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According to this study, cited by many popular science magazines, jellyfish and other, smaller animals can contribute to the vertical ocean stir as much as currents do.

Black Sea chemistry and biodiversity is highly coupled with the lack of vertical currents, present in oceans or bigger seas. (Some ) effects are limited salinity in the top layers - and many ocean fish species cannot live in black sea - and limited space for life (under 50 meters there is no oxygen)

Can the jellyfish population (which is exploding now) change the stratification of water layers enough that the highly saline waters from bottom go up, and oxygen goes down, allowing a completely new ecosystem here?

If yes, what are the outcomes of such prospects? Besides the potentially catastrophic release of hydrogen sulphide form the bottom, can it be an improvement for the biodiversity here?

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At least they have an influence in the sense of butterfly effect. Which can be small, or huge. – skymningen Aug 27 '13 at 8:02

To quote the PI from the BBC article:

But, Dr Dabiri explained, the jellyfish were unlikely to be the "primary ocean mixers". "Crustaceans - like copepods and krill - are likely the primary biogenic mixers, because there are so many of them," he explained.

"But much of the open ocean is like desert," he added, "and the density of these animals is unlikely to be sufficient (to cause mixing)."

My guess therefore is no, jellyfish would not be sufficient to dramatically change the current structure of ocean layers. Animals in general, as the paper and its references suggest, do contribute significantly, on-par with tides and wind.

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While in the ocean they are not the primary mixers, their influence is not negligible. My questions is about their effect in a sea where other vertical movement vectors are missing. A change of 10% in water movement is not much in an ocean, but in a closed sea may mean a dramatic change. – sammy Aug 27 '13 at 8:57

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