Internal Phosphorus Loading and Alum Effectiveness in Shallow Lakes

ABSTRACT

Internal phosphorus loading in shallow aerobic lakes appears to be controlled by iron redox, much as it is in deep, stratified anaerobic lakes. Lack of stratification in shallow lakes, permitting greater oxygen transport to sediments than in stratified lakes, may be offset by higher temperature; this promotes greater demand for electron acceptors (O₂ and NO₃) and anoxic conditions at the sediment-water interface, as well as high pH that can maintain phosphorus in a soluble state under oxic conditions. Thus, it is not surprising that alum treatments in shallow lakes have been rather successful, with results lasting for more than a year. Of the nine treatments of shallow lakes for which data are adequate for evaluation, four can be considered unsuccessful (three of these failures were due to other sources of phosphorus). Treatment longevity is known for only one lake, however. If an alum longevity of four to five years turns out to be typical for shallow lakes, alum could be less cost effective than in deeper, stratified lakes, where longevity usually exceeds a decade. Possible control of other mechanisms of internal phosphorus loading (high sediment fractions of weakly sorbed phosphorus and overwintering buoyant blue-green algae) are also discussed.