Investigating Drivers of Long-Term Trends in Algal Growth on Utah Lake using Earth Observation and Local Process Data

Since the 1970s, remotely sensed (RS) data have provided valuable estimates of surface water quality parameters globally . Unlike in-situ water samples, which have limited spatial and temporal scope, RS data can provide long time-histories of spatially comprehensive water quality data. We investigated historical water quality trends on Utah Lake, a shallow, turbid, eutrophic lake in the semi-arid Utah Valley in Utah, USA. Utah Lake is a vital natural resource embroiled in controversy because much of the lake's current and historical ecological functioning is poorly understood, and there are differing views on how best to manage threats to the lake's health, including high nutrient loads, frequent algal blooms, and invasive species. We used data from the Landsat series of satellites to construct a 40-year time-series of estimated water column chlorophyll-a (chl-a) concentrations, an index for general water quality, every 8 or 16 days. We used the Mann-Kendall statistical test to determine trends in chl-a concentrations for each individual pixel over the 40-year study period and estimated the magnitude of that trend. We analyzed spatial patterns in the lake both by examining trends for each image pixel in the lake and for regions associated with various inflows and processes. This included inter- and intra-seasonal variation in chl-a concentrations to investigate potential impacts of climate change on algal growth in the lake. As shown in the figure, we found a small decreasing trend in chl-a concentrations since 1984, with high inter and intra-seasonal variability. To characterize driving factors, we compared the chl-a data with other RS data, including turbidity and temperature, as well as local process information such as lake level measurements, weather data, lake mixing models, and estimates of nutrient inflows to identify correlations that will improve our understanding of how the lake functions and what factors drive algal growth in the lake. Although this study is focused on Utah Lake, we anticipate that the tools, statistical tests, and analysis methods we used can serve as a template for other researchers using RS data to investigate historical water quality trends in surface water bodies around the world.