National Geographic

Jellyfish shift ocean food webs by feeding bacteria with mucus and excrement

Despite its name, the Gulf of Mexico’s Dead Zone is full with life. This region stretches over 22,000 square kilometres, an area the size of Israel. Its waters are choked by a combination of fertiliser, sewage and industrial run-offs, flowing down from the businesses that line the Mississippi. These pollutants feed large blooms of algae that ultimately rob the water of oxygen, depriving it of the ability to support fish and other typical residents. Instead, the Dead Zone has become the dominion of jellyfish.

Jellyfish congregate here in their thousands. Locals like moon jellies and sea nettles are joined by foreign travellers like the Australian spotted jellyfish. These gelatinous swarms stretch for miles, covering the ocean in a web of pulsating umbrellas and stinging tentacles. At their densest, you could scoop up a hundred jellies within a single cubic metre of water. They shut down beaches, fisheries, industries and ecosystems.

The reasons for these swarms are unclear. By overfishing, we could have removed the jellies’ main competition for food. By sinking man-made debris like vehicles and rigs, we could have created habitats for their larvae. By raising the temperature of the oceans and pumping them with pollutants, we could have created warm, oxygen-poor waters that only they can thrive in.

The jellyfish blooms are a natural phenomenon but marine biologists suspect that they are becoming increasingly common. Reports are constantly flooding in of unusual thick and large swarms, not just in the Gulf of Mexico, but all over the world from the Mediterranean to the Japanese coast. The worry is that we are witnessing a transition from an ocean full of scales, shells and flippers to one that’s ruled by jelly.

Robert Condon from the Dauphin Island Sea Lab has found evidence to confirm those fears. By studying jellyfish blooms in Chesapeake Bay, he has found that the blooms instigate major changes in the ocean’s food webs. By tracing the flow of carbon through the web, Condon found that the jellies shunt this resource to bacteria, which in turn release it into the air. As a result, the oceans lose valuable food energy that would otherwise have gone to larger animals like fish.

Jellyfish are often viewed as an energetic dead end. They voraciously eat plankton but few animals eat them. As a result, all the carbon that ends up in jellyfish is trapped, never travelling to other parts of the food web. But Condon found that this view is too simplistic.

The jellies may not be easily eaten, but they do release large amount of organic matter into the surrounding water – a combination of mucus and the jellyfish version of excrement. This jellyfish waste contains 25 to 30 times more carbon than nitrogen; for comparison, most organic matter in the ocean has just 6 times more carbon than nitrogen. The sound of mucus and excrement may be unappetising to us, but these chemicals provide a delectable bonanza for bacteria.

To measure the value of their excrement, Condon collected two species of jellyfish that bloom from an estuary in Chesapeake Bay – the stealthy sea walnut and the stinging sea nettle, both of which form problematic blooms worldwide. He incubated them in buckets of water, removed them, and measured the growth of bacteria in the waste-filled water. Within six to eight hours, the microbes had consumed anywhere from half to all of the jelly waste.

This wouldn’t be a problem for the ecosystem if the bacteria were growing fat off the carbon in the jellyfish waste. They would then be eaten by plankton, which could provide more fuel for fish and other animals.

But that’s not the case. Condon found that the bacteria weren’t growing or reproducing any faster in the jelly-filled waters; if anything, they grew more slowly. Instead, they quickly metabolised any extra carbon into carbon dioxide, releasing it into the atmosphere. Rather than channelling the carbon up the food web, the bacteria were rerouting it away from the ocean entirely in what Condon describes as the “jelly carbon shunt”.

The shunt is a conveyor belt that ensures the jellyfish’s dominance, by drastically shifting the flow of energy around the ocean. Condon thinks that its harmful effects will only get worse as ocean temperatures continue to rise. After all, jellyfish thrive in warmer waters. For example, in Chesapeake Bay, spring temperatures have gone up over the last 40 years, and the sea walnut has reacted by blooming earlier in the year and more persistently.

In the open ocean, things are less clear – at least these waters are home to predators like turtles and swordfish that can actually eat jellyfish. Either way, with jellyfish blooms predicted to become bigger and more common, Condon’s results foretell a worrying future for our seas.

