A preliminary investigation of orthophosphate concentration and the uptake or orthophosphate by seston in two Canadian Shield lakes
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Past investigators have proved that conventional chemical techniques do not give accurate estimates of the biologically-active phosphorus in lake water. Soluble reactive phosphorus, long believed to be orthophosphate-phosphorus, the form of phosphorus most readily available to plankton, includes large amounts of phosphorus which are hydrolyzed from colloidal compounds during the molybdate-blue analysis. This colloidally-bound phosphorus is not removed by phytoplankton at rates comparable to PO4-P uptake. During the current study several previously untested methods for estimating PO4-P concentration were tried. None of these methods proved to be very satisfactory. The chemical methods which were investigated either lacked the sensitivity necessary for PO4P measurement or suffered from interference by colloidal phosphorus or other compounds in lake water. Many of the radiochemical methods which were considered required isotopic equilibrium. Colloidal phosphorus acquires labe1 so slowly that meeting this condition was impractical. Based on the above results, it was decided that Rigler's bioassay (Rig1er, 1966) was the most reliable method available for estimating PO4-P concentration. This method was used during seasonal studies of an oligotrophic and a fertilized lake in the Experimental Lakes Area, Northwestern Ontario. Rigler's bioassay gives only maximum concentration estimates. During summer stagnation, these estimates were very low (from less than 1 to 180 ng/1) for both epilimnion and hypolimnion samples. PO4-P concentration was only slightly higher in the hypolimnion than in the epilimnion during the summer, although the hypolimnions of both lakes were aoxic. The flux of PO4-P to seston was substantial (about 3 ug/1/ day, on average) and was nearly equal in the epilimnion and hypolimnion. This suggests that organisms, rather than inorganic reactions, control PO4-P concentration at all depths in a lake. The highest concentration estimates were obtained for epilimnion samples during the winter. Although the concentrations of particulate phosphorus, ATP, total dissolved phosphorus, and chlorophyll-a were much greater in the fertilized lake, the PO4-P concentrations in the fertilized and unfertilized lakes were similar. Apparently, the added PO4-P was removed by seston before samples were taken. It then either was sedimented or was converted to colloidal phosphorus.