A whale fall is the carcass of a cetacean that has fallen into the bathyal or abyssal zone (i.e. deeper than 1,000 m, or 3,300 ft) on the ocean floor. 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.
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 food falls to remain intact and sink to great depths.
Contribution to the biological pump
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 (at 33°13.0′N 118°32.5′W / 33.2167°N 118.5417°W). 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.
Another whale fall was discovered accidentally by a team of marine biologists led by University of Hawaii oceanographer Craig Smith in 1987. The DSV Alvin identified the remains using scanning sonar at 1,240 m (4,070 ft) in the Catalina Basin.
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.
A consistent group of organisms inhabits whale falls in all oceans. The mussels and vesicomyid clams from Alvin's discovery belonged to groups that harbor chemosynthetic bacteria which can draw energy from inorganic chemicals. Their only known habitats were sunken wood and hydrothermal vents. The lucinid clams' only other habitats were seeps and anoxic seafloor sediments. Similarly, a snail inhabited only anoxic sediments.
Similar ecosystems exist when other large volumes of nutrient-rich material fall to the sea floor. Sunken beds of kelp create kelp falls, and large trees can sink to create wood falls. In more recent years, shipwrecks have also provided bases for deepwater communities.
Scientists have concluded from the data recorded from Santa Catalina, as well as other cases off the California coast, that there are at least three stages of decomposition associated with a whale fall.
The initial period begins with "mobile scavengers" such as hagfish and sleeper sharks actively consuming soft tissue from the carcass. Consumption can be at a rate of 40–60 kilograms (88–132 lb) per day over a two-year period.
The second stage introduces the "enrichment opportunists". These are animals which colonize the bones and surrounding sediments that have been contaminated with organic matter from the carcass and any other tissue left by the scavengers. This process can take up to two years.
Finally "sulfophilic bacteria" anaerobically break down the lipids embedded in the bones. Instead of oxygen, they reduce dissolved sulfate SO2−
4 and excrete hydrogen sulfide. Due to the toxicity of H
2S, only resistant chemosynthetic bacteria survive. The bacterial mats provide nourishment for mussels, clams, limpets and sea snails. As whale bones are rich in lipids, representing 4–6% of its body weight, the final digestion stage can last between 50 and possibly 100 years.
A process called methanogenesis can also occur around whale falls. Archaea that produce methane can be abundant in anoxic sediment, but is typically not found in co-occurrence with the sulfur reducing bacteria found at whale falls. Whale falls do however support both sulfur reducing bacteria and methane producing archaea, leading to the conclusion that the area is not electron donor limited, and/or there is minimal or no competition for suitable substrate. Concentration gradients of both sulfide and methane can be found around whale falls, with the highest concentration coming within one meter of the carcass, which is several orders of magnitude higher than the surrounding sediment concentrations. Methanogenesis appears to only occur in sediments as opposed to sulfur reduction, which occurs both in sediments and on the bones of the carcass. The addition of sulfur reduction in both sediments and high lipid whale bones is a key factor for why whale falls are able to sustain deep-sea communities for extended periods of time.
Variance in decomposition rates
The differences in stage patterns between whale falls is likely to be related to carcass size. Large, intact whale falls appear to pass through the three decomposition stages, while the stages on smaller or partial carcasses may be truncated. Researchers believe the presence of 1 cm (0.39 in) Osedax (Latin: bone-devourer) may be a contributing factor in the successional differences. Also known as zombie worms—so called because they have no eyes or mouth—they were first discovered in the deep Submarine Monterey Canyon off central California in 2002. Other examples have since been discovered off Sweden, Japan, and Antarctica. Osedax larvae have no digestive tract; instead they float through the oceans until they encounter a carcass. The worms then grow appendages resembling tiny trees that absorb oxygen. A root-like structure grows into the bone; these contain symbiotic bacteria which make an acid that breaks down proteins within the bone. The process supplies nutrients for the worms. When these worms sexually mature, eggs are released to start the cycle over.
In 1989, scientists hypothesized that some species (such as Osedax) might use whale falls as stepping-stones to extend their range across multiple chemosynthetic communities. They estimate that 690,000 carcasses/skeletons of the nine largest whale species are in one of the three stages at any one time. This estimate implies an average spacing of 12 km (7.5 mi) and as little as 5 km (3.1 mi) along migration routes. They hypothesize that this distance is short enough to allow larvae to disperse/migrate from one to another.
Another polychaetae worm that is known to be abundant at whale falls is Bathykurila guaymasensis. This worm however does not have a symbiotic relationship with bacteria that breakdown proteins within the whale bones; alternatively, Bathykurila is a specialist feeder on the sulfur-reducing bacteria Beggiatoa, that grows in abundant mats around a whale fall and hydrothermal vents.
Whale fall fossils from the late Eocene and Oligocene (34–23 MYA) in Washington and from the Pliocene in Italy include clams that also inhabited non-chemosynthetic environments. Chemosynthetic-only animals do not appear until the Miocene (23–5 MYA) in California and Japan. This may be because the lipid content of early whale bones was too low.
The discovery of the limpet Osteopelta in an Eocene New Zealand turtle bone indicates that these animals evolved before whales, including possibly inhabiting Mesozoic (251–66 MYA) reptiles. They may have survived in seeps, wood-falls and vents while waiting out the 20 million year gap between the reptiles' extinction and whales' emergence. Another possibility is that these fossils represent a prior, dead-end evolutionary path, and that today's whale fall animals evolved independently.
Contrast with other large food-falls
There have also been studies based on the carcasses of other, non-mammalian marine vertebrates that have fallen to the deep sea. In particular, the chance discovery of a whale shark carcass and three mobulid ray carcasses led to observations on the communities that form surrounding large elasmobranch falls as opposed to whale falls. Whale sharks inhabit waters of roughly 1,000 meters depth regularly, which suggests it could be a regular form of food fall in areas where it is abundant. Many eelpouts (Zoarcidae) were found surrounding the whale shark with some evidence of direct feeding as boreholes were observed on the carcass. Another theory suggests that the eelpouts were waiting for their main prey, amphipods and other small benthic animals. The three rays found were at varying stages of decomposition, leading to varying assemblages found surrounding the individuals. A higher abundance of scavengers was found surrounding the more intact individuals, including scavengers typical of whale falls like hagfish. Around the least intact individual a bacterial mat was observed in the zone of enrichment, but no clams or mussels typical of this environment (i.e. whale falls) were seen.
Overall, the four carcasses observed showed no evidence of progression past the scavenger stage (Stage 1 below). The size limitations, as well as physiological differences between large elasmobranchs and whales more than likely causes the changes observed in the communities surrounding their respective carcasses. Osedax worms have the ability to extract collagen from bones as well as lipids, enabling them to sustain themselves on bones other than the lipid-rich remains of whales. Although no Osedax were found on the non-mammalian remains in this study, their absence may have been due to the timing of observation, and the Osedax had not yet colonized the carcasses. Various studies on smaller cetaceans and other marine vertebrate food falls come to similar conclusions that these falls bring a large amount of new organic material to depth, but support mostly a scavenger community, as opposed to the diverse assemblage seen at whale falls. This conclusion can be drawn based on the knowledge that large whales have much higher lipid content in their bulk composition and bone marrow, which supports the diverse communities present in succession at whale falls. (Witte, 1999; Janssen et al., 2000; Kemp et al., 2006; Higgs et al., 2014).
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