Salting our freshwater lakes

Abstract

The highest densities of lakes on Earth are in north temperate ecosystems, where increasing urbanization and associated chloride runoff can salinize freshwaters and threaten lake water quality and the many ecosystem services lakes provide. However, the extent to which lake salinity may be changing at broad spatial scales remains unknown, leading us to first identify spatial patterns and then investigate the drivers of these patterns. Significant decadal trends in lake salinization were identified using a dataset of long-term chloride concentrations from 371 North American lakes. Landscape and climate metrics calculated for each site demonstrated that impervious land cover was a strong predictor of chloride trends in Northeast and Midwest North American lakes. As little as 1% impervious land cover surrounding a lake increased the likelihood of long-term salinization. Considering that 27% of large lakes in the United States have >1% impervious land cover around their perimeters, the potential for steady and long-term salinization of these aquatic systems is high. This study predicts that many lakes will exceed the aquatic life threshold criterion for chronic chloride exposure (230 mg L^-1), stipulated by the US Environmental Protection Agency (EPA), in the next 50 y if current trends continue.

Keywords: chloride; ecosystem services; impervious surface; limnology; road salt.

Fig. 1.

Chloride trends for North American freshwater lakes (circles and squares, n = 371). The states and province included in the NALR are outlined in black. Points are colored by the slope value of linear regression models (red, positive slope; yellow, negative slope; purple, zero or nonsignificant slope). Squares denote lakes with at least biennial chloride concentrations recorded from 1985 to 2010 (n = 56). These LTC datasets are a subset of lakes in the NALR, which is a region of dense sampling (n = 284). Upper Inset of chloride time series from 1985 to 2010 are colored by slope value. Road salt application rates for North American provinces and states range from 0 to 35 US tons per mile and are shown in blue. No salt application rates were available in areas with hatched lines. The lengths of all individual datasets (dark green) as well as the lengths of LTC datasets (light green) are shown in the Inset histogram.

Fig. 2.

(A) LTC lakes (n = 56) with biennial chloride data from 1985 to 2010 grouped into three clusters using a hierarchical cluster analysis. In general, the three clusters show a neutral/decreasing (cluster 1), oscillating (cluster 2), or increasing (cluster 3) pattern. Thick black lines are GAMs fit to all lakes within each cluster, to represent the average pattern. (B) Histograms display the number of lakes in each cluster by linear slope (yellow, negative slope; purple, zero slope; red, positive slope).

Fig. 3.

Scatterplots of linear regression slope values versus (A) impervious surface within a 500-m buffer, (B) road density within a 500-m buffer, (C) rate of atmospheric salt deposition, and (D) mean in-lake chloride concentration over the entire time series for all NALR sites (n = 284). In all plots, the size of the symbol is scaled by lake area. Squares with black borders denote LTC lakes. In A and B, zero values have been adjusted to fit on the x axis and are highlighted in gray.

Fig. 4.

(A) Distribution of impervious land cover within a 500-m buffer of all lakes >4 ha in the lower 48 United States (n = 149,350). Black squares represent the median impervious land cover percentage in each state. Thick horizontal black lines denote the interquartile range of the distribution, and thin black lines extend to 1.5 times the interquartile range. The vertical dashed line is shown at impervious land surface = 1%. Circles represent lakes included in this study, colored by slope (yellow, negative slope; purple, zero slope; red, positive slope). Due to the frequency of zero values on the x axis, circles are spread out within the gray rectangle. Percentages following y axis labels represent the percent of lakes in that state with greater than 1% impervious land cover within a 500-m buffer. In states with >10 lakes present in the dataset, an asterisk denotes that the sampling distribution in our dataset was significantly different from statewide distribution (Mann–Whitney test, P < 0.05), and ^ denotes that the sampling distribution was not significantly different from statewide distribution. (B) Chloride trends, as represented by linear regression model fits, are shown for four states with relatively large sample sizes (New York, Minnesota, Wisconsin, and Rhode Island). The dotted gray line demarcates the EPA’s aquatic life criterion of 230 mg L⁻¹.