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As decommissioning of oil and gas (O&G) installations intensifies in the North Sea, and worldwide, debate rages regarding the fate of these novel habitats and their associated biota—a debate that has important implications for future decommissioning of offshore wind farms (OWFs). Calls to relax complete removal requirements in some circumstances and allow part of an O&G installation to be left in the marine environment are increasing. Yet knowledge regarding the biological communities that develop on these structures and their ecological role in the North Sea is currently insufficient to inform such decommissioning decisions. To focus debate regarding decommissioning policy and guide ecological research, we review environmental policy objectives in the region, summarize existing knowledge regarding ecological aspects of decommissioning for both O&G and OWF installations, and identify approaches to address knowledge gaps through science–industry collaboration. We find that in some cases complete removal will conflict with other policies regarding protection and restoration of reefs, as well as the conservation of species within the region. Key ecological considerations that are rarely considered during decommissioning decisions are: (i) provision of reef habitat, (ii) productivity of offshore ecosystems, (iii) enhancement of biodiversity, (iv) protection of the seabed from trawling, and (v) enhancement of connectivity. Knowledge gaps within these areas will best be addressed using industry infrastructure and vessels for scientific investigations, re-analysis of historical data held by industry, scientific training of industry personnel, joint research funding opportunities, and trial decommissioning projects.

The aim of this document is to provide a resource to researchers in marine conservation by giving summaries of the international instruments that may have implications for marine conservation in the MarCons region. These instruments include international agreements and conventions, regional agreements and intergovernmental sectoral agreements, and European directives and policies, which directly or indirectly determine marine conservation policies and management. This provides the basis for an inherently interdisciplinary cross-sectoral approach to conservation policy analysis.

Offshore wind farms are expected to contribute significantly to the Belgian 2020 tar- gets for renewable energy. As of 2016, an installed capacity of 870 Megawatt (MW), consisting of 232 offshore wind turbines, is operational in the Belgian part of the North Sea. In 2017 and 2018, an additional capa- city of respectively 275 and 320 MW will be added (fig. 1), with three other projects sche- duled for the next few years after that. With 238 km2 reserved for offshore wind farms in Belgium and 344 km2 in the adjacent Dutch Borssele, cumulative ecological im- pacts may however be expected. These im- pacts both positive and negative, triggered an environmental monitoring programme focusing on various aspects of the marine ecosystem components, but also on the human appreciation of offshore wind farms. This chapter provides an overview of the offshore renewable energy development in the Belgian part of the North Sea.

Piling driving sound is known to impact harbour porpoise (Phocoena phocoena) distribution, but to date detailed knowledge on the combined spatial and temporal components of this impact over longer time periods remains lacking. From May to September 2016, pile driving was taking place at the Nobelwind wind farm located on the Bligh Bank in Belgium. In this period, porpoise activity was recorded using passive acoustic monitoring (PAM) using Continuous Porpoise Detectors (C-PoDs), at various distances from the construction site (1 – > 55 km). In this study, we compared porpoise detections before, during and after pile driving. During piling, porpoise detections decreased at stations located up to 20 km from the location of the piling event. At larger distances (20-55 km), porpoise detections either remained the same or increased slightly during piling events, which may be due to displaced porpoises entering the area. Underwater sound levels were extrapolated for the different locations. Pile driving sound levels at the furthest distance where reductions in porpoise detections were observed were ~159 dB re 1μPa (Lsub>z-p), which is close to the threshold level for major disturbance for harbour porpoise proposed in literature.

The harbour porpoise (Phocoena phocoena) is the most abundant cetacean in the Belgian part of the North Sea. We developed a mooring system for static passive acoustic monitoring (PAM) of this species using c-PoDs at locations of opportunity. Data of moorings between 2010 and 2015 at two locations were analysed. They revealed a significant seasonal trend in detections, assessed by month, with peaks in late winter - early spring and late summer, consistent with the results of aerial surveys and with strandings data. At one location there were significant differences in detections between years, with higher detection rates in every year between 2011 and 2014, and the highest detection rates in 2013 and 2014. The experiences gained are used to design a subsequent study strategy to monitor harbour porpoise presence in Belgian waters, including possible effects on their presence due to the construction of offshore windfarms.

