Control and treatment type experimental investigation
with pre and post dredge comparisons. Two
spatially separated sites exposed to a single
dredge with subsequent benthic sampling.
Site A, Lavan Sands NW Wales 3m above chart
datum substrate very fine sand, extensively
rippled, compact and firm, well oxygenated
sediment. Site B, Blackshaw Flats, Solway
Firth 5m above chart datum well sorted very
fine sand, extensively rippled, compact and
firm, well oxygenated sediment. Two experimental
Experiment 1: Effects of a single dredging
Habitat effects - Dredging had no
significant impact on the measured sediment
characteristics due to the small percentage
of fine material and the high degree of sorting.
Species and community effects - Rapid
recovery of benthic infaunal communities as
sediment exposed to regular disturbance from
water movement - community already adapted
to disturbance. Hydrobia ulvae, surface
grazing gastropod, significantly affected
Experiment 2 at Lavan Sands
80 sampling stations over an area of 400x300m
used to assess the effects of a 3 month licensed
commercial dredging operation using pre and
post dredging data.
Habitat effects - No severe erosion
of sediments occurred.
Species and community effects - Impacts
appear to be small and for the most part not
statistically significant. Significant decrease
in the population of tube dwelling polychaete
Pygospio elegans whose tubes may be
destroyed by dredging. Lanice conchilega
has tough tubes apparently not greatly affected
by the dredging operation. Also they can retract
into tubes below the maximum depth disturbed
by the dredge and can regrow head tentacles.
Numbers of Cerastoderma edule and Macoma
balthica reduced significantly resulting
in a significant reduction in the total macrofaunal
biomass (these molluscs contribute to about
70% of the biomass wet weight). Author concludes
hydraulic cockle dredging unlikely to have
a significant impact on non-target infaunal
species at the site as the sediments are moderately
mobile with a low silt content.