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Abstract

Filtration of planktonic algal cells from the water column into permeable sublittoral sed-iment and the fate of the cells in the shallow sands were studied during a red tide produced by thedinoflagellate Peridinella catenataat Hel Peninsula/Baltic in May 2004. Advective porewater flowsassociated with ripple topography of the bed caused rapid transport of algal cells down to 5 cm sedi-ment depth. Sedimentary concentrations of algal cells mirrored algal concentrations in the overlyingwater column with increases and decreases within the upper 3 cm of the bed occurring within a fewhours. Sedimentary algal uptake and release significantly differed between stations only 15 m later-ally apart. Laboratory sediment-column experiments with 13C-labeled algal cells revealed algaldecomposition at rates of up to 0.2% 13C h–1in percolated sands originating from the study site. Thiswas 2 orders of magnitude lower than observed decreases in sediment algal cell C abundance of upto 23% C h–1after a drop in cell concentrations in the water column. Because bioturbation and ripplemigration were negligible, we conclude that advective flushing of the uppermost sediment layercould rapidly remove cells from the sediment. Our results demonstrate close spatial and temporalcoupling between algal cell concentrations in the boundary layer and those in the upper 6 cm of per-meable sand sediment, and suggest that permeable beds can act as short-term storage buffer forphytoplankton. During passage through the sediment, planktonic algae may benefit from the highernutrient concentrations available in the porewater.