Hydrodynamic alterations induced by floating solar structures co-located with an offshore wind farm

Marine renewable energies are part of the current energy transition strategy in Europe. Offshore wind farms (OWFs) in the North Sea currently supply around 25.8 GW of power and are aimed to reach at least 117 GW by 2030. Yet, on its own, wind energy supply remains partially unreliable for a consistent energy generation. Offshore photovoltaic (PV) installations are increasingly considered a suitable technology to complement the intermittent energy supply of OWF.  In the North Sea, installation of offshore photovoltaics within OWFs offers two significant advantages: (1) space optimization in an already busy North Sea, and (2) the possibility of utilizing and integrating an existing power network.However, the installation of such systems comes with significant environmental challenges. In particular, solar technologies currently involve more submerged structures per produced energy unit. These floating structures induce hydrographic changes, particularly in terms of current velocity slowdown and turbulence production. Also, the floaters act as artificial hard substrates that are quickly colonized by organisms, potentially altering the biogeochemical dynamics of the water column and, ultimately, affecting the sediments.This study provides a first assessment of the impact of PV structures on key hydrodynamic variables (e.g. current velocity fields, bottom shear stress, turbulence production), both in the near-field and far-field around an OWF using the 3D hydrodynamic model COHERENS (https://doi.org/10.5281/zenodo.11654795). A 3D computational grid around the Mermaid OWF in the Belgian part of the North Sea was implemented, with a grid resolution of 50m x 50m. We first present the impact of floating solar panels on the surrounding circulation and turbulence field,  assessed using a sub-grid scale parameterization. Results from different scenarios will be presented and compared.Second, we present a first estimate of the enrichment of organic carbon flux to the sediments due to the presence of colonizing organisms (mainly Mytilus edulis) on the submerged parts of PV structures. Our aim is to assess the areas of the seabed impacted by the deposition of faecal pellets due to the installation of PV structures within the OWF, considering the hydrodynamic perturbations presented above. This part uses a 3D Lagrangian particle tracking model (OSERIT; Dulière et al., 2012), faecal pellet characteristics gathered from laboratory experiments (e.g. sinking velocity, production rate and carbon content) and literature data on colonization of wind turbine foundations (Mavraki et al., 2020). In this model, each numerical particle represents a certain quantity of faecal pellets and, consequently, organic carbon.Maps of faecal pellet deposition patterns will be presented for several scenarios of PV structures distribution in the Mermaid OWF. Our simulations show that the footprint affected by faecal pellet depositions could reach up to 18 times the surface area of the OWF and that the amount of carbon deposited could reach up to 1454 gC.km-² per day (worst-case scenario). These maps illustrate the causal relationship between PV farm design and the surface area of sediment affected by the faecal pellet deposition and thus exposed to organic carbon enrichment.

Functional trait responses to different anthropogenic pressures

Short-term effects of fishery exclusion in offshore wind farms on macrofaunal communities in the Belgian part of the North Sea

Bird collision assessments are generally made at the scale of a single wind farm. While especially in offshore situations such assessments already hold several assumptions, even bigger challenges exist on estimating the cumulative impact of multiple wind farms and the impacts at population level. In this paper, the number of collision victims at Belgian offshore wind farms was estimated with a (theoretical) collision risk model based on technical turbine specifications, bird-related parameters and bird density data of both local seabirds and passerine migrants. Bird density data were gathered by visual censuses and radar registrations. The outcome of the model was extrapolated to future development scenarios in the Belgian part of the North Sea and in the entire North Sea, and then further used for a preliminary assessment of the impact at population level for the species at risk. The results indicate that the cumulative impact of a realistic scenario of 10,000 turbines in the North Sea might have a significant negative effect at population level for lesser and great black-backed gull. We further show that during a single night of intense songbird migration, the number of collision victims among passerine migrants might be in the order of magnitude of several thousands in the entire North Sea. We argue that it is of great importance to further develop methods to quantify the uncertainties and to minimise the assumptions, in order to assure more reliable cumulative impact assessments.

