Abstract
The North Sea food web is a network of trophic relationships and ecological interactions between organisms such as plankton, fish, birds, and marine mammals. The food web can be altered by environmental changes resulting from factors such as large-scale offshore wind farm construction, changes in fishing pressure, eutrophication, climate warming, as well as nature restoration. These changes in the ecosystem may benefit some species while negatively affecting others, leading to shifts in species interactions and, consequently, changes in the food web. In this project, the potential additional value for marine management of applying relevant food web indices to monitor changes in food webs will be investigated. When analysing food webs, it is necessary to consider how food web components vary across space and time. By applying food web indicators to assess the environment, the functional role of species within an ecosystem or food web can be clarified. Taking differences regarding functional roles or sensitivities of the included species into account, habitats where food web dynamics are more vulnerable may be identified. This may help management protect species and ecosystem functions and stimulate development of ecosystem-based management according to visions for nature and responsible resource use. The main research questions for this study are: • How can marine food web assessments be used to support ecosystem-based management and guide environmental policy? • Which food web indicators are most suitable for assessing the impacts of human activities and environmental change on marine food webs, including both bottom-up- and top-down-effects? • Which indicator species or functional groups can serve as effective indicators of food web change, and how can they be integrated into management and policy frameworks? For ecosystem assessments (e.g. under MSFD), the use of simplified food‑web indicators requires explicit decisions on food‑web boundaries and aggregation levels. Complex food‑web approaches, such as applying Ecopath with Ecosim modelling, Ecological Network Analysis and cross‑habitat connectivity analyses, can be used to inform on boundaries and aggregation levels by identifying the functional components, trophic pathways, and spatial linkages that are essential for maintaining food‑web structure and functioning. In this way, complex indicators can support the development of policy‑ready indicators that remain sensitive to pressures on trophic interactions, thereby strengthening ecosystem‑based management and the assessment of Good Environmental Status. In chapters and 3, we outline the principles of North Sea food web categorization in the context of MSFD Descriptor 4 ‘Food webs’. Operational food web indicators are listed, categorized, and evaluated in chapter 4, where recommendations of complementary indicators are given and motivated. In chapter 5, we present two contrasting case studies that illustrate the added value of complex food-web indicators. In the Dutch Voordelta, such indicators can reveal changes in benthic-pelagic coupling that may be masked by functional redundancy and apparent stability at higher trophic levels. In the Dutch coastal zone and wider North Sea, they can help identify ecological mechanisms underlying seabird population trends, including changes in prey availability, trophic energy transfer, and connectivity between habitats, when changes in the food web can otherwise remain undetected when shifts in species composition and distribution balance each other out. Together, these case studies show that complex food-web indicators provide mechanistic insight that complement simpler policy indicators. They can reveal trophic bottlenecks, distinguish ecological resilience from compensatory dynamics, and provide early warning signals of ecosystem change, thereby improving the interpretation of environmental status and strengthening the ecological basis for ecosystem-based management. While the relatively strong monitoring available in these systems enables detailed food-web analyses, challenges remain in quantifying interaction strengths and disentangling trophic effects from environmental drivers such as climate change, hydrodynamics, and species redistribution.