Benthic Communities at Horns Rev Before, During and After Construction of Horns Rev Offshore Wind Farm: Final Report


Title: Benthic Communities at Horns Rev Before, During and After Construction of Horns Rev Offshore Wind Farm: Final Report
Authors: Vattenfall
Publication Date:
May 31, 2006
Document Number: 2572-03-005
Pages: 134

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Vattenfall (2006). Benthic Communities at Horns Rev Before, During and After Construction of Horns Rev Offshore Wind Farm: Final Report. Report by BioConsult SH and Vattenfall. pp 134.

In 1997, the Horns Rev area was designated as one of five areas suitable for future offshore wind farm development pursuant to the Danish Action Plan for Offshore Wind Farms. In 2002, the Horns Rev Offshore Wind Farm was established with a production capacity of 160 MW.


As part of the demonstration project, a monitoring programme was initiated to investigate the effects on the environment before, during and after the completion of the wind farm. The objective of the programme was to ensure that offshore wind power does not have damaging effects on natural ecosystems and to provide a solid basis for decisions for further offshore wind power development. The present study describes the effects on the benthic fauna and flora communities in the area designated for and later developed into the Horns Rev Offshore Wind Farm.


Horns Rev – the site of location Horns Rev is an extension of Blåvands Huk, which extends more than 40 km to the west into the North Sea. The width of the reef varies between 1 km and 5 km and forms the northern extremity of the European Wadden Sea. Horns Rev was formed by sediment deposited during earlier geological periods. Today these deposits are covered by huge accumulations of marine sand and are subject to continuous sand deposition.


Horns Rev is constantly adjusting to variations in hydrography and sea level changes. It is an area of relatively shallow water, tidally influenced and dominated by waves. The hydrographic conditions are mainly a result of intrusions of Atlantic water into the southern part of the North Sea. The tidal current is mainly in a north south direction with a prevailing current toward NNE and a mean current speed of 0.5-0.7 m/s. The water depth and the prevailing mixing of the water do not favour stratified conditions or oxygen depletion.


The Horns Rev Offshore Wind Farm was established approximately 14 km WSW off Blåvands Huk. A total of 80 turbines were installed with the last turbine set in place in August 2002. The wind turbine foundations are monopiles that are 4 m in diameter. In order to minimise erosion, scour protections that are approximately 25 m in diameter were established using large stones around the foundations.


Within the wind farm area, the water depth varies from 6.5 m to 13.5 m. From the baseline study, the sediment consisted of medium to coarse sand with a great variability in grain size distribution. The seabed was generally characterised by migrating bed forms.


Objectives The possible and expected effects on benthic communities from the wind farm establishment were outlined in the Environmental Impact Assessment. Loss of pre-existing habitats, the physical presence of the wind turbines and the introduction of hard substrate habitats were considered as the main and most important impacts to the benthic communities.


In compliance with the objectives of the demonstration programme monitoring of effects on the benthic and epifouling communities was required. The monitoring targeted the potential impacts from the introduction of hard substrate to benthic communities and the succession in the epifouling communities. The monitoring was performed from 1999 to 2005, before and after the erection of the wind farm.


Methodology The sampling methodology for the monitoring programme was designed based on an evaluation of the data from 1999 solely to enable detection of major changes in the community structure of the infauna and to monitor the introduced hard bottom communities which was in compliance with the requirements set by the national authorities.


The monitoring on infauna and sediment included collection of samples by SCUBA divers in the wind farm area and in designated reference areas outside the wind farm. The stations in the wind farm were situated 5, 25 and 100 metres in a leeward direction from the scour protections around six turbine sites.


The hard bottom substrate monitoring of epifouling communities was performed at six turbine sites to investigate the horizontal and vertical distribution on scour protections and monopiles. Quantitative samples were collected from stone blocks at all six turbine scour protection sites; whereas the monopiles were only sampled at three turbine sites. Semiquantitative (not precisely counted records) observations on the horizontal and vertical distribution of the flora and fauna fouling communities were made at all six turbine sites. The observations were classified according to a modified Braun-Blanquet scale along transects on both the scour protections and the monopiles. Epifouling communities exposed to different current regimes were studied both on the monopiles as well as on the scour protection. Some additional sampling was performed on specific communities in the splash zone. Fish species were observed, in addition to standard gill nets being used for specific test fishing. For documentation, underwater video recordings were made.


