Sediment and modelled hydrodynamical variables
An increase in mud content was observed after the cessation of sand extraction (fig. 7). Locations with a high mud value also have a high sediment organic matter content (SOM). Modelled time-averaged bed shear stress is high at the crests of natural occurring sand waves in the reference areas. The northern edge of the northern sand extraction site shows the highest bed shear stress values. The southern part of the northern sand extraction site has the lowest bed shear stress values. Differences in bed shear stress are also visible at the ecosystem-based sand bars (fig. 12 and 13).
Figure 12: A selection of abiotic variables at infauna sample locations: median grain size (micron) and mud and sediment organic matter (2012) (%) and salinity (g/kg) and bed shear stress (N/m2) for the year 2010 (De Jong et al., 2015b).
Figure 13: Modelled bed shear stress (N/m2) in the Maasvlakte2 sand extraction site (Tonnon et al., 2013).
Two years after the cessation of sand extraction, macrozoobenthic biomass increased five-fold in the deepest areas. Species composition changed significantly and white furrow shell (Abra alba) became abundant (fig. 14, 15 and 16). Several sediment characteristics also changed significantly in the deepest parts (fig. 12). Macrozoobenthic species composition and biomass significantly correlated with time after cessation of sand extraction, sediment and hydrographical characteristics. Ecosystem-based landscaped sand bars were found to be effective in influencing sediment characteristics and macrozoobenthic assemblage. Significant changes in epifauna occurred in deepest parts in 2012 which coincided with the highest sedimentation rate.
Figure 14: Characteristics of infaunal recolonization(De Jong et al., 2015b).
Figure 15: Characteristics of epifaunal recolonization (De Jong et al., 2015b).
Figure 16: Characteristics of macrozoobenthos per location (De Jong et al., 2015b).
Demersal fish species
One and two years after cessation, a significant 20-fold increase in demersal fish biomass was observed in deep parts of the extraction site (Fig. 17). The average demersal fish wet weight biomass in the reference areas was 20.9 kg WW ha-1 whereas in the deep areas of the MV2 sand extraction site biomass increased up to 522 kg WW ha-. The most abundant fish species in the extraction site is plaice (Pleuronectes platessa) whereas in the reference areas, dab (Limanda limanda) is most abundant.
In the troughs of a landscaped sand bar however, a significant drop in biomass down to reference levels and a significant change in species assemblage was observed two years after cessation (fig. 7). The fish assemblage at the crests of the sand bars differed significantly from the troughs with tub gurnard (Chelidonichthys lucerna) being a typical species of the crests. This is a first indication of the applicability of landscaping techniques to induce heterogeneity of the seabed although it remains difficult to draw a strong conclusion due the lack of replication in the experiment. An ecological equilibrium is not reached after 2 years since biotic and abiotic variables are still adapting.
Fig. 17: Demersal fish biomass in- and outside the sand extraction site in 2010, 2011 and 2012. Values are proportional to the radius of the circles in the bubble plot with maximum values converted to bubbles with 1000 m radius. The highest biomass value was found in 2012 at the trough of the oblique sand bar (522 kg WW ha-1).
Significant differences in demersal fish species assemblages in the sand extraction site were associated with variables such as water depth, median grain size, fraction of very fine sand, biomass of white furrow shell (Abra alba) and time after the cessation of sand extraction (Fig. 18). Large quantities of undigested crushed white furrow shell fragments were found in all stomachs and intestines of plaice, indicating that it is an important prey item.
Figure 18: nMDS ordination with demersal fish samples and significant associations with variables. Continuous variables are depicted with arrows and categorical variable time after the cessation, only with text (Tref: reference in black, Trecent in white, T1 in light grey and T2: dark grey. Sub-locations are denoted with symbols, reference as black bullets, slope of the reference area as large open circles, crests of sand bars as squares, trough of sand bars as diamonds, deep (SE) as point-up triangles and deep (NW) as point-down triangles. In 2012, the significant association of the ordination and infaunal white furrow shell biomass is denoted with an arrow and surface plot to show the non-linear property of the relationship. Stress of all ordinations was below 0.07.