Longshore patterns of distribution of macroinfauna on a Uruguayan sandy beach: An analysis at different spatial scales, and of their potential causes

Abstract

Spatial distribution of macrofauna along the longshore axis was studied on an exposed (Uruguayan) sandy beach in order to assess (1) its dependence on beach topography, and (2) the validity to extrapolate local distribution patterns to larger spatial scales (i.e. km). The distribution of the isopods Excirolana armata and Excirolana braziliensis, the sand crab Emerita brasiliensis and several species of insects was analyzed at 3 spatial scales. A grid sampling was used at small scale (intersample distance: 4 m; extension sampled: 30 to 40 m), a transect design at meso scale (intersample distance 4 to 20 m; extension sampled: 100 to 120 m), and random sampling at large scale (intersample distance: 100 m; extension sampled: 3000 m). Spatial distribution at small and meso scales were described using autocorrelation functions, and tested for effects of topography at meso and large scales using ANOVA and paired t-tests. We found that at small and meso scales the distribution of E. armata and E. brasiliensis was patchy and affected by cusp topography. At large scales, the effect of cusp topography was restricted to E. armata. E. armata and E. brasiliensis showed large-scale aggregations, and E. braziliensis and insects showed large-scale patchiness associated with longshore variability in sediment water content and dune characteristics. We conclude that cusp topography affects longshore distribution patterns, and that it is not valid to extrapolate local longshore distribution to larger scales for every species. We suggest that different processes affecting spatial distribution must be operating at different scales. At small scales, patterns of distribution may be affected by swash transport of food and/or organisms, involving factors such as beach topography, swash and animal movements. At larger scales, in addition to larval and food supply, sediment transport by the wind would play an important role. Spatial patterns of sand transport by the wind may be affected by longshore changes in vegetation cover on the dune field. We further suggest that there may be a link between the beach and the dune habitats operating at large spatial scales.