Changes in ocean conditions influenced by climatic fluctuations have lead to changes in individual species distributions, which alter the diversity, communities and species interactions across marine ecosystems worldwide. Assessing the species composition and identifying regions and habitats that can safeguard the persistence of biota are critically important. In this dissertation, I present multiple analyses on two long-term fishery–independent survey datasets; one along the Skagerrak coastline of southeastern Norway, and another in the epipelagic zone of the Northern California Current in the northeastern Pacific Ocean. Chapter 1 presents an overall discussion of general biodiversity, and how community ecology in marine ecosystems can be integrated into marine ecosystem based management.
Chapter 2 presents an analysis of an assemblage of juvenile nearshore fishes based on nearly eight decades of highly standardized Norwegian survey records. Using multivariate statistical techniques, I (i) characterize the change in taxonomic community composition through time, (ii) determine whether there has been an increase in warm-water affinity species relative to their cold-water affinity counterparts, and (iii) characterize the temporal changes in the species functional trait assemblage. The results strongly indicate a shift toward a novel fish assemblage between the late 1990s and 2000s. The context of changes within the most recent two decades is in stark contrast to those during the 1960s and 1970s, but similar to those during the previous warm period during the 1930s and 1940s. This novel assemblage is tightly linked to the warming temperatures in the region portrayed by the increased presence of warm-water species and a higher incidence of pelagic, planktivorous species. The results indicate a clear influence of ocean temperature on the regions juvenile fish community that points to climate-mediated effects the species assemblage of important fish nursery area.
Chapter 3 addresses the current interdisciplinary challenge of effectively tracking multiple facets of biodiversity of the marine ecosystem from a remote perspective with the ultimate goal of developing ecosystem indicators for management. In this study, I use generalize additive mixed models to quantify the relationship between spatially and temporally explicit community data using both in-situ and remotely sensed oceanographic data over three different months and across 10 years. From the modeling approach as well as with matchups of satellite data with species specific positive catch locations, I show the utility of using MODIS-Aqua Rrs555 data field in order to understand the distribution of higher trophic levels community data associated with freshwater input into this region. Using these community gradients projected onto satellite data, I develop a new community-level temporally and spatially explicit indicator that assesses variations in the epipelagic community. This index of community differences is useful for regional ecosystem condition reports and could be useful within a broader ecosystem based fisheries management context.
Chapter 4 presents results from a study using a unique and long-term (1999-2015) standardized survey occurring in the month of June of pelagic marine and anadromous species off Oregon and Washington in the northern California Current. The objectives of this study were to identify geographic locations with a stable species compositions off of Oregon and Washington that are also relatively buffered from climate fluctuations observed over the past two decades. Specifically, I used species richness and a metric of temporal beta diversity (change in species composition) to assess locations with high species richness and community persistence relative to local and basin-scale environmental fluctuations. Also, using spatially variant generalized additive mixed models, I identified areas with species communities that are more influenced by basin-scale climatic fluctuations than others. I have identified a specific region along the continental shelf off Washington State to be a potential climate change refugia for pelagic nekton and some gelatinous taxa. This potential climate change refugia off of Washington contrasts with adjacent areas to the south (e.g. the Newport Hydrographic transect) and offshore regions that have generally lower species richness, and higher temporal change in species composition. I propose that upwelling regions with retentive topographic features, such as wide continental shelves, can function as marine refugia for pelagic fauna, whereas offshore locations are potentially more climatically sensitive and experience higher temporal change in species composition.
Finally, Chapter 5 includes the main conclusions of this thesis and provides discussion of other studies that could follow.