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Thousands of artificial (‘human-made’) structures are present in the marine environment, many at or approaching end-of-life and requiring urgent decisions regarding their decommissioning. No consensus has been reached on which decommissioning option(s) result in optimal environmental and societal outcomes, in part, owing to a paucity of evidence from real-world decommissioning case studies. To address this significant challenge, we asked a worldwide panel of scientists to provide their expert opinion. They were asked to identify and characterise the ecosystem effects of artificial structures in the sea, their causes and consequences, and to identify which, if any, should be retained following decommissioning. Experts considered that most of the pressures driving ecological and societal effects from marine artificial structures (MAS) were of medium severity, occur frequently, and are dependent on spatial scale with local-scale effects of greater magnitude than regional effects. The duration of many effects following decommissioning were considered to be relatively short, in the order of days. Overall, environmental effects of structures were considered marginally undesirable, while societal effects marginally desirable. Experts therefore indicated that any decision to leave MAS in place at end-of-life to be more beneficial to society than the natural environment. However, some individual environmental effects were considered desirable and worthy of retention, especially in certain geographic locations, where structures can support improved trophic linkages, increases in tourism, habitat provision, and population size, and provide stability in population dynamics. The expert analysis consensus that the effects of MAS are both negative and positive for the environment and society, gives no strong support for policy change whether removal or retention is favoured until further empirical evidence is available to justify change to the status quo. The combination of desira

Switching from fossil fuels to renewable energy is key to international energy transition efforts and the move toward net zero. For many nations, this requires decommissioning of hundreds of oil and gas infrastructure in the marine environment. Current international, regional and national legislation largely dictates that structures must be completely removed at end-of-life although, increasingly, alternative decommissioning options are being promoted and implemented. Yet, a paucity of real-world case studies describing the impacts of decommissioning on the environment make decision-making with respect to which option(s) might be optimal for meeting international and regional strategic environmental targets challenging. To address this gap, we draw together international expertise and judgment from marine environmental scientists on marine artificial structures as an alternative source of evidence that explores how different decommissioning options might ameliorate pressures that drive environmental status toward (or away) from environmental objectives. Synthesis reveals that for 37 United Nations and Oslo-Paris Commissions (OSPAR) global and regional environmental targets, experts consider repurposing or abandoning individual structures, or abandoning multiple structures across a region, as the options that would most strongly contribute toward targets. This collective view suggests complete removal may not be best for the environment or society. However, different decommissioning options act in different ways and make variable contributions toward environmental targets, such that policy makers and managers would likely need to prioritise some targets over others considering political, social, economic, and ecological contexts. Current policy may not result in optimal outcomes for the environment or society.

The increasing need for renewable energy has led to the transition of renewable energy devices to the marine environment. Currently, mainly offshore wind farms have been completely developed and are operational in the North Sea. The solar energy sector is also rapidly evolving and floating photovoltaics are continuously created and deployed. In this study, we investigated the colonisation patterns and community changes with time of fouling fauna on the first floating photovoltaics in the coastal Dutch North Sea. Samples were collected by divers from the underwater side of 4 floaters, coated with different anti-fouling techniques (Intersleek, GreenPowerNano PPDura, Finsulate and Pato) at two different moments, shortly after the deployment of the floaters and approximately a year later. In total, 72 fouling taxa were identified on the floaters, from which ca. 11% are known to be non-indigenous species for the region. The anti-fouling coating Intersleek seemed to work the most efficiently against fouling colonisation, since the fouling community sampled from this floater contained the least taxa. However, the small number of samples collected from the different floaters did not allow for a direct comparison between the anti-fouling coatings. The communities evolved with time, with young communities accommodating a larger number of individuals and old communities having less individuals but higher biomass, indicating that the organisms become bigger in size and compete for the available space. Nevertheless, the communities had not reached a stable climax yet, while this process might take multiple years due to the dynamic environment in which floating photovoltaics are deployed. Monitoring the fouling communities occurring on floating photovoltaics in the North Sea for a long-term is necessary to understand the effects of these new man-made structures on the marine environment, especially since floating photovoltaics are moving to offshore locations and will be possibly

