Xinyu Sun , Kendra Spence Cheruvelil, Patrick Hanly, Katherine Webster, Patricia Soranno

Specific conductance (SC) and major ion composition are important for understanding and predicting lake water quality and ecosystem responses to global changes and human disturbances. However, little is known about SC and ionic composition for populations of lakes at the continental scale, nor their relationships with natural and human factors operating at multiple spatial scales. We examined the spatial patterns in SC (N=9,784 lakes) and major anion and cation concentrations (N=1,218 lakes) across the conterminous United States, and quantified their relationships with a wide range of multi-scaled natural and human factors. We found substantial spatial variation in ion composition and that lakes with similar SC values can have very different ion composition. Most lakes had relatively low SC (median=206μS/cm), with high-SC lakes mainly located in the Plains, Desert Southwest, and Florida. Calcium and bicarbonate were the most common ions in 61% of the study lakes, with the remaining lakes dominated by the cations magnesium or sodium and the anions sulfate or chloride. Lake SC was associated with natural factors including elevation, watershed soils, and hydrology and was influenced by watershed land uses. Ion composition was associated with similar natural factors along with surface connectivity and precipitation, but also strongly affected by road density and urban development. Our results suggest that while geological, hydrological, and climate processes control the ion inputs from natural sources, human disturbances can cause SC and major ions to deviate from their background levels.