Cross Sands broadscale survey 1998

The purpose of this study was to determine whether there was any evidence of a large-scale cumulative impact on benthic macro-invertebrate communities as a result of the multiple sites of aggregate extraction located off Great Yarmouth in the southern North Sea. Forty 0.1m2 Hamon grab samples were collected from across the region, both within and beyond the extraction area, and analysed for macrofauna and sediment particle size distribution in order to produce a regional description of the status of the seabed environment. In addition, the data were analysed in relation to the area of seabed impacted by dredging over the period 1993-1998. Areas subject to ‘direct’ impacts were determined through reference to annual electronic records of dredging activity and this information was then used to model the likely extent of areas potentially subject to ‘indirect’ ecological and geophysical impact. Results showed the study area to be characterised by sands in the northern half of the survey area, and sandy gravels in the south. The low diversity communities found across much of the survey area were typical of mobile sandy sediments. However, stations located in the southern half and northern extreme of the survey area tended to support higher numbers of species and individuals. This may be due to marginally enhanced stability arising from the higher proportion of gravel found in samples to the south of the extraction licences and to the presence of Sabellaria spinulosa reef in the north. Analysis of data in relation to areas of predicted dredging impact revealed proportionally less gravel and more sand within the ‘direct’ impact zone, compared to the ‘indirect’ impact zone. Whilst multivariate analyses of macrofaunal data were unable to discriminate between dredging impact zones, a comparison of univariate measures revealed significantly lower numbers of species and individuals in areas which have been subject to ‘direct’ dredging impacts in comparison with ‘reference’ areas. This provides good evidence of the near-field consequences of dredging. Values of these measures in the ‘indirect’ zone were intermediate, although not significantly different from the ’reference’ zone. We conclude that, although the dominant influence on assemblages in the region is that of sediment instability induced by tidal currents, we cannot dismiss the possibility of a subsidiary influence of dredging activity in the near vicinity of the licensed block and further investigation is warranted.