Stable isotope analysis reveals differences in domoic acid accumulation and feeding strategies of key vectors in a California hotspot for outbreaks.

Given the effects of harmful algal blooms (HABs) on human and wildlife health, understanding how domoic acid (DA) is accumulated and transferred through food webs is critical for recognizing the most affected marine communities and predicting ecosystem effects. This study combines stable isotopes of carbon (δC) and nitrogen (δN) from bulk muscle tissue with DA measurements from viscera to identify the foraging strategies of important DA vectors and predators in Monterey Bay, CA. Tissue samples were collected from 27 species across three habitats in the summer of 2018 and 2019 (time periods without prominent HABs). Our results highlight an inshore-offshore variation in krill δC values and DA concentrations ([DA]; ppm) in anchovies indicating differences in coastal productivity and DA accumulation. The narrow overlapping isotopic niches between anchovies and sardines suggest similar diets and trophic positions, but striking differences in [DA] indicate a degree of specialization, thus, resource partitioning. In contrast, krill, market squid, and juvenile rockfish accumulated minimal DA and had comparatively broad isotopic niches, suggesting a lower capacity to serve as vectors because of potential differences in diet or feeding in isotopically distinct locations. Low [DA] in the liver of stranded sea lions and their generalist foraging tendencies limits our ability to use them as sentinels for DA outbreaks in a specific geographic area. Collectively, our results show that DA was produced a few kilometers from the coastline, and anchovies were the most powerful DA vector in coastal-pelagic zones (their DA loads exceeded the 20 ppm FDA regulatory limits for human consumption), while mussels did not contain detectable DA and only reflect in situ DA, δC, and δN values. Our study demonstrates the efficacy of combining multiple biogeochemical tracers to improve HAB monitoring efforts and identify the main routes of DA transfer across habitats and trophic levels.