Characterization of suspended matter size and composition in the St. Lawrence Estuary (eastern Canada)

Abstract

The St. Lawrence Estuary (SLE), in eastern Canada, is a dynamic coastal environment where no recent description of the particles’ composition and size distribution has been completed. In this study, we characterize the particle size distribution (PSD) of suspended particulate matter in the SLE, both vertically and horizontally, and evaluate its variability during ice-free (May 2010) and ice-covered (February 2019) conditions. The results showed the importance of sedimentation in the Lower Estuary as coarse particles, larger than 63μm, in May 2010 and particles between 4μm and 63μm in February 2019 were less present than in the Upper Estuary. In the Lower Estuary, coarse particles had a higher number density in ice-covered than in ice-free conditions, possibly due to ice transport and the higher suspended matter load of regulated rivers. Alternatively, larger particles could also be flocs composed of finer matter. In the Upper Estuary, no substantial vertical variability is present during the ice-free season and in the zone of maximum turbidity during the ice-covered season due to vertical mixing. The mineralogical and elemental geochemical composition of the suspended detrital particles during ice-covered conditions was similar throughout the estuary and indicated a primary origin from the Canadian Shield with the Appalachian province as a secondary source. While the PSD variability between the two contrasting environmental periods was greater than the spatial variability, this was not the case for the mineralogical and elemental geochemical composition which was similar both spatially and temporally. These results provide new information on the dynamics of suspended matter for a large estuarine and subarctic environment.