Annual and inter-annual variability in the diffuse attenuation coefficient and turbidity in an urbanized Washington lake from 2013 to 2022 assessed using Landsat-8/9

Water clarity, defined in this study using measurements of the downwelling diffuse light attenuation coefficient (K_d) and turbidity, is an important indicator of lake trophic status and ecosystem health. We used in-situ measurements to evaluate existing semi-analytical models for K_d and turbidity, developed a regional turbidity model based on spectral shape, and evaluated the spatial and temporal trends in Lake Washington from 2013 to 2022 using Landsat-8/9 Operational Land Imager (OLI). We found no significant trends from 2013 to 2022 in K_d or turbidity when both the annual and full datasets were considered. In addition to the spring peak lasting from April through June, autumn K_d peaks were present at all sites, a pattern consistent with seasonal chlorophyll a and zooplankton concentrations. There existed no autumn peak in the monthly turbidity dataset, and the spring peak occurred two months before the K_d peak, nearly mirroring seasonal variability in the Cedar River discharge rates over the same period. The K_d and turbidity algorithms were thus each more sensitive to different sources of water clarity variability in Lake Washington.