Benthic community assemblages in the San Juan Islands: Do abiotic factors influence community composition at small spatial scales?

Abstract

There are many factors contributing to the composition of biological communities in coastal marine habitats worldwide. Operating over large regional spatial scales, abiotic variables such as temperature and oceanic current patterns have been shown to form patchy, complex distributions of organisms. At much smaller local scales, inter- and intraspecific interactions such as competition for resources and space as well as predation pressures have been shown to structure the distribution of organisms. For this study, seven study sites were selected in close proximity to the UW Friday Harbor Laboratories to investigate the contributions of select abiotic factors (depth, slope, aspect, and flow rates) on community compositions at small spatial scales. Ordination of sample groups by non-parametric MDS and ANOSIM analysis using percent cover data showed significant differences in community composition across all depths (Global R= 0.561, p=0.001) with a distinct transition at the 50fsw depth (Global R=0.702, p=0.001). MDS ordination and one-way ANOSIM analysis of the horizontally and vertically oriented transects in the shallow and deep depths showed significant differences between these orientation classifications (R=0.17, p=0.025; R=0.359, p=0.001, respectively). MDS and ANOSIM analysis of the GIS-derived slope classifications in the shallow and deep depths showed no significant differences between the slope classifications (R=0.065, p=0.618; R=0.037, p=0.354, respectively). MDS and ANOSIM analysis of the GIS-derived aspect classifications in the shallow and deep depths showed no significant differences between the aspect classifications (R=0.054, p=0.292; R=0.108, p=0.204, respectively). To the best of our knowledge, no published studies to date have assessed the influence of the aspect of a habitat to community structure, either qualitatively or quantitatively. Studying the small-scale effects of these abiotic factors, accompanied with studies of biotic factors regulating community composition, provide a more complete assessment of coastal ecosystems and can enhance conservation and management of these important habitats.