There is concern about possible effects on the marine ecosystem of high levels of underwater noise generated during pile driving for the construction of offshore wind farms. As a result, various national governments in Europe have identified limits of underwater sound levels, as such imposing in many cases the use of noise mitigation measures. In this paper we compare the regulations with regard to impulsive underwater noise in the Belgian wind farm zone with those in the Dutch wind energy zone of Borssele. These (planned and existing) wind farms are situated at opposite sides of the maritime border between both countries. These regulations are quite different and at times even contradictory and developers could benefit from an alignment of regulatory practices on a regional basis. Measurements of piling noise from constructed wind farms are used to extrapolate the anticipated noise levels of the next two wind farms to be constructed, and these are evaluated in relation to the new regulations on underwater sound. Wind farm developers are already developing strategies for costeffective piling noise reduction but uncertainty remains with regards to both the level of underwater noise produced during piling as well as with the effectiveness of the noise mitigation measures being applied. Our results indicate that a combination of noise mitigation measures may need to be used to comply with the new regulations.

Offshore wind farms are expected to contribute significantly to the Belgian 2020 targets for renewable energy. Today, 182 turbines are operational in the Belgian part of the North Sea. In the next few years, an additional 234-342 turbines may be installed. With 238 km² reserved for offshore wind farms in Belgium and 344 km² in the adjacent Dutch Borssele, cumulative ecological impacts may however be expected. These impacts both positive and negative, triggered an environmental monitoring programme focusing on various aspects of the marine ecosystem components, but also on the human appreciation of offshore wind farms. This report provides an overview of the offshore renewable energy development in the Belgian part of the North Sea.

Since the installation of the wind farm foundations and associated scour protection in an area previously characterized by soft bottom sediments, the number of hard substrate associated fish and benthic species has increased markedly. At the level of a single turbine footprint a nearly 4000-fold increase in autumn biomass was observed, whereas at the level of the entire wind farm a 14-fold increase was observed. Further development of the entire Belgian wind energy zone may increase benthic biomass by as much as 3% of the current estimated benthic biomass in the Belgian part of the North Sea.

The construction, operation and dismantling of offshore wind farms generate noise both above and under water that may be of environmental concern. The maximum detected sound power level of the above water pin piling noise for example, reached 145 dB(A), while the operational sound power level amounted to 105-115 dB(A) at high wind speed. Underwater construction noise was close to ambient noise levels for gravity based foundations (about 115 dB re 1 µPa RMS), while pin piling and especially monopile piling produced excessive levels of underwater noise up to 194 dB re 1 µPa (zero to peak level at 750m), attenuating to ambient noise levels at a distance of up to 70 km. Whether or not such noise levels are to be considered acceptable will depend on the future implementation of proposed regulations into the Belgian legislation.

In 2002 and 2009 two sociological seascape surveys took place in Belgium. These surveys focused on both the visual and overall experience. People’s general opinion on wind energy and on the local planned wind farms were asked. Results show that in 2002 there was already a majority in favor of wind farms and this number still increased by 10% in 2009. A future survey is proposed to take place in the summer after the first wind turbines of the wind farms closest to the land have been installed. At that time at least three other wind farms will also be operational.

Offshore wind farms are expected to contribute for about 43% of the Belgian 2020 targets for renewable energy. Today, 109 turbines are operational in the Belgian part of the North Sea. In the next few years, several hundred of turbines will be up and running. With 238 km² reserved for offshore wind farms, major ecological impacts may however be expected. These impacts both positive and negative, triggered an environmental monitoring programme focusing on various aspects of the marine ecosystem components, but also on the human appreciation of offshore wind farms. This report targeting marine scientists, policy makers and managers, provides an overview of the major scientific achievements of six years of monitoring.

The introduction of hard substrata in the mainly sandy environment of the Belgian part of the North Sea provides a new habitat to species that could previously not establish in the area. The community is dominated by the amphipod Jassa herdmani, the hydroids Tubularia larynx and T. indivisa and Actiniaria species, of which the plumose anemone Metridium senile is the most dominant. Only few species, especially those arriving early in the colonization process, are able to establish a viable population. The vertical foundations and the complex three-dimensional structure of the scour protection harbour different fouling communities.

At present, everybody agrees that offshore wind farms do impact the natural environment. Whether or not these impacts should be valued positive or negative, or ecologically and societally acceptable, however remains an open question. While boosting local species richness, the artificial hard substrata may for example also open the door to non-indigenous and even invasive species. Some fish and seabirds are further known to be attracted to wind farms, but fish do not necessarily take profit from these structures and seabirds may suffer from an increased collision risk. The true impact will therefore be valued only if local observations are up scaled to the ecoregion level.

