Whole Lake Oxygen Dynamic Studies: A Prelude to Engineering Controls of Internal Phosphorus Loading

A study of six urban lakes in the western suburbs of Minneapolis, Minnesota investigated the relationship between lake oxygen dynamics and internal loading of phosphorus. In five of six lakes significant internal phosphorus loading occurred when redox values at the sediment surface were +100 mV or less (reference standard hydrogen electrode). Water was anoxic at redox values less than +275 mV in lake hypolimnia. In-situ sediment oxygen demand studies on three of the six lakes revealed values ranging from 2.16 to 3.84 g O2/m2/d. Planktonic community oxygen demand studies conducted under the ice on those three lakes revealed values of 1.04 to 1.26 g O2/m2/d. Sediment samples revealed iron to total phosphorus mass ratios of 15:1 or greater, indicating that under oxic conditions ferric iron binding of soluble reactive phosphorus (SRP) controls internal phosphorus loading. Results of this study support oxygen titration of sediments as a method to prevent release of SRP by maintaining an oxidized layer of ferric iron at the sediment surface.