Lessons from a disturbance experiment in the intertidal zone of an exposedsandy beach

Exposed sandy beaches are important, sensitive and widespread coastal habit ats. Although they have been studied for more than 50 years, investigators have been reluctant to attempt manipulative experiments due to the dynamic nature of these environments. Consequently, the ecology of exposed sandy be aches remains relatively poorly understood. We conducted a community-level, manipulative experiment involving a simulated anthropogenic disturbance on an exposed microtidal sandy beach in the Eastern Cape, South Africa; the f irst of its kind and scale. This study comprised pre- and post-impact sampl ing at an experimental site and two control sites. The impact involved exca vating and removing a 200 m(2) quadrat of sand from the mid-intertidal of t he experimental site to a depth of 0.3 m. The intention was to address the prediction that anthropogenic disturbances would be detectable if appropria te spatial and temporal scales were investigated. The following variables w ere monitored: transect gradient; species richness; macrofaunal abundance; and both the abundance and biomass of the dominant infaunal species, the be ach clam Donax serra Roding. Analyses revealed significant differences in t emporal patterns of all response variables amongst sites. Some evidence lin ked these changes to the experimental disturbance, although impacts appear temporary, being ameliorated within, at most, one semi-lunar cycle. This co nfirms that it is possible to successfully conduct manipulative experiments on exposed sandy beaches. However, the uncontrollable, natural dynamics of the beach face, as expressed by intertidal gradient, contributed significa ntly to the description of spatio-temporal variation in biotic response var iables. It is concluded that to isolate treatment effects from those of nat ural variation, two advances are necessary on the current research approach . First, experimental designs must take cognizance of the fact that exposed , microtidal sandy beaches have little in common with other intertidal habi tats; and second, large-scale treatments must be replicated in space. (C) 2 000 Academic Press.