Among-fjord comparison of species and trait composition reveals the influence of dispersal and stochasticity on macrobenthic community assembly

Fundamental ecosystem properties are often linked to biodiversity. Biodiversity can be analysed from a taxonomic point of view as a measure of coexisting species, and from a functional point of view as the range of functional niches occupied by these species. Biological traits can serve as a proxy between the 2 approaches, as they are related to resource use and the functional niche of a species. Macrobenthic functional diversity and community assembly have been rarely addressed in fjords, which limits our understanding of ecosystem processes in these depositional habitats. In the present study, we combined data on 8 deep (290-725 m) sub-Arctic basins in northern Norway to assess the community differentiation of neighbouring fjords. There was a significant difference in species composition between fjords with deep (>200 m) and shallow sills (26-120 m), which were characterised by distinct bottom water masses. Moreover, further community differentiation was observed primarily at the basin scale, with occasional extension into adjacent basins. Biological trait composition revealed that, generally, basins with shallow sills have a higher proportion of deep-dwelling subsurface deposit-feeding fauna, indicating differences in benthic functioning. We suggest that differentiation on a basin scale regarding species composition may result from stochastic community assembly, low connectivity among basins, and, in particular, the presence of dispersal barriers (shallow sills), while water exchange with surrounding waters mediates organic matter fluxes to the seafloor and, consequently, the functional structure of macrobenthos. Overall, our findings suggest that stochastic and deterministic processes in the community assembly might operate simultaneously, but on different levels of community organization.