Whale fall

A whale fall is the carcass of a cetacean that has fallen onto the ocean floor at a depth greater than 1,000 m (3,300 ft), in the bathyal or abyssal zones. They can create complex localized ecosystems that supply sustenance to deep-sea organisms for decades. This is unlike in shallower waters, where a whale carcass will be consumed by scavengers over a relatively short period of time. It was with the development of deep-sea robotic exploration that whale falls were first observed in the late 1970s. Organisms that have been observed at deep-sea whale falls include giant isopods, squat lobsters, bristleworms, prawns, shrimp, lobsters, hagfish, Osedax, crabs, sea cucumbers, and sleeper sharks. Whale falls are able to occur in the deep open ocean due to cold temperatures and high hydrostatic pressures. In the coastal ocean, a higher incidence of predators as well as warmer waters hasten the decomposition of whale carcasses. Carcasses may also float due to decompositional gases, keeping the carcass at the surface. In the deep-sea, cold temperatures slow decomposition rates, and high hydrostatic pressures increase gas solubility, allowing whale falls to remain intact and sink to great depths. The amount of carbon tied up in a typical single whale carcass (about two metric tons of carbon for a typical forty-ton carcass) is roughly equivalent to the amount of carbon exported to a hectare of abyssal ocean floor in 100–200 years. This amount of organic material reaching the seafloor at one time creates a pulse equivalent to about 2000 years of background carbon flux in the 50 square metres of sediment immediately beneath the whale fall. This helps to sustain the community structure that develops around a whale fall, but it also has potential implications for the biological pump, or the flux of organic material from the surface ocean to depth. Whales and some other large marine animals feed on and follow large aggregations of zooplankton for sustenance. Based on simple trophic structure, this would mean whales and other large zooplankton feeders can be found at higher abundance around areas of high primary production, potentially making them important exporters of carbon to depth through food falls (Higgs et al., 2014). Biological pump models indicate that a large amount of carbon uptake by the deep sea is not supplied by particulate organic carbon (POC) alone, and must come from another source. Lateral advection of carbon, especially in coastal areas contributes to this deficit in the model, but food falls are also another source of organic carbon for the deep ocean (Higgs et al., 2014). Various percentages of the food fall contribution to the total carbon flux to the deep ocean have been hypothesized, ranging from 0.3% (Smith 2006) to 4% (Higgs et al., 2014). There is growing evidence that the contribution of food falls to the deep ocean carbon flux is larger than originally proposed, especially on the local scale in areas of high primary productivity. Unfortunately, contributions of food falls to the biological pump are hard to measure and rely on a few serendipitous studies on discovered falls (Smith et al., 1989; Higgs et al., 2014) as well as planted carcasses (Janssen et al., 2000; Kemp et al., 2006; Smith et al., 2009), with much of the deep sea carbon flux studies relying on sediment traps (Robison et al., 2005; Miller and Wheeler, 2012). One study even suggested that the whaling industry has had an effect on the biological pump through the elimination of many large whales, as well as a large number of whales, reducing the amount of whale falls (Pershing et al., 2010). The earliest indication that whale carcasses could host specialized animal communities occurred in 1854 when a new mussel species was extracted from a piece of floating whale blubber. By the 1960s, deep sea trawlers unintentionally recovered other new mollusc species including limpets (named Osteopelta) attached to whale bones. The first recorded abyssal whale fall was discovered by US Navy bathyscaphe pilots LT Ken Hanson, Master Chief George Ellis and LT Tom Vetter diving in bathyscaphe Trieste II (DSV-1) on 19 February 1977. The skeleton of the carcass, which was completely devoid of organic tissue, remained intact and collapsed flat on the seafloor. The submersible recovered a jawbone and phalanges. The whale was considered to be a gray whale based on the size of the bones and the skeleton, the lack of teeth and its location west of Santa Catalina. The first whale-fall ecosystem, which included a chemoautrophic assemblage living on the anaerobic breakdown of organic material in whale bones, was discovered by a team of scientists led by University of Hawaii oceanographer Craig Smith in 1987. The DSV Alvin observed the remains using scanning sonar at 1,240 m (4,070 ft) in the Catalina Basin and collected the first photographic images and samples of animals and microbes from this remarkable community. Many other whale falls have since been found by more researchers and deep-sea explorers as well as naval submarines. The increase in detection is largely due to the use of cutting-edge side-scan sonar which can minutely examine the ocean floor for large aggregations of matter.