European oysters (Ostrea edulis) once covered large areas of the North Sea, but have disappeared due to a combination of overexploitation and the destruction of benthic habitats including hard settlement substrate. Offshore wind parks offer an opportunity for oyster restoration as fishing is banned inside these parks and scour protection provides hard settlement substrate. However, ecological restoration of marine systems is capital-intensive. The success of restoration projects is mainly determined by the choice of methods and techniques and consequently costs. Costs and cost-effectiveness information are therefore key in decision making processes concerning the selection of restoration efforts and techniques. So far, economic viability of marine ecosystem restoration have mainly focused on near-shore shallow habitats. The aim of this study was to provide insight into the most cost-effective deployment options to create a European flat oyster reef in an offshore wind farm in the North Sea. Within the current policy and legislation framework, several deployment scenarios were identified based on best practices, expert knowledge, and preliminary results of several pilots. The 9 scenarios included 'adults placed loose on the seafloor, 'adults glued on granite, 'spat settled on shells, 'spat settled on granite and a combined adult and spat scenario. Cost-effectiveness of the different scenarios was determined by modelling the expected reef biomass post-deployment both with and without the option to add additional settlement substrate post-deployment. The main conclusions from this exercise were that: 1. based on investment value, the scenarios adult loose on the seafloor, 'adults in cages and 'spat on shells had the highest revenues per Euro invested; 2. adding substrate in the years post-deployment increased cost-effectiveness in the model for all scenarios, and 3. the time post-deployment to reach a self-sustaining adult oyster population was, with 8-10 yr, shortest for the scenarios 'spat settled on shells' and the combined scenario of 'adults placed loose on the seafloor' and 'spat settled on shells'.

The flat oyster Ostrea edulis L., once common in the North Sea, declined rapidly due to intensive fisheries in the late 19th century and disease outbreaks at the beginning of the 20th century and is now listed as ‘threatened’ or ‘declining,’ with restoration of oyster beds now included in European directives and national plans. For oyster restoration, availability of suitable substrate is required to ensure successful settlement of oyster larvae. Off-shore windfarms are good candidates for restoration as bottom disturbance is not allowed and hard substrate is present in the form of so-called scour protection. This can provide settlement substrate for oyster larvae. In addition to the rock material that currently makes up the scour protection, studies focus on finding alternative and moldable materials that stimulate settlement. The aim of this study was to identify flat oyster larvae settlement preferences for different substrate materials. Oyster settlement on conventional scour protection rock (granite and eclogite), and currently used sandstone and concrete were compared to new types of scour protection rock (marble and limestone). In addition, three new substrates were included in the tests: a coating based on fine ground oyster shells (BESE-reef paste), substrate made of sandy dredged sediment (Geowall) and a bioinspired glue that binds crushed oyster shell fragments together (SeaCrete). Flat oyster larvae were exposed to the substrates in two hatchery experiments as well as under realistic, challenging field conditions. Flat oyster larvae settled on all substrates, with the lowest spat density on eclogite, granite and Geowall and the highest spat density on the two novel substrates SeaCrete and BESE-reef paste. These results promise to enhance native European oyster bed restoration with limited environmental impact as the novel substrates have low CO₂ footprints and make use of wasted shells from the seafood industry.

