Toxic algal bloom off Washington, US. Left is the natural colour, right has been enhanced to reveal chlorophyll concentrations (Image: SeaWiFS Project, NASA/Goddard Space Flight Center/ORBIMAGE)

Toxic algal blooms are bad enough on the ocean surface, but now it turns out that the toxin in them sinks to the ocean floor – where it persists for weeks.

Far from degrading soon after the bloom, as previously assumed, new research suggests that the neurotoxin that causes shellfish poisoning, domoic acid, sinks to the ocean floor and could poison marine mammals, birds and humans.

“The first signs of an algal bloom are often birds washing up on the shore or seals acting funny, aggressive and twitching, looking as if they were drunk,” says Claudia Benitez-Nelson of the University of South Carolina.


“We used to think that once the bloom died, the danger was over, but now it turns out that domoic acid is a ‘gift’ that just keeps on giving.”

Benitez-Nelson’s team are the first to look for the chemical in algae particles sinking through the ocean, as well as in sediment samples on the ocean floor, up to 800 metres down. They found copious amounts of the neurotoxin, reaching concentrations eight times the US federal limit for the substance in shellfish.

Toxic shock

The team also compared the peak of domoic acid levels from the sediment with those of algae blooms at the surfaces. Their findings indicate that the toxin reaches the bottom of the ocean in only three days but stays there for much longer – at least several weeks.

The speedy trip to the bottom is probably driven by dead algae clumping together at the surface to form heavier aggregates, says the team, a process that also protects the toxin from degradation.

Domoic acid gets broken down easily in water and by sunlight, but once the clumped algae are buried in the sediment, the toxin may stay protected until a bottom-dwelling organism eats it.

“Domoic acid is a rich amino acid that will be tasty to worms and other critters, who may suffer no ill effects from it,” says Benitez-Nelson.

Raphael Kudela at the University of Santa Cruz in California says that the new work is “the missing link to explain why domoic acid also shows up in bottom-dwelling organisms like crabs and flatfish. These contain lots of commercially important species, but they are not yet monitored for domoic acid.”

The new data warrants studies to test if these species should be included in future monitoring efforts, according to Kudela. So far, only shellfish that live close to the water surface are monitored.

To Benitez-Nelson, the most important next step now is to work out in more detail how much longer the domoic acid sticks around and into how many more organisms it gets.

“It is clearly a lot more prevalent and spread out than we thought before and this problem affects many areas, not just California. On top of this, all signs seem to point to further increases in the future as people dump more and more algae feeding nutrients into the ocean.”

Journal reference: Nature Geoscience, DOI: 10.1038/NGEO472 (in press)