Environmental discrimination among soft-bottom mollusc associations off Lagoa dos Patos, South Brazil

Abstract

Sublittoral benthic molluscs established in the outlet of Lagoa dos Patos (South Brazil) were sampled with a rectangular dredge and the malacological associations delineated by cluster analysis (WPGA) using the Baroni-Urbani and Buser similarity index. Three main associations were characterized in bands parallel to the beach with relative concordance with specific sedimentological facies. The multiple discriminant analysis showed that there was a high correlation between these associations and the following environmental parameters: depth, average grain size, skewness of the sediment, medium sand, coarse sand, total sand and total mud. The effect of reducing the multidimensionality of the data to two factors, ecologically explained, brings out the importance of depth, average grain size, total sand and total mud. The first appears to be a bathymetric gradient of stability/disturbance; the disturbance from storms becomes less frequent with increasing depth. This is partially supported by the results of the Caswell neutral model, and is in agreement with the intermediate disturbance hypothesis predictions. The second sedimentological factor is the relative percentages of sand and mud in determining the specific composition of these associations.