Whale fall

A whale fall occurs when the

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.[1] Carcasses may also float due to decompositional gases, keeping the carcass at the surface.^[6] The bodies of most great whales (which includes sperm whales and many species of baleen whale^[7]) are slightly denser than the surrounding seawater, and only become positively buoyant when the lungs are filled with air.^[8] When the lungs deflate, the whale carcasses can reach the seafloor quickly and relatively intact due to a lack of significant whale fall scavengers in the water column.^[1] Once 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 even greater depths.^[6]

Contribution to the biological pump

The amount of carbon tied up in a typical single whale carcass (about two tonnes of carbon for a typical 40-tonne carcass) is roughly equivalent to the amount of carbon exported to a hectare of abyssal ocean floor in 100–200 years.^[9] 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 meters of sediment immediately beneath the whale fall.^[9] 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.^[10] 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.^[10] Various percentages of the food fall contribution to the total carbon flux to the deep ocean have been hypothesized, ranging from 0.3%^[11] to 4%.^[10]

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^[12][10] as well as planted carcasses^[13] with much of the deep sea carbon flux studies relying on sediment traps.^[14]

Discovery

Many other whale falls have since been found by more researchers and deep-sea explorers, as well as by 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 2022 study identified 45 known natural whale falls, 38 implanted ones, and 78 fossil ones, mostly in the Pacific, but a significant number, particularly of fossil ones, in the Atlantic.^[17]

Ecology

At whale fall sites it is common to see between three and five trophic levels present, with two main nutritional sources constituting the base of the food web. Adult whale carcasses can house up to five trophic levels, whereas juveniles more typically have three.[19]

Recent studies also show a possible trend of "dual niche partitioning", in which scavengers tend to reach peak densities on the carcass during the day and predators are more present during the night, reducing competition between the two trophic groups.^[4] There is also a possible trend in tidal patterns and species occurrence, indicating that tides play a role in niche partitioning as well.^[4]

Similar ecosystems exist when other large volumes of nutrient-rich material fall to the sea floor. Sunken beds of

There are four stages of decomposition associated with a whale fall.[1] These stages vary in duration and overlap with one other with the size of the carcass, water depth, and other environmental variables, such as tidal flow.^[6] Large, intact whale falls appear to pass through the four decomposition stages, while the stages on smaller or partial carcasses may be truncated.^[20] Smaller cetaceans, such as porpoises and dolphins, do not undergo the same ecological succession stages due to their small size and lower lipid content.^[20] Researchers believe the presence of Osedax worms may also be a contributing factor in the observed successional differences.^[21]

Stage 1

The initial period begins with "mobile scavengers" such as

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.[5] This stage can last months up to 4.5 years.^[4]

Stage 3

In the third stage,

Some scientists postulate a fourth stage of ecological succession at whale fall sites, called the "reef stage".[1] A whale fall enters this stage once the organic compounds have been exhausted and only minerals remain in the bones, which provide a hard substrate for suspension and filter feeders.^[20]

Methanogenesis

A process called methanogenesis can also occur around whale falls. Archaea that produce methane can be abundant in anoxic sediment, but are 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 or there is minimal or no competition for suitable substrate.^[22] 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.^[22] 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.^[22]

Paleontology

Whale fall fossils from the late

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.^[24] 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.^[5]

Anthropogenic effects

It has been suggested that the whaling industry has had an effect on the biological pump through the elimination of many large whales, reducing the amount of whale falls. The effects of this on benthic whale fall community assemblages is not well understood.^[25] However, it is suggested that the removal of large whales might have reduced the total biomass of the deep sea by more than 30%.^[25] Whales stored massive amounts of carbon that were exported to the deep sea during whale fall events. Whaling has thus also reduced the ability of the deep sea to sequester carbon.^[25] Carbon can be sequestered for hundreds to thousands of years in the deep sea, supporting benthic communities.^[25] It is estimated that, in terms of carbon sequestration, each whale is equivalent to thousands of trees.^[26]

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

Overall, the four carcasses observed showed no evidence of progression past the scavenger stage. 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.[27] 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.^[29] 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.^[27] 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.^[13][10]

Researchers have compared sauropod carcasses to modern whale-fall events, recognizing that sauropods reached the size of modern cetaceans. Sauropod carcasses may have been energy rich reservoirs, and because they did not float great distances, may have been the primary resources of many terrestrial carnivorous dinosaurs.^[30][31]

See also

• Deep-sea community
• Marine snow
• Detritivore
• Jelly-falls
• Whaling
• Osedax
• Carbon sequestration
• Deep-sea wood

References