The mesozooplankton community of the Belgian shelf (North Sea)

The development of offshore wind farms (OWFs) in the North Sea has increased considerably to create alternatives for fossil fuel energy. Activities related to the construction of OWFs, in particular gravity-based foundations (GBFs), are mainly associated to dredging, causing direct effects to the macrofauna in the seabed. The sediment characteristics and macrofauna were studied before and after construction (2005–2010) of six GBFs in an OWF in the Belgian part of the North Sea. We distinguished natural from anthropogenic-related fluctuations in macrofaunal communities by analysing a long-term dataset (1980–2012). The analysed sandbanks are characterised by sandy substrates and a community with low species abundance (180–812 ind m−2) and diversity (6–15 species per 0.1 m2). Strong temporal variations were observed possibly related to variable weather conditions in the area. Significant differences in community composition were observed due to the installation of six GBFs in the construction year of the OWF followed by a rapid recovery a year later and confirmed by the benthic ecosystem quality index BEQI. Even though the construction of GBFs creates a physical disturbance to the seabed, the macrobenthic community of these sediments have illustrated a fast recovery potential.

Marine phytoplankton biomass dynamics are affected by eutrophication, ocean warming, and ocean acidification. These changing abiotic conditions may impact phytoplankton biomass and its spatiotemporal dynamics. In this study, we used a nutrient–phytoplankton–zooplankton (NPZ) model to quantify the relative importance of the bottom-up and top-down determinants of phytoplankton biomass dynamics in the Belgian part of the North Sea (BPNS). Using four years (2014–2017) of monthly observations of nutrients, solar irradiance, sea surface temperature, chlorophyll-a, and zooplankton biomass at ten locations, we disentangled the monthly, seasonal, and yearly variation in phytoplankton biomass dynamics. To quantify how the relative importance of determinants changed along a near–offshore gradient, the analysis was performed for three spatial regions, i.e., the nearshore region (<10 km to the coastline), the midshore region (10–30 km), and the offshore region (>30 km). We found that, from year 2014 to 2017, the phytoplankton biomass dynamics ranged from 1.4 to 23.1 mg Chla m−3. Phytoplankton biomass dynamics follow a general seasonal cycle, as is the case in other temperate regional seas, with a distinct spring bloom (5.3–23.1 mg Chla m−3) and a modest autumn bloom (2.9–5.4 mg Chla m−3). This classic bimodal bloom pattern was not observed between 2003 and 2010 in the BPNS. The seasonal pattern was most expressed in the nearshore region. The relative contribution of factors determining phytoplankton biomass dynamics varied spatially and temporally. Throughout a calendar year, solar irradiance and zooplankton grazing were the most influential determinants in all regions, i.e., they jointly explained 38–65% of the variation in the offshore region, 45–71% in the midshore region, and 56–77% in the nearshore region. In the near- and midshore regions, nutrients were the greatest limit on phytoplankton production in the month following the spring bloom (44–55%). Nutrients were a determinant throughout the year in the offshore region (27–62%). During winter, sea surface temperature was a determinant in all regions (15–17%). By the high-resolution spatiotemporal analysis of the relative contributions of different determinants, this study contributes to a better mechanistic understanding of the spatiotemporal dynamics of phytoplankton biomass in the southern North Sea. This detailed understanding is anticipated to contribute to the definition of targeted management strategies for the BPNS and to support sustainable development in Belgium’s blue economy.