Reference: Condon, Steinberg, del Giorgio, Bouvier, Bronk, Graham & Ducklow. 2011. Jellyfish blooms result in a major microbial respiratory sink of carbon in marine systems. PNAS http://dx.doi.org/10.1073/pnas.1015782108

There are 12 Comments. Add Yours.

  1. Elyse
    June 6, 2011

    “These pollutants feed large blooms of algae that rob the water of oxygen…”

    I hate to be a stickler for detail, but this is wrong. The algae don’t remove the oxygen because they are photosynthesizers – they actually release oxygen. When they die and sink to the bottom of the water, bacteria respire their organic matter and use up the oxygen, creating the dead zone.

    The exception is a diurnal oxygen depletion, where a very dense algal bloom can use up the oxygen at night, before releasing it again after sunrise. But this isn’t what creates the nearly-annual Gulf of Mexico dead zone.

  2. James
    June 6, 2011

    This was predicted as a possible problemthat might be exaggerated after the Deepwater Horizon spill. I hate it when these predictions are spot on.
    http://diseaseoftheweek.wordpress.com/2010/06/16/the-wednesday-post-16610/

  3. Sam
    June 6, 2011

    Very interesting article. also is Elyse correct in saying this?

  4. Matt Gruner
    June 6, 2011

    Do you think these findings have a connection to the hypothesis that climate change events can lead to mass extinctions? I remember reading an article about paleontologist Peter Ward and his theories about the explosion of microbial life leading to toxic alterations to ecosystems during mass extinction events. Is there fossil evidence of previous jelly explosions?

  5. Emmy
    June 6, 2011

    And here I was thinking that tropical rainforests would go first. Coral reefs are being destroyed by about 5 different types of pollution. Mangroves as well. Plastic pollution and synthetic hormones, acidification and warming temperatures. Oil spills. Now this. Do we know the implications of the ocean turning into a toxic wasteland?

  6. Azkyroth
    June 6, 2011

    There’s a Fox News joke or three in here somewhere…

    Otherwise, fascinating. I wonder what the prospects for farming/fishing jellyfish are like…

  7. rdiac
    June 7, 2011

    @Elyse, wasn’t the point that jellyfish transmit significantly less fixed nitrogen that would other wise form the basis of a more normal ecosystem? My read of it is that the bacteria are consuming the equivalent of empty calories and just exhaling (?) stuff that would otherwise be food for others. On the other hand, I don’t actually know enough to figure out if I just have no idea….

  8. rdiac
    June 7, 2011

    @Azkyroth I think we may find out what the prospects for farming/fishing jellyfish are like whether we like it or not. Perhaps Fox News could scope that out for us?

  9. Ed Yong
    June 7, 2011

    @Elyse – added “ultimately” above to indicate that this isn’t a direct process

    @Matt – For obvious reasons, jellies don’t fossilise well. There are however exceptions: see here and here for a whole group of them

  10. Tamakazura
    June 8, 2011

    I like jellyfish…they are beautiful, mesmerizing and elegantly simple creatures.
    I am not a biologist but I worry that given how we are just now discovering how jelly blooms impact the ecosystem via bacteria, we may be being too quick to factor them into our doomsday scenarios–they may not impact CO2 levels at all due to a yet unknown part of the food chain.
    Regardless, the jellyfish were here long before us or anything we’d recognize as a higher life form. They’ll most likely outlast us.

  11. Liz in PA
    June 10, 2011

    Another issue is the global shipping industry introducing non-native species by discharging bilge and other waste at ports of call. As for the jellyfish blooms they may well be an early stage symptom of repair even if our scientists do not understand the process.

  12. Bernd Eickmann
    June 12, 2011

    The algae bloom ultimately leads to oxygen depletion, which in turn leads to an explosion in the jellyfish population, as jellyfish need less oxygen than their competition – having all the plankton more or less for themselves. But jellyfish are carnivores, so they depend on the herbivorous zooplankton feeding from the algae. I am wondering if and why the oxygen depletion is not affecting the diverse bunch of other animals the zooplankton consists of?

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