Jellies Fast Track Carbon to the Seafloor
Mario Lebrato and Dr. Daniel Jones of the National Oceanography Centre, Southampton have published an account of the rapid mass transport of carbon to the deep-sea floor in the bodies of jellyfish-like creatures called pyrosomes. Their findings provide new insights into the way we understand carbon processes at large-scales, and demonstrate the importance of gelatinous zooplankton carcasses in the functioning of benthic (seafloor) ecosystems.
Because of the absence of light, deep-sea benthic ecosystems largely depend on the export of organic carbon from the sunlit surface waters. Much of this carbon comes from the remains of phytoplankton, the tiny plant-like micro-organisms that dominate primary production in the oceans, as well as the dead bodies and faecal pellets of the animals - zooplankton - that graze on them. The input of carbon to the deep sea is usually estimated using sediment traps, but they miss carbon from other organisms such as jellyfish and other gelatinous marine creatures. The ecological and biogeochemical significance of such additional carbon inputs has been difficult to estimate, but should not be ignored. It has been overlooked in biogeochemical models, but the present evidence suggests a much careful consideration.
Between February and March 2006, there was a mass deposition of thousands of moribund carcasses of the pyrosome species Pyrosoma atlanticum at the seafloor in the region of the Ivory Coast (West Africa). The name pyrosome translates as ‘bodies on fire’ - they are famous for their bioluminescence. They are not related to the true jellyfish, but are closely related to the sea squirts. Each pyrosome is a free-drifting colony made up of 100s to 1000s of sea squirt-like zooids. They live by filter feeding on smaller members of the plankton. The group belongs to the Thalicea, a class of marine animals within the chordates that also includes salps and doliolids. Thaliaceans only represent a minor part of the zooplankton biomass in the Gulf of Guinea, but they bloom periodically, then dominating the standing biomass.
The continental shelf off the Ivory Coast is narrow, covering some 10,000 square kilometers, with the slope normally starting at about 120 m, gaining depth relatively fast. In the study area, from the coast down to 2000 m, there is a predominantly gently sloped seafloor, which few irregularities except for a marine canyon - ‘‘le Trou-sans-fond’’ (the bottomless hole).
The new research was conducted by PhD student Mario Lebrato and Dr Daniel Jones of the National Oceanography Centre, Southampton, UK. The work was done when Mr Lebrato was affiliated with the University of Southampton’s School of Ocean and Earth Science based at the Centre; he is now at the Leibniz Institute of Marine Science (IFM-GEOMAR) in Germany.
The researchers used a Sonsub work-class remotely operated vehicle (ROV) operated from the vessel DP Reel to survey the seafloor along oil pipelines serving the Baobab oil field, which lies in 1484 m of water off Ivory Coast. ROV surveys were conducted between 18 February and 19 March 2006, the ROV making several traverses of the continental shelf and slope between 20 and 1275 m depth. The work was done as part of the ‘scientific and environmental ROV partnership using existing industrial technology’ (SERPENT) project, hosted by the Centre.
Dr Jones said that “these unique observations were made possible through the SERPENT project, which provides researchers with access to deep-water ROVs used by the oil industry. There is a very high demand for time on the NOCS deep-water science ROV Isis, so SERPENT provides a great alternative for new deep-sea discoveries like the jellies off Ivory Coast.”
Underwater cameras and video on the ROV were used to estimate the distribution, density, and size of Pyrosoma atlanticum carcasses on the seabed. From the video images, the researchers were also able to record the interactions between other marine creatures and the gelatinous material. They observed large accumulations (up to 4000 per 100 square metres) of pyrosome bodies (5-20 cm long) covering extensive areas of seabed on the continental slope, with minor aggregations on the shelf. Eight different types of animal (four echinoderms, three cnidarians, and one arthropod) were seen to feed on the decomposing carcasses, one of the first deep-water observations of this, suggesting that the gelatinous carbon is an important component of the seabed food web in this region. Also, bacterial mats were observed covering some carcasses, probably respiring the carbon.
Back in the lab, the researchers used dried samples of Pyrosoma atlanticum to calculate their carbon content. Pyrosoma atlanticum was found to have the highest content of carbon of all gelatinous animals studied to date (almost one third of its body). They calculate that the average standing stock of organic carbon associated with the carcasses was above 5 g of carbon per square metre in the whole slope and canyon (over 13,000 m2), with values as high as 22 g of carbon per square metre in certain areas. This contrasts with the ‘normal’ carbon supply of 1 g of carbon per square meter in the form of phytoplankton, zooplankton and detritus. These jellies are clearly very important in the local carbon cycle, making a significantly larger contribution to the total carbon input than from any other source measured in the region.
Although the importance of the mass blooming of pyrosomes in the global ocean remains very poorly understood, Lebrato and Jones conclude from their findings that such blooms lead to significant transport of corpses from the coastal margins to the deep sea, and that “gelatinous zooplankton populations have a major effect on large-scale processes, adding new evidence into the fate of their biomass once dead and their importance in the carbon cycle.”
This is important because data on episodic export of carbon from the carcasses of gelatinous zooplankton populations are sparse, and also because gelatinous animals have been considered not the preferred prey item for other species. However, the findings are broadly in line with earlier research by other scientists, also at the National Ocean Centre, Southampton (David Billett, Brian Bett, Colin Jacobs, Ian Rouse and Ben Wigham) that has shown a similar role for jellyfish in the transport of carbon to the deep-sea floor in the Gulf of Oman. Here the jellyfish supplied up to 78 g of carbon per square metre compared to a ‘normal’ supply of 5 g carbon per square metre in this region. Together, the two studies represent the first direct quantifications of the input and deposition of material derived from dead gelatinous zooplankton, even though it has been suspected to be an important component of the marine carbon cycle.
Commenting on their new work, Lebrato and Jones say: “This study highlights the limited understanding of major processes associated with gelatinous zooplankton blooms and the carbon cycle at large scales, especially when mass deposition events seem to be a major feature of the world oceans. The work stresses the importance of organism-level measurements in carbon budget calculations regionally and globally.”
It is clear from the research that rapidly sinking gelatinous carcasses off the Ivory Coast will have made a substantial contribution to benthic energy supply. “Over large areas, the ultimate fate of gelatinous carbon will certainly be its incorporation to the detrital food web and the sediments through microbial pathways,” say the researchers. Moreover, “future changes in abundance, and biomass linked to the projected global anthropogenic and climatic influence, may change their role and importance in nutrient and element cycling.”
The paper, ‘Mass deposition event of Pyrosoma atlanticum carcasses off Ivory Coast (West Africa)’ is published in the May issue of Limnology and Oceanography.