Organic carbon in suspended particulate material in the North Sea: Effect of mixing resuspended and background particles

Intensive sampling of suspended particles was undertaken at two stations in the North Sea on three separate occasions during 1989 as part of a seasonal study of the influence of wave and tidal disturbances on bed sediments and suspended particle composition. Measurements of carbon content indicated that the composition of the particle population changed non-linearly with concentration. This is interpreted as mixing of bed sediment of relatively uniform and low carbon content with more permanently suspended, carbon-rich particles of biological origin, present at low concentration. A mixing model is used to describe the change in composition of the suspended particle population with concentration as bed material is mobilised into the water column. The model clearly shows that a two-component mixing scheme can account for the observed decrease of particulate carbon concentration with increasing suspended particle load. In addition, the model suggests that resuspended material is mobilised selectively since the mobilised material is richer in carbon than bulk surface sediment. The existence of a surficial layer of detrital ‘fluff’, similar to that observed on the deep ocean floor following the spring bloom, is postulated.