Distribution of Suspended Sediment in the Gulf of Gdansk off the Vistula River mouth (Baltic Sea, Poland)

Abstract

The Vistula River mouth is situated on the southern coast of the Gulf of Gdańsk (Poland). Considering the average flow and draining area of the Vistula River, it is one of the largest rivers in the Baltic Sea catchment area. It is also one of the largest sources of terrigenous material supplied to the Baltic Sea. The particle size analysis of the material transported in suspension shows that it consists of fractions ranging in size between 1 and 600 μm. The silt fraction constitutes the biggest share (55%), while the medium and fine sands amount to 30%. Close to 15% of the material transported in suspension belongs to the clay fraction. During field research in April 2014, many parameters were measured at the stations in the Gulf of Gdansk. The variation in suspension characteristics and water parameters was determined with the LISST-25X sensor and CTD probe, respectively. The flow and current directions were assessed by means of ADCP. In the Vistula River mouth, the mean concentration of suspended matter in the surface layer (0–3 m) varied from 30 to 1 099 μL/L. The Sauter mean diameter of suspended sediment ranged from 44 to 172 μm, wherein the smallest and the biggest particle sizes were 22 μm and 360 μm, respectively. At a depth of 10 m, differences in mean diameter (36–54 μm) and concentration (16–47 μL/L) were much lower. The changes observed in these waters occurred over time, reflecting changes in the intensity of the Vistula River discharge. As distance from the river mouth increases, at the surface and at depth of 10 m, the average particle concentration and the mean particle diameter were lower. In deeper water layers (below the developing thermocline), the mean concentration of suspended sediment was clearly lower (2.5 μL/L), while the mean particle diameter was higher (32 μm). Different characteristics of water masses containing suspension were recorded in the marine waters, which are not directly influenced by the Vistula River freshwater inflow. In marine waters, the mean values of suspension concentration ranged from 1 to 2 μL/L, with relatively large particle sizes between 50 and 60 μm. As the distance from the Vistula River mouth increased, the influence of riverine waters on the particle size distribution and suspension concentration became increasingly less visible. The observed distributions of salinity, temperature and suspension concentration indicate that freshwater is spreading in the surface layer in a fan-shaped form. The spreading of suspension occurs along the dominant directions of the river water movement. Changes in the suspension concentration and particle size in the water column are caused by flocculation.