Structuring features of lake districts: landscape controls on lake chemical responses to drought

1. Within a lake district of relatively homogeneous geomorphology, the resp onses of lakes to climate are influenced by the complexity of the hydrogeol ogic setting, position in the landscape, and lake-specific biological and p hysical features. We examined lake chemical responses to drought in surface water- and groundwater-dominated districts to address two general question s. (1) Are spatial patterns in chemical dynamics among lakes uniform and sy nchronous within a lake district, suggesting broad geomorphic controls; var iable in a spatially explicit pattern, with synchrony related to landscape position, suggesting hydrologic flowpath controls; or spatially unstructure d and asynchronous, suggesting overriding control by lake-speficic factors? (2) Are lake responses to drought a simple function of precipitation quant ity or are they dictated by more complex interactions among climate, unique lake features, and hydrologic setting? 2. Annual open-water means for epilimnetic concentrations of chloride, calc ium, sulfate, ANC, DOG, total nitrogen, silica, total phosphorus, and chlor ophyll a measured between 1982 and 1995 were assembled for lakes in the Red Lake and ELA districts of northwestern Ontario, the Muskoka - Dorset distr ict in south-central Ontario, and the Northern Highland district of Wiscons in. Within each district, we compared responses of lakes classified by land scape position into highland or lowland, depending on relative location wit hin the local to regional hydrologic flow system. Synchrony, defined as a m easure of the similarity in inter-annual dynamics among lakes within a dist rict, was quantified as the Pearson product-moment correlation (r) between two lakes with observations paired by year. To determine if solute concentr ations were directly related to interannual variations in precipitation qua ntity, we used regression analysis to fit district-wide slopes describing t he relationship between each chemical variable and annual (June to May) and October to May (Oct-May) precipitation. 3. Among lakes in each of the three Ontario districts, the pattern of chemi cal response to interannual shifts in precipitation was spatially uniform. In these surface water-dominated districts, solute concentrations were gene rally a simple function of: precipitation. Conservative solutes, like calci um, and chloride, tended to be more synchronous and were negatively related to precipitation. Solutes such as silica, total phosphorus, and chlorophyl l a, which are influenced by in-lake processes, were less synchronous and r elationships with precipitation tended to be positive or absent. 4. In the groundwater-dominated Northern Highland lakes of Wisconsin, we ob served spatial structure in drought response, with lowland lakes more synch ronous than highland lakes, However, there was no evidence for a direct rel ationship between any solute and precipitation. Instead, increases in the c oncentration of the conservative ion calcium during drought were not follow ed by a symmetrical return, to pre-drought conditions when precipitation re turned to normal or above-average values. 5. For calcium, time lags in recovery from drought appeared related to hydr ologic features in a complex way. Zn the highland Crystal Lake, calcium con centrations tracked lake stage inversely, with a return to pre-drought conc entrations and lake stage five years after the drought. This pattern sugges ts strong evaporative controls. In contrast, after five years of normal pre cipitation, calcium in the lowland Sparkling Lake had not returned to predr ought conditions despite a rebound in lake stage. This result: suggests tha t calcium concentrations in lowland lakes were controlled more by regional groundwater flowpaths, which. track climatic signals more slowly. 6. Temporal dynamics driven by climate were most similar among lakes in dis tricts that have a relatively simple hydrology, such as ELA. Where hydrolog ic setting was more complex, as in the groundwater-dominated Northern Highl and of Wisconsin, the expression of climate signals in lakes showed lags an d spatial patterns related to landscape position. In general, we expect tha t landscape and lake-specific factors become increasingly important in lake districts with more heterogeneous hydrogeology, topography or land use. Th ese strong chemical responses to climate need to be considered when interpr eting the responses of lakes to other regional disturbances.