Alejandra Mora-Soto ^1* Sarah Schroeder ¹ Lianna Gendall ^1,2 Alena Wachmann ¹ Gita Roshni Narayan ³ Silven Read ¹ Isobel Pearsall ⁴ Emily Rubidge ⁵ Joanne Lessard ⁶ Kathryn Martell ⁷ Maycira Costa ¹

• ¹ University of Victoria, Victoria, Canada
• ² University of Western Australia, Perth, Western Australia, Australia
• ³ Vancouver Island University, Nanaimo, British Columbia, Canada
• ⁴ Pacific Salmon Foundation, Vancouver, British Columbia, Canada
• ⁵ Institute of Ocean Sciences, Fisheries and Oceans Canada (DFO), Sidney, British Columbia, Canada
• ⁶ Pacific Biological Station, Fisheries and Oceans Canada (DFO), Nanaimo, British Columbia, Canada
• ⁷ Islands Trust Conservancy, Victoria, Canada

The Salish Sea, a dynamic system of straits, fjords, and channels in southwestern British Columbia, is home to ecologically and culturally important bull kelp (Nereocystis luetkeana) forests. Yet the long-term fluctuations in the area and the persistence of this pivotal coastal marine habitat are unknown. Using very high-resolution satellite imagery to map kelp forests over two decades, we present the spatial changes in kelp forest area within the Salish Sea, before (2002 to 2013) and after (2014 to 2022) the 'Blob,' an anomalously warm period in the Northeast Pacific. This analysis was spatially constrained by local environmental conditions. Based on nearshore sea surface temperatures (SSTs) from four decades (1984-2022), we found two periods of distinct increases in SST, one starting in 2000 and another in 2014. Further, the highest SST anomalies occurred on warmer coastlines in the enclosed inlets and the Strait of Georgia, while smaller anomalies were found on colder coastlines near the Strait of Juan de Fuca and the Discovery Passage. The total area of bull kelp forests from 2014 to 2022 has decreased compared to 2002 to 2013, particularly in the northern sector of the Salish Sea. Using the satellite-derived kelp data, we also present an analysis of kelp persistence compared with historical distribution of kelp forests depicted on British Admiralty Nautical Charts from 1858 to 1956. This analysis shows that warm, sheltered areas experienced a considerable decrease in persistence of kelp beds when compared to satellite-derived distribution of modern kelp, confirming a century-scale loss. In particular, the presence of kelp forests in the Strait of Georgia and on the warmest coasts has decreased considerably over the century, likely due to warming temperatures. While the coldest coasts to the south have maintained their centennial persistence, the northern Salish Sea requires further research to understand its current dynamics. This research contributes to a wider understanding of temporal and spatial factors for kelp from the regional perspective of the Salish Sea.