A monitoring campaign was designed and alligned with the survey campaign of PoR to maximise intercompatibility and amount of field data output and to reduce costs. During the campaign focus was placed on the short-term effects in the sand extraction site on macrozoobenthos (infauna and epifauna), demersal fish, sediment characteristics, sedimentation-erosion and hydrodynamics (Fig. 10).
Figure 10: Used monitoring techniques: top from left to right: a boxc corer for infauna sampling, sediment sampling from a box corer sample, sieving of sediment to extract infauna: bottom from left to right: bottom sledge to sample epifauna, 4.5 m wide beam trawl to sample demersal fish and multibeam measurements for bathymetry and changes in sedimentation-erosion.
Macrozoobenthos short-term monitoring campaign
To ensure comparable data, sampling was carried out using identical protocols as during the baseline study (De Jong et al., 2015a; Perdon and Kaag, 2006) and during the recolonisation study of the Environmental Impact Assessment (EIA) of the Port of Rotterdam for the construction of Maasvlakte 2 (runs from 2014-2018) . A box corer with a surface area of 0.077 m2 was used to sample sediment and infauna, organisms larger than 1 mm and mainly living in the seabed. A bottom sledge was used to sample macrobenthic in- and epifauna with a size range of 0.5 – 10 cm. Bottom sledge samples are hereafter called epifauna (EP) beacause the largest part is epifauna although infauna was collected as well (De Jong et al., 2015b).
Sampling with the box corer was executed by the Royal Netherlands Institute for Sea Research (NIOZ) on 29–30th June 2010, 2–5th May 2011 and 23–25th April 2012. In 2010 and 2011, 45 and in 2012, 64 box corer samples were collected. To reach a higher spatial resolution in- and outside the extraction site in 2012, a subsample of the box core sample was analysed which reduced the sampled surface area to 0.015 m2. Four samples were collected in the deep parts of the extraction site, 14 samples in the reference area (near and far field) and four samples in the shipping lane area. No maintenance dredging was executed in the shipping lane according to Rijkswaterstaat. Specimens were identified up to species level when possible and ash-free dry weight biomass (g AFDW m-2) was analysed by means of loss on ignition, 2 days at 80 °C followed by 2 h at 520 °C.
Sampling with the bottom sledge for epifauna was conducted by the Institute for Marine Resources & Ecosystem Studies (IMARES Wageningen UR) on 7–8th July 2010, 14–15th June 2011 and 6–7th June 2012. The bottom sledge was equipped with a 5-mm mesh cage. On average, a surface area of 15 m2 was sampled during each sledge haul of approximately 150 m length, 10 cm width and a maximum penetration depth of 10 cm. In 2010 and 2011, 26 and in 2012, 32 bottom sledge samples were collected. In 2010, 11 bottom sledge samples were collected in the reference area. In 2012, three samples were collected in the shipping lane area of the Port of Rotterdam and one sample in the reference area. Specimens were identified up to species level when possible. Wet weight of epifauna was directly measured after sorting (g m-2 WW). Biomass of Atlantic jackknife clam Ensis sp. was determined by using regression equations based on previous IMARES field surveys. Sampling locations are visualised in figure 11 and 12.
Samples from the upper 5 cm of sediment were collected from untreated boxcorer samples and kept frozen until analysis. Sediment samples were freeze dried, homogenised and analysed with a Malvern Mastersizer 2000 particle size analyser. Percentile sediment grain size (D10, D50, D90) and sediment grain size distribution among the different classes; clay (<4 mm), silt (4-63 mm), mud (<63 mm), very fine sand (63 mm-125 mm), fine sand (125 mm-250 mm), medium sand (250 mm-500 mm) and coarse sand (>500 mm) were measured as percentage of total volume. Sediment organic matter (SOM) was analysed in 2012 by means of loss on ignition as percentage of sediment mass (freeze-dried sediment samples were placed for 2 hours at 520 °C) (De Jong et al., 2014).
Modelling of abiotic variables
The sand extraction site and surrounding area have complex hydrodynamic conditions. Due to the region of fresh water input (ROFI) from the Rhine River, periods of strong haline stratification, up- and downwelling, wind-driven flow, baroclinic cross-shore flows and wind and wave-induced mixing occur frequently. This may lead to considerable fluctuations in salinity and bottom shear stress. Delft 3D DD ZUNO has been used, a hydrodynamical model of the southern North Sea consisting of a coarse curvilinear horizontal grid with two grid refinement towards the Dutch coast through domain decomposition (DD). The nested model grid covers an area of about 7.5 km by 7.0 km and the horizontal grid size is about 45 m by 38 m for the research area. To generate appropriate boundary conditions for the nested model (Maasvlakte 2 sand extraction site), four open boundaries were used with tangential velocity (Tonnon et al., 2013). Twelve vertical σ-layers were specified, the relative thickness of these has been chosen in such way that near-bed and near-surface vertical gradients were better resolved. From top to bottom, these layers represent respectively 4.0%, 5.6%, 7.8%, 10.8%, 10.9%, 10.9%, 10.9%, 10.9%, 10.8%, 7.8%, 5.6%, and 4.0% of the water depth. The bathymetry of the nested model was interpolated from multibeam measurements performed by the dredging companies in October 2010. The model was forced with measured meteorological and riverine discharge data for the specific period. In order to keep the calculation time manageable, one single spring-neap cycle with relatively high river discharges was used for validation. During December to April, the largest riverine discharges can be expected and stratification is more likely to occur. The period between 2 March and 17 March 2007 was simulated using a time step of 15 seconds. Mean and maximum values of bed shear stress (N m-2) and near-bed salinity (ppt) were modelled.
A commercial fishing vessel was used, the Jan Maria, GO 29, with a length of 23 m, less than 300 horsepower and equipped with a standard commercial 4.5 m beam trawl. The beam trawl was equipped with four tickler chains, five flip-up ropes and diamond mesh size of 80 mm, which was applied at 4 knots fishing speed. The ship’s GPS-system logged the position of the sampling locations and water depth was determined with the ship's depth sounder. The maximum haul distance was one nautical mile in the reference area. Shorter hauls were planned within the sand extraction site; at the landscaped sand bars, hauls of approximately 700 m length were applied. Some of the hauls ended before the planned end coordinates because of difficulties with fishing inside the sand extraction site due to large changes in seabed topography and sediment composition. In surrounding reference areas, fishing direction was generally perpendicular to the direction of naturally occurring seabed patters to ensure heterogeneous sampling of crests and troughs of sand waves. In the sand extraction site, fishing direction was generally parallel to the seabed structures to enable comparisons between the different locations. Sampling was made in the reference area, at the slope of the sand extraction site, two locations in the deep parts of the extraction site i.e. the south-east and north-west, in the troughs and at the crests of the sand bars (fig. 6).
Figure 11: Fish tracks and macrozoobenthos samples (picture: Maarten de Jong).