Polycyclic Aromatic Hydrocarbons (PAHs) in Marine Sediments: Distribution, Sources, and Biological Risks in the Habitat of Resident Killer Whales and their Prey in British Columbia, Canada

Polycyclic aromatic hydrocarbons (PAHs), a class of persistent, bioaccumulative, and toxic contaminants, represent a threat to at-risk populations of Resident killer whales (Orcinus orca) in the northeastern Pacific Ocean, and their primary prey, Chinook salmon (Oncorhynchus tshawytscha). Concentrations of 76 PAHs in subtidal surface sediments (1 to 435 m depth) from 98 sites along the coast of British Columbia (BC), Canada, were analyzed to evaluate their distribution patterns and potential sources. A subset of parent and alkylated PAHs, including the 16 priority PAHs, was evaluated to assess biological risks to Resident killer whales and Chinook salmon. Principal component analysis (PCA) revealed site-specific PAH patterns in sediments, characterized by heavier parent PAHs (4-6 aromatic rings) and lighter alkylated PAHs (2-3 aromatic rings). Polycyclic aromatic hydrocarbon distribution and profiles were strongly influenced by octanol-water partition coefficients (Log Kow) and total organic carbon (TOC) (p < 0.001), with water depth exhibiting a gradient from heavier PAHs in shallow sediments to lighter PAHs in deeper sediments (p < 0.001). Hotspots of pyrogenic and petrogenic sources were identified along the BC coast, with most areas being contaminated by pyrogenic inputs primarily associated with combustion of lignite and bituminous coal, and petrogenic inputs mainly associated with coal and diesel oil. Biological risk, based on sediment quality guidelines (SQGs), risk quotients (RQs), and toxic equivalents (TEQs), suggested varying degrees of direct and indirect impacts for Chinook salmon in nearshore Conservation Units (CUs), as well as for Resident killer whales within their Critical Habitats. Among Resident killer whale Critical Habitats, the greatest impacts were expected in the Southern Resident Killer Whale (SRKW) Critical Habitat around southern Vancouver Island, from both potential direct and indirect effects, with direct impacts from alkylated PAHs requiring special consideration. These findings highlight the urgent need for targeted management actions to reduce PAH emissions from both point and non-point sources in coastal environments to support conservation efforts for Chinook salmon and Resident killer whales. In particular, for Resident killer whales, such efforts should be prioritized within the SRKW Critical Habitat surrounding southern Vancouver Island.