Wildfire interacts with land cover to influence mercury concentrations in juvenile steelhead (Oncorhynchus mykiss) in coastal California watersheds

Wildfire is a growing driver of ecological change in freshwater ecosystems, yet how fire interacts with land cover to affect contaminant exposure in aquatic organisms remains poorly understood. Mercury is a particular concern; concentrations can increase to harmful levels following wildfire. We leveraged the 2020 wildfires in coastal California as a natural experiment to examine how human development and land cover influence total mercury concentrations in juvenile steelhead (Oncorhynchus mykiss). We predicted that human development and forest cover would both increase mercury levels in juvenile steelhead. We measured mercury in fish muscle tissue from 192 fish across paired burned and unburned sub-watersheds in four watersheds. Fire significantly altered mercury in three of four watersheds, but the direction of change depended on development and land cover. As predicted, the watershed with substantial human development showed elevated mercury following fire (+65%). Contrary to expectations, forest cover appeared to reduce fire-driven mercury in wildland watershed. Our results demonstrate that post-fire mercury risk to juvenile steelhead depends on landscape context and highlight the wildland-urban interface as a priority for post-fire contaminant monitoring.