Seepage Lake Chemistry as an Indicator of Climate Change

A lake\u27s chemistry reflects its connection to the landscape via hydrologic flowpaths. Stronger hydrologic connections equate with higher inputs of solutes to lakes. Both interannual and long-term changes in climate influence these connections and their transport of solutes to lakes. Properties of seepage lakes, in particular their longer water residence times and lack of permanent surface-water inlets, make them useful for interpreting the influence of climate on lake hydrologic connections. Despite their apparent isolation from surface connections, seepage lakes receive material input from adjacent wetlands. Groudwater connections to these hydraulically isolated lakes groundwater are small, but can be a major source of solutes. This research investigates whether precipitation-dominated seepage lakes have the potential to provide clear signals of climate change. Specific research objectives were: 1) determine if seepage lake chemistry could signal interannual shifts in climate that subsequently affect wetland and groundwater connections, and 2) determine if seepage lakes with stronger hydrologic connections were better indicators of climate shifts. Long-term seepage lake data from Maine and Wisconsin were used to address these objectives. Lakes were grouped into classes according to strength of their hydrologic connection to wetlands and groundwater. The number and size of wetlands adjacent to lakes determined wetland classes. Lake Si concentration, which reflects the magnitude of groundwater inputs, was used to classify lakes by groundwater interaction strength. Lake DOC and color were used as indicators of wetland connections while Si and Ca provided indicators of groundwater connections. These chemical indicators were analyzed using ANOVA first to determine if the classification scheme reflected chemical lake inputs during average climate periods. ANOVA was used to determine whether lake chemistry significantly changed in response to climate shifts, and assumed climatically-altered hydrologic connections. Climate periods were identified using annual precipitation data. In both Maine and Wisconsin, DOC, color and Si clearly reflected shifts in precipitation. Concentration increases occurred during higher rainfall and decreases during drought. Ca did not clearly signal climatic shifts in groundwater inputs. Evapoconcentration during drought caused Ca to increase in both groundwater classes. Strong wetland connections also interfered with the Ca groundwater signalin that concentrations reflected precipitation shifts more clearly in lakes with strong wetland connections, despite the strength and shifts in groundwater connections. Maine lakes with stronger groundwater connections experienced a lag in Ca response to groundwater shifts, as concentrations did not decrease until the second year of drought-induced low groundwater levels. Overall, lakes with the strongest hydrologic connections signaled precipitation shifts more clearly and were the better climate indicators. More weakly-connected lakes had little response to climatic shifts in hydrologic connections. In these lakes, chemical indicators were influenced more by a long-term DOC trend and evapoconcentration/dilution effects. Essential to using seepage lakes as climate signals is accurately identifying strength of hydrologic connection. Chemical shifts in lakes with strong hydrologic connections more clearly signaled shifts in precipitation. Results support the contention that precipitation-domintated sepage lakes have potential as indicators of climate change and its effects on freshwater resources