Elemental concentrations in surface microlayer and subsurface water samples from Lake Michigan and from its major fluvial sources were subjected to separate R mode factor analyses to define the geo-chemical phases and mechanisms which influence the composition of the surface microlayer. The associations revealed by these analyses indicate that the composition fluvial microlayers is controlled by localized factors related to the geology of individual drainage basins, while open lake microlayers are influenced by broad scale physiochemical interactions. Depletions of both dissolved an particulate phase constituents in fluvial microlayers are attributed to upward diffusion of ions in sediment pore fluids and resuspension of bottom sediments. Enrichments of these constituents in open lake microlayers are due to organic complexation, biological uptake, and bubble flotation of fine-grained mineral particles.