Experimental studies on some factors influencing phosphorus solubilization in connexion with the drawdown of a reservoir

The periodical drawdown of reservoirs affects the phosphorus cycle. During the dry period the sediments are in immediate contact with the atmosphere, while part of them are resuspended when the water rises again. Experimental studies on the sediments of the Puyvalador reservoir (Pyrenees, France), which is subjected to a considerable drawdown every year, show that the quantity of phosphorus solubilized after resuspension of these sediments varies according to the speed with which the water rises, the preceding desiccation of the sediments and the origin of the water used in the experiments. The intensity with which the sediments are stirred depends on the refilling rate. The observed differences related to the origin of the water of the affluent streams are attributed to their initial ortho-phosphate content and their difference in pH. These variations related to drawdown make it necessary to take this important phenomenon into account when studying the solubilization of phosphorus in reservoirs.