Long-term trends and drivers of water color in Missouri reservoirs

Contrasting water quality trends are occurring within and across North America, with waterbodies experiencing increasing phytoplankton blooms, increasing dissolved organic matter, or both. Simultaneously, other waterbodies are becoming clearer and bluer; dramatically changing water color. To assess the spatial and temporal variability in water color, we quantified trends in satellite-derived dominant wavelength (λd) from 1984 to 2020 from the LimnoSat-US for 478 reservoirs in Missouri, USA. We also analyzed trends in summer water quality (WQ) parameters from two long-term monitoring programs to compare with observed water color changes. We demonstrate that λd is a robust indicator of water quality, including nutrients (total nitrogen and total phosphorus) that are not typically associated with satellite-derived data. Currently, the vast majority of Missouri reservoirs (94%) are classified as green and within a range (538−555 nm) that lies closer to the brown, rather than blue, color endmember. Nearly one-third of reservoirs (n = 155) experienced significant temporal shifts in water color, with more (n = 92) negative (e.g., bluer) than positive (n = 63) λd trends; although shifts were largely confined to the green region of the visible spectrum. This result agrees with observed WQ trends within individual reservoirs that show indices of eutrophication and nutrient reductions. Linear mixed-effect models indicate that periods of extreme wetness and drought are associated with browner and bluer waters, respectively, and boosted regression trees further reveal that waterbody and watershed characteristics are important predictors for water color trends. Our results help explain some of the previously observed heterogeneous controls on water color and emphasize the importance of integrating water quality data alongside commonly used landscape and morphological features. This is important not only to better understand regional trends in water color, but also to link these trends to changes in watershed characteristics and their impact on waterbody-specific processes.