LB 2-18 - The browning of Adirondack Lakes: Past patterns, effects on thermal stratification and future research

Surface waters of the Adirondack region of New York were historically impacted by atmospheric acid deposition but have recovered in recent decades due to controls on fossil fuel emissions and shifts in energy use. Dissolved organic matter (DOM) is a key attribute of lakes, regulating many characteristics including thermal stratification. Here, we explore spatial and temporal patterns in DOM, related water chemistry and the role of changing DOM in the attenuation of light and thermal stratification of Adirondack lakes by leveraging long-term monitoring programs. This analysis is also conducted to inform future research on browning and its effects on Adirondack lakes through a future Survey of Climate change and Adirondack Lake Ecosystems (SCALE). Adirondack lakes are “browning” (i.e., increasing in DOM). Most lakes of the Adirondack Long-Term Monitoring (ALTM) program (46 of 52) have experienced increases in dissolved organic carbon (DOC) concentrations, ranging from modest (0.11 μmol C/L-yr) to high (16.5 μmol C/L-yr) rates of increase. Increases in lake DOC have coincided with decreases in sulfate plus nitrate concentrations and specific conductance and increases in pH, suggesting this response is likely driven by decreases in acid deposition though changing climate could also contribute. The rate of lake color also increases with DOC, but in a non-linear manner indicating possible shifts in carbon quality. Based on a November 2022 survey of ALTM lakes, we find that lake DOM has greater abundance in terrestrial vs microbial DOM components, has greater aromaticity, has higher molecular weight, and shows less microbial processing with increases in DOC. ALTM lakes will be sampled again in early and late summer 2023 for further insight on patterns in DOM quality. Concentrations and changes in DOM also influence the thermal stratification of Adirondack lakes. The light extinction coefficient (Kd) is strongly related to lake DOC and increases in lake DOC coincide with increases in Kd. although relationships vary considerably among lakes. Long-term increases in DOC and Kd are strengthening the thermal stratification of Adirondack lakes and amplifying effects of climate warming.