Rapid downward transport of the neurotoxin domoic acid in coastal waters

Toxic phytoplankton blooms threaten coastlines worldwide by diminishing beach quality and adversely affecting marine ecosystems and human health1,2. The common diatom genus Pseudo-nitzschia consists of several species known to produce the neurotoxin domoic acid3. Recent studies suggest that algal blooms dominated by Pseudo-nitzschia are increasing in frequency and duration owing to changes in coastal nutrient regimes1,4,5. However, few studies have examined the persistence or long-term biogeochemical cycling of domoic acid in marine waters6–8. Here, we measure the concentration of domoic acid in surface waters and sediment traps—up to 800 m in depth—off the coast of Southern California. We show that peaks in Pseudo-nitzschia abundance and domoic acid concentrations in surface waters coincide with peaks in diatom and toxin abundance at depth, suggesting rapid downward transport of the toxin. In some cases, the sinking particles contain over five times the United States federal limit of domoic acid. Detection of domoic acid in bottom sediments indicates that the toxin may persist long after the Pseudo-nitzschia blooms. Our results indicate that vertical fluxes of domoic acid are a substantial source of the toxin to deep-ocean food webs, and could explain high levels of domoic acid previously observed in benthic organisms9,10. Pseudo-nitzschia species have been implicated in toxic blooms throughout coastal waters of Europe (for example Scotland, Spain), Asia (South Korea) and along the coasts of North America3. In western US waters, where Pseudo-nitzschia blooms seasonally, toxic cells are commonly consumed by sardines and anchovies, and the corresponding trophic transfer to higher organisms has resulted in mortalities of cetaceans, California sea lions and marine birds11–13. Consumption of seafood contaminated domoic acid (DA) can also affect humans, causing symptoms frommild gastrointestinal illness and headaches to memory loss, disorientation, seizures and even death14. Preventative beach closures help avert human illness, but closures are becoming increasingly routine and result in millions of dollars in economic losses2. The factors that govern the inception and demise of Pseudonitzschia blooms are poorly understood, particularly with respect