Concentrations of dissolved organic carbon (DOC) and dissolved oxygen (DO) are changing globally in freshwater lakes. Using a long-term dataset of over 50 years from the IISD Experimental Lakes Area (IISD-ELA), in Ontario, Canada, we investigated the dynamics of thermal stratification, DO, and DOC levels in lakes 224, 239, 373 and 442 with trend analysis, early warning indicators and breakpoint analyses. We found that hypolimnetic DO concentrations are significantly declining, with annual decreases ranging from -0.0176 to -0.064 mg/L per year (p < 0.05) from 1969-2022, during the ice-free season. This trend mirrors significant increases in both epilimnetic and hypolimnetic DOC concentrations over time, notably in lakes 239, 224 and 373. Preliminary analyses of early warning indicators of DOC and DO have identified breakpoints of increasing autocorrelation and standard deviation, highlighting critical transitions toward low-oxygen environments. This study will deepen our understanding of lake ecosystems' responses to environmental stressors and improve predictive models of ecological change and will contribute to informed conservation and strategic management of these important freshwater resources. Keywords: Lake ecosystems, Dissolved organic carbon, Dissolved oxygen, Early warning indicators, breakpoint, Ice-free season, epilimnion, hypolimnion, predictive models, ecological change

Authors:

Sherryann Prowell, Trent University  (sherryannprowell@trentu.ca)

Scott Higgins, IISD Experimental Lakes Area  (shiggins@iisd-ela.org)

Michael Paterson, IISD Experimental Lakes Area  (mpaterson@iisd-ela.org)

Marguerite Xenopoulos, Trent University  (mxenopoulos@trentu.ca)