VARIABILITY OF ZONATION PATTERNS IN TEMPERATE MICROTIDAL URUGUAYAN BEACHES WITH DIFFERENT MORPHODYNAMIC TYPES

Early studies on sandy beach zonation patterns have shown rather rigid schemes. Recent work has suggested that zonation changes with morphod ynamic type and, in dissipative beaches, it also changes through time. This last finding leads to the conclusion that at least 1 yr of study is necessary to understand zonation patterns in dissipative sandy bea ches. Here we report a 1 yr study (from March 1994 to March 1995) of 4 microtidal beaches with different morphodynamic types situated on the Atlantic coast of Uruguay. We show that zonation patterns (number of zones or belts) can change through time regardless of beach type. Howe ver, the morphodynamic characteristics of the beach seem to affect how frequently the zonation pattern can change by fusion or subdivision o f zones, and which zones are involved in those processes. In beaches w ith flat slopes (toward the dissipative extremity), the lower zones we re frequently fused and divided. The same occurred with the medium and upper zones in the beaches with steeper slopes (towards the reflectiv e extremity). Our results suggest that spatial variability of the macr ofauna is related to variability in the position of the swash zone, wh ich in turn depends on beach slope and width. Variability of species d istribution would also depend on morphological adaptations of organism s to move in such environments. We suggest 2 mechanisms of species mov ement to explain the variability in species distribution observed in t his study: Swash Mediated Active Movement (SMAM) and Autonomous Active Movement (AAM). The first mechanism involves both an active and a pas sive component. AAM is independent of swash movements and affects spec ies distribution on the upper levels of the beach.