Acidification and Climate Linkages to Increased Dissolved Organic Carbon in High-Elevation Lakes

Increasing concentrations of dissolved organic carbon (DOC) in the northeastern U.S. have been attributed to two potential mechanisms: recovery from acidification and changing climate. Maine's high-elevation lakes (>600 m) could potentially provide unique insight into the response of surface water chemistry to declining acidic deposition and interannual climate variability. The geochemical response in 29 lakes was analyzed during 30 years of change in sulfate (SO42-) deposition and climate. All 29 lakes exhibited positive trends in DOC from 1986 to 2015, and 19 of 29 lakes had statistically significant increases in DOC throughout the study period. These results illustrate a region-wide change from low-DOC lakes (<5 mg/L) to moderate DOC lakes (5-30 mg/L). Increasing DOC trends for these high-elevation lakes were more consistent than for lower elevation lakes in the northeastern U.S. A linear mixed effects model demonstrated that lake water SO42- and climate variables describe most of the variability in DOC concentrations (r(2) = 50.78), and the strongest predictor of DOC concentration was an inverse relationship with SO42-. Due to SO42- concentrations trending toward preacidification levels and projections of a warmer, wetter, and more variable climate, there is uncertainty for the future trajectory of DOC trends in surface waters. Long-term monitoring of Maine's high-elevation lakes is critical to understand the recovery and response in surface water chemistry to a changing chemical and physical environment in the decades ahead. Plain Language Summary Increasing concentrations of dissolved organic carbon (DOC) in the northeastern U.S. have been attributed to two potential mechanisms: recovery from acidification and changing climate. Maine's high-elevation lakes provide unique insight into the response of surface water chemistry to declining acidic deposition and climate change. Surface water chemistry was monitored in 29 high-elevation lakes from 1986 to 2015 and analyzed in the context of declining acidic deposition and climate change. All 29 lakes exhibited positive trends in DOC, which illustrates a region-wide change from low-DOC lakes (<5 mg/L) to moderate DOC lakes (5-30 mg/L). The implications for this could be far reaching, for HELM lakes currently provide refugia for aquatic and macroinvertebrate communities that have declined in other places due to fish stocking, anthropogenic stressors, and a changing climate (Schilling et al., 2008, https://doi.org/10.1111/j.1365-2427.2007.01949.x). Due to sulfate concentrations trending toward preacidification levels and projections of a warmer, wetter, and more variable climate, there is uncertainty for the future trajectory of DOC trends in surface waters. Long-term monitoring of Maine's high-elevation lakes is critical to understand recovery and response in surface water chemistry to a changing chemical and physical environment in the decades ahead.