COMPARISON OF THE SEDIMENTATION OF A DIATOM SPRING BLOOM AND OF A SUBSURFACE CHLOROPHYLL MAXIMUM

The seasonal pattern of sedimentation was determined over a 8-mo inves tigation period covering the productive season at a permanent station in the Southern Kattegat (Denmark) in 1990. The phytoplankton successi on was characterised by a 4-mo long subsurface maximum associated with the pycnocline which was entirely dominated by the dinoflagellate Gyr odinium aureolum. The bulk sedimentation of organic matter took place during this period and accounted for >60% of the annual particulate or ganic carbon (POC) and particulate organic nitrogen (PON) sedimentatio n. The spring bloom period contributed 60% of the sedimentation of int act phytoplankton cells, but only 20% of the POC and PON sedimentation . A minor fraction of the sedimenting matter from the subsurface phyto plankton maximum consisted of intact phytoplankton (<20%), suggesting that the phytoplankton was processed by heterotrophs and that it was m ainly products from this activity which contributed to the vertical fl ux of organic matter. The variation in oxygen concentration below the pycnocline coincided with the pattern of sedimentation with a delay of 3 to 6 wk.