Mechanisms of the Deoxygenation of the Hypolimnia of Lakes

The deoxygenation of the hypolimnia of lakes and reservoirs normally occurs as a combined result of the sediment oxygen demand and the oxygen demand of the algae and other plant materials that grow in the epilimnion. For some waterbodies, there may be appreciable amounts of oxygen-demanding material brought in from the watershed, but this is usually a relatively unimportant source of oxygen demand in the hypolimnion of lakes and reservoirs. This report reviews the work that has been done as part of the US OECD Eutrophication Study during the 1970's on the relationships between the phosphorus loads to waterbodies and their hypolimnetic oxygen depletion rates. In the US OECD Eutrophication Study and follow-on work done by the authors, data for approximately 750 US lakes and reservoirs have been evaluated for P loads and eutrophication-related water quality response. Regressions have been developed for these waterbodies between the normalized phosphorus loads to waterbodies and the planktonic algal chlorophyll, Secchi depth, and areal hypolimnetic oxygen depletion rate. The phosphorus load is normalized based on the waterbody's mean depth, hydraulic residence time and surface area. Examination of the oxygen profiles in the hypolimnia of many lakes has shown that the primary mechanism of deoxygenation is the oxygen consumption associated with the decomposition of dead algae raining down from the epilimnia. In many waterbodies, the sediment oxygen demand is controlled primarily by inorganic reactions of ferrous iron and sulfide with dissolved oxygen. This demand is normally exerted within 1 to 2 m of the sediment in the waterbody's watercolumn. The relative significance of sediment and algal-related oxygen demand to hypolimnetic oxygen depletion can be assessed through the use of the US OECD load response models.