Abstract
The Offshore Wind farm Egmond aan Zee (OWEZ) was built between April and August 2006 and is in operation since January 2007. An extensive Monitoring and Evaluation Program (NSW-MEP) has been designed in which the economical, technical, ecological and social effects of the OWEZ are studied. This report presents an ecological study on the development of marine flora and fauna communities on the new hard substrates introduced by the wind farm: the monopiles and the rocks of the scour protection layer. Three turbines were selected to cover different areas of the OWEZ: turbines 7, 13 and 34. These turbines were visited in February and September 2008.
Using video footage and samples collected by divers, qualitative (species composition and covering percentages) and quantitative assessments (numbers and biomasses of species present) of the communities that have colonised the hard substrates of turbines 7, 13 and 34 were carried out. Results were compared with growth on hard structures of the turbines in the Horns Rev offshore wind farm in Denmark and with growth on other hard structures in the North Sea.
In February 2008, a total of 30 different species were identified on the video footage and/or collected samples from turbines 7, 13 and 34. In September 2008 three additional species were identified, a species of green algae, the pullet carpet shell (Venerupis senegalensis) and the Northsea crab (Cancer pagurus) resulting in a total of 33 species. Fish species were not included in this number, but fish species seen during the surveys in February and September 2008 included schools of pouting (Trisopterus lucus), longspined bullhead (Taurulus bubalis) and Northsea cod (Gadus morhua).
In February 2008 and September 2008 two clear zones consisting of two hard substrate communities could be distinguished on the monopiles of turbines 7, 13 and 34:
- An upper zone consisting of a community dominated by the common mussel (Mytilus edulis) and associated species like barnacles (Balanus crenatus and Balanus balanoides), the common starfish (Asterias rubens), several species of worms and crabs and the encrusting sea mat (Conopeum reticulum). Covering percentages of mussels within the first few metres from the surface varied between 80-100%. Bare patches in between the mussels were colonised by anemones (mainly Metridium senile and Sargartia spp.) and (tubes of) the small crustacean Jassa spp.
- A deeper zone dominated by a community consisting of (tubes of) Jassa spp., several species of anemones (Metridium senile, Sargartia spp. and Diadumene cincta) and patches of the ringed tubularia Tubularia larynx. Green sea urchins (Psammechinus miliaris) and common starfish (Asterias rubens) were also present in this zone, but occurred in low numbers. This community occupied the entire surface of the monopiles (covering percentage 100%) from the zone below the mussels to the sea floor.
Covering percentages of the most dominant species of these communities varied between turbines and different depths. Other less common species identified on the 8 monopiles include the Japanese oyster (Crassostrea gigas), Titan acorn barnacle (Megabalanus coccopoma), skeleton shrimp (Caprella linearis), hairy crab (Pilimnus hortellus), common brittlestar (Ophiotrix fragilis), aquatic sowbug (Idotea balthica), porcelain crab (Pisidia longicornis), velvet swimming crab (Necora puber) and Northsea crab (Cancer pagurus).
Two main differences between the assessment carried out in February 2008 and the assessment carried out in September 2008 were recognised:
- The abundance of mussels increased. Coverage of mussels has become denser in September and bare patches still present in February were colonised.
- The hard substrate community dominated by mussels expanded to greater depths in September 2008, especially on the monopiles of turbines 7 and 13.
The most dominant species on the rocks of the scour-protection layer of turbines 7, 13 and 34 are the sea mat Conopeum reticulum, the plumose anemone Metridium senile, Sargartia spp. anemones, (tubes of) the crustacean Jassa spp. and the ringed tubularia Tubularia larynx. Clumps of mussels (fallen of the monopile) between the rocks attract the common starfish Asterias rubens. Other notable less abundant species include the Japanese oyster (Crassostrea gigas), slipper limpet (Crepidula fornicata), barnacles (Semibalanus balanoides and Balanus crenatus), the hydroid Obelia spp. and orange crust (Cryptosula pallasiana).
In February 2008 the rocks collected at turbines 7 and 34 were almost fully overgrown, but rocks collected at turbine 13 (figure 8) were relatively bare. This was also expressed in the total number of species found on the rocks around the base of the three turbines (12 species at turbines 7 and 34 and 5 species at turbine 13 (see table 1).
In September 2008 growth on the rocks collected at turbines 7, 13 and 34 was comparable (respectively 14, 17 and 11 species). Different from the February 2008 survey was the presence of large schools of pouting around the base of the three turbines and the presence of the Northsea crab (Cancer pagurus) between the rocks.
The new hard substrate communities provide a valuable food source for fish species like the North Sea cod (Gadus morhua) and pouting (Trisopterus lucus). In September 2008 schools of pouting were present at the bases of turbines 7, 13 and 34. The North Sea cod is an important commercial species that showed a strong decline as a result of over-fishing. If the new hard substrate communities do attract North Sea cod to the OWEZ and if they reside within the park where fishing is prohibited, the OWEZ could contribute to the recovery of this species.
Within the Danish offshore Horns Rev wind farm a significant increase in the abundance of Common scoter (Melanitta nigra) was observed between 1999 and 2006, and the Common eider (Somateria mollisima) was also a numerous species within this wind farm. This increase of both species within this wind farm could be a reflection of changes in the availability of food as a result of establishment of new hard substrate communities on the monopiles and rocks of the scour-protection layer, but 9 no conclusive explanation was provided by the researchers. Bird observations carried out by Bureau Waardenburg and IMARES in the OWEZ wind farm since mid 2007 show that cormorants are foraging for fish in the wind farm on a regular basis, especially during the summer months. Other bird species that forage on fish or molluscs are only seen in the wind farm occasionally. For instance, divers are seen incidentally near or in the wind farm, and recently a pair of common eiders was seen foraging (for molluscs) within the wind farm (Karen Krijgsveld personal comments).
Causal relationships between the presence of the new hard substrate communities and fish and/or birds within OWEZ cannot be demonstrated at the moment. Monitoring the effects on birds and fish is still work in progress and further research into the development of the hard substrate communities will also be carried out in 2011. A recommendation for future work is to carry out an analysis to study effects of offshore wind farms on the entire North Sea ecosystem integrating results of separate monitoring and evaluation programmes.
A comparison of the results obtained in the OWEZ with results from the Horns Rev wind farm, indicates that the growth on the hard structures of the turbines in the two wind farms is comparable. Notable differences are: 1) a more distinct presence of an algal zone in Horns Rev, 2) the presence of an almost monoculture population of the giant midge Telmatogeton japonicus in the splash zone in Horns Rev and 3) the much higher abundance of Jassa spp. in Horns Rev.
The observed growth on the hard structures of the OWEZ also seems comparable to growth on other similar hard structures (e.g. steel platforms) in the North Sea.