The native European flat oyster (Ostrea edulis) is an ecosystem engineer providing important ecosystem services, but became nearly extinct from the North Sea due to diseases and overfishing. There's a growing interest to restore these oyster reefs for their valuable contribution in re-establishing a rich ecosystem in the North Sea. In order to reintroduce the flat oyster population, the availability of hard substrate is crucial for initial settlement and reef development. Such substrate is offered by the infrastructure in offshore wind farms, by means of quarried rock placed at the base of the wind turbine foundations and on top of cable crossings to prevent scouring of the seabed. Further anthropogenic disturbances of the seabed are largely restricted, making wind farm areas promising sites for oyster reef restoration. For successful oyster reef initiation, offering a suitable type of substrate for larvae settlement is important. Here, we assess the settlement preference of flat oysters on 9 different types of substrate, by comparing total settlement, spat densities and spat survival. Oyster larvae settlement preference based on the total number of spat per surface area of the substrate was the highest for granite, a rock type conventionally used as scour protection in offshore wind farms. The lowest settlement preference was observed for steel and the biodegradable polymer BESE. The experiments were performed in a spatting pond and in a natural bay to be able to compare spat collection under controlled and natural conditions. Settlement rates in the spatting pond were much higher than in the natural environment, though survival rates were lower. Our results provide insight in the settlement preference of the European flat oyster for different types of substrate under controlled and natural conditions. Knowing these favorable substrates and conditions for oyster larvae settlement allows for the selection of pro-active measures that contribute to flat oyster reef restoration in the North Sea.

In the past, a large part of the seabed of the southern North Sea was covered by hard substrates, including oyster beds, coarse peat banks, and glacial erratics. Human activities, particularly bottom trawl fisheries, led to the disappearance of most of these hard substrates, resulting in the loss of its associated diverse benthic life as well. However, the introduction of human-made structures such as offshore wind farms in the North Sea, offers a chance to provide habitat of similar functionality as the former hard substrates. The offshore wind farm infrastructure generally contains layers of rock material deployed at the base of the wind turbine foundations and cable crossings, so-called scour protection, aiming to prevent seabed erosion. The scour protection offers a unique habitat for rock-dwelling benthic organisms in an otherwise mostly soft-bottom environment. By designing the scour protection to be more nature-inclusive, the biodiversity of benthic life can be increased. In this study we examined the effect of substrate material and grading of the scour protection on the epibenthic biodiversity in situ. This was done by deploying research cages containing crates (n = 15) with different types of substrates (concrete, granite, and marble) on the scour protection within an offshore wind farm in the Dutch North Sea. The study revealed a significant (p < 0.05) positive relation between available substrate surface (pebble size) and taxonomic richness. Furthermore, a biological trait assessment of living habits (Tube dwelling, Burrowing, Free living, Crevice dwelling, Epi/endobiotic, and Attached) revealed variations in habit modes across substrate types, with marble and concrete samples showing greatest divergence. Marble samples contained a higher prevalence of tube dwelling organisms, whereas concrete samples contained a relatively higher prevalence of free living, epi/endobiotic and crevice dwelling organisms. The findings support the value of nature-inclusive scour protection designs, emphasizing that both taxonomic and functional diversity can be enhanced by increasing the available surface area of the scour protection and incorporating a variety of substrate types. By adopting these nature-inclusive design components, the coexistence of renewable energy production and a diverse marine benthic community can be further optimized.

Cross-habitat facilitative processes can enhance seascape restoration outcomes but there is uncertainty around the spatial dependencies of these processes across habitats. We synthesised the influence of environmental parameters on six processes underpinning cross-habitat facilitation and identified the linear distances over which they operate between habitats. All six process types occur at distances commonly used in seascape restoration demonstrating how harnessing facilitation can scale-up restoration to meet national and international goals.

Incorporation of ecology and ecosystem services into marine infrastructural developments has gained interest over the last decades. Growing attention is given to combine the massive roll-out of offshore wind farms in the North Sea with reinstating the once rich but nowadays nearly extinct European flat oyster (Ostrea edulis). However, the practical upscaling of these pilots is hindered by the absence of clear management objectives and the lack of quantitative knowledge on the effect of technical interventions that could stimulate oyster reef development. Consequently, it is unclear what scale of intervention would actually be required to achieve overall management objectives. This paper presents a stepwise procedure designed in particular to guide the selection of appropriate measures and their required scale for pro-actively facilitating flat oyster reef development in offshore wind farms, in order to reach a desired state for oyster reef inclusive wind farms. The stepwise procedure addresses the historical and current situation of the physical system and social environment, provides options for intervention that stimulate oyster reef development at a range of scales, from micro-scale (materials used) to mega-scale (connectivity between wind farms), and quantitatively assesses the potential effect of applying these interventions. Assumptions have been made in quantifying the effort required for developing oyster reefs in offshore wind farms, and refinement is obviously needed. However, this is a first attempt to make such estimates. The outcomes provide direction in identifying research needs to fill knowledge gaps, as well as in decision-making during the design process for inducing oyster reef development in offshore wind farms. Herewith, application of the stepwise procedure supports authorities in restoration management for the successful reinstatement of flat oyster reefs in the southern North Sea.

