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Free keywords:
German bight; permeable shelf sediments; pore water flow; planktonic diatoms; benthic diatoms; C-13-labeling; remineralization; carbon cycling
Abstract:
This contribution addresses the incorporation and degradation of diatoms in coastal fine, medium and coarse North Sea sands. During 3 cruises in 2001 to a highly dynamic, non-depositional area in the southern German Bight, the transport of 13C-labeled diatoms into these different permeable sand beds was assessed by in situ and on-board chamber experiments. Enhanced advective transport of diatom frustules and 13C-enriched diatom carbon into sandy sediments with increasing permeability was demonstrated. Highest transport rates were observed in medium and coarse sand, where 6% of the added algae were found below 1 cm after 20 h incubation. In the coarse sand, the high ratio between sand grain and particle size enhanced the delivery of algae to the sediment, but seemed to reduce the filtration efficiency and thus algal retention. Broken frustules of Thalassiosira sp., the diatom which dominated the diatom spring bloom in 2001, were found in the medium and coarse sand in autumn. This indicates that advective transport and, to some limited extent, bioturbation, deposits phytoplankton into these sandy sediments, where strong bottom currents theoretically would prevent the sedimentation of low-density organic material. The trapped cells are rapidly degraded, as observed in our chamber experiments, where 28% of the added diatom carbon was released as dissolved organic carbon (DOC) per day after the third incubation day. We conclude that permeable sediments represent expansive coastal filter systems, where high advective flushing rates boost remineralization of trapped algal cells. These processes promote a fast recycling of organic matter and, thus, may be important for maintaining high primary production rates in shelf environments.
