by internal mechanisms related to their sexual cycle. The fairly rapid build-up of the diatom bloom, in spite of low irradiation, was aided by water column stratification. Nutrients were reduced but not depleted by this bloom, which abruptly sank out of the water column after a period of extremely low global irradiation. Mortality did not precede sinking of this bloom and it is suggested that mass sinking represents the transition from a pelagic to a benthic stage in the life history cycle of these diatoms. The transition is presumably triggered by an unfavourable growth environment. Zooplankton biomass increased following both blooms but was insignificant relative to that of phytoplankton.