Acropora palmata is one of the major reef-building coral species in the Caribbean. The species has suffered drastic declines in abundance and sexual recruitment over the past decades. One method for active rehabilitation of A. palmata reefs is by assisting the production of sexual recruits under controlled lab conditions. Within this study, the effect of different aquaculture regimes and culturing periods on the survival rates of these recruits was investigated. In August 2016, coral spawn was collected on a reef nearshore New Providence, Bahamas, cross-fertilized, and reared in mobile laboratory facilities from Van Oord Dredging and Marine Contractors. Larvae were settled on pre-conditioned aragonite plugs. Sexual recruits were cultured under four different aquaculture conditions: ambient vs. high Total Alkalinity (TA) (∼2.8 mEq L⁻¹ vs. 4.8 mEq L⁻¹) and with vs. without feeding Artemia nauplii. Recruit size was monitored by tracking living tissue area and the number of polyps of a subset of recruits. Plates with recruits were outplanted to a nursery on the reef after 4, 9 and 14 weeks of aquaculture. Survival was determined during the aquaculture phase (at 4, 9 and 14 weeks after settlement), and after outplanting (at 27 and 44 weeks after settlement). During the aquaculture phase, survival was significantly lower in seawater with increased TA compared to ambient seawater conditions. The average number of polyps per recruit was significantly higher in the treatments with feeding. After outplanting to the reef, both survival and recruit size were highest in the feeding treatments. The most successful aquaculture treatment in this study was a combination of increased TA and feeding during 9 weeks of aquaculture, which resulted in a doubling of survival and recruit size at 10 months after settlement compared to ambient conditions. Ambient conditions did not enhance survivorship nor recruit size at 10 months after settlement, as compared to the other aquaculture treatments. Nevertheless, the success of ambient aquaculture conditions exceeded natural conditions, as no natural recruitment of A. palmata was observed in this study. We conclude that feeding during and ex-situ culture period enhances ex-situ growth rates and in situ recruit survival of A. palmata juveniles. No positive effects of the aquaculture treatment with only increased TA were found. Building on these results, recommendations are provided for future reef rehabilitation efforts using ex-situ rearing of sexually reproduced A. palmata recruits.

The North Sea was once abundantly covered with hard substrates such as oyster beds, coarse peat banks and glacial erratics, providing habitat to a rich community of marine species. Most of these habitats were destroyed by bottom-trawl fisheries over the past century, and today, the seabed hosts a relatively poor species community. Emerging offshore windfarms include the re-introduction of hard substrate by means of scour protection around the foundation of wind turbines. It is assumed that the new habitat will contribute to marine biodiversity, and this study aims to demonstrate that. Video data were collected using a Remotely Operated Vehicle in four wind farms in the southern North Sea. A quantitative assessment was made to determine the effect of scour protection on community structure. The assessment revealed distinct community clusters for geographic location and seabed type. Windfarms closely located to each other had a more similar epibenthic community compared to those further away. The epibenthic community at the rocky armour layer of the scour protection had a different species composition and a higher species abundance than the one at the sandy seabed surrounding it. Species diversity by means of richness, evenness and the Shannon diversity index was not consistently higher or lower for the communities at the different seabed types. This study shows that marine life inhabits scour protection in offshore wind farms and that it is different from the community living at the surrounding seabed. Knowing the potential epibenthic community structure at and around a scour protection supports the development of new wind farms that include components to enhance their ecological value. Herewith, our study contributes to efforts to restore biodiversity in the North Sea.