RESUSPENSION PROCESSES AND SESTON DYNAMICS, SOUTHERN NORTH-SEA

Coupling of physical, biological and chemical processes associated wit h particle resuspension and seston flux was investigated at three site s in the North Sea with contrasting water column (mixed/stratified) an d seabed (cohesive/non-cohesive) characteristics. Seston concentration was determined by a combination of local resuspension and advection o f a regional horizontal concentration gradient. Model simulations of o bservations show that fair weather, the bed erosion rate was limited b y the availability of suitable bed material. The resuspended particles were derived from a surficial veneer of material (fluff) that was rel atively enriched in organic carbon. Sediment from the bed itself was t herefore not resuspended by tidal currents even at a shallow water, sa ndy site. Bioturbation of the seabed by infauna significantly modified the properties of muddy sands at a deep water site in summer, but thi s was insufficient to cause tidal entrainment of the bed sediment. Res uspension increased under combined wave/current flows during storms. H owever, model simulations predict that self-stratification of the boun dary layer by resuspended fine sediment during storms reduces bed stre ss and limits further resuspension, so that storm resuspension of fine sediments may be self-limiting. Seston was a mixture of: (1) particle s relatively rich in organic carbon, with low settling velocities, in long-term suspension; (2) particles with less organic carbon (though s till greater than that of the bed material), faster settling velocitie s, periodically resuspended; (3) particles that were very rich in orga nic carbon, with fast settling velocities, produced during plankton bl ooms. Particles in category 3 scavenged those in category 1 as they se ttled, so that seston concentrations diminished and deposition rates i ncreased after blooms. In stratified waters during blooms, deposition of organic-rich detritus gave rise to seabed anoxia and efflux of trac e metals (Fe and Mn) from pore waters. Differential rates of metal exc hange altered the particulate Fe/Mn ratio below the thermocline. Settl ing, deposition, and resuspension of fluff were therefore important co ntrols of metal exchanges in the boundary layer.