Fast-sinking jellyfish drag carbon to seafloor
13 June 2013, by Harriet Jarlett
When jellyfish die they sink to the ocean floor faster than other marine organisms, allowing the oceans to absorb carbon dioxide, new research shows.
The study, published in Limnology and Oceanography, is the first ever to look at how quickly some gelatinous life in the oceans sinks.
This sinking biomass is an important part of the process by which carbon is exported from the ocean surface to the seafloor. Understanding how quickly dead organisms sink means scientists can make better estimates of how much carbon the oceans can absorb in the future.
Around 25 per cent of the carbon dioxide emitted from human activities dissolves into the oceans, where billions of tiny plankton start to transform some of it into organic carbon through photosynthesis.
As larger organisms, like jellyfish and pelagic tunicates – small transparent filter-feeders – eat these plankton, the carbon passes through the food chain until the animals die and sink to the sea floor.
As they sink the carbon is dragged down through the water column, away from the surface waters, where it is either ingested by scavengers, or stored in the deep water. This means more CO2 can be absorbed into the oceans at the surface.
Previous studies had shown that plankton and marine snow – the organic detritus that falls out of the water column – are the main sources of carbon transport to the seafloor. But this study showed jellyfish sink much faster and so may be able to transport even more carbon away from the surface.
'We are just starting to comprehend the fundamental properties that will allow us to better understand the role of jellyfish and pelagic tunicates in the global carbon cycle,' explains Dr Mario Lebrato of the GEOMAR Helmholtz Centre for Ocean Research, who led the study.
The team collected samples of different jellyfish, salps – a barrel shaped pelagic tunicate – and comb jellies from the Baltic, the Mediterranean, the Atlantic and the Southern Ocean to conduct their experiments. They then placed them in large clear cylinders filled with seawater and observed their sinking speeds in the field and lab.
'The speed which jelly remains sink at is much, much higher than we had expected – about 500 to 1600 metres each day,' says Dr Bethan Jones, who is now at Oregon State University but was a researcher at NERC's National Oceanography Centre in Southampton when she co-authored the study. 'What was more surprising is that the jellies do not remineralize in the water column – they reach the seafloor almost completely intact, as we observe in field studies. So, they not only take carbon away from the atmosphere but they also maintain their nutritional value for the organisms on the sea floor that feed on them.'
While the carbon content of jellies is less than that of other plankton or marine snow, their large populations coupled with the high sinking speed mean they may be capable of delivering huge quantities of carbon down to the seabed. It may be possible that any increasing numbers of gelatinous plankton could help in mitigating the rising carbon dioxide problem.
Lebrato, M., de Jesus Mendes, P., Steinberg, D. K., Cartes, J. E., Jones, B. M., Birsa, L. M., Benavides, R. and Oschlies, A. (2013) Jelly biomass sinking speed reveals a fast carbon export mechanism Limnology and Oceanography, 58 (3). pp. 1113-1122. DOI 10.4319/lo.2013.58.3.1113.
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