Mass Strandings of Marine Mammals Attributed to Harmful Algal Blooms

mass strandingsA research team, headed by Nick Pyenson, curator of fossil marine mammals at the Smithsonian’s National Museum of Natural History is reporting on an archaeological site containing more than 40 fossils from ancient mass strandings of marine mammals.  The fossils, including three types of whales, two seals, a marine sloth, and a predatory bony fish, are distributed over four separate layers at the site on Cerro Ballena (“Whale Hill”) in Chile, suggesting that the mass strandings occurred repeatedly between six and nine million years ago.  From the orientation of the fossils, it is likely that the mammals died at sea and were washed ashore, where the carcasses remained largely intact due to absence of predators that might have dis-articulated the skeletons.

Mass strandings of marine mammals have been documented previously; including a more recent incident in the late 1980’s when 14 humpback whales washed up on the shore in Cape Cod.  These events are something of a mystery.  They can be attributed to human causes such as pollution and injury from navy sonar, or to natural phenomena such as tsunamis, viral infection, and harmful algal blooms (HABs).

Evidence points to Harmful Algae Blooms (HABs) as the culprit in the Chilean mass strandings. Human causes, such as pollution and injury from sonar are obviously ruled out, due to the age of the fossils.  A viral infection is unlikely: viruses tend to be species-specific, and thus would not account for the diversity of fossils at the site.  Tsunamis are ruled out for a number of reasons, including absence of tsunami sediment deposits at the site, physical orientation of the fossils, unlikelihood of four tsunamis occurring at the same site, and absence of any other sorts of fossils.

HABs used to be known as “red tides,” but because they vary greatly in density and color, they have since been renamed.  HABs can be deadly in two different ways.  First, through sheer over-proliferation, they can deplete all the oxygen in the area, suffocating the fish trapped within the bloom.  Some types of algae produce toxins that can poison marine vertebrates through eating contaminated prey, or through inhalation.

Two vital pieces of evidence led to algae being identified as the prime suspect in the marine mammal deaths. The whales were found belly-up, indicating a death at sea, and fossil algae mats with high levels of iron surrounded the bones, which indicated a massive algae bloom.  The team also found orange-colored rock associated with the skeletons suggesting the degradation of the algal mats over time. Under microscopic examination, the researchers found tiny spheres in the rock similar to the dinoflagellates that produce HABs today.  Recovering relic DNA from the fossils for comparison with contemporary dinoflagellates would be most conclusive, but according to Dr Pyenson, there is no real chance of this.  Although the recovery of relic DNA from fossils has been documented (the real-life basis for the Jurassic Park movies), the age of the Chilean fossils and the highly oxidizing environment of the tidal flats where they were found make this impossible.

So what causes HABs?  Algal blooms can arise from entirely natural causes, such as the movement of certain ocean currents.  They can result from increased levels of nitrates and phosphates from agricultural run-off.  Warm waters and currents that cause the algae to aggregate densely, also favor the production of the blooms.  Iron-rich runoff from the Andes along the coast of western South America may have caused the HABs that led to the Chilean strandings; iron oxidation would account for the orange-colored rock associated with the fossils.

Future plans for the research team include a re-visit to the site of the mass strandings, to uncover what may be hundreds more skeletons.  Also, the research team employed a 3D imaging team to study the fossils, which will enable them to replicate the fossils in their entirety by making use of a 3D printer.

By Laura Prendergast

Scientific American
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