Shallow sand extraction
In general, sand extraction has direct impacts. Benthic organisms are damaged or removed and bathymetry and sediment composition may change considerably. Indirect effects vary from increased suspended matter and turbidity and detrimental effects on filter-feeders, predatory fish and the food web (Fig. 2).
Figure 2: Ecological effects of sand extraction, varying from direct effects (alteration of habitat and removal of benthos), to nearby indirect effects (increase of suspended matter, possible effects on filter feeders, sedimentation and resuspension, increased turbidity and detrimental effects on phytoplankton, predatory organism relying on eyesight and the food web).
Until a few years ago, the focus of ecological research was mainly on the on the direct effects of sand extraction. For shallow sand extraction depths, up to -2 m below the original seabed on the Dutch Continental, in general recovery of macrozoobenthos back to pre-disturbance conditions occurs in 2-4 year. For surrounding areas similar recovery times are reported.
Differences in recovery time exists, for shallow coarse sediments with weak tidal bed shear stress a recovery time of 11 year was found (Wan Hussin et al. 2012). Recovery times for dredged estuarine muddy areas are found to be around 6-8 months (Newell et al. 2008). Recovery times of areas with reef building organisms such as the sand mason worm (Lanice conchilega), honeycomb worm (Sabellaria alveolate) or long-living organism such as the ocean quahog (Arctica islandica) would be in the order of decades even after shallow sand extraction.
In general, demersal fish are closely linked to macrozoobenthos which can be severely influenced by sand extraction. In France, it was found that aggregate extraction can also lead to new seabed habitats which may favour certain macrozoobenthic and demersal fish species (Desprez et al. 2000). Fishing fleets near aggregate extraction sites were not deterred and fishing effort of potters and English dredgers were positively related to extraction intensity Marchal et al. 2014). Sand extraction sites may even attract marine mammals (Todd et al. 2014).
Deep sand extraction
It appeared that in the Netherlands there is little experience with the effects of deep sand extraction sites on existing values and user-functions, due to Dutch legislation prior to 2000. In 2000, possibilities of extraction depths larger than 2 m below the seabed were explored (Boers 2005). It became clear that in water depths of less than 40 m, the chance of reduced seawater oxygen content is rather small and that re-establishment of organisms on the seabed is possible.
Baseline data from the Port of Rotterdam which was collected in 2006 and 2008 in a 2500 km2 large research area showed that the benthic system can be changed considerably by sand extraction and sediment disposal and that these changes persists in time (De Jong et al. 2015).
More information on the effects of sand extraction can be found in Tool – Ecosystem-based Design Rules for Sand Extraction Sites and Case - Ecosystem-based design of sand extraction sites – Pilot MV2 Sand Extraction site.