In the North Sea, the offshore wind farm (OWF) industry is rapidly developing, with new wind farms being under construction, licenced or planned. These installations induce changes to the marine environment, by adding artificial hard substrates into soft-bottom areas. Multiple vertebrate and invertebrate species are attracted to these structures altering the biodiversity, arising concerns about the impacts of OWFs on the environment, including effects on ecosystem functioning.
In this PhD thesis, the effects of offshore wind turbines on the local food web properties were investigated at two levels: (a) detailed food web structure on one gravity-based foundation (Chapters 2 and 3), and (b) local (Belgian part of the North Sea) effects on primary productivity and on fish (Chapter 4 and 5). Colonising assemblages along the entire depth gradient of a gravity-based foundation in the Belgian part of the North Sea (BPNS) and fish species in close proximity to the same turbine were sampled to get insights in the in situ food web structure. Laboratory experiments with fully colonised PVC panels obtained from a tripod inside an OWF in the BPNS allowed for detailed ex situ observation of the carbon assimilation by colonising species.
In Chapters 2 and 3, the food web structure of the colonising assemblages along the depth gradient of the offshore wind turbine, its scour protection layer (SPL) and the surrounding soft substrate were investigated. The objective of Chapter 2 was to investigate whether structural differences in community composition would be reflected in the food web complexity and whether resource partitioning could contribute to the co-existence of such dense communities. This chapter, therefore, focused on the local food web properties between and within different communities along the depth gradient to investigate resource partitioning at both levels (i.e. community-specific study). The aim of Chapter 3 was to investigated whether trophic plasticity is one of the mechanisms contributing to the wide distribution and survival of invertebrate organisms occurring at different depths along the depth gradient of the gravity-based foundation (i.e. species-specific study). For the purposes of these two chapters, stable isotope analysis was performed on the organisms collected from the turbine foundation. The results (Chapter 2) showed that structural community differences are reflected in the food web structure of communities occurring in different depth zones. Resource partitioning was detected both between and within the assemblages. The highest food web complexity was found at the SPL and the soft substrate, where a strong accumulation of organic matter is expected. This high food web complexity was further supported by the results of the species-specific study (Chapter 3), demonstrating that the organisms occurring in these two zones exploited a wider range of resources compared to the organisms found higher up at the turbine. Moreover, this study indicated that most of the investigated invertebrate species found at the offshore wind turbine are trophic generalists, with depth-specific resource use strategies. This suggests that trophic plasticity contributes to the co-existence of invertebrate species within and across the depth zones. In these two chapters, the importance of the SPL and the soft substrate near the turbine foundation is highlighted, since in these zones high resource quantity is accumulated, supporting the presence of organisms of multiple trophic levels.
In Chapter 4, the carbon assimilation by colonising assemblages that typically occur at offshore wind turbines in the North Sea was investigated. While the role of colonising species in reducing the primary producer standing stock has previously been modelled for the southern North Sea, real data to validate these models are still scarce. Therefore, an ex-situ pulse-chase experiment was conducted to track the processing of organic matter by colonising organisms on the wind turbine foundations. The results indicated that the blue mussel Mytilus edulis showed the highest biomass-specific carbon assimilation, while the local amphipod Jassa herdmani population as a whole assimilated the highest amount of carbon. This study showed that J. herdmani and M. edulis contributed the most to the local consumption of the primary producer standing stock, since their assimilation was ca. 97 % of the total faunal carbon assimilation. The results of this experiment were upscaled to the total number of all the currently installed turbines (264 monopiles, 48 jackets, and 6 gravity-based foundations) in the BPNS. The total amount of carbon assimilated by the total number of J. herdmani and M. edulis individuals on every type of foundation was calculated and compared with the total annual primary production in the BPNS. It was estimated that 1.3 % of the local annual primary producer standing stock is grazed upon by M. edulis and J. herdmani. This value was compared with the amount of carbon that is not assimilated by the soft sediment macrofauna due to the loss of habitat by the installation of the different foundation types. The data suggest that the presence of offshore wind turbines and their subsequent colonisation by colonising fauna remarkably increases the carbon assimilation compared to the permeable sediment macrofauna inhabiting the same surface area (i.e. footprint of the turbines). As compared to the soft sediments, the presence of jacket foundations causes the highest and the gravity-based foundations the lowest increase in the local carbon assimilation.
Chapter 5 aimed at understanding the feeding ecology of fish species that are attracted to artificial reefs, such as OWFs, in the BPNS. Two pelagic (Scomber scombrus and Trachurus trachurus), two benthopelagic (Gadus morhua and Trisopterus luscus) and one benthic (Myoxocephalus scorpioides) species abundantly present close to a gravity-based foundation were sampled. Stomach content and stable isotope analyses were performed to respectively investigate the short- and the long-term dietary composition of these fish species. Both short- and the long-term analyses showed that the benthic and benthopelagic species (species highly associated with the SPLs) feed on the colonising species J. herdmani and Pisidia longicornis. These results imply that these species utilize artificial reefs, such as OWFs, as feeding grounds for a prolonged period. The short-term dietary analysis of Trachurus trachurus indicated a diet based on J. herdmani, but the long-term analysis suggested that this species feeds on zooplankton. Thus, this species feeds only occasionally on the colonising fauna, using the artificial reefs as oases of enhanced resources. Scomber scombrus in contrary does not utilize the artificial habitats of OWFs as feeding grounds at all, since both analyses indicated a diet based on zooplankton. The dietary results of this chapter on the benthic and benthopelagic species corroborate the hypothesis that their local production could potentially be increased. However, this study did not support such statement for the pelagic species.
In conclusion, OWFs do influence the local food web properties, with the occurrence of colonising organisms slightly lowering the water column primary producer standing stock, but also being an important resource for organisms of higher trophic levels. Altogether, the major role of suspension feeding organisms with key roles M. edulis and J. herdmani was highlighted in this thesis. These organisms were responsible for the reduction of the local annual primary producer standing stock and they increased the local food web complexity, mainly through their biodeposition process. Furthermore, the importance of SPLs as newly introduced habitats was highlighted throughout this thesis: (a) a high food web complexity was found in this area; (b) their invertebrate species exploit a wide range of resources; and (c) fish species associated with SPLs remain in the area to feed for a prolonged period. Moreover, an over-representation of trophic generalists and an under-representation of trophic specialists were observed, suggesting that more generalist organisms will occur in the North Sea in the future due to the development of more OWFs. Finally, it was shown that the introduction of jacket foundations causes the highest increase in carbon assimilation compared to the other types of foundations, reducing significantly the carbon content of the water column. From the above, it is implied that foundations with SPLs, such as gravity-based and monopile foundations, are possibly more beneficial for the local food webs, but further investigation is necessary to completely understand the effects of OWFs on the marine food webs.