We will employ a three-tiered strategy to acquire PACE OCI validation data in the Great Lakes that includes recurring seasonal and weekly monitoring cruises, a dedicated “super-site”, and other cruises of opportunity. All validation activities will be linked to externally funded monitoring programs, leveraging ship-time and lab processing capabilities. All measurement activities will follow best practices established in the IOCCG protocol series and will include documented uncertainty.  Specifically, we will collect a full suite of inherent and apparent optical properties along with paired laboratory measured biochemical parameters. Discrete station vertical profiles along with flow-through surface observations will be made.