Multivariate analysis of the combined input from each species with respect to biomass and abundance was used to enhance the sensitivity of the statistical analysis.


Sediment The wind farm area and the reference area are characterised by relatively uniform bottom conditions consisting of pure medium-fine to coarse sand with no organic matter. The particle size, measured as median grain size, of the sediment in the wind farm area has increased significantly from 350 μm in 2001 to 509 μm in 2005. A proportional increase was found in the reference areas with no differences being found in sediment parameters between reference and wind farm areas at each survey. In an expanded reference area, great variability in the grain size distribution was found. This variability is likely due to temporal changes and spatial differences in the sediment parameters in the Horns Rev area, which are attributable to natural variations in the seabed sediments.


Infauna The natural benthic fauna in the Horns Rev area can be characterised as a Goniadella- Spisula community named after characteristic species in the community. Character species in the Horns Rev area consist of bristle worms (Goniadella bobretzkii, Ophelia borealis, Pisione remota, and Orbinia sertulata) and mussels (Goodallia triangularis and Spisula solida). The most abundant species were Goniadella bobretzkii and Goodallia triangularis. Like the fauna at other sublittoral sandbanks in the North Sea, the fauna at Horns Rev was very variable, heterogeneous and difficult to compare with other sandbanks and adjoining deeper waters. Mobile epifauna like the edible crab (Cancer pagurus) and the hermit crab (Pagurus bernhardus) could often be found on the seabed.


Considerable and statistically significant changes in the community structure were found in the infauna community in the wind farm area from 2001 to 2003 and 2004, although no changes were found in the community structure between 1999 and 2005. In general, no statistical significant changes in abundance and biomass distribution were found from 1999 to 2005 for most of the designated indicator organisms. The changes are considered as natural variations and are not attributable to the wind farm construction. No differences were found in benthic communities between sites at different distances from the wind turbine structures.


Effects from the wind farm No effect was found on the sediment distribution pattern from the changes in hydrodynamic regimes due to the establishment of the turbine foundations.


No statistically significant differences were found in community structures on the scour protections between the leeward and the current side of the monopiles. At the base of the monopiles, a statistical difference was found in community structures indicating an impact from different hydrodynamic regimes on each side of the monopiles. Differences in community structure on the scour protections between overlapping zones at the leeward side of the monopiles might also reflect the effect of turbulence in the hydrodynamic regimes.


Differences in the distribution pattern of mussels inside and outside the wind farm area might be an effect from differences in the feeding behaviour of sea birds.


The most significant effect attributable to the construction of the offshore wind farm was the loss of pre-existing habitats and the introduction of hard substrate habitats into a community that originally was dominated by infauna in sandy sediments.


Hard bottom structures. Vegetation The seaweeds introduced on the hard bottom structures displayed a distinct variation in temporal and spatial distribution. The vegetation was more frequently found on the monopiles compared to the scour protections. Only a few species were found on stones on the scour protections and if found these were predominantly at turbine sites in the shallowest sites, but an increase in total coverage was found since 2003.


A typical vertical zonation was found on the monopiles with species of Ulva (Enteromorpha) being the most frequent. Considerable changes in the vegetation community were observed since 2003, especially in the splash zone and at the upper part of the monopiles. Apparently the initial vegetation cover of filamentous algae was replaced by more or less permanent vegetation consisting of different species of green algae (Ulva). The red algae (Polysiphonia fibrillose), the purple laver (Porphyra umbilicalis) and the green algae (Chaetomorpha linum) were introduced at the latest in 2005. Succession in the vegetation cover of green algae at the monopiles was found with an increased depth distribution since 2003.


Hard bottom structures. Epifauna Great variations were found in temporal and spatial distribution between species and communities. In general, community structure between turbine sites was statistically different. Differences in abundances of the dominant species of amphipods (Jassa marmorata and Caprella linearis) were the main factors to the vertical and spatial differences. The cosmopolitan (Jassa marmorata) was most frequently found on the monopiles in densities of more than 1 million ind./m2.


