Increasing dissolved organic carbon concentrations in northern boreal lakes: Implications for lake water transparency and thermal structure

We evaluated trends in dissolved organic carbon (DOC) and associated changes in water transparency and epilimnion thickness to better understand the implications of regional increases in DOC concentration in lakes. Long-term monitoring of a suite of physical, chemical, and biological data from six to 12 lakes in Acadia National Park in Maine was paired with high-frequency sensor monitoring of one lake as a model system. Water transparency declined across study sites since 1995 as DOC increased and chlorophyll remained stable, suggesting that this was not a signal of increased eutrophication. As clarity declined, some lakes experienced reduced epilimnion thickness. The degree to which transparency changed across the lakes was dependent on DOC concentration, with a larger decline in transparency occurring in clear water lakes (-0.3myr(-1)) than brown water lakes (-0.1myr(-1)). DOC concentration was an important explanatory variable for reduced epilimnion thickness in short-term sensor measurements. A regional decline in water transparency across all lakes and reduction in epilimnion thickness in a limited number of systems appeared to be acting as a sentinel for changes in atmospheric deposition and regional weather that modified the delivery of DOC from the watershed.