Effects of erosion-accretion processes on a community dominated by Brachidontes rodriguezii (Mollusca: Mytilidae) on a rocky outcrop of the South Atlantic coast

Abstract

Strong winds are powerful modifiers of coastal areas, altering sedimentary balance on sandy beaches and causing either loss (erosion) or gain (accretion) of sediment. Sand inundations can impact intertidal organisms by smothering or scouring and also affect community development and organization. The rocky outcrop where the mussel bed of Brachidontes rodriguezii develops is exposed to sand burial disturbance. To investigate the effect of this disturbance on the mussel population and its associated community, seasonal sampling was carried out on two adjacent areas within a rocky outcrop, one less exposed (A1) to sand disturbance than the other (A2). The seasonal variation in the area of the mussel bed was estimated using GPS. Biological samples were taken from A1 and A2 and additional samples were taken from buried and unburied sites in A2. In addition, a microcosm experiment was designed to assess the effect of burial duration on mussel behaviour and mortality. Local data on wind speed and direction were obtained and analyzed in relation to changes in the rocky outcrop areas. The erosive period was associated with strong, persistent winds whereas during the period of accretion no such wind events were recorded. The gain of sediment was associated with a higher reduction in the mussel bed area in A2. The mussel population in A1 showed a higher abundance and recruitment percentage, and organisms reached a larger size. Diversity was also higher in A1 but only during the erosive period. Mussel mortality was higher for buried than unburied samples while richness and dominance were higher in unburied samples. During the microcosm experiment, mussel mortality was higher in buried trials than in the controls and the inability of the species to emerge from the buried trial was also observed. This underlines the importance of sand dynamics for the mussel bed community.