Long-term changes (1977 to 1987) in a muddy fine sand Abra alba - Melinna palmata community from the Western English Channel: multivariate time-series analysis

Abstract

of the muddy fine sand community of the Bay of Morlaix (N France) has allowed us to determine the principal stages of structural succession following pollution by the hydrocarbons of the 'Amoco Cadiz' (1978). Abundance of 30 main species and biomasses of 30 categories were investigated. Regular series were obtained by averaqng values from each season, resulting in 40 sequential observations. General trends and seasonal variations were extracted by the Eigen-Filtering method whch takes into account the shortness of the annual sampling. Principal component analysis of the covariance matrix of the general trends e x l b i t s 7 successive periods of temporal community changes. T h s succession is a function of changes in duection in gradients within the populations. The community developed through a successional phase following the oil spill, with establishment at first of a community of opportunists, followed by a colonisation of the polychaete Lanice conchilega and finally by reestablishment of the original community pre-Amoco Ca&z. An attempt to classify the species according to their seasonal variations is proposed using a semiquantitative approach. Results show a concentration of maximal abundances in summer and autumn, and of minimal abundances in winter. The dissimilarity correlograms of the densities and the biomasses were used after rearranging the specles Into 3 trophic groups. The different behaviour of trophc groups is related to recolonization after the oil spill and to organic matter in the sediment.