Abstract
In 2006 the OWEZ Wind farm was constructed along the Dutch coast 10 -17 km offshore Egmond aan Zee just north of IJmuiden. The OWEZ Wind farm with a surface area of approximately 5*5 km encloses 36 wind turbines with distances of 600-900 m between them. According to the NSW-MEP the environmental impact of the Wind farm on the marine ecosystem is monitored.
Our main objective is to study the impact of the Wind farm on the macrobenthic fauna i.e. the invertebrate in- and epifauna. Considering the possible effects of the OWEZ Wind farm on this soft bottom fauna we decided to focus on the fact that this 5*5 km Wind farm is a fishery free area. According to many studies, commercial trawling leads to an increased mortality in various benthos species (Bergman & Santbrink, 2000) and to long-term effects on the composition of the benthic community (Duineveld et.al, 2007). Therefore, we assumed that in the non-fished Wind farm the species composition of benthic fauna and their settlers might become different from the surrounding normally trawled coastal zone. In the absence of trawling mortality, higher abundances of species vulnerable to trawling might be expected. Besides this direct effect, we assume that if trawling is banned there could be also indirect effects on benthos. Such indirect effects of the fishery-stop may include a lower frequency in resuspension of particles from the seabed. As stated by Witbaard et al. (2001), lower suspended matter concentrations can have an advantage for growth and survival of filter feeding fauna, as their filter efficiency is reduced by high loads. Another indirect effect is a change in sediment composition due to the lower rate of resuspension. This may have consequences for benthic settlers, who are selective with respect to the sediment type.
In our sub project we decided to focus on the impact of the non-fished Wind farm on the settlement of juvenile benthos. We focus especially on juvenile bivalves, which adults stages are a major food supply for fish and diving birds in the shallow coastal zone and a dominant factor in filtering particles from the water column enabling deposition and burial of organic material into the sediments.
To determine differences in settlement we compared the autumn densities of settled juvenile benthos in the non-fished OWEZ with the densities in the 5 surrounding reference areas which have been trawled normally during 2006 and 2007. We focused on filter feeding bivalve species. Possible differences in densities of juvenile bivalve species between the Wind farm and the reference areas will be discussed. As a contribution to the discussion, the expected densities of juvenile Spisula subtruncata (a bivalve species) in the Wind farm will be estimated from age converted length distributions of pre-OWEZ populations.
Differences in juvenile densities between non–fished Wind farm and trawled reference areas can possibly be explained by differences in environmental variables (e.g. turbidity). To measure these environmental variables in situ we deployed a submerged lander frame in the OWEZ Wind farm and another in one of the southern reference areas from February till October 2007. Autonomous instruments mounted at these landers measured current speed and direction, fluorescence, turbidity, salinity and temperature in five minute intervals. Their recordings are discussed in view of existing knowledge of particle transport and mud (<63 µ) dynamics along the coast (Kleinhans et al 2005). Differences in sediment composition between Wind farm and reference areas are measured by analysing median grain size and mud content in sediment collected during the juvenile benthos survey.
Differences found in juvenile densities between OWEZ Wind farm and reference areas might also be explained by changes in the sediment composition of the seabed in OWEZ due to the fishery-stop. To examine effects of sediment type on settlement of larval benthos we mounted manipulated mesocosms at the landers. In these mesocosms we offered next to normal sized coastal sediment (0.5 mm-1mm) also finer (0.2-0.5 mm) and coarser sized (>1 mm) fractions in adjacent trays. With these mesocosms we performed two in situ experiments during the major settling period in summer 2007.
This Interim Report presents the progress in the subproject Benthos-Recruitment up to 31 January 2008. The objectives of the study are described in the Introduction. The section Material and Methods includes a description of the study area, the set-up of the field survey, and the landers and their instruments. In the chapter Results the progress so far of each component of the study is described, and in the Discussion the results are discussed and possible explanations are offered. In the Final Report (Q1- 2009) all results will be presented and discussed.