Historical data reveal a half-century of kelp forest extirpation and community deborealization in a geographically isolated microclimateLong-term Temporal Trends

Brian Timmer^{1, 2}, Julia Baum¹, Luba Reshitnyk³, Chris Neufeld^{4, 5}

¹University of Victoria, Canada - ²National Geographic, USA - ³Hakai Institute, Canada - ⁴LGL limited, Canada - ⁵University of British Columbia Okanagan, Canada

Historical records of nearshore habitat are rare, as are accurate long-term historical baselines from which to compare modern communities and understand long-term climate effects. Recent marine heatwaves have resulted in the extirpation of habitat-forming foundation species, however, multi-decadal climate warming is also impacting marine habitats and their associated nearshore communities. In temperate coastal waters, complex landscapes of glacial fjords and inland seas can create microclimates in which rapid warming can occur compared to the open coast. Using archival data, we investigated a half-century of nearshore habitat change within a geographically isolated, rapidly warming microclimate on the coast of British Columbia, Canada. First, we created detailed historical baselines for bull kelp (Nereocystis luetkeana) extent, derived from historical aerial imagery, and compared them with modern aerial surveys. Then, at the community level we resurveyed historical subtidal transects to understand how the composition of temperate nearshore communities and their community temperature index (CTI) have shifted under climate change. Together, these data show large-scale extirpation of more than 500 hectares of bull kelp forests occurred decades earlier than previously recorded, increasing the historical baselines of kelp extent in this region by more than ten times previous estimates. Moreover, shifts in the CTI of associated nearshore seaweed communities have been due primarily to deborealization rather than topicalization, meaning the abundance of cold affinity species is decreasing faster than warm affinity species are increasing. These changes are likely due to the geographic isolation common in coastal microclimates. This work underscores the importance of accurate historical baselines for understanding climate-related community changes in coastal microclimates.