Clonality, connectivity, and conservation: Genomic insights into eelgrass (Zostera marina) across the Baltic and Scandinavian Seas

We are currently facing an extreme loss of biodiversity on a global scale, affecting many species. A critical aspect of this crisis is the erosion of genetic diversity, as it provides insight into a species adaptive potential in a changing environment. Investigating the genetic variation, particularly of foundation species (species that provide a habitat for many other species), is therefore important. The marine flowering plant eelgrass (Zostera marina, Linnaeus 1753) is a habitat provider in the Northern Hemisphere, sustaining a large number of associated species, and plays a significant role in sediment stabilization and carbon sequestration. This thesis aims to explore genetic variation, clonality, connectivity, and aspects of conservation of eelgrass along the Scandinavian coastlines. To achieve this, population genomic analyses were conducted at local, countrywide, and across- country levels covering wide areas of the eelgrass distribution in Norway, Sweden, Germany, Estonia, and Finland. Patterns of genetic variation and differentiation were examined, along with the environmental factors driving this variation. On a local scale (1s - 10s km) within Sweden, we found genetic differences between eelgrass meadows in sheltered compared to exposed sampling sites. Expanding to a regional scale (10s - 1,000s km), eelgrass meadows in the Skagerrak and Kattegat exhibit higher genetic diversity and lower clonality compared to those in the Baltic Sea, and at the species northern distribution limit in Norway. This pattern appears to be driven by the extreme environment of the Arctic and the Baltic Sea, with cold temperatures and short summers in the Arctic and the salinity gradient in the Öresund acting as a barrier to gene flow into the Baltic Sea. While the increased clonal reproduction can be a strategy for surviving in extreme environments, it can also lower genetic diversity and increase vulnerability. The thesis therefore further addresses how clones from facultative reproductive organisms influence population dynamics, genetic diversity, and the adaptive potential. Across the study area, genetic diversity within meadows increased when clones were included, suggesting that somatic mutations may contribute to adaptation, particularly within the isolated Baltic and northern Norway eelgrass meadows. Based on these results, more frequent application of direct comparisons between clone-included and clone-removed datasets is required. For conservation, meadows with high genetic diversity should be prioritized and monitored for shifts towards dominant clonal reproduction, which can signal environmental stress and a loss of adaptive potential.