Heterogenous controls on lake color and trends across the high-elevation U.S. Rocky Mountain region

Abstract

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.

Long-term trends in productivity across Intermountain West lakes provide no evidence of widespread eutrophication

“…Lake-specific characteristics reveal that lakes improving in water quality were in more developed and less forested catchments, as well as at lower elevations. These results are consistent with studies on water clarity , lake color (Oleksy et al, 2022), and chlorophyll-a (Wilkinson et al, 2022), that highlight improvements in water quality metrics over the same time period. These trends have been hypothesized to be the result of management actions or restoration projects (Wilkinson et al, 2022), although we lacked the information to make conclusions about the mechanisms of these trends.…”

“…manuscript submitted to Water Resources Research lakes have been shown to be increasing in the same metric over the past 40 years (Wilkinson et al, 2022). In addition, a minority of lakes (13%) in the Rocky Mountain region have shown to be shifting from blue to greener wavelengths during this time (Oleksy et al, 2022). With our analysis, we show that concerns regarding the widespread intensification of algal blooms are not captured in our analysis of chlorophyll-a and trophic state.…”

“…However, in shallow lakes, warmer temperatures and higher light absorption have been found to be more significant drivers of productivity (Kosten et al, 2012). In other words, the combination of manuscript submitted to Water Resources Research factors that drive rapid increases in lake productivity may differ between individual water bodies or geographic regions, hence smaller and more focused state and regional scale studies may be more useful in describing changes in lake productivity dynamics (Oleksy et al, 2022).…”

“…However, this increase in awareness and reporting has the potential to create a perception that blooms are already increasing in intensity and frequency (Hallegraeff et al, 2021). Recent work in the region highlights that lakes are experiencing roughly equal trends of changing from blue to green or changing green to blue, indicating there is not overwhelming evidence that they are getting more eutrophic, where eutrophic lakes are generally more green manuscript submitted to Water Resources Research (Oleksy et al, 2022). It remains unclear whether this is a result of representative increases in intensity or a result of heightened monitoring.…”

Shifts, Trends, and Drivers of Lake Color Across China Since the 1980s

“…Satellite images have been utilized to investigate spatial distributions of lake color (Giardino et al., 2019; Lehmann et al., 2018; Topp et al., 2021; Yang et al., 2022). However, temporal shifts and trends in lake color and relevant drivers are rarely documented (Kuhn & Butman, 2021; Lehmann et al., 2019; Oleksy et al., 2022).…”