Spatial variability of climate and land‐use effects on lakes of the northern Great Plains

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 = 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 = 0.13‐0.58). Furthermore, >25% of sites exhibited increased nitrogen influx (as d15N) 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.