Are large jellyfish swarms a threat to oceans?

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Jellyfish are ecological vampires, sucking carbon away from marine ecosystems, according to a study to be published in a forthcoming issue of the Proceedings of the National Academy of Sciences of the USA.

Jellyfish, like many marine organisms, excrete organic compounds as metabolic wastes and as slime covering their bodies, and understanding the eventual fate of these organic compounds prompted Robert Condon and coauthors to undertake this study.

After sampling blooms of Warty comb jellies (Mnemiopsis leidyi) and Atlantic stinging sea nettles (Chrysaora quinquecirrha) in the York River (a tributary of lower Chesapeake Bay), the authors then carried out a series of incubation experiments in closed containers that tracked the flow of food energy by measuring the amount of dissolved organic material (carbon, nitrogen and phosphorus), ammonium and phosphate released by jellyfish and consumed by bacteria.  

The authors also analysed potential changes to the bacterial community composition as a result of the large quantities of dissolved organic material released by these jellyfish blooms.

The authors found that when bacteria consumed dissolved organic matter from jellyfish, they tended to shunt the carbon towards respiration rather than growth.  This has the effect of converting the carbon from the jellyfish back to carbon dioxide, rather than using it for the bacteria to grow larger or to reproduce.  The carbon is thus lost to the food web as organic energy, since it can no longer by utilised by other organisms further up the food chain.

In addition to this “jelly carbon shunt”, the jellyfish also removes carbon from the food chain by capturing plankton that would otherwise be eaten by fish and converting that food energy into gelatinous biomass.

As other predators do not readily consume jellyfish, this severely restricts the amount of carbon available for other organisms further up the food chain.

The results also showed that the dissolved organic materials excreted by the jellyfish enabled gammaproteobacteria, a class of microbes that is otherwise rare, to thrive. Therefore, jellyfish blooms have an additional effect of changing the composition of the bacterial communities, with gammaproteobacteria predominating.

With the predicted rise in frequencies and severities of jellyfish blooms due to anthropogenic activities, “…our results suggest fundamental shifts in the biological structure and biogeochemical functioning of the marine systems affected, with potentially significant environmental, societal, and economic implications”, concludes the paper.

For more information, see the paper: Condon RH, DK Steinberg, PA Del Giorgio, TC Bouvier, DA Bronk, WM Graham and HW Ducklow (2011) Jellyfish blooms result in a major microbial respiratory sink of carbon in marine systems. Proceedings of the National Academy of Sciences doi:10.1073/pnas.1015782108

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