Ochre sea stars washed up on the California coast as part of a mass mortality event in August 2011. (Photo by Ashley Robart/PLOS One)

In August 2011, a sudden mass die-off of marine organisms off the coast of California, including sea stars, chitons and abalone, left scientists scratching their heads.

University of California Davis scientists first caught on to the strange event when they found thousands of purple sea urchins and other organisms dead in their laboratory tanks. The tanks were full of seawater that came straight from the Pacific Ocean, so whatever killed them was likely an outside disturbance, the scientists suspected. Sure enough, they found, thousands of marine organisms were washing up dead on nearby beaches at the same time.

Now, in a study published Wednesday in open-access science journal PLOS One, the UC Davis researchers, along with colleagues at other University of California branches, seek to explain what caused the disconcerting die-off. Their theory: The mass deaths were caused by a huge bloom of algae.

An algal bloom is a sudden, rapid growth in a water system’s algae population. They’re sometimes brought on when extra nutrients make it into the water — for example, if chemical pollution from a nearby factory dumps an excess of phosphates into a lake — and feed the algae. But while the algae are having a field day, other organisms in the water can suffer.

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The tiny organisms that compose an algal bloom, known as phytoplankton, can release toxic chemicals into the water that kill fish and other animals. In the 2011 event, the researchers observed discolored seawater and high levels of a chemical called chlorophyll-a in the water, both strong signs of an algal bloom. In fact, the chlorophyll-a data suggest that two algal blooms took place in August 2011 — one at the beginning of the month, and a second one toward the end.

The researchers measured salt content, air temperature and water temperature to make sure some other natural event wasn’t causing the die-off, and they found all these other factors to be normal. As a result, it’s a pretty good bet that the algae was to blame. The factors that caused the bloom in the first place, on the other hand, remain another question. Ocean blooms are thought to occur under a variety of conditions, such as slow water circulation or pollution drifting down river into the sea.

The researchers also examined the effects of the event on a handful of marine organisms by conducting surveys along the California coast and comparing these numbers with pre-die-off population counts. The two most badly affected species were the purple sea urchin and the six-armed sea star. These two organisms were both essentially eradicated from the die-off’s impact zone, which spanned about 100 kilometers (that’s more than 60 miles) of coastline. The scientists note that for the purple sea urchin, there’s never been such a severe mass mortality event over such a large area before.

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The gumboot chiton and the ochre sea star were two other organisms that were observed in large numbers washed up along the shore during the die-off. But while they obviously experienced an increase in mortality during the event, the researchers’ surveys showed that their populations did not suffer any significant declines overall.

For the purple sea urchin and the six-armed sea star, however, recovery from the event could be a long and slow process. The researchers note in the paper that “recolonization is likely to rely heavily on immigration from sources outside the region,” meaning the die-off was so complete that the only real chance for recovery is if urchins and sea stars from other places start migrating into the area.

This may be an easier process for the purple sea urchin than for the six-armed sea star. Urchin larvae tend to disperse away from their homes, meaning larvae from other areas might make it into the die-off zone and start to settle there. In fact the purple sea urchin has already slowly begun to show back up in the area.

The sea star’s larvae, on the other hand, tend to stick close to the spots where they were born, making immigration less likely. “If someone were to come to this area, they wouldn’t know these six-armed sea stars existed here, even though this has been a main part of their species range,” said lead author Laura Jurgens, a researcher at UC Davis’s Bodega Marine Laboratory, in a statement.

This paper doesn’t focus on understanding the cause of the algal bloom. However, the researchers do note that dormant phytoplankton “can persist for years in sediment and may bloom again under favorable conditions,” meaning it may be useful in the future to conduct more research on the causes behind algal blooms in the ocean and ways to predict when they might happen.

How intertidal communities will be affected by the change in population dynamics in the future remains a bit unclear, although the researchers expect some changes in food webs structure to occur. For instance, the six-armed sea star is a key predator in the intertidal zone examined in the study, and its loss could cause the organisms that it normally feeds on, such as snails, to flourish.

And studying the event can provide other useful insights to scientists. Marine mass mortality events are known to occur from time to time, but unless they’re caused by a sudden and catastrophic weather event, like a hurricane, they tend to happen slowly over a period of months or even years. This event’s suddenness makes it highly unusual. “The unique attributes of the mass mortality we describe here therefore provide a rare opportunity for subsequent investigations into response and recovery dynamics following a strong and discrete perturbation,” the authors write.

But most notably, the study gives scientists a glimpse into future mortality events that may happen as climate change causes more environmental disturbances, such as extreme weather events, ocean acidification, temperature changes and alterations to ocean currents.

The authors write, “Documenting such disturbance events and their manner of recovery will become increasingly important if and when global changes escalate the magnitude and frequency of local to regional-scale environmental perturbations.”

In other words, mass die-offs are rarely a good thing — but studying these kinds of events is good practice since there’s a possibility they may become more common in the future.