Isabella Anna Oleksy, Sarah Collins, Samuel J. Sillen, Simon Topp, Xiao Yang, Edward K. Hall, Miles T. Austin, Catherine M. O'Reilly, Matthew R.V. Ross

Global change may contribute to ecological changes in high-elevation lakes and reservoirs, but a lack of data makes it difficult to evaluate spatiotemporal patterns. Remote sensing imagery can provide more complete records to evaluate whether consistent changes across a broad geographic region are occurring. We used Landsat surface reflectance data to evaluate spatial patterns of contemporary lake color (2010-2020) in 940 lakes in the U.S. Rocky Mountains, a historically understudied area for lake water quality. Intuitively, we found that most of the lakes in the region are blue (66%) and were found in steep-sided watersheds (>22.5º) or alternatively were relatively deep (>4.5m) with mean annual air temperature (MAAT) <4.5ºC. Most green/brown lakes were found in relatively shallow sloped watersheds with MAAT ≥4.5ºC. We extended the analysis of contemporary lake color to evaluate changes in color from 1984-2020 for a subset of lakes with the most complete time series (n=527). We found limited evidence of lakes shifting from blue to green states, but rather, 55% of the lakes had no trend in lake color. Surprisingly, where lake color was changing, 32% of lakes were trending toward bluer wavelengths, and only 13% shifted toward greener wavelengths. Lakes and reservoirs with the most substantial shifts toward blue wavelengths tended to be in urbanized, human population centers at relatively lower elevations. In contrast, lakes that shifted to greener wavelengths did not relate clearly to any lake or landscape features that we evaluated, though declining winter precipitation and warming summer and fall temperatures may play a role in some systems. Collectively, these results suggest that the interactions between local landscape factors and broader climatic changes can result in heterogeneous, context-dependent changes in lake color.