Responses of lakes to drought: geomorphic and landscape controls
I evaluated geomorphic and landscape controls on biogeochemical responses to drought in long-term research lakes from the Wisconsin Long-Term Ecological Research Program, the Experimental Lakes Area in Ontario and the Dorset Research Centre in Ontario. Landscape position refers to spatially explicit locations along gradients defined by features such as hydrology or watershed cover. I apply it as a conceptual framework explaining differences in dynamics among lakes. In northern Wisconsin lakes, dynamics during drought depended on position along local hydrologic flowpaths. In precipitation-dominated lakes high in the landscape, calcium mass declined as reversals in flow direction reduced inputs of calcium-rich groundwater. In lowland lakes with stronger connections to groundwater, calcium mass increased, suggesting a proportional increase in groundwater inputs from longer flow paths during drought. Lake groundwater interactions introduced lags in calcium recovery following drought. These lags were linked to water level changes in highland lakes and to more complicated interactions with groundwater flowpaths in lowland lakes. The detection of trends related to declining rates of acid deposition in sensitive highland lakes was hindered by these drought-induced responses. In contrast to the spatially-explicit responses of the groundwater-dominated lakes in Wisconsin, geomorphology appeared to control the dynamics of lakes in surface-water settings in Ontario, Canada. Here the response to drought was uniform among lakes. Further, lake chemistry exhibited no lags, but was directly related to fluctuations in precipitation drivers. Among 50 Wisconsin lakes, chlorophyll a responses to the 1988 drought were spatially specific to the landscape features of each lake. In lakes with forested watersheds, chlorophyll was higher in 1988 than average, likely a physiological response to increased temperature and light availability. In contrast, chlorophyll tended to be lower in lakes with agricultural watersheds, highlighting the strong effect of external sources of nutrients on lake water quality. Together, these results support the contention that landscape and geomorphic factors interact to control lake responses to strong climate signals like drought. The landscape perspective provides a powerful context for interpreting heterogeneity in average and dynamic properties of lakes.