Ecological impacts of offshore structures on North Sea biodiversity and food webs

Offshore structures including oil and gas (O&G) platforms and offshore wind turbines induce spatially and temporally extensive anthropogenic changes that profoundly influence the North Sea ecosystem. However, there are currently inadequate understandings of the biodiversity and food webs associated with offshore structures and their likely responses to future changes. This limits the development of ecologically sound management policies to address emerging issues such as decommissioning and the expansion of offshore renewables. This thesis utilises a series of approaches to assess ecological impacts of offshore structures on the North Sea biodiversity and food web structure. First, the links between O&G-associated sediment contamination and benthic invertebrate biodiversity and food web structures are assessed in a before-after control- impact (BACI) design, highlighting loss of larger individuals, shorter food chains and simpler food webs with more dietary generalists in the high contamination areas. Second, three O&G decommissioning scenarios are analysed to assess benthic ecological recovery, showing a non- linear trajectory from reduced diversity and food web complexity in the short term to a greater density of trophic interaction and more intermediate species in the long term. Third, the effects of both O&G platforms and wind turbines on marine fish biodiversity and food web responses are examined at the North Sea scale using Bayesian INLA models, revealing that wind turbines benefit total fish biomass. The study shows the biggest piscivores are negatively affected whilst small planktivores benefit. Cumulative effects of both structures in the past three decades were estimated to inform cumulative changes at each fish response. This thesis synthesises the major contributions of new insights in the context of wider application and informing future decision-making process and provides future research directions for biodiversity and food web studies in the context of offshore structures.