Organic Enrichment Associated with Outwash from Marine Aggregates Dredging: A Probable Explanation for Surface Sheens and Enhanced Benthic Production in the Vicinity of Dredging Operations

Most recent studies of dispersion of sediment plumes generated by marine aggregates dredging, including those reported here, suggest that the zone of settlement of fine material temporarily suspended by the dredging and screening process is smaller than estimates based on Gaussian diffusion models. There is, however, often a relatively larger zone of visible impact which can extend for several kilometres downstream from a dredger during normal loading operations. This paper presents evidence which suggests that the “far field” visibility of the dispersing plume is associated with organic enrichment derived from fragmented marine benthos discharged with the outwash water. The values which we have recorded for unexploited deposits off Southwold, Suffolk are as high as 1.454 g/l AFDW(ash-free dry weight) of which 0.007 g/l (0.48%) comprises lipids. Such material appears to be of sufficient concentration to match the likely removal of benthos from the sediments by the dredgehead. Even allowing for the dispersion which must occur downstream from the dredger, it seems likely that the organic enrichment derived from fragmented invertebrates in the dredger outwash may account for the enhanced species diversity and population density of benthic invertebrates recorded by others beyond the boundaries of dredged areas.