Six years of monitoring triggered a reflection on how to best continue with the monitoring programme. The basic monitoring has to be rationalised at the level of the likelihood of impact detection, the meaningfulness of impact size and representativeness of the findings. Targeted monitoring should continue to disentangle processes behind the observed impact, for instance the overarching artificial reef effect created by wind farms. The major challenge however remains to achieve a reliable assessment of the cumulative impacts. Continuing consultation and collaboration within the Belgian offshore wind farm monitoring team and with foreign marine scientists and managers will ensure an optimisation of the future monitoring programme.

Offshore wind farms generate underwater noise during construction, operation and decommissioning. Different foundation designs are in use for offshore wind farms. Steel monopiles, jackets made of four steel pinpiles and gravity-based foundations are applied in the Belgian part of the North Sea. Both monopiles and jacket pinpiles are hammered into the bottom and as such excessive noise is generated during their installation. This paper aims at comparing the emitted underwater noise generated during the piling activities of two wind farms, each using a different type of pile foundation: steel monopiles were used in the Belwind wind farm (Bligh Bank), while jacket pinpiles were chosen for the C-Power project (phases 2 and 3) on the Thorntonbank. Underwater noise was measured from a rigid hull inflatable boat at various distances from the pile driving location during the installation of steel monopiles and pinpiles. Analysis focused on the quantification of the discontinuous impulsive pile driving-generated underwater noise. Zero to peak sound pressure level (Lz-p), unweighted sound exposure level (SEL), cumulative SEL and 1/3 octave spectra were computed in order to quantify and compare the underwater noise. Lz-p and SEL were normalized to 750 m distance from the piling location. Piling of the monopiles generated a Lz-p between 179 and 194 dB re 1µPa. Although piling of the smaller pinpiles was expected to be less noisy, no significant differences with monopiling could be demonstrated (pinpiling Lz-p: 172-189 dB re 1 µPa). Similarly, also SEL, varying between 145 and 168 dB re 1 µPa2s, exerted no statistical difference between mono- and pinpiling and furthermore near identical spectra were measured for both types of piling. The rather wide variability within and broad overlap between both types of piling can possibly be attributed to the (small scale) differences in bottom type as well as in the setup used. However, the radius of discomfort in the harbour porpoise Phocoena phocoena was estimated at 30 km and 19 km for monopiles and pinpiles, respectively. Furthermore, the pinpiling (four pinpiles per foundation) took about 2.5 times more time than the monopiling and will as such have a prolonged impact onto the marine fauna. When standardised to megawatt installed (3 MW turbines on the monopiles and 6 MW turbines on the jacket foundations) both types of piling will score about equally.

In this contribution, the species composition and ecological succession of the biofouling on the subtidal zone (circa littoral) of selected foundations of two offshore wind farms in the Belgian part of the North Sea, the C-Power farm on the Thorntonbank and the Belwind farm on the Bligh Bank, are studied through time. As observed in many other studies, the colonization of the bare substratum has been very fast in the two studied wind farms. No less than 50% of the total species pool (41 taxa out of 78 taxa) was shared between both wind farms and both wind farms were dominated by the amphipod Jassa herdmani with up to 90000 ind./m2 (i.e. about 70% of the enumerable fauna). Other dominant species in both wind farms comprised the starfish Asterias rubens and the amphipod Stenothoe valida. The first year of the succession also showed a highly similar trajectory, with particularly mobile species, e.g. J. herdmani and Corophium acherusicum displaying a typical summer – winter oscillation pattern in both wind farms. This similarity was however less obvious for the sessile species such as Pomatoceros triqueter and Clytia hemisphaerica, and seemed to strongly diverge after the first year. The concrete gravity based foundations (GBFs) at the Thorntonbank hosted more species than the steel monopiles at the Bligh Bank (70 taxa versus 49 species). The presence of some coastal, soft sediment species such as the bivalves Abra alba and Mysella (Kurtiella) bidentata, only in the C-Power farm, further contributed to the dissimilarity between both wind farms. The biofouling on both wind farms exhibits direct influence by the same pool of species originating from the surrounding artificial and natural hard substrata. As for shipwrecks, specific features of the studied structures such as verticality, substratum composition or lower depths may explain specific patterns of taxonomic composition such as an impoverished sessile epifauna compared to the surrounding natural hard substrata. Predatory activities are an important factor controlling the succession of sessile species on the substratum. The high degree of patchiness suggests that the colonization process is still ongoing and the mature state, the Metridium senile biotope, characteristic for this type of substrata in the North Sea is not fully reached yet. Apart from the slipper limpet Crepidula fornicata, no introduced species have been observed thus far in the permanently submerged part of the wind turbine foundations. This observation contrasts with the intertidal and splash zone, where many introduced species were present in the biofouling community.