Abstract
In 2006 the Offshore Windfarm Egmond aan Zee (OWEZ) was constructed in the Dutch coastal zone, 10 -18 km offshore Egmond aan Zee just north of IJmuiden. The OWEZ Wind farm with a surface area of approximately 8*4 km encloses 36 wind turbines with distances of 650-1000 m in between. Part of the OWEZ project was an extensive monitoring program, the Monitoring and Evaluation Program (NSW-MEP 2006-2012). According to this program, the environmental impact of the Wind farm on the marine ecosystem was monitored. The possible impact of the construction of the Wind farm on the benthic ecosystem was examined in two separate studies: 1) effects on the macrobenthos community (in- and epifauna > 1mm), covered in a report by Daan et al. (2009), and 2) effects on the recruitment of bivalves. The second study will be described in this report. In this NSW-MEP study we focused on the impact of the Wind farm on the settlement of juvenile benthos, being the start for the benthos populations in the coastal ecosystem. Potential impacts of the construction and operation of OWEZ during the 1-1.5 years preceding our field studies could be expected to show up in recently settled bottom fauna. We focused especially on the recruitment of juvenile bivalves, as their adult stages are a major food supply for fish and diving birds in the shallow coastal zone. Next to this, bivalve populations are a dominant factor in filtering particles from the water column enabling deposition and burial of (organic) material into the sediments.
In this NSW-MEP study we focused on three questions: 1) do the autumn densities of bivalve settlers show differences between OWEZ and the reference areas, 2) can possible differences in density of settlers be explained by environmental conditions and 3) do pelagic bivalve larvae respond differently to various sediment types offered in mesocosms.
To determine differences in settlement of young bivalve species we compared autumn-2007 densities in the non-fished OWEZ with densities in the 5 surrounding reference areas which were, just as before the construction of OWEZ, subject to regularly trawling. Median grain size and mud content of the sediment cores collected during this survey were also determined. From this dataset we used densities of Spisula subtruncata as an additional test to reveal the impact of OWEZ on settlement by comparing the actual numbers of juveniles in OWEZ Wind farm with the expected density of recruits required to maintain the present-day coastal density. This expected density was estimated from the 2007-density of adults found in OWEZ and surrounding reference areas (Daan et al., 2009) and the mortality estimate based on age converted length distributions of pre-OWEZ populations of S. subtruncata. If the fishery-stop in OWEZ led to substantial higher settlement of S. subtruncata their juvenile densities in 2007 should be at least some orders of magnitude higher than the expected density.