Dissolved Organic Phosphorus Compounds: Do They Satisfy Planktonic Phosphate Demand in Summer?

Abstract

The rate of release of phosphate from dissolved organic phosphorus (DOP) compounds generally accounted for <1% of the phosphate uptake rate by seston in the open water of five diverse freshwater systems in summer. Surface water samples were taken during July and August 1984 from a eutrophic lake (East Twin Lake, OH), an acid bog lake (Triangle Bog Lake, OH), a freshwater estuarine marsh that empties into the western basin of Lake Erie (Old Woman Creek, OH), and two large mainstem reservoirs on the Savannah River (R. B. Russel Reservoir and Clarks Hill Lake, GA). In each of these, phosphatase hydrolysable phosphomonoesters (PME) often were the major fraction of DOP; phosphate release from photosensitive DOP was not detected in any of these systems at this time. The rate of release of phosphate from PME was calculated from Michaelis–Menten kinetics, and phosphatase activity was estimated spectrophotometrically using p-nitrophenyl phosphate as a model substrate. Radiometric analysis of the rate of phosphate uptake by seston showed that phosphate was sorbed to seston by at least two different processes. The total uptake rate by all uptake processes exhibited an apparent first-order dependence on the concentration of available phosphate. Typically, the velocity of uptake was 1–10 nmol∙L−1∙min−1, and the velocity of release from PME was 0.01–0.06 nmol∙L−1∙min−1.