Grain-size composition of the suspended particulate matter in the marginal filter of the Severnaya Dvina River

The quantitative distribution and grain-size composition of the suspended particulate matter (SPM) in the marginal filter of the Severnaya Dvina River during the summer low-water periods of 2001–2005 were first analyzed in seawater on board of the vessel immediately after its sampling (without preliminary treatment) using a Coulter counter. This analysis revealed the main regularities in the transformation of the grain-size spectra at successive salinity steps of the marginal filter as well as the boundaries between these steps based on the data obtained by direct complex studies of the SPM dispersion. It is established that the water salinity is the main factor that controls the changes in the grain-size distribution and the composition of the particulate matter in the marginal filter. The concentrations of the pelitic fraction and the salinity demonstrate negative correlations between each other. It is shown that the areas characterized by the mass development of phytoplankton are located along the outer boundary of the marginal filter (at the biological step), where the salinity amounts to 23–24 psu. The content of the suspended forms of some chemical (lithogenic) elements and the C_org indicating the SPM’s genetic composition and their relations with the grain-size composition of the latter and the environments are studied.