Distinct vertical zonations and changes in distribution pattern and abundances were observed in the faunal assemblages on the monopiles since the initial colonization in 2003. In the splash zone, the almost monoculture population of the “giant” midge (Telmatogeton japonicus) increased markedly since 2003. Dense aggregations of either spat or larger individuals of Mytilus edulis were found in the sublittoral zone just beneath the sea surface at the monopiles. Changes in population structure since 2003 clearly demonstrate growth of the common mussels. In 2005, successful establishment of Mytilus edulis was found at more turbine sites than previously. Clear discrepancies in the distribution and abundance between the common mussel (Mytilus edulis), the barnacle (Balanus crenatus) and the predator (Asterias rubens) indicated that the starfish was the main keystone predator controlling the vertical and horizontal distribution of its prey species.


At the base of the monopiles, the keelworm (Pomatoceros triqueter), an initial colonizer, was more abundant than in the upper zones. This species has decreased in abundance since September 2003. The apparent stagnation in population size of Pomatoceros triqueter might be the result of competition for space from other species. Similarly, another primary coloniser, the hydrozoan (Tubularia indivisa), displayed a rather fluctuating distribution pattern and was less abundant in 2004 compared to 2003. This could be a result of lack of space or predation from sea slugs (Facelina bostoniensis), which among others were new to the Horns Rev fauna in 2004.


Impact from predation, recruitment and competition for space will contribute to a continuously repeating succession process until a relatively stabile community is reached.


Hard substrates were found being used as hatchery or nursery grounds for several species after construction of the wind farm. The new introduced habitat was an especially successful nursery for the edible crab (Cancer pagurus). The number and biomass of Cancer pagurus juveniles at the turbine sites has increased markedly from 2003 to 2005.


Succession in the epifaunal community was demonstrated but the community will continuously undergo changes due to ecological succession enabling a climax community to be formed. A climax community is not expected within 5-6 years after hard substrate deployment. Occasional disruption of community succession due to effects from storm events and hard winters may even prolong this process until a stable community is attained.


Introduced and designated species The introduction of more fouling species in the Horns Rev area is directly attributable to the deployment of hard bottom structures. Two species, the initial colonizers Jassa marmorata and Telmatogeton japonicus, have not previously been recorded in Danish waters. In the material of Caprella linearis, some of the specimens were identified as Caprella mutica in 2005, which is an alien species introduced from the Japanese Sea. Occasionally some of these species were introduced to the faunal communities on the sanded seabed.


Special attention should be given to the ross worm (Sabellaria spinulosa) and the white weed (Sertularia cupressina), which in the Wadden Sea area are regarded as threatened or red listed species. Although more common on hard bottom substrates the ross worm can generate biogenic reef structures on mixed sediments or shells.


Fish community A marked increase in the number of fish and fish species was observed from the March surveys to the September surveys each year. This might be a result of seasonal migrations of fish species to the turbine site for foraging. Bip (pouting) was observed presumably feeding on crustaceans on the scour protection together with schools of cod. The goldsinny-wrasse was often found in numbers at the turbine sites. Individuals of the rock gunnel and the dragonet were commonly found inhabiting caves and crevices between the stones.


It seems that noise and vibrations from the turbine generators have not impacted the fish and other mobile organisms attracted to the hard bottom substrates for foraging, shelter and protection.


By comparing the average biomass of the infauna on the sand bottom between the turbines, it was estimated that the availability of food for fish at the wind turbine sites has increased by a factor of approximately 50 after the introduction of the hard substratum at Horns Rev. Taking the whole wind farm area into account, the estimated increase in biomass is only 38 tonnes or about 7% of the total biomass in the area. An increase in fish production related to the presence of the hard substratum is considered possible.


Results in perspective to the monitoring objectives The establishment of the wind farm resulted only in insignificant loss of natural seabed, which was replaced by hard bottom structures.


Results from the investigations on the benthic communities showed that only negligible impacts on the native communities are attributable to the wind turbine structures. The effects of the introduced hard substrates increased local biodiversity and increased local food availability. Cumulative effects of reduced trawling activities might be beneficial to local biodiversity by enabling benthic communities to mature and generally improve environmental conditions in areas of more wind farms.

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