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.

The need to understand patterns and processes in the marine environment has never been so profound as today, particularly as anthropogenic pressures upon coastal regions are drastically affecting habitats and species across a vast range. One approach to further understand these patterns and processes is through the use of seascape ecology methods. Pertinently, fish are ideal candidates to use in many seascape ecological studies due to their mobility and potential to connect a multitude of patches and habitats throughout their life cycle. They also serve as fundamental components in coastal food webs and are of economic benefit. This thesis strives to answer how fish assemblages are affected by ecological and environmental patterns and changes in temperate seascapes throughout the Swedish Skagerrak and the Baltic Sea. 

Initially, the spatial arrangement of benthic habitat patches in coastal Skagerrak was investigated in relation to the fish community inhabiting seagrass meadows. Seascape structure and complexity was shown to create optimal or sub-optimal areas for certain parts of the fish community. For instance, simpler seascapes (e.g. less habitat patches and edges) were found to have a higher density of juvenile fish, while wrasse densities were related to more complex seascapes. This offers insights into the consequences of spatial patterning in the marine environment and possible effects of habitat loss in the ecosystem (paper I). Through surveying fish assemblages in common, shallow-water habitats, the more structurally complex habitats, i.e. seagrass and macroalgae, were found to harbour a greater fish abundance compared to the less complex unvegetated soft bottoms. However, all three habitats were deemed important for their role in supporting juvenile fish species, thus suggesting that embayments in this environment might function as seascape nurseries (paper II). The importance of connectivity of a marine predator was discovered using acoustic telemetry and network analysis. This study demonstrated that sea surface temperature was of major importance for Atlantic cod movement dynamics within a fjord system as well as revealing the significance of localised connectivity at varying spatial and temporal scales (paper III). Finally, spatial pattern relationships and fish assemblages were explored in Baltic seagrass meadows. Fish assemblages were dominated by meso-predators (i.e. three-spined stickleback) both during summer and autumn, with a noticeable lack of larger piscivorous species throughout both seasons. Correlative analysis showed that fish densities were influenced by seagrass habitat structure (negatively), area of bare sediment (negatively) and habitat patch diversity (positively) (paper IV).  

This thesis has lifted a central role in addressing important seascape ecology questions and tools in the temperate marine environment. Specifically, it highlights the importance of analysing patterns and processes at multiple scales to gain a more comprehensive understanding of the relationships between fish and their environments, which is relevant for marine spatial planning and conservation.