Often it's the tiniest organisms that do the most harm. One example is microscopic algae, which can grow rapidly to form harmful algal blooms. Such blooms (some are called "red tides") create unhealthy water conditions or produce toxins that kill other organisms in the water. In 2013, hundreds of Florida manatees died from eating toxic red algae, which also killed off their usual seagrass food. That same year, more than 200 dead sea turtles washed ashore in El Salvador, also killed by eating toxic algae. In 2012, it was jumbo squid on the California coast, and the year before that it was sardines.
While such mass mortality events (as they're often called) are common nowadays, it's hard to study whether they happened in the past and how frequently. Paleontologists find fossils of long-dead animals all the time, but it's not possible to tell if they were killed by toxic algae or if they died for a different reason.
Nick Pyenson, the curator of fossil marine mammals at the Smithsonian National Museum of Natural History, thinks he found the oldest example of a toxic algae bloom—between 6 and 9 million years old—that killed off large whales and other animals. He led the excavation of a graveyard of more than 40 now-extinct large baleen whales (rorquals), walrus-whales (extinct dolphins with tusked walrus-like faces), penguins, seals, and aquatic sloths at a site in Chile named Cerro Ballena, Spanish for "whale hill."
If finding such a stockpile of ancient marine mammals wasn't exciting enough, Nick had other surprises in store. Nearly all of the skeletons were complete. They all lay belly-up, suggesting that they had died at sea and washed ashore where they were quickly buried. And the skeletons were arranged in four layers, as if a series of similar mass deaths had occurred in the same place.
What could have killed many different types of marine mammals at sea, and have done so on four separate occasions? Nick could only think of one plausible explanation: toxic algal blooms. He can't be sure, since he didn't find any fossilized algae in the sediment around the fossils. But he did find tiny orange splotches on the bones that are exactly the right size to be dinoflagellates, a common culprit behind harmful algal blooms.
Harmful algal blooms form when there is extra fertilizer in the water, which creates the perfect conditions for algae to thrive. Nowadays, such blooms are becoming more common as fertilizers from farms, golf courses, and people's front lawns wash into the sea. But there are also natural fertilizers that cause blooms. If toxic algae killed the marine mammals at Cerro Ballena, they likely bloomed when water flowing through the Andes carried iron from the mountains' rocks into the ocean. Iron is a vital nutrient that is hard to come by at sea, so would have been the perfect fertilizer for the toxic algae.
Such large whales are rarely the victims of toxic algal blooms today; there is only one known case, when fourteen humpbacks were found dead with algal toxin-laden fish in their stomachs in 1987. If this happened four times in just one spot over 10,00-16,000 years, why do we not see it now? Nick says it's because people hunted so many whales that the chances of a large group of whales crossing the path of a harmful algal bloom are very low. "We reduced their abundances by upwards of 95 percent," he told Smithsonian Science. "So we are living in a very altered world today where baleen whales are not nearly as abundant as they used to be.”
Read more about the expedition to Cerro Ballena at Nick Pyenson's blog, and see more photos and 3D scans of the fossils at the expedition website/. Also read his blog posts about other research here on the Ocean Portal.