Rapid biodiversity loss is raising concerns about potential declines in the functioning of Earth's ecosystems. Although decades of research explore biodiversity - ecosystem function (BEF) relations, empirical BEF assessments have lagged behind theoretical advances, particularly in marine benthic systems. Here, we incubate intact sediment push cores to examine the relationship between macroinfaunal community composition and benthic nutrient cycling in three nearshore sub-Arctic sites. First, we quantitatively assess potential effects of taxonomic and functional diversity, as well as community-weighted trait means, on oxygen and nutrient fluxes. Second, we examine fluxes in relation to macrofaunal abundance, oxygen consumption (a proxy for total core metabolism), and abundance of key functional groups, to test fundamental expectations based on ecological theory. We report distinct macrofaunal communities and contrasting benthic fluxes among sites, with oxygen and ammonium largely driving multivariate inter-site flux differences. Diversity indices and community-weighted trait means collectively explained similar to 76 % of the variation in multivariate fluxes but provided little insight into the mechanistic links between diversity and functioning. In fact, we find that total macrofaunal abundance was the key driver of benthic fluxes at our sites, rather than functional community composition, which may have important implications for coastal conservation planning. Overall, our findings emphasize the highly context-dependent nature of BEF relationships and highlight the need to improve empirical understanding of these patterns in complex, natural ecosystems.
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