Dissolved phosphorus fractions present in the surface waters of a eutrophic reservoir lake in northcentral Oklahoma were characterized by chemical, enzymatic, and chromatographic techniques. Simultaneous measurements of alkaline phosphatase activity (APA) were also conducted on each lakewater sample. An inverse biphasic linear relationship was found between soluble reactive phosphorus concentrations (SRP) and APA on a seasonal basis. Phosphomonoesters (PME) comprised from 0–65 percent of the soluble unreactive phosphorus (SUP) pool and an inverse linear relationship was found between PME concentrations and APA on a seasonal basis. Gel filtration chromatography of filtered lake water from the above system and a mesotrophic lake exhibiting high clay turbidity supported the view that the majority of complex phosphorus in these systems existed as low molecular weight phosphate esters.