Spatial variability in benthic macrofauna communities and associated ecosystem functions across coastal habitats

The rapid rates of global biodiversity loss and the serious anthropogenic pressures currently affecting our marine ecosystems have increased the interest in and importance of understanding the role of biodiversity for ecosystem functioning. It is well known that biodiversity contributes to a plethora of ecosystem functions, and that benthic macrofauna have an influence on several functions that sedimentary ecosystems provide. Biodiversity-ecosystem function relationships can however be very context dependent, which complicates our ability to generalize on the role of biodiversity and to predict the consequences of environmental change for ecosystem functions and services. In this thesis, biodiversity-ecosystem functioning (BEF) relationships were examined in a variety of field studies in different ecosystems to elucidate the context-dependence of these relationships in heterogeneous real-world coastal zones. This included testing the effects of hypoxia, habitat characteristics and background biodiversity on the link between the fauna and the functioning in terms of benthic nutrient recycling. Hypoxia is a serious threat both in open and coastal waters, and its consequences for the benthic macrofauna-function link was investigated through sampling along natural gradients of increasing hypoxia, in a low-diversity as well as a higherdiversity coastal system. In both systems, the benthic macrofaunal communities were clearly decimated with declining oxygen conditions, although some species did show a higher tolerance of low oxygen concentrations. These species were likely important for the solute fluxes as long as the species could prevail, but during severe hypoxia and anoxia nutrient recycling processes were heavily altered, with markedly larger effluxes of ammonium and phosphate. A large variation in macrofaunal communities and solute fluxes was also observed between normoxic sites, with indications that even small variations in sediment organic material content and carbon/nitrogen ratio affected the relationships. The effects of environmental context in mediating ecosystem functioning were further assessed through sampling 18 sites on a gradient of grain size, from coarse sand to silty sediments, with varying organic material content and vegetation. Benthic macrofaunal community abundance, biomass and species richness was higher in coarser sediments and in habitats with more vegetation. Biomass and abundance of a few dominant species together with organic content, amount of roots and vegetation were indicated to contribute to nutrient recycling processes across the sedimentary gradient. Closer analyses suggested that the benthic macrofauna had a stronger influence on the ecosystem functions in muddy and medium sediments than in sandy sediment, despite the richer communities in the sandy sediments. Species redundancy is hypothesised to be important for the stability of ecosystems. Therefore redundancy patterns over space and their ability to reflect natural biodiversity-ecosystem function relationships across an extensive sandflat were explored. Redundancy over space was observed within the investigated functional groups, but the dominant species were indicated to drive the spatial distribution of the functional groups and the ecosystem functioning. In summary, these correlative field studies indicated that abundance and biomass of benthic macrofauna are important for nutrient recycling processes at the sediment-water interface, but the relationships are significantly mediated by environmental context. Hypoxic conditions in coastal zones are especially problematic because the heavily altered nutrient recycling processes and decimated macrofauna communities, can have severe consequences for overall functioning of the ecosystems. Furthermore, a few dominating species were suggested to be especially important for the investigated ecosystem functions regardless of large variations in species richness across studies. Hence, it is important to consider natural variability, as well as several measures of biodiversity, not only species richness, in BEF studies in order to obtain a more realistic understanding of the biodiversity-ecosystem function relationships in heterogeneous coastal areas. An improved understanding of the complex links within coastal ecosystems is a prerequisite for improved management and conservation.