Biodiversity patterns and blue carbon characteristics of Zostera marina meadows in the Oslofjord

Sammendrag

Seagrass meadows are coastal foundation habitats that support biodiversity and store organic carbon in sediments, a process often referred to as blue carbon storage. However, both biodiversity and sediment carbon storage can vary strongly among meadows, and it remains unclear how local meadow structure, hydrodynamic exposure, organic-matter sources, and associated fauna interact in temperate eelgrass systems. This thesis investigated biodiversity and blue carbon characteristics of Zostera marina meadows in the Oslofjord, Norway. The study combined two complementary approaches: citizen-science-based environmental DNA (eDNA) water sampling across eelgrass meadows and seasons, and researcher-led field sampling at three meadows along an inner-outer Oslofjord gradient. The field-sampled meadows represented contrasting coastal settings: an inner-fjord meadow at Fornebu, a sheltered meadow near Sætre/Sandspollen, and a more wave- exposed outer-fjord meadow at Holmestrand. The water eDNA dataset provided broad spatial and seasonal coverage, but ecological interpretation was limited by uneven sampling and read-depth loss after filtering. After taxonomic partitioning of the cytochrome c oxidase I (COI) metabarcoding dataset, only 20.5% of reads were retained as faunal reads, while 79.5% were classified as non-faunal reads. The retained faunal water eDNA dataset did not show clear winter-spring or Oslofjord-section structure. Instead, it contained a mixed coastal signal, including pelagic planktonic taxa and benthic or soft-sediment-associated taxa. In contrast, the non-faunal fraction showed clearer winter-spring structuring, largely associated with microalgal and protist groups. This indicates that broad COI water eDNA can capture useful biodiversity signals, but also that faunal and non-faunal datasets should be separated before ecological interpretation. The strongest ecological patterns were found in the field-measured meadows. The sheltered Sætre/Sandspollen meadow had continuous eelgrass cover, low sediment bulk density, high and consistent organic carbon stocks, and the highest apparent seagrass-like carbon contribution. The more wave- exposed Holmestrand meadow was patchier and had higher bulk density, lower organic carbon stocks, and the lowest apparent seagrass-like carbon contribution. The inner-fjord Fornebu meadow had intermediate sediment carbon stock, bulk density, and apparent seagrass-like carbon contribution, but showed greater within-meadow variability. Stable isotope composition supported these meadow-level differences and indicated variation in both carbon-source characteristics and assimilated nitrogen among meadows. Benthic eDNA showed clear separation between seagrass-blade epifaunal communities and sediment-core infaunal communities. Infaunal communities were most strongly associated with sedimentary gradients and were dominated by sediment-associated annelids, including lugworms and other ii taxa linked to deposit feeding, burrowing, sediment reworking, and bioirrigation. In contrast, the epifaunal signal was dominated more strongly by sessile or surface-associated taxa, including barnacles, bryozoans, and hydrozoans, rather than by mobile mesograzers. Overall, this thesis shows that eelgrass meadows in the Oslofjord can differ substantially in meadow structure, sediment carbon characteristics, stable isotope composition, and associated benthic fauna. The results support a descriptive interpretation in which blue-carbon characteristics and benthic biodiversity vary together at local meadow scale, particularly through the association between sediment conditions and infaunal community structure. Broader inference about Oslofjord-wide seasonal biodiversity patterns will require more balanced repeated sampling but the study demonstrates the value of combining citizen-science water eDNA with targeted field measurements when assessing biodiversity and blue carbon in eelgrass meadows.