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 Ϫ3 d Ϫ1 . Oxygen decay rates were ϳ4 times higher in shallow polymictic lakes compared to deep, stratified lakes. Within shallow lakes, those classified as turbid had decay rates ϳ1.5 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 ϳ150 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.Dissolved oxygen (O 2 ) is a common water quality indicator. Nutrient enrichment enhances primary production, increases O 2 depletion rates, and leads to decreased water quality (Dillon et al. 1978;Snodgrass and Ng 1985;Lind 1987;Gelda and Auer 1996). When low O 2 concentration results in anoxia, fish die. Fish kills have resulted in a need to predict anoxia to proactively manage lakes with goals of sustaining productive fisheries and maintaining ecosystem stability. Loss of a top predator has consequences to lower trophic levels; hence anoxia can affect community structure. The frequency and extent of fish kills determine the structure of fish assemblages (Tonn and Magnuson 1982;Rahel 1984) with implications to lower trophic levels (Carpenter and Kitchell 1993).In lakes, winter O 2 budgets are a function of the O 2 mass at freezing, the duration of ice cover, and the balance of processes that produce and consume O 2 . Winter O 2 depletion rates depend on the particulate organic matter (POM) mass decomposing in the lake (Lasenby 1975;Welch et al. 1976;Cornett and Rigler 1980;Mathias and Barica 1980;Jackson and Lasenby 1982), which is composed of senesced aquatic macrophytes and phytoplankton produced during the previous summer. Microbial decomposition of POM and chemical oxidation of reduced inorganic chemical species results in biological O 2 demand (BOD) and chemical O 2 demand (COD). Oxygen consumption in the water and sediments is primarily a function of lake productivity, since BOD and COD are proportional to the supply of oxidizeable material 1 Present address: Arizona Game and Fish Department, 2221 West Greenway Road, Phoenix, Arizona 85023-4399.2 Corresponding author (ljackson@ucalgary.ca). AcknowledgmentsWe thank S. Herman and Alberta Environment, Department of Sustainable Resources, for use of equipment and background data for Caroline lakes, and landowners who graciously allowed us access to lakes. We also thank C. Solohub, S. Wilson, and J. Grayson ...

We compiled data for 23 North American temperate zone lakes to assess three alternative winter O2 depletion models for estimating O2 dynamics from freezing to thawing. Dissolved O2 concentrations were constant or declined slightly for an average of 40 days after freezing and then declined rapidly. Once O2 concentrations reached 1–3 mg·L–1, consumption slowed. No model that we fit captured O2 dynamics shortly after freezing. The best fit was a one-pool exponential decay model after one to four initial data were removed. Photosynthesis and freeze-out estimates suggest that O2 inputs are more important in shallow than in deep lakes. Oxygen decay rates (k) correlated with morphometry in shallow lakes and chlorophyll a, Secchi depth, and the sediment surface area to volume ratio in deep lakes. We hypothesize that the failure of chlorophyll a to correlate with k in shallow lakes is because macrophytes are the primary source of decaying organic matter but have not been included in assessments of winter O2 depletion. Thus, some processes in deep lakes cannot simply be scaled to smaller scales in shallow lakes.

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