Spatial and temporal variations in copepod community structure in the lower St. Lawrence Estuary, Canada

Abstract

The lower St. Lawrence estuary (LSLE) is a narrow and deep basin under the influence of both estuarine and marine processes. As a result, the LSLE is characterized by strong spatiotemporal heterogeneity in biological and physical environmental conditions. Previous studies have indicated a bias in the summer composition of the copepod population for predominance of late-development stages of Calanus spp., in comparison with adjacent waters of the Gulf of St. Lawrence (GSL) and northwest Atlantic. In this study, we use a 22 mo spatial survey of the LSLE to determine spatial and temporal variations in the copepod community and the relative contribution of Calanus spp. (C. finmarchicus and C. hyperboreus). Multivariate analysis revealed 4 copepod assemblages associated with seasonal patterns in temperature and chlorophyll a. Calanus spp. dominated the early summer community whereas the seasonal increase in abundance of the more surface-dwelling species (Acartia spp., Oithona spp. and Pseudocalanus spp.) explained the mixed Calanus spp./surface species community observed in late summer-autumn and winter. Three recurrent geographic zones can be discriminated in the LSLE on the basis of specific composition and abundance. The copepod community in the upper-central Laurentian Channel was dominated by Calanus spp. Downstream, in late summer-autumn and winter, a mixed Calanus/surface species assemblage was observed. In the shallow region, a very low abundance of Calanus spp. resulted in dominance of surface species after the early summer period. The genus Calanus spp. was the dominant component of the copepod community in LSLE as a whole because of its consistently high abundance along the Laurentian Channel during different seasons. A change in environmental conditions in the 1990s associated with a decadal climate variation in the GSL-LSLE region appears to be related to a dramatic increase in abundance of midwater copepod species (mainly Metridia longa) and, subsequent decline in the relative abundance of Calanus spp. We develop the hypothesis that the observed patterns in copepod species composition in the LSLE are primarily a consequence of the combined effect of the export in the residual surface outflow of early life stages of Calanus spp. and small surface-dwelling species and the upstream advection of late-development stages of Calanus spp. and midwater species in the inflowing deep water of the Laurentian Channel.