Assessing the recovery of functional diversity after sustained sediment screening at an aggregate dredging site in the North Sea

The effects of dredging the seabed for aggregate on benthic functional diversity were assessed using a suite of suitable indices on a recovering macrofaunal assemblage. Recovery was assessed as the return of a dredged assemblage to a state found in neighbouring undisturbed (reference) sites. In situ sediment screening was permitted during dredging operations; a difference in the sedimentary profile of the seabed between dredged and undisturbed reference sites was also observed. At sites of relatively high and low dredging intensity the sediment appeared more homogenous than reference sites after the selective removal of the coarser component. Initial assessment of the macrofaunal assemblage using univariate analytical techniques suggested a recovery of functional diversity at the low dredging intensity site after two years (according to the Infaunal Trophic Index, Taxonomic Distinctness index and Rao’s Quadratic Entropy coefficient). However, multivariate analyses of the same data and of all indices except Taxonomic Distinctness indicated that assemblages at both high and low dredging intensity sites remained statistically indistinguishable from each other yet markedly different to the assemblage present in the reference area during the four-year study. The study concluded that recovery of functional diversity to a level found in a neighbouring undredged habitat had not occurred at either dredged site five years after the cessation of dredging. It is thought that the damage by dredging to functional diversity and to the capacity of the macrofaunal assemblage to recover is immediate and not so dependent on dredging intensity. The cumulative and wider ranging effects of sediment screening cannot be ignored or dismissed as a contributing factor to the similarities observed. The wider significance of these findings on the regulation of dredging activities is discussed.

► Recovery of benthic functional diversity is assessed at an aggregate extraction site.
► A suite of traditional and functional diversity indices is compared.
► Recovery of functional diversity is not apparent 5 years after cessation of dredging.
► Findings are in contrast to those from other aggregate extraction sites.
► Sediment screening is likely responsible for differences observed between sites.