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

Abstract

Physical stress is known as the prevailing environmental force structuring many intertidal communities globally. Here, we present for the first time the vulnerability to disturbance of a bivalve assemblage situated in the middle intertidal in the Atlantic coast of the Southern tip of South America (ca 53º 36´S, 67º 58’W). This intertidal zone is characterized by a high level of sand movement preventing any establishment of sessile organisms on vertical sides of rock outcrops. The benthic assemblage inhabiting rock surfaces of this physically dynamic intertidal zone has been poorly studied although this coast is prone to potential spills from nearby hydrocarbon marine platforms. In February 2001, we exposed the assemblage to two different experimental conditions: a complete removal and a physical disturbance, and tracked its recovery during the following 4 years. The disturbance lowered the diversity of associated fauna and made disappear the sediment layer trapped among bivalves, which was not restored any longer. Mytilus chilensis recruited massively 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 slower than M. chilensis probably due to its lower growth rate compared to M. chilensis and dependence of an adequate byssus matrix for settlement. Moreover, the main apparent stressor was the irruption of sand that can cover the bivalve assemblage for variable periods of time. This assemblage is characterized by low predatory pressure and therefore we hypothesize that this bivalve assemblage is predominantly controlled by competition for space.

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