Enrichment and shifts in macrobenthic assemblages in an offshore wind farm area in the Belgian part of the North Sea

In the next 10–20 years, thousands of wind turbines will be present in the North Sea. In this paper, we investigate the impact of these windmill artificial reefs (WARs) on the ecology of benthopelagic fish. More specifically we will try to resolve the attraction-ecological trap-production issue for Atlantic cod and pouting at WARs and link the information to opportunities for fisheries activities. From 2009 until 2012 the behavioural ecology of Atlantic cod and pouting was investigated at WARs in the Belgian part of the North Sea (BPNS). Information on length-frequency distribution, diet, community structure and movements were combined to gain insights on the behavioural ecology and to unravel whether production occurs. We demonstrated that specific age groups of Atlantic cod and pouting are seasonally attracted towards the WARs, that they show high site fidelity and feed upon the dominant epifaunal prey species present. Growth was observed throughout the period the fishes were present. Production on a local scale can be assumed. On a regional scale however, no changes were observed yet. Based on the acquired knowledge we judged that no fisheries activities should be allowed inside the offshore wind farms in the BPNS.

Several bat species are known to migrate long distances between summer and winter roosts. During migration, many bats even cross the North Sea. The developments of offshore wind farms in the North Sea could therefore pose a collision risk for migrating bats. While bats have been observed inside offshore wind farms, their activity at turbine rotor height yet remains unknown. We therefore installed acoustic bat detectors at wind turbines in the Belgian part of the North Sea. Seven detectors were installed on the service platform of the transition piece (16 m above mean sea level) and four were installed on the nacelle of the turbines, in the centre of the rotor swept area (93 m above mean sea level). A total of 151 recordings of call sequences of Pipistrellus nathusii (Nathusius' pipistrelle) were made during 20 nights over an entire autumn migration season (8 August – 30 November 2017). 45 recordings contained more than 10 calls. These were further investigated for behavioural clues. We identified 32 recordings of animals in transit and 10 sequences of animals passing by while simultaneously exploring. Only three detections contained feeding buzzes and/or intense exploratory behaviour. The number of recordings at 93 m were around 10% of the number of recordings made at 16 m. This indicates that the activity of P. nathusii at our study site, measured at that particular altitude is low. Our observations therefore suggest that the collision risk might be lower than what could be expected from low altitude observations. However, a low number of recordings at nacelle height does not necessarily mean that only a low number of bats will collide with the turbines. The activity in the outer parts of the rotor swept zone, outside the detection range of our acoustic detectors, remains unknown and should be further investigated.

Simple SummarySome species of bats migrate over longer distances between their summer roosts and winter areas. During migration, many man-made obstacles, such as wind farms, can pose a collision risk for bats. As it is known that bats can fly over open sea during migration, and offshore wind farms can also be problematic. We studied the presence of bats during migration at several North Sea locations with the aim of understanding the weather conditions triggering bat migration at sea. Our results show a decrease in bat activity with distance from the coast and a correlation between bat migration and wind speed (negative), wind direction, temperature (positive), and atmospheric pressure (positive). Understanding these relationships can help in reducing the effects of offshore wind farms by periodically idling the blades when optimal meteorological conditions prevail and by opting for wind farm locations where bat activity is less prevalent.Bats undertaking seasonal migration between summer roosts and wintering areas can cross large areas of open sea. Given the known impact of onshore wind turbines on bats, concerns were raised on whether offshore wind farms pose risks to bats. Better comprehension of the phenology and weather conditions of offshore bat migration are considered as research priorities for bat conservation and provide a scientific basis for mitigating the impact of offshore wind turbines on bats. This study investigated the weather conditions linked to the migratory activity of Pipistrellus bats at multiple near- and offshore locations in the Belgian part of the North Sea. We found a positive relationship between migratory activity and ambient temperature and atmospheric pressure and a negative relationship with wind speed. The activity was highest with a wind direction between NE and SE, which may favor offshore migration towards the UK. Further, we found a clear negative relationship between the number of detections and the distance from the coast. At the nearshore survey location, the number of detections was up to 24 times higher compared to the offshore locations. Our results can support mitigation strategies to reduce offshore wind farm effects on bats and offer guidance in the siting process of new offshore wind farms.

