Small scale variability of benthic assemblages: biogenic neighborhood effects

In this study, patterns of community development were investigated within v s. outside 'habitats'. These habitats represented five different monospecif ic assemblages of one of the following species: the brown alga Fucus serrat us, the red alga Delesseria sanguinea, the green alga Enteromorpha intestin alis, the seagrass Zostera marina and the blue mussel Mytilus edulis. Natur al assemblages were allowed to develop on paired artificial substrata-separ ated by ca. 1 m -within (treatment) vs, outside (control) of habitats. The same colonizer species settled on treatment and control substrata for given habitats. However, after 5 months of settlement and post-settlement dynami cs, their proportional abundance and the structure of treatment and control assemblages differed in many instances. Variability among replicates of a given treatment, seperated by up to 50 m, was large, indicating a patchy sp atial distribution of organisms. Despite this spatial heterogeneity among w ithin-treatment replicates, analysis of similarity revealed that in most in stances significantly different assemblages developed between treatments on a small spatial scale depending on whether substrate were positioned withi n as compared to outside a, given habitat. Consequently, the algae, seagrass or mussels constituting a habitat seem to control the structure of the benthic assemblage developing in their vicini ty by one or more possible mechanisms: reduction of larval advection, exuda tion of metabolites that influence settlement and/or post-settlement surviv al, and/or-in the case of mussel assemblages-predation on larvae. In addition to spatial variability in larval supply, stochasticity in succe ssion, substratum heterogeneity, competition and predation effects, this in vestigation reveals the potential of a further assemblage structuring facto r: the impact of neighboring organisms. (C) 2001 Elsevier Science B.V. All rights reserved.