Wednesday, 16 September 2015

Whale Fall – More than just a hilarious existential crisis

Whale fall. It may sound like the rejected title of a James Bond movie or a Douglas Adams reference, but it is actually a surprisingly important and relatively little known concept in the study of the world’s oceans. It all begins with one of the saddest moments in the story of any whale family. Grandpa whale, let’s call him Moby, after a long a fulfilling life of eating ship captains’ legs, dies. 

As sad as this is for the rest of the whale family, odds are that if Old Moby died from natural causes he lived a pretty long life. Research beginning in the 1990’s revealed that a significant portion of the population of many whale species is over 100 years old. Questions started being asked when whale carcasses were pulled from the ocean containing stone harpoon heads, which fell out of fashion around 1860. Tissue samples from a group of Bowhead whales subsequently revealed that several were in their mid-100’s and one male was pushing 200. The point is whales can live a long time.

When a whale dies without human intervention its body either washes up on shore, occasionally with hilarious consequences, or it sinks to the bottom of the ocean. The latter is much more common and the result is something pretty remarkable.

When a whale fall, as these things are known, touches down the animals that live on the sea floor throw a party. The deep ocean is a surprisingly poor environment in terms of available food and energy with animals living mostly on small particles of so-called “marine snow” that drift down from above or by eating each other. A whale carcass, as you can imagine, is a welcome break from this kind of life.

The first phase of a whale fall food chain involves getting all that delicious, rancid flesh off the bones. That job is taken care of by animals like ratfish, sharks, crabs, and hagfish. The work goes surprisingly quickly. These scavengers can liberate up to 60 kg (132 lbs) of meat in a day. After a few short months (whales are big animals) only a skeleton remains. At first glance it would seem like the show is over, but in reality things are just heating up.

As the crabs and sharks pack up and move on to greener pastures, worms move in. Lots of worms. Polychaete worms, to be precise. Up to 45,000 worms per square meter blanket the sea floor about one year after the whale buffet opens its doors. They, along with a few other species of invertebrates, feast on the organic material in the whale bones. Although this stage of whale decay brings huge numbers of animals, they represent only a few species. The real fun begins when the microbes arrive on the scene.

Somewhere between one and two years into the show, most of the low hanging fruit (to use a more pleasant sounding metaphor) has been harvested. At this point sulphur-reducing bacteria arrive to feed on the fats and oils left in the skeleton. As they do this they release sulphur into the surrounding water, which attracts sulphophilic (sulphur-loving) bacteria. These bacteria, remarkable, form the basis for a very unique food web.

Most food webs on Earth begin with photosynthesis as plants turn sunlight into food. The web around this stage of a whale fall by comparison is chemosynthetic, meaning that it’s basis is chemical reactions by bacteria. Larger and larger animals feed on the bacteria and on the creatures that feed on the bacteria until eventually you have a booming community that includes up to 190 different species of visible animals. The only other place where this type of system exists is around deep sea vents where the energy comes from within the Earth itself.

These Stage Three whale fall communities can last a surprisingly long time. We're talking decades. Research has shown that some whale falls can sustain an ecosystem for over 50 years! And these are not rare systems. Given the number of whales alive today and the length of time that these communities last, scientists estimate that there may be a whale fall every 5 to 16 km (3 to 10 miles) along the sea floor, meaning that when the nutrients finally do run dry, the creatures that depended on them don’t have far to travel until they find their next home.

Whale falls are islands of biodiversity in one of Earth’s least bountiful places. Yet another reason why humans should do our part not only to protect whales, but to protect the fish and oceans that they depend on.