The forgotten dimension in sandy beach ecology: Vertical distribution of the macrofauna and its environment

Spatial and temporal environmental heterogeneity influences biodiversity distribution patterns and dynamics on sandy beaches. In these ecosystems, the microscale vertical distribution adds a dimension to the analysis of biological and physical processes, whose ecological patterns have been rarely described. An intensive across-shore sampling in a dissipative beach was conducted, using a stratified corer to analyze small-scale vertical variations in macrofaunal and physical variables. Sediment temperature decreased towards the swash zone and at deeper sediments. Conversely, moisture, grain size and organic content increased seaward, concurrently with the highest vertical variability. Species richness and density increased from the dunes to the swash zone, being highest at the surface, whereas biomass was higher in sediments deeper than 15 cm as a consequence of larger individual sizes. Sediment moisture and temperature showed a consistent correlation with biological descriptors. Deconstructed by taxonomic group, a significant vertical segregation of body size was observed, suggesting different microhabitat preferences and burrowing abilities. Species spatial patterns varied according to life history traits and differential susceptibility to variations in environmental conditions. This study provides novel insights on vertical environmental and biological variations across the dune-shore axis in a dissipative beach. Expanding the findings of this research to larger spatio-temporal scales is a short-term need to decipher the processes underlying the community and population patterns outlined here.