Particulate and dissolved phase concentrations of heavy metals (Fe, Mn, Cu, and Ni) and organic carbon were determined in surface microlayer and subsurface (1m) samples collected in the vicinity of a river plume. Observed variations in dissolved Ni and Mn concentrations are attributed to mixing between river and lake waters. Heavy metal concentrations in the particulate organic carbon show a progressive decrease between the river mouth, a down-plume station, and the open lake area outside the river plume. These trends are due to segregation between mineralized and organic particulates as well as differential transport of different mineralogical groups. The pronounced influence that these physical processes have on the composition of the surfsce microlayer raised a serious doubt concerning our ability to identify compositional variations due to strictly chemical processes within the microlayer.