Biotic, chemical, and morphometric factors contributing to winter anoxia in prairie lakes

To assess how features of lakes and their watersheds influence winter oxygen decay rates and the frequency of anoxia in shallow prairie lakes, we measured lake and watershed characteristics for 21 south-central Alberta lakes and related these to measured oxygen decay rates during 1998-2000. Oxygen decay rates were functions of macrophyte biomass, percentage littoral area, and total phosphorus and ranged from 0.006 to 0.216 mg O-2 m(-3) d(-1). Oxygen decay rates were similar to4 times higher in shallow polymictic lakes compared to deep, stratified lakes. Within shallow lakes, those classified as turbid had decay rates similar to1.5 times higher than those classified as clear. Chlorophyll a was not a predictor of the oxygen decay rate in shallow lakes; however, macrophyte-derived carbon averaged similar to150 times more than phytoplankton-derived carbon in the shallow lakes we examined. Reasons that lakes frequently or never become anoxic are related to productivity and morphometry; however, processes explaining occasional anoxia appear not to be related to factors we measured.