Effects of physical disturbance on a sub-Antarctic middle intertidal bivalve assemblage

Abstract

We present for the first time the vulnerability to disturbance of a bivalve assemblage situated in the middle intertidal zone of the Atlantic coast of the Southern South America (53°36′S, 67°58′W). This intertidal zone is characterized by a high level of sand movement preventing any establishment of sessile organisms on the vertical sides of rock outcrops. The coast is prone to potential spills from nearby hydrocarbon marine platforms, but this benthic assemblage has been poorly studied. In February 2001, we exposed the assemblage to two different experimental conditions: a complete removal of bivalves and a physical disturbance and tracked its recovery during the following 4 years. The disturbance lowered the diversity of associated fauna and made the sediment layer trapped among bivalves disappear, which was not restored. Mytilus chilensis was recruited into crevices or between byssi in the following settlement season, i.e. summer 2002. The recuperation of mussel coverage to predisturbance levels took 3 years after the total removal. In April 2004 the size frequency distributions, density and biomass/number ratio of M. chilensis were similar to those at the start of the experiment. Perumytilus purpuratus recovered more slowly than M. chilensis, probably due to its slower growth rate compared to M. chilensis and dependence on an adequate byssus matrix for settlement. The main apparent stressor was the irruption of sand, covering the bivalve assemblage for variable periods of time. This bivalve assemblage is characterized by low predatory pressure and therefore we hypothesize that it is predominantly controlled by competition for space. © 2012 Copyright Taylor and Francis Group, LLC.