Lakes integrate natural and anthropogenic signals from climate and the surrounding landscape. However, in the U.S., our understanding of large-scale changes in these systems is limited because of historical biases in when and where ground-based monitoring of lake water quality occurred. Here, we use 35 years of Landsat imagery (1984-2018) to generate time series of summertime lake water clarity for two representative samples of U.S. lakes: the statistical sample used in the 2012 National Lake Assessment (n = 1,029), and a large random sample of lakes from the National Hydrography Dataset (NHD) (n = 13,362). We aggregate these time series at the ecoregion scale and estimate trends in clarity over the past 35 years.
Results/Conclusions
Results show an increase in water clarity across at least 7 of the 9 ecoregions, with the largest increases ( >0.8 cm yr-1) in the Xeric West and Northern Appalachians. These increases in clarity are concentrated in lakes smaller than 10 km2, which show a median increase of 0.27 cm yr-1 compared to 0.08 cm yr-1 for lakes larger than 10 km2. The observed patterns suggest that local and federal management efforts have decreased nutrient and sediment loading to freshwater resources over the study period, with greater impacts in smaller waterbodies due to their higher responsiveness to watershed-scale biogeophysical changes. These results highlight the need for systematic monitoring programs that target a broad array of biophysically different lakes to better understand how to protect freshwater resources in the future.