Structural changes in marine ecosystems – the application of novel network approaches to understand marine regime shift

Abstract: Anthropogenic drivers, such as climate change and fishing, have severe, often unexpected impacts on marine ecosystems. Species responses to drivers may have cascading effects through ecological communities, potentially changing the structure and functioning of the food web. The aim of this thesis is to examine how the structure and functioning of marine food webs change over time and space when exposed to perturbation. Applying novel structural network analysis tools, I studied changes and consequences of marine regime shifts in a regional (Baltic Sea) and global context.The study of ecological network analysis indicators on the long-term food web dynamics describes the late-1980s abrupt regime shift as well as two topologically different food web states in the central Baltic Sea. However, using a larger food web and examining the change in ecological functioning as represented by motifs in Exponential Random Graph Modeling (ERGM), no change in dominant ecological processes was found in the central offshore Baltic Sea region, despite the food web reorganization. On the contrary, the study suggests that the coastal region is currently significantly altered. These results expand the knowledge of the Baltic Sea food web responses to major drivers and show the role of the coastal zone as the interface ecosystem between land and sea, indicating potentially increased vulnerability to functional change in the offshore Baltic Sea.The global analysis on the co-occurrence of drivers potentially causing regime shifts identifies three clusters of drivers primarily responsible for all marine regime shifts. The findings indicate sets of drivers that should be prioritized in management through coordinated actions across multiple scales, and support the notion that sets of ecosystem services change simultaneously.Examining the impacts of large-scale changes in marine systems from the perspective of network science, this thesis argues that marine resource management needs to account for multiple stressors and their synergistic effects on species interactions in an ecosystem context so that marine resources can support ecosystem services for the future.