This report is an assessment of key physical, chemical, and biological attributes that influence the natural and cultural resources of western mid-lake Lake Michigan. It is a synthesis of existing scientific literature and a collection of new ecological data that substantially expands the body of ecological knowledge within Lake Michigan. The impetus of this report when conceived in 2015 was to provide ecological information that would support the designation and management of the proposed Wisconsin-Lake Michigan National Marine Sanctuary. The sanctuary was nominated to protect a nationally-significant collection of shipwrecks and join a network of underwater parks protected in part by the Office of National Marine Sanctuaries (ONMS). For over 30 years the National Centers for Coastal Ocean Science (NCCOS) has supported the ONMS with information to develop effective conservation and management tools for many of the nation’s marine sanctuaries. This report was designed to inform the development of an environmental impact statement, a sanctuary management plan, and to serve as a baseline for future sanctuary condition reports. For example, many of the ecological questions asked in a future ONMS-required sanctuary condition report (e.g., what is the eutrophic condition of sanctuary waters?) can be answered with the information provided in this report. In addition, this report can serve as an instrument for engaging local communities, research partners and resource managers in a common, accessible, and scientifically sound body of knowledge. Since the proposed Wisconsin-Lake Michigan National Marine Sanctuary was nominated to protect a nationally significant collection of historic shipwrecks and maritime-related underwater cultural sites, this report focuses in part on ecological factors which influence the preservation and conservation of underwater archaeological resources. Accordingly, this report characterizes the status and trends of the lakebed, water quality, lake ice, invasive mussels, and fish community within the proposed sanctuary. These ecological components are linked to physical preservation and conservation, site access, and public perception of submerged cultural resources. Equally important, however, characterizing these aspects of the proposed sanctuary can serve a much broader purpose in the overall conservation of Lake Michigan.