Epifauna dynamics at an offshore foundation – Implications of future wind power farming in the North Sea

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Abstract

In the light of the introduction of thousands of large offshore wind power foundations into the North Sea within the next decades, this manuscript focuses on the biofouling processes and likely reef effects. The study explores the macrozoobenthos (biofouling) colonization at an offshore platform which is comparable to offshore wind turbine foundations. A total of 183 single samples were taken and the parameters water depth and time were considered comparing biofouling masses and communities. The blue mussel Mytilus edulis, Anthozoa and the Amphipoda Jassa spp. were the dominant species. The community from the 1 m zone and those from the 5 and 20–28 m zones can clearly be differentiated. The 10 m zone community represents the transition between the M. edulis dominated 1 m and 5 m zones and the Anthozoa dominated 20–28 m zone. In the future offshore wind farms, thousands of wind turbine foundations will provide habitat for a hard bottom fauna which is otherwise restricted to the sparse rocky habitats scattered within extensive sedimentary soft bottoms of the German Bight. However, offshore wind power foundations cannot be considered natural rock equivalents as they selectively increase certain natural hard bottom species. The surface of the construction (1280 m²) was covered by an average of 4300 kg biomass. This foundation concentrates on its footprint area (1024 m²) 35 times more macrozoobenthos biomass than the same area of soft bottom in the German exclusive economic zone (0.12 kg m−2), functioning as a biomass hotspot. Concerning the temporal biomass variation, we assume that at least 2700 kg biomass was exported on a yearly basis. 345 × 104 single mussel shells of different sizes were produced during the study period. It is anticipated that the M. edulis abundance will increase in the North Sea due to the expansion of the offshore wind farm development. This will result in the enhanced production of secondary hard substrate (mussel shells) and its associated fauna and will intensify filtration rates of the seawater. This predicted ecological system change is coined the ’Mytilusation’ of the German Bight.

Highlights

► Wind power farms result in 0.8% more macrozoobenthos biomass in the German Bight. ► Wind power foundations provide new intertidal stepping-stones in the open North Sea. ► The ‘Mytilusation’ of the North Sea will result in ecological system changes.

Introduction

In the near future the wind energy industry will expand on a large scale into offshore regions of western European shelf seas. Thousands of large steel turbine foundations will function as artificial reefs within areas which are naturally characterised by extensive sedimentary soft bottoms. It is expected that the turbine foundations will affect marine life through noise emission (Wahlberg and Westerberg, 2005; Madsen et al., 2006; Lindeboom et al., 2011), changed seafloor topography and sediment regimes (Wilson et al., 2010), and barrier effects (Masden et al., 2009). Additionally, strong implications for the subtidal ecosystem are expected from the settlement of macrozoobenthos on the artificial solid surface of the turbine foundations (Lindeboom et al., 2011). The macrozoobenthos communities on artificial hard substrata (biofouling or fouling) differ from natural macrozoobenthos communities on natural hard substrata (People, 2006; Wilhelmsson and Malm; 2008; Andersson et al., 2010) and on soft bottoms (Barros et al., 2001 Fabi et al., 2002; Langlois et al., 2006; Langhamer, 2010). In particular, in areas were natural hard substrata are rare, high numbers of artificial constructions favour the establishment of taxa such as cnidarians and mussels whose life histories include temporary or permanent attachment to solid substrates (Richardson et al., 2009). Increasing numbers of filtrating mussels (Winter, 1973; Clausen and Riisgård, 1996) may influence particle and nutrient fluxes between the water column and the sediment, thereby potentially affecting the plankton biomass (Wilhelmsson and Malm, 2008). Mussels, in turn, provide secondary hard substrate attractive for other epifaunal organisms (Norling and Kautsky, 2007). Continuous mussel shell litter fall modifies the grain size of the sediment where shells aggregate at the seafloor, providing new habitats for hemi- and holo-sessile organisms such as Anthozoa and Hydrozoa which require solid attachment sites, and typical hard bottom crabs (Wolfson et al., 1979; Freire and González-Gurriarán, 1995; Riis and Dolmer, 2003). Aggregations of marine biota at wind turbines will change the benthic biomass and provide food for a variety of predators (Wolfson et al., 1979; Freire and González-Gurriarán, 1995; Page et al., 1999; Wilhelmsson et al., 2006; Krone et al., submitted). Accordingly, benthic invertebrate communities and the local physico-chemical conditions are expected to change around the structures (Wolfson et al., 1979; Falcão et al., 2007). Finally, artificial reefs such as wind turbine foundations have been found to act as stepping-stones for the dispersal of hard bottom organisms facilitating the spread of both exotic and indigenous species (Connell, 2001; Bullerie and Airoldi, 2005; Glasby et al., 2007; Bulleri and Chapman 2010; Zintzen and Massin, 2010; Kerckhof et al., 2012).

