Context dependency of relationships between biodiversity and ecosystem functioning is different for multiple ecosystem functions Jan Geert Hiddink, Thomas Wynter Davies, Matthew Perkins, Margarita Machairopoulou and Simon P. Neill J. G. Hiddink (j.hiddink@bangor.ac.uk), T. W. Davies, M. Perkins, M. Machairopoulou and S. P. Neill, School of Ocean Sciences, College of Natural Sciences, Bangor Univ., Menai Bridge, Anglesey, LL59 5AB, UK. 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 NO x 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 a 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. Increasing concern over current rates of biodiversity loss have highlighted the need to evaluate the consequences of species extirpation on the functioning of key ecosystem processes such as nutrient cycling and primary productivity (Hooper et al. 2005). There is, however, uncertainty as to how results obtained in ecological experiments in general and biodiversity  ecosystem functioning experiments in particular scale up to landscape and regional levels (Carpenter 1996, Schindler 1998, Loreau et al. 2002). A number of recent studies have shown that the relationship between biodiversity and ecosystem functioning can be context dependent in natural ecosystems (Mermillod- Blondin and Rosenberg 2006, Tylianakis et al. 2008). For example, processes such as physical disturbance and flow have been identified as potentially important moderators of biodiversityecosystem function relationships in aquatic ecosystems (Cardinale and Palmer 2002, Biles et al. 2003, Cardinale et al. 2005). Cardinale et al. (2005) showed that increasing periphyton species richness increased primary production in streams characterised by unpredictable flow regimes, but not in streams with predictable flow regimes. This has important implications for the management of ecosystems, as species that seem redundant under natural conditions may be important for ecosystem functioning when ecosystems are disturbed. Several mechanisms have been proposed to explain the relationship between biodi- versity and ecosystem functioning. The most important of these are resource partitioning, positive interactions between species and sampling effects (Hooper et al. 2005). Positive interactions have been forwarded as a potential mechanisms underpinning BEF effects in deep- sea benthos (Danovaro et al. 2008), whilst resource partitioning has been shown to be important in a model shallow water marine habitat (Griffin et al. 2008). Marine soft-sediments are the largest ecosystem on earth, and the oceans account for approximately two thirds of global ecosystem services (Snelgrove et al. 1997). Most important among these are nutrient recycling and secondary production. The biodiversity of invertebrate communities in coastal ecosystems is affected by anthropogenic distur- bances, such as bottom trawling, dredging, aggregate extraction and pollution. In this study, the relationship between a measure of the biodiversity of benthic macro- invertebrates and several measures of ecosystem functioning in soft sediments was determined over a natural gradient of wave bed shear stresses in the Irish Sea, as a measure of disturbance. Wind shear at the sea surface leads to the generation of waves, and in relatively shallow water or for large wavelengths, these waves translate into wave-induced bed shear stress. High shear stresses can cause sediment Oikos 000: 000000, 2009 doi: 10.1111/j.1600-0706.2009.17556.x, # 2009 The Authors. Journal compilation # 2009 Oikos Subject Editor: Ulrich Brose. Accepted 26 May 2009 1