In The Netherlands, the first offshore wind farm (OWF) became fully operational in 2007. This OWF was followed by several others. The establishment of wind farms is accompanied by a rapid colonisation by fouling species, which attach on the newly introduced hard substrates. Monitoring programmes of the communities on wind turbine foundations in the Netherlands have not yet been conducted on a long-term basis (>5 years) after construction. The ‘Monitoring and Research, Nature Enhancement and Species Protection’ (MONS) aims, among many other things, to monitor hard substrate communities in OWFs during the next 10 years. The focus is on differences in succession in OWFs with different characteristics in the various habitats and monitoring long term succession of benthic communities on the turbine foundations and scour protection layer (SPL). Since the execution of hard substrate sampling campaigns in OWFs is challenging and costly, an advice was requested on how to come to a feasible monitoring programme in MONS. For this purpose, the available and to-be-developed sampling methods and legal & safety requirements were evaluated. Multiple methods are available that are or may become suitable but no single method meets all criteria. On the very short term, the only fully suitable method to obtain samples from the turbine foundations, is collection by divers using manual scraping tools and sample nets as has been performed in past research in OWFs and on other offshore installations. However, OWF operators prefer to replace diving work with remotely operated vehicles (ROVs) wherever possible, as part of their legal obligations to minimise health and safety risks. In the next 2 years, an ROV mounted sampling tool in development by Bluestream and Wageningen Marine Research may become a suitable replacement for diver sampling. To collect small rocks from the scour protection layer, multiple methods exist, but for some of the options may not be accepted by OWF operators. The s

Changes to fisheries that result from offshore wind farm (OWF) installations may be considered good or bad depending on various stakeholders’ perspectives. OWFs can act as artificial reefs that may benefit secondary fish production, but such effects may also have ecological consequences. The fisheries exclusion effect that turns some OWFs into no-go areas, hence effectively no-take zones, could provide resource enhancements or redistribution. However, the displacement of fishing effort may have consequences to fisheries elsewhere. Changes in the sensory environment related to sound, as well as electromagnetic fields and physical alterations of current and wind wakes, may have as yet unknown impacts on fisheries resources. Understanding the interactions among effect type, OWF development phase, and spatiotemporal population dynamics of commercial and recreational species remains chal-lenging, exemplified by the commercial fishery lobster genus Homarus in European and North American waters. While knowledge of the interactions between resource species and OWFs is improving, there remain questions on the wider interaction between and consequences of OWFs and fisheries. Studies of this wider relevance should aim to improve understanding of the economic and societal impacts of OWFs linked to ecosystem services that support fisheries. Furthermore, assisting fisheries management and providing advice requires monitoring and survey data collection at appropriate spatiotemporal scales. This information will help to determine whether OWFs have any meaningful impact on regional fisheries, and increased investments will be needed to target scientifically appropriate monitoring of OWFs and fisheries, which is supported by better integrated policy and regulation.

Artificial structures such as offshore oil and gas platforms can significantly alter local species communities. It has been argued that this effect should be considered during decisions over their removal during decommissioning. In the North Sea, leaving such structures in place is prohibited but derogations are allowed for large concrete installations. To assess removal options for one such installation, the Halfweg GBS (gravity-based structure) a concrete platform foundation off the Dutch coast, we studied the resident fouling macrofauna community. The faunal structure, biomass and trophic composition of the Halfweg was then compared with those from the surrounding seabed sediments, other local artificial structures and a natural rocky reef. In total, 65 macrofaunal species were observed on the concrete (52 species), steel legs (32) and surrounding rock dump (44) of Halfweg. Mean Simpson diversity per sample was highest on the rock dump (0.71) but concrete (0.54) and steel (0.60) of the GBS were lower than seabed (0.69). Ten of the species observed on the concrete were not reported on other substrates while 10 of the species were also observed in the surrounding seabed. The GBS structure was numerically dominated by Arthropoda which comprised 98% of the total abundance. Mean ash free dry weight (AFDW) was significantly higher (p < 0.001) on the Halfweg substrates (204 g AFDW per m2) than in the surrounding seabed (65 g AFDW per m2). Over 94% of the biomass on Halfweg consisted of the plumose anemone Metridium senile. While common on other reefs, this species was absent from the surrounding seabed. Macrofaunal feeding mechanisms of the concrete and rock dump communities on the GBS were similar to those of nearby sediments, although these differed from those on the Halfweg steel legs. Therefore, the presence of Halfweg alters the local community feeding modes. Multivariate analysis revealed that taxonomic structure of the GBS and other artificial structures signific