IMIS

Methods: To better understand sediment variability during human-induced disturbance of the seabed, both sandbanks have been monitored using multibeam bathymetry and backscatter, and Reineck box cores on board of the Research Vessel Belgica A962. Acoustic and sediment data were preferably jointly collected within a one-week period to substantiate the interrelationships.

Result: Very well-sorted medium sands with low backscatter values (−28 to −33 dB) are detected near the top of the sandbank, while (moderately) well-sorted coarser sands and shell fragments with high backscatter values (−20 to −24 dB) are detected in the deeper parts of the sandbank slope. Although natural conditions such as sandwave movement and storms may mask early sediment changes, 7 years of intermittent and intensive sand extraction on the upper parts of a gentle sandbank slope caused a seabed deepening of at least 2 m, a backscatter decrease between 5 and 8 dB, a reduction in carbonate content by 1%–5% and an increase in organic matter content by 1%–2%. Two years after the cessation of the operations, the carbonate content increased and organic matter content decreased by 1%–2%.

Discussion: Natural variability of sediments remains dependent on depth and geomorphology in both undisturbed, as well as recently and longer disturbed sandbank systems. However, a spreading of the more homogeneous, very well-sorted medium sands with limited carbonate content from the sandbank crest to the upper parts of the sandbank slope was observed, as sand extraction progressed. Subsequently, possible first signs of a slow return to the original sediment characteristics were detected.