Abstract
Abstract Dissolved oxygen controls important processes in lakes, from chemical reactions to organism community structure and metabolism. In shallow lakes, small volumes allow for large fluctuations in dissolved oxygen concentrations, and the oxygen regime can greatly affect ecosystem‐scale processes. We used high frequency dissolved oxygen measurements to examine differences in oxygen regimes between two alternative stable states that occur in shallow lakes. We compared annual oxygen regimes in four macrophyte‐dominated, clear state lakes to four phytoplankton‐dominated, turbid state lakes by quantifying oxygen concentrations, anoxia frequency, and measures of whole‐lake metabolism. Oxygen regimes were not significantly different between lake states throughout the year except for during the winter under‐ice period. During winter, clear lakes had less oxygen, higher frequency of anoxic periods, and higher oxygen depletion rates. Winter oxygen depletion rates correlated positively with peak summer macrophyte biomass. Due to lower levels of oxygen, clear shallow lakes may experience anoxia more often and for longer duration during the winter, increasing the likelihood of fish winterkills. These observations have important implications for shallow lake management, which typically focuses efforts on maintaining the clearwater state.
Plain Language Summary In lakes, the amount of oxygen dissolved in the water has a profound impact on lake processes, from chemical reactions to the varieties and quantities of organisms present. In shallow lakes, the amount of dissolved oxygen can vary greatly due to the differences in the rates of production, mostly through photosynthesis, and consumption, mostly through respiration. In this study, we compared dissolved oxygen availability seasonally between two common states found in shallow lakes; a turbid, low clarity state dominated by phytoplankton, and a clear state dominated by submersed aquatic plants. Patterns of oxygen were similar between the two lakes states in all seasons except winter. During the winter under‐ice period, clear lakes had significantly less oxygen compared to turbid lakes, and lost oxygen at a faster rate through the winter. The lower levels of oxygen in clear lakes during the winter could affect many lake processes, such as the winterkill of fish. Lake managers typically try to maintain shallow lakes in the clear state because of better water quality and increased wildlife diversity. Winter fish kills could help maintain lakes in the clear state, but may also select for rough fish that can drive shifts to the turbid state.
Key Points Winter oxygen regimes differed between clear and turbid shallow lakes Clear shallow lakes had significantly higher oxygen depletion rates under ice cover than turbid shallow lakes Oxygen depletion rates were highly correlated with summer macrophyte biomass