Vertical distribution and settling of spring phytoplankton in the offshore NW Baltic Sea proper

We studied the vertical distribution and settling of dominant diatoms and dinoflagellates during the 1996 spring phytoplankton bloom in the offshore NW Baltic Sea proper. We sampled phytoplankton at 11 depths (to 80 m) and collected settling cells in sediment traps at 25, 50 and 100 m depth, every week from March 26 to May 7. Phytoplankton were counted and sized from both water and trap samples, to estimate the share of phytoplankton in bulk settling carbon. Diatoms, mainly Chaetoceros spp. and Achnanthes taeniata, dominated the early bloom, but were replaced by the dinoflagellates cf. Scrippsiella hangoei and Peridiniella catenata when inorganic nitrogen was depleted above the seasonal pycnocline at ca. 10 m depth. By late April, vertically migrating dinoflagellates had depleted inorganic nitrogen down to 30 m, well below the seasonal pycnocline. We found clear species-specific sedimentation patterns. Scrippsiella hangoei and Chaetoceros spp., which dominated in the water column, were clearly underrepresented in the traps, while Thalassiosira baltica and T. levanderi, which were sparse in the water column, were overrepresented in sediment traps. Only 4, 3 and 0.5 g C m(-2) (or 16, 12 and 2% of phytoplankton primary production) settled as intact phytoplankton cells at 25, 50 and 100 m, respectively, although these numbers may be overestimated due to migrating P. catenata. The settling bulk carbon was similar to3 g C m(-2) or 12% of the primary production at all depths. This is low compared to other estimates from coastal waters and suggests additional loss mechanisms, e.g. disintegration in the water column and grazing by zooplankton overwintering in the permanent halocline area.