Investigating shifts in epifaunal communities in Zostera marina meadows and Fucus serratus beds in the Norwegian Skagerrak

Abstract

Coastal ecosystems are under increasing pressure from climate change and other anthropogenic stressors, including along the coast of Norway. Seagrass meadows and macroalgal beds are two highly productive coastal ecosystems that provide valuable ecosystem services, including carbon sequestration, nutrient cycling, shoreline stabilization, and play a crucial role in supporting marine biodiversity. In coastal ecosystems, epifauna living among these macrophytes are essential in driving ecosystem and trophic functioning, however epifauna communities are still understudied in Norway. In this study we resampled epifauna in two stations (both with subtidal Z. marina and F. serratus) along the Norwegian Skagerrak in 2023 that were first sampled in 1999, looking at faunal composition, potential shifts, and the effect of certain environmental variables (temperature, exposure, salinity, nutrients, and Secchi depth). Significant shifts in the epifaunal communities of both Z. marina meadows and F. serratus beds were found. In 2023, both macrophytes had considerably lower densities of Mytilus edulis, with a mean decrease of 95%. Concurrently, there were substantially higher densities of gastropods (mean +440%) and amphipods (mean + 400%). This shift suggests a possible regime change in the dominant species within these communities, possibly as a result of higher temperatures, lower salinities, and increased coastal darkening. Despite changes in, the trophic network and biodiversity in these ecosystems in Norway seems mostly in relatively good health, compared to similar studies in Sweden. However, faunal shifts here and in other areas, indicate broader ecological changes in the Skagerrak area. By revisiting historical sampling sites, this thesis provides valuable insights into the responses and dynamics of epifaunal communities in the Norwegian Skagerrak when faced with increased anthropogenic stressors, such as climate change and overfishing.