Marine litter is recognized as a global environmental concern. Seafloor litter can provide important information to help assess the status of the marine ecosystem and is relatively easy to collect on a regular basis. The Belgian fisheries area covers different parts of the OSPAR Greater North Sea region and the Celtic Seas. In these regions, seafloor litter data were gathered by quantifying the litter items caught in the trawl net during two different fisheries surveys to investigate litter distribution on both regional and local scales. In the international beam trawl survey (BTS), covering essentially the OSPAR Greater North Sea and Celtic Seas, an average of 2.2 ± 0.05 items.ha-1 were caught with a median of 1.4 items.ha-1. In the environmental monitoring survey (EMS) only the Belgian part of the North Sea was covered and a smaller cod-end mesh size was used, resulting in 12.7 ± 1.7 litter items.ha-1 in the coastal zone (< 12 nm) and 2.8 ± 0.2 items.ha-1 in the more offshore zone (> 12 nm). In both surveys plastic items were predominant, representing up to 88% of the collected litter in the Belgian part of the North Sea. The impact of human activities at sea such as fisheries, sand extraction, wind farms and dredge disposal was investigated. A significant correlation was found between fishing activities and the amount of litter registered in the Belgian part of the North Sea, but not for the OSPAR Greater North Sea and Celtic Seas.

Marked changes in diatom and dinoflagellate biomass, composition and seasonality in the Belgian Part of the North Sea between the 1970s and 2000s

Seasonal and spatial fatty acid profiling of the calanoid copepods Temora longicornis and Acartia clausi linked to environmental stressors in the North Sea

In aquatic ecosystems, plankton communities generally form the base of trophic webs, and environmental changes are often reflected, or even amplified, in these communities. In the context of anthropogenic climate change, plankton communities could be drastically impacted by rising temperatures and the increase of extreme climate events. The Belgian Part of the North Sea (BPNS) is already heavily influenced by human activity, and extreme climate events could greatly alter the dynamics of the ecosystem. Recent years have seen an uptick in marine heatwaves in the BPNS, which have been associated with unusually prominent Bellerochea sp. blooms and temporary copepod die-offs. As the frequency of these marine heatwaves is projected to increase, it is necessary to gain an understanding of how phytoplankton and zooplankton communities may respond not only to stable temperature change, but also to rapid change. This research therefore aimed first to characterize BPNS marine heatwaves, and second to analyze the corresponding plankton community dynamics. Initially, 30 years of satellite data were used from the National Oceanic and Atmospheric Administration Optimum Interpolation Sea Surface Temperature dataset (NOAA OI SST V2 High Resolution Dataset). Temperature data was used to establish a 90th percentile threshold for marine heatwave detection. Marine heatwaves were then characterized based on intensity relative to the 90th percentile threshold, cumulative intensity (deg. C x days), duration, peak temperature reached, and timing of peak temperature. These results were compared with those from underway temperature measurements at ~3 m depth as well as with CTD temperature measurements, all collected on the RV Simon Stevin. Additionally, we defined and analyzed temperature regions based on both geographical and biological zones. For plankton data, samples were collected monthly (nine coastal stations) and seasonally (with eight additional offshore stations) on board the RV Simon Stevin, with zooplankton data from 2014 onwards and phytoplankton data from 2017 onwards. ZooScan and FlowCam automated imaging sensors were used to quantify zooplankton and phytoplankton, respectively. Plankton dynamics were then analyzed in terms of bloom timings and abundances (dominant groups, diversity indices). Overall temperature data from the BPNS showed similar marine heatwave trends using NOAA satellite data and RV Simon Stevin underway data, and highest temperatures were reached in the summers of 2018 and 2022. At several nearshore stations, the key plankton group Appendicularia had a delayed bloom during the 2022 marine heatwave compared to the 2018 marine heatwave. Ultimately, this parallel characterization of marine heatwaves and plankton dynamics offers insight into the health of the BPNS ecosystem. Furthermore, it offers potential for predicting the responses of phytoplankton and zooplankton to future marine heatwave events.