Swedish seagrass ecosystems in a changing climate : Coastal connectivity and global change sensitivity

Sammanfattning: Coastal shallow-water ecosystems are essential for providing several goods and services globally, with seagrasses as an important contributor for maintaining high biodiversity and productivity within the nearshore seascape. The temperate species Zostera marina serves as a vital habitat for many species, including ecologically and economically important juvenile fish. Ecological patterns and processes within the shallow-water seascape are driven by a multitude of factors, for instance food-web dynamics, species interactions, habitat configuration, oceanographic hydrodynamics, and influenced by human impacts, all occurring at different spatial and temporal scales. A complex interworking of abiotic and biotic processes takes place within the coastal environment with the system expected to be impacted by future climate changes. This scientific work contributes to the ecological understanding of coastal marine ecosystems by examining connectivity and disturbance effects on multiple spatiotemporal scales. The thesis consists of two main themes: 1) evaluation of the influence of seascape structure on seagrass fish communities at different scales, and 2) understanding species’ physiological responses to multiple global change stressors in Z. marina meadows, and the regional implications of these results. The work focused on temperate Swedish coastal waters. To address these themes a variety of methods were performed including a seascape ecology field approach, experimental laboratory work and spatial modeling. The results contribute to the understanding of seascape connectivity and the impact of disturbance from climate-related stressors on the shallow-water ecosystem and associated fish communities. The results highlight the importance of evaluating fish assemblages at multiple spatial scales, from within-meadow characteristics to region-wide geographical features. Generally, fish with higher site fidelity were found to be influenced by smaller scale (meters) habitat characteristics, while broader ranging, more migratory species showed impacts on a larger scale (kilometers). It was also shown that the shallow-water environment has a fish assemblage overlap, with the same species found within multiple coastal habitats, dominated by juvenile fish (in summer), thus constituting a shallow-water seascape nursery. Regarding the consequences of global change the thesis showed that, while individual global change stressors can have either positive, negative or neutral affects depending on the species in question, all trophic levels of the mesocosm study showed a deleterious stress response to multiple stressors combined. With the significance of these laboratory results in mind, the final risk assessment identified three high-risk regions for seagrass meadows along parts of the Swedish coast that are expected to be exposed to a high degree of change from multiple coinciding global stressors by the end of the century.