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Increasing concern over the loss of biodiversity has led to attempts to quantify relationships between biodiversity and ecosystem functioning. While manipulative investigations have accumulated substantial evidence to support the notion that decreasing biodiversity can be detrimental to the functioning of ecosystems, recent investigations have identified the potential importance of physical processes in moderating biodiversity – ecosystem function relationships at larger geographical scales. In this study, the relationship between the genus richness of benthic macro-invertebrates and five measures of ecosystem functioning (macrofaunal biomass, depth of the apparent redox discontinuity, fluxes of ammonium and NOx and the abundance of nematodes) was determined over a large scale wave-induced bed shear stress gradient on the seabed of the northern Irish Sea. Ecosystem functioning was significantly correlated to genus richness for four out of five ecosystem functions. However, wave stress moderated the genus richness – ecosystem functioning relationship for only one of the ecosystem functions; genus richness had a positive effect on the depth of the apparent redox discontinuity in the sediment at high wave stress but not at low wave stress. These results indicate that the effects of biodiversity on some ecosystem functions may be sufficiently strong to generate patterns in ecosystems where other factors are also affecting ecosystem processes, but that the biodiversity–ecosystem function relationship for can be dependent on environmental conditions for specific ecosystem functions.