The Laurentian Great Lakes provide extensive optical and trophic variability across diverse ecosystems, from environments challenged by current and legacy nutrient pollution and continuing water quality impairments, e.g., harmful algal blooms (HABs) and hypoxia, to relatively pristine aquatic systems with emerging water quality concerns such as Lake Superior. This project will conduct high density sampling of the Great Lakes in space and time to provide a diversity of reflectance spectra broadly representative of the fundamental inherent optical properties (IOPs) and biogeochemical conditions observed across inland and coastal systems globally. Station locations are responsive to in situ conditions and potential for PACE validation, but are generally located in Lake Eries western and central basins, southwestern Lake Michigan and Green Bay, and the central basin and the western arm of Lake Superior. Sampling uses coastal class vessels that enable rapid transit between stations and range in size from 25 to 70 ft. Data collected includes above water radiometry (SVC HR-512i), discrete spectral absorption (ap, aph, aNAP, aCDOM), hyperspectral backscattering (Sequoia hyper-bb), physical and optical biogeochemical variability (YSI EXO2 water quality sonde), and discrete biogeochemical parameters (HPLC pigments, DOC, POC, and SPM). Data will be collected in every month of the year over the course of a three-year period, ensuring seasonal matchups for PACE OCI science data products in the winter and shoulder seasons that remain chronically under observed.