Macroinfauna Zonation in Microtidal Sandy Beaches: is it Possible to Identify Patterns in Such Variable Environments?

Prior studies on sandy beach macroinfauna have been directed to elucidate generalizable zonation patterns. These studies, usually based on a reduced temporal scale (i.e. one sampling date), have often highlighted the discrepancies between their results and the classical, static zonation schemes. This paper demonstrates that snapshot studies might fail to obtain conclusive results about zonation patterns in microtidal sandy beaches, because these constitute dynamic environments subjected to many sources of uncertainty. For this purpose, a yearly study (February 1988–January 1989) of the macroinfauna was carried out on a dissipative beach on the Atlantic coast of Uruguay. An average zonation pattern with three main belts was recognized between the sand dunes and the lower levels of the swash zone when considering annual mean abundances. This pattern roughly matched traditional zonation schemes, whereas monthly patterns did not always fit them—with the exception of some species. Important spatial variability of the macroinfauna was observed, with aperiodic and seasonal components. The former was associated with unpredictable movements of tidal levels, with the macroinfauna changing its position, tracking the high water level. Concerning seasonal periodicity, the species tended to occupy the upper levels of their respective distribution areas during spring and summer, whereas in autumn and winter they inhabited the lower levels. The fact that faunal zones are dynamic, and track sea levels and seasons, diminishes the applicability of static traditional patterns. It was concluded that a yearly study is needed to give a more representative picture of the zonation patterns in microtidal sandy beaches.