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. 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
experiencing fish winterkill. These observations have important
implications for shallow lake management, which typically focuses
efforts on maintaining the clearwater state.