Resuspension processes and seston dynamics, southern North Sea

Coupling of physical, biological and chemical processes associated with particle resuspension and seston flux was investigated at three sites in the North Sea with contrasting water column (mixed/stratified) and seabed (cohesive/non-cohesive) characteristics. Seston concentration was determined by a combination of local resuspension and advection of a regional horizontal concentration gradient. Model simulations of observations show that fair weather, the bed erosion rate was limited by the availability of suitable bed material. The resuspended particles were derived from a surficial veneer of material (fluff) that was relatively enriched in organic carbon. Sediment from the bed itself was therefore not resuspended by tidal currents even at a shallow water, sandy site. Bioturbation of the seabed by infauna significantly modified the properties of muddy sands at a deep water site in summer, but this was insufficient to cause tidal entrainment of the bed sediment. Resuspension increased under combined wave/current flows during storms. However, model simulations predict that self-stratification of the boundary layer by resuspended fine sediment during storms reduces bed stress and limits further resuspension, so that storm resuspension of fine sediments may be self-limiting. Seston was a mixture of: (1) particles relatively rich in organic carbon, with low settling velocities, in long-term suspension; (2) particles with less organic carbon (though still greater than that of the bed material), faster settling velocities, periodically resuspended; (3) particles that were very rich in organic carbon, with fast settling velocities, produced during plankton blooms. Particles in category 3 scavenged those in category 1 as they settled, so that seston concentrations diminished and deposition rates increased after blooms. In stratified waters during blooms, deposition of organic-rich detritus gave rise to seabed anoxia and efflux of trace metals (Fe and Mn) from pore waters. Differential rates of metal exchange altered the particulate Fe/Mn ratio below the thermocline. Settling, deposition, and resuspension of fluff were therefore important controls of metal exchanges in the boundary layer.