"We have toxic algae events that result in shellfish closures off the Washington and Oregon coast every three to five years or so, but none of them have been as large as this one," said lead author Ryan McCabe, a research scientist at the UW's Joint Institute for the Study of the Atmosphere and Ocean, a collaborative center with NOAA.
"This one was entirely different, and our results show that it was connected to the unusual ocean conditions."
"This paper is significant because it identifies a link between ocean conditions and the magnitude of the toxic bloom in 2015 that resulted in the highest levels of domoic acid contamination in the food web ever recorded for many species," said co-author Kathi Lefebvre, a marine biologist at NOAA's Northwest Fisheries Science Center.
"This is an eye-opener for what the future may hold as ocean conditions continue to warm globally."
The authors found that the 2015 harmful algal bloom, which set records for its spatial extent and the level of toxicity, was dominated by a single species of diatom, Pseudo-nitzschia australis, normally found farther south off California.
Warm water not only allowed this species to survive, it also created an environment favoring its growth. By early 2015 the warm "blob" had moved toward shore and spread all along the West Coast. Warmer water creates less dense surface water that is more likely to stay floating on the surface, where it can become depleted in nutrients.
Previous laboratory studies by co-author William Cochlan of San Francisco State University showed that P. australis can take up nitrogen very quickly from a variety of sources, and appear to outcompete other, nontoxic phytoplankton in nutrient-depleted warm water.
For the new study, Cochlan's lab performed experiments with P. australis from the 2015 bloom. They showed that when these cells experience warmer temperatures and get more nutrients they can double or triple their cell division rates, allowing them to potentially bloom into a large population fairly quickly at sea.
"When springtime shifts in wind direction brought deeper, nutrient-rich water upward near the coast, a small population of P. australis became a big population, which was then washed ashore along the West Coast by late spring storms," said co-author Barbara Hickey, a UW professor of oceanography.
This was especially damaging in a year dominated by P. australis.
