Optical turbidity surveys combined with pigment,plankton,and current measurements were used to investigate the vertical andhorizontal dynamics of suspended particulate matter (SPM)in the Laptev Sea,one of the largest Siberian shelf seas,during the ice-free period.Optical measuring devices prove to be an excellent tool to measure SPM distribution in real time.SPM concentrationswere quanti .ed owing to the high correlation of water samples and optical backscatter.Thus,the formation and distribution of thebottom nepheloid layer,a layer of increased SPM concentration,and its signi .cance for the sediment transport on the Laptev Seashelf can be described.Two nepheloid layers exist in the eastern and central Laptev Sea.Formation and concentration of the surface layer are mainlyrelated to the amount of phytoplankton and zooplankton occurrence.However,in the vicinity of the Lena Delta,the concentrationis strongly dependent on riverine discharge.The bottom nepheloid layer is suggested to develop during and brie .y after the springbreakup,when about 60%of the mean annual sediment input is discharged onto the shelf.SPM spreads over the shelf and is kept insuspension within the bottom layer.Especially during the ice-free period,almost no sedimentation takes place.However,bottommaterial is resuspended due to wind-induced increased bottom currents,mainly in paleo-river valleys and on shoals.Valleys actas transport conduits during the ice-free period and SPM is shifted within them.An intermediate layer near Stolbovoy Bank is probably caused by the displacement of the bottom layer from the topographic highs into the valleys.The combined turbidity and current measurements indicate that most of the sediment transport on the Laptev Sea shelf takes place in the bottom nepheloid layer.