From macrophytobenthos to invertebrates: Community responses across coastal habitats in the Northern Baltic Sea

The Baltic Sea is an enclosed brackish water body with an extensive coastline. Due to an isostatic land uplift, the northern Baltic Sea archipelago consists of geological features like islands, lagoons, straits and inlets. As such, the seascape is made up of a mosaic comprised of different vegetated habitats such as Fucus vesiculosus belts, Zostera marina meadows, charophyte meadows, and Phragmites australis beds. These shallow coastal areas are very productive habitats and offer a wide range of ecosystem services. Apart from acting as primary producers, macrophytobenthos also provide structural complexity and act as nursery- and spawning grounds for higher trophic levels like fish and invertebrates, thereby increasing secondary production. These vegetated habitats, however, are subject to anthropogenic drivers, like eutrophication and climate change, as well as natural drivers, like wave exposure, sediment grain size and ecological interactions, which change the invertebrate abundance and the community composition.

In this thesis, I aim to investigate the effects of these drivers on the Baltic Sea macrophytobenthos and its associated invertebrate communities by applying a seascape ecology approach, which incorporates temporal and spatial scale changes. Temporal changes were examined by (I) mapping the macrophytobenthos at an exposed Island in the Northern Baltic Sea and comparing the species composition with those from earlier studies (1956 & 1993) at the same location. The effects of large-scale drivers (wave exposure and nutrient levels) and small-scale drivers (sediment properties, distance from the seaward edge of the reed bed, stalk density and epiphyte biomass, rhizome biomass and associated macrophytobenthos species) on the invertebrate community were examined. This was done by sampling the invertebrate community in (II) shallow, soft-bottomed bays around the Åland Islands and (III) coastal P. australis beds around the Åland Islands and the Archipelago Sea and analysing their effects on the community composition, total abundance, taxon richness and diversity.

My results show that the macrophytobenthos community underwent substantial changes in 62 years (between 1956, 1993, and 2018). While filamentous brown and green algae increased in percentage cover, the depth range of F. vesiculosus and overall species richness declined. These temporal changes are indicative of eutrophication and climate change. Regarding spatial changes in the invertebrate community, it is clear that the large-scale drivers, wave exposure or isolation and nutrient levels are the most impactful in shaping the invertebrate community, affecting the species composition, abundance, taxon richness, and diversity. The effects of the small-scale drivers are, however, far more bays), from which macrophytobenthos species, the invertebrate community was sampled appeared to be of little effect. However, when examined on a smaller within-bay scale, the associated macrophytobenthos species did affect the composition of the invertebrate community. Other small-scale drivers, like sediment grain size, distance from the seaward edge of the reed bed, stalk density and epiphyte biomass, appeared highly region-dependent with varying effects on the invertebrate community.

The effects of wave exposure and nutrient levels on Baltic Sea invertebrate communities are well known. This thesis demonstrated that their impacts also affect shallow, soft-bottomed bays and coastal reed beds. However, the more intricate responses of the invertebrate community to the small-scale drivers invite the application of more broad-scale seascape ecology-based studies to unravel geographical differences and interaction between these drivers to allow for better ecological understanding and more applied management of Baltic Sea habitats.