Multivariate Approach to Study Species Assemblages at Large Spatiotemporal Scales: The Community Structure of the Epibenthic Fauna of the Estuary and Gulf of St. Lawrence

We investigated hierarchical organization and spatiotemporal discontinuities in species abundances in the epibenthic community of the Estuary and Gulf of St. Lawrence. Quantitative samples were obtained from 1975 to 1984, on 161 collectors (navigation buoys) moored yearly from May through November. Maximum biomass values of the dominant species, common to all regions studied, were used to assess epibenthic community structure. Numerical methods were used to characterize spatial structure and temporal variability of the dominant assemblage. Spatially constrained clustering and ordination techniques revealed six broad biogeographic zones whose limits vary yearly. However, spatially unconstrained clustering and ordination techniques showed two major sets of non-continguous localities, each characterized by a singular biotic structure. Further, spatial autocorrelation analyses showed a significant relationship between biomass and geographic distance. The resulting spatial structure of biomass was dependent on the species considered. The multidimensional Mantel technique showed an 8-yr period of variation in community structure at large (whole system, Gulf) and intermediate (North Shore plus Lower North Shore) spatial scales. The amplitude and asymmetry of this temporal cycle increased as the spatial scale decreased. In spite of the observed discontinuous spatial patterns, the temporal oscillations in community structure detected at different spatial scales suggest that the Estuary-Gulf system responds to the external input of auxiliary energy as an integrated system.