|value||Water temperatures are warming in many aquatic ecosystems, resulting in range shifts and extirpations of native fishes. Lakes and streams provide diverse thermal habitat for many fish species, and connections between these ecosystems can be exploited by mobile biota as a climate change adaptation strategy. Given clear passage, coldwater stream fishes can take refuge in headwater streams providing suitable thermal habitat when higher order streams become too warm. Likewise, many temperate lakes thermally stratify and their deeper cold waters offer refugia from warming surface waters and connected streams. These intra- and inter-ecosystem connections are dynamic and influenced by multiple factors, including the spatial arrangement of thermal conditions, the degree of fragmentation in the hydroscape (e.g., road culverts and dams), and cycles of drought or heavy precipitation. To date, assessments of the effects of climate change on fishes have ignored the coupled nature of lakes and streams and instead have focused on each component individually. Because lentic and lotic ecosystems represent an interconnected network of aquatic habitats, an integrated assessment of thermal refugia under climate change is necessary for informed decision making regarding the preservation of lake/stream linkages, prioritization of fish passageways and restoration strategies, and targeted stocking efforts for high-value game fish. Fishery biologists, biogeochemists, and ecologists recognize temperature as a master factor of aquatic ecosystem processes. While existing NE CSC and UMGL LCC projects have generated regional databases of stream connectivity and historical stream water temperatures, a complementary assemblage of lake temperature data does not exist. Similarly, previous work has projected suitable fish habitat for streams throughout the NE CSC region under various climate change scenarios, allowing managers to prioritize stocking, riparian habitat restoration, and land acquisition activities to most efficiently and effectively conserve stream fish habitat under climate change. The absence of complementary lake temperature data means that similar prioritization efforts to understand and conserve lake thermal habitat is impossible, and that connections between stream and lake habitats are ignored. At present, the three state agencies in Wisconsin, Minnesota, and Michigan are using three different approaches to estimate future lake temperatures and to fill historical information gaps. These partner states are excited to collaborate on this proposal, which will generate lake temperature data using standardized, transparent, and repeatable methods that can be easily extended to other regions in the future. Our efforts will focus on a subset of the NE CSC region (the states of Minnesota, Wisconsin, and Michigan) where we have engaged with research partners and are familiar with challenges to accessing the regional data required for this analysis. This project will employ state-of-the-science methods to mechanistically hind-cast and forecast thermal habitat for nearly ten thousand lakes. These data will be combined with existing observations of fish occurrence and abundance, stream connectivity, and stream temperature data to predict suitable fish thermal habitat across lakes and streams. Fish species will include representation from different thermal guilds and lentic/lotic preferences. The integrated assessment performed will answer the following stakeholder-driven questions: How does the spatial arrangement of thermal characteristics in connected streams and lakes influence existing patterns of fish distribution? How will climate change affect fish thermal habitat in lakes and streams? Can connectivity among aquatic habitats result in recolonization of ecosystems projected to be periodically unsuitable due to warming? Our team is uniquely qualified to address these research questions, and includes hydrodynamic modelers, informaticists, aquatic ecologists, and fisheries biologists. We will be actively leveraging existing funding from the WI DNR, NCCWSC, and salary support from each institute and agency. We will generate and disseminate an integrated hind- and forecasted dataset of lake temperature profiles and associated fish-specific derivatives (e.g., growing degree days). These data, combined with the existing NE CSC and UMGL LCC datasets, will represent a substantial management tool and resource for the development of State Wildlife Action Plans.The results of this project will be used by partners and involved stakeholders to prioritize adaptation and restoration strategies across the region to preserve and enhance fish habitat while accounting for lakes and streams and source/sink dynamics between populations in connected systems. We are exploring the possibility of hosting data generated by this proposal with the NorEaST data portal. Regardless, we will provide these data products openly and in machine-readable formats.