Evaporation and groundwater fluxes are thought to regulate hydrologic variability in lakes of the northern Great Plains, but little is known of how the relative importance of these processes may vary in time or space. To address this issue, we measured the isotopic composition of water (d 18 O, d 2 H) from 70 closed-basin lakes in southern Saskatchewan, Canada. All lakes occupied endorheic basins along a long gradient of salinity (0.2-115 g total dissolved solids L 21 ). Lakes exhibited synchronous seasonal changes in salinity (synchrony, S 5 0.78) and d 18 O (S 5 0.84) during the dry summer of 2003 (,195 mm rain), whereas coherence was reduced to 0.56 and 0.22, respectively, during the wet summer of 2004 (,295 mm rain). However, despite evaporative enrichment of isotopic ratios during dry summers, hydrologic balances were regulated mainly by changes in water inflow (I) rather than evaporation (E) in both wet and dry years, with particularly strong influence of inflow (lowest E : I ratio) in dry southwestern regions. Analysis of isotopic composition also identified winter precipitation or groundwater as the most influential source of water to most lakes, despite only ,30% of annual precipitation being delivered during winter. Therefore, although seasonal variability in lake chemistry was influenced by evaporation during summer, long-term mean chemical characteristics of prairie lakes were regulated mainly by changes in winter precipitation or groundwater influx.Lakes are abundant in the northern Great Plains (Last 1992) despite intense evaporation (Pham et al. 2008) and net precipitation deficits of 40 cm yr 21 to 60 cm yr 21 (Laird et al. 1996). In general, inter-annual variability of meteorological conditions that affect the persistence of lakes (temperature, seasonal precipitation) is regulated by the interplay between air masses arising over the Arctic, Pacific Ocean, and Gulf of Mexico (Bryson and Hare 1974), as well as global atmosphere-ocean systems such as the El Niñ o-Southern Oscillation (ENSO; Trenberth and Hurrell 1994), North Atlantic Oscillation (NAO; Hurrell 1995), and the Pacific Decadal Oscillation (PDO; Mantua et al. 1997). In addition, at a regional scale, the spatial and temporal variability of prairie lake hydrology is also affected by groundwater fluxes (van der Fritz et al. 2000) and by an unusually high supply of runoff from large catchments, most of which is derived from winter precipitation (Steppuhn 1981;Akinremi et al. 1999). However, despite these generalities, little is known of the specific conditions under which evaporation or water influx may control changes in lake chemistry and persistence, nor of the relative importance of summer and winter precipitation in the hydrologic budget of local and regional lakes. Consequently, an improved understanding of the basic hydrology of prairie lakes is needed to both forecast the effects of future climate change on lakes of the northern Great Plains, and to better interpret paleolimnological records of past climate variability in th...

Evaluation of the effects of climate change and human activities on lakes requires improved understanding of how stressors interact and the degree to which individual sentinel lakes represent broad spatial patterns of ecosystem response to disturbance. We surveyed modern water chemistry (major ions, conductivity, salinity, lake volume) and sediments (algal pigments, stable isotopes) in 21 lakes that surround Humboldt Lake, Saskatchewan, site of a 2,000-yr climate reconstruction, to quantify spatial synchrony (S, the mean among-lake correlation coefficient) of prairie lake response to climate variability, land use, and their interactions. Whole-lake mass balances of total dissolved substances constructed at each site revealed that evaporation of water controlled seasonal changes in salt content only in years with dry summers (2003), leading to widespread spatial coherence of ecosystems (S 5 0.78). In contrast, variations in hydrologic inputs (precipitation, groundwater) and solute fluxes regulated salt balances of lakes during years with wet summers (2004, 2005) and substantially reduced lake synchrony (S 5 0.13-0.58). Furthermore, .25% of sites exhibited increased nitrogen influx (as d 15 N) and cyanobacterial production (as fossil pigments) between ca. 1920 and 2003, with particularly strong effects of land use recorded for northeastern sites, where evaporative forcing was greatest. Finally, principal component and canonical ordinations with redundancy analysis both explained ,50% of the variance in lake sensitivity to climate and land use and revealed that the effects of climate and land use interacted strongly, but that the unique effects of each factor remained identifiable in modern lake surveys.

Endorheic lakes of the northern Great Plains encompass a wide range of environmental parameters (e.g., salinity, pH, DOC, Ca, nutrients, depth) that vary 1000-fold among sites and through the past 2000 years due to variation in basin hydrology and evaporative forcing. However, while many environmental parameters are known to individually influence zooplankton diversity and taxonomic composition, relatively little is known of the hierarchical relationships among potential controls or of how regulatory mechanisms may change in response to climate variation on diverse scales. To address these issues, we surveyed 70 lakes within a 100 000 km 2 prairie region to simulate the magnitude of environmental change expected to occur over 100-1000 years and to quantify the unique and interactive effects of diverse environmental parameters in regulating pelagic invertebrate community structure at that scale. Multivariate analyses showed that salinity was the principal correlate of changes in invertebrate composition among lakes, with a sequential loss of taxa between salinities of 4 and 50 g total dissolved solids L À1 until one to two species predominated in highly saline systems. In contrast, changes in the concentrations of Ca 2 1 and other mineral nutrients exerted secondary controls of invertebrate assemblages independent of salinity, whereas lake depth provided a tertiary regulatory mechanism structuring species composition. In contrast to these large-scale hierarchical patterns, seasonal surveys (May, July, September) of a subset of 21 lakes in each of 2003-2005 revealed that annual meteorological variation had no measurable effect on pelagic invertebrates, despite large differences in temperature, precipitation, and evaporation arising from regional droughts. Together these findings show that pelagic invertebrate communities in saline lakes are resilient to interannual variability in climate, but suggest that lakes of the northern Great Plains may provide a sensitive model to forecast centennial effects of future climate change.