In the German Exclusive Economic Zone (EEZ) of the North Sea at least 5000 single turbines are envisaged to be built within the next 20 years (IEA, 2008; BMU, 2010). To date, 22 wind farms with 1540 turbines are authorised for construction (BSH, 2012) and one wind farm with 12 turbines is operating. The potential impacts of the massive biofouling associated with the large-scale introduction of numerous turbine foundations into the North Sea are of concern and the resulting ecological processes are not well enough understood (Inger et al., 2009; Gill, 2005). Studies on specific effects of biomass accumulations on artificial structures are costly and often not feasible in offshore waters. Accordingly, ecological implications have to be derived from the qualitative and quantitative composition of fouling communities sampled in the course of baseline monitoring programs. Previous studies indicate that the composition of the fouling assemblage and, thus, the ecological implications of offshore constructions depend on a variety of factors such as the material and the size of the construction, the time of exposure, distance from the shore, the wind and current regime, and the water depth (Kingsbury, 1981; Butler and Conolly, 1999; Whomersley and Picken, 2003; People, 2006; Zintzen et al., 2008a; Andersson et al., 2010).

Most biofouling studies on offshore constructions in the North Sea have been conducted on oil and gas rigs. The biomass and the composition of the epifauna varies between rigs in coastal waters of the North Sea and those under Atlantic influence with growth rates of some species differing by up to 50% (Kingsbury, 1981). Similarly, the fouling communities varied between scattered ship wrecks in Belgian waters indicating spatial variation depending on the water mass (Zintzen et al., 2006, 2008a, 2008b, 2010). Fouling communities on offshore constructions are often completely dominated by either mussels or Anthozoa. On four North Sea oil platforms (45–67 m depth) blue mussels, Mytilus edulis, dominated the fouling assemblages in the shallow subtidal while Anthozoa occurred mainly in the deeper sections (Whomersley and Picken, 2003). Eleven years after construction, the fouling communities on the rigs did not yet reach a climax stage. The young wind power projects in the southern North Sea have also been investigated from the beginning in 2002 with first results emerging. In Belgian North Sea waters, six concrete gravity foundations of offshore wind turbines were erected at water depths of 25 m. Within the first two years after implementation, the fouling community displayed strong seasonal variations and lower numbers of taxa than that on older shipwrecks in the same region (Degraer and Brabant, 2009, Kerckhof et al., 2010). On turbine foundations of a Dutch wind farm (water depth: 21 m), 80–100% of the construction surface was covered by mussels down to a depth of 10 m while deeper sections were fully covered by Anthozoa and Hydrozoa (Lindeboom et al., 2011). As for most other North Sea constructions, the Amphipoda Jassa spp. (further also referred to as Jassa) and its tubes occurred all over the pylons. M. edulis dominated the fouling assemblages also on wind turbines in shallow (max. 14 m depth) coastal waters of the Danish North Sea (Leonhard and Pedersen, 2006). However, the mussel abundances varied substantially among the foundations within the wind farm.

The aim of the present study was to investigate the biofouling community on the steel foundation of an offshore research platform in the south-eastern North Sea and to apply it as the first available basis to calculate the impacts of large scale offshore wind farming on marine ecosystems in the German Bight. We hypothesise that the blue mussel M. edulis will form large persistent stocks in the upper sublittoral as such colonies have been detected in neighbouring North Sea regions as well. We further hypothesise that large biomasses will accumulate at these artificial construction.

Section snippets

Material and methods

The investigated foundation of the platform is similar in size and shape and thus equivalent to common wind turbine foundations. Depth zone typifying fouling communities were identified and their temporal and spatial development was addressed. Dominant taxa abundances were compared between the construction and the rocky island of Helgoland. The biomass balance and the production of secondary hard substrate by mussel shell litter fall were calculated.

Results

A total of 58 taxa were identified to species level. To achieve a homogeneous taxonomic resolution among all samples, some species had to be combined on a higher taxonomic level resulting in a data set for the analysis which consisted of 35 taxa.

Discussion

In future offshore wind farms, thousands of wind turbine foundations will provide habitat for a hard bottom fauna which otherwise restricted to the sparse rocky habitats scattered within extensive sedimentary soft bottoms of the German Bight. For the German Bight it was proofed that an offshore construction functions as a biomass hotspot within extensive soft sediment seafloor terrains. Such constructions also produce secondary artificial hard substrates by mussel shell litter fall and most

Conclusion

Because scientific offshore-diving projects are highly time-, staff- and money consuming, methodological constraints need to be taken into consideration and must be acknowledged when interpreting the results and predicted scenarios. Since weather conditions hardly allow for offshore sampling in winter, this season was not included. However, the present study provides the most comprehensive biofouling data set from offshore artificial constructions for the south- eastern North Sea. It is a basis

Author contributions

Dr. Roland Krone

Sampling, Sample processing, Taxa identification, Analysis, Article Preparation

Dr. Lars Gutow

Sample processing, Analysis, Article preparation

Dr. Tanja J. Joschko

Sample processing, Article preparation

Dr. Alexander Schröder

Sampling, Article preparation, Statistics

Acknowledgement

The study was carried out within the project BeoFINO II funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU, grant number 0329974A). The conclusions of this paper are those of the authors and do not necessarily represent the views of the BMU. We would like to thank M. Gusky and numerous volunteers for preserving the samples on board of the research vessel ‘FS Heincke’ and the many assistants for pre-sorting the samples in the laboratory. A

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