Limited evidence of interactive disturbance and nutrient effects on the diversity of macrobenthic assemblages.

The causes and consequences of the coexistence of a number of species in a given habitat have attracted ecological research for several decades. Numerous theories have been developed in this context, including the ‘intermediate disturbance hypothesis’ (IDH), but supportive experimental evidence remains scarce and contradictory, leading scientists to propose the existence of an interaction between disturbance and productivity. This study assessed the interactive effects of disturbance and nutrient enrichment at 1 oligotrophic (Arraial do Cabo) and 1 eutrophic (Guanabara Bay) site on the coast of Brazil. At each site, an epibenthic assemblage was allowed to establish on settlement panels (PVC) for 3 mo prior to a 6 mo manipulation phase comprising nutrient enrichment and disturbance (biomass removal). Our results revealed site-specific diversity-driving processes in the absence of disturbance. Nevertheless, diversity and species richness peaked at both sites at some intermediate level of disturbance, corroborating the IDH. Nutrient enrichment increased total species richness and algal species richness in particular, but only at the oligotrophic site. Only here, did nutrient enrichment eliminate the unimodal species richness pattern observed along the disturbance gradient under ambient nutrient concentrations. Such interactive effects of disturbance and productivity on diversity confirm the general predictions of advanced IDH models, e.g. the Kondoh model. The present study indicates that interactive effects of ‘bottom-up’ and ‘top-down’ processes may explain more of the variation in community diversity than the separate models of disturbance–diversity and productivity–diversity relationships.