Ocean Conditions Contributed to Unprecedented 2015 Toxic Algal Bloom

A study led by researchers at the University of Washington and the National Oceanic and Atmospheric Administration connects the unprecedented West Coast toxic algal bloom of 2015 that closed fisheries from southern California to northern British Columbia to the unusually warm ocean conditions -- nicknamed "the blob" -- in winter and spring of that year.

"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."

The study is now online in Geophysical Research Letters, a journal of the American Geophysical Union.  "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.
...
"There's a significant connection there," McCabe said.  "The toxic events also tend to coincide with previously established marine ecosystem shifts.  We had not made that connection before, and I think it's fascinating."  Ocean climate cycles could help understand and better predict the emergence of toxic algal blooms.  And while the blob was a one-time event that was not due to global warming, it provides a window into what climate change might look like.

Read more at Ocean Conditions Contributed to Unprecedented 2015 Toxic Algal Bloom