Linking marine benthic biodiversity and ecosystem functions related to carbon cycling in a continental mud depocenter

Abstract. The importance of carbon storage in continental seafloor sediments is increasingly recognized, yet the role of benthic macrofaunal biodiversity in the regulation of these processes remains poorly understood. Benthic macrofauna contributes to organic carbon cycling through respiration and secondary production, while the sediment reworking (bioturbation) and ventilation (bioirrigation) of infauna promote the redistribution and remineralization of organic matter in sediments. Here, we investigated how benthic community structure, functional traits, and the relationship between biodiversity and ecosystem functions related to carbon cycling vary along environmental gradients in muddy sediments of the southeastern North Sea. Based on 171 macrofaunal taxa collected from 50 stations, a cluster analysis revealed a clear spatial structuring of the benthic macrofauna communities across the study region. The community composition was primarily structured by bottom shear stress, salinity, and sediment characteristics. Further, a functional trait analysis showed a clear shift in community composition with water depth. Communities in the deeper sections of the study area were dominated by mobile biodiffusors and subsurface filter feeders, whereas shallower communities were characterized by less mobile, surface-modifying bivalves and polychaetes. These contrasting patterns led to pronounced differences in ecosystem functioning: bioturbation and bioirrigation potentials were significantly higher in deeper communities, whereas community secondary production and respiration were higher in shallow communities. Across all stations, community secondary production and respiration increased with taxonomic and functional diversity, while bioturbation and bioirrigation potentials were negatively related to diversity and community evenness, reflecting a dominance by key bioturbating taxa. Our findings demonstrate that environmental gradients fundamentally shape both benthic community structure and the nature of the link between biodiversity and essential ecosystem functions. These results contribute to our understanding of the role macrofauna can play in processes related to carbon sequestration in marine deposition centers with fine-grained sediment and organic matter in shelf sea systems.