Are jellyfish a harbinger of dying seas?

By Kate Spinner Herald Tribune, Sarasota FL
18 June 2011


Jellyfish, common in the seas for eons, suck so up so much food — and give back so little — that a dramatic population increase would gravely threaten the future of oceans worldwide, according to a new study.

Jellyfish could send once-productive seas, including the Gulf of Mexico, back to a more primitive state, if theories pointing to striking increases in the gelatinous creatures prove true.

They assault the base of the food chain, creating conditions where little can survive but jellyfish and bacteria, new scientific findings published this month reveal.

Scientists had already considered jellyfish a biological dead-end for their voracious appetites and low taste appeal to other animals. The creatures, renowned for their irritating sting, remove more food energy from the seas than they put in, pushing the oceans into an altered state that is much more hostile to other life.

The findings are a cause for concern because reports of jellyfish blooms are increasing, leading many scientists to speculate that water pollution, global warming and overfishing may be tipping the scales toward conditions more favorable for jellyfish.

Until now, jellyfish were considered little more than a nuisance. But Robert Condon, lead author of the new jellyfish study and faculty research scientist at Dauphin Island Sea Lab in Alabama, said their impact on the food web is serious and should be considered in regulating struggling fish populations.

Condon’s findings in the Proceedings of the National Academy of Sciences have other ominous implications. Jellyfish thrive in low-oxygen waters where most other species perish, raising concern that the northern Gulf could see a jellyfish spike this year. Scientists are predicting a larger-than-ever dead zone in the area, following a glut of pollution-laden floodwaters from the Mississippi.

The system has already sustained damage from last year’s Deepwater Horizon oil spill. A large jellyfish bloom could further damage a system that supports most of the nation’s domestic seafood catch.

Stiff competition

Jellyfish gobble the most valuable food source in the ocean: plankton, made up of microscopic plants and animals. In return, they release a sugary goo that only bacteria consume.

When feasting on the goo, bacteria grow more slowly and convert most of the food energy into carbon dioxide, taking "the food web back to square one," said Condon. The blow to the food web is temporary, but hurts fish because they need larger prey than bacteria.

Jellyfish compete with grazing fish, such as sardines, and fish larvae for the same food: microscopic shrimp, crabs and other small creatures.

When fish eat that food, they, in turn, become food for larger fish and marine mammals, such as dolphins. Jellyfish offer less to the food chain because most creatures do not like to eat them. Those that do, such as loggerhead sea turtles, are in decline.

They knock the system further out of whack by providing food to bacteria that nourish single-cell critters that fish don’t favor. The jellyfish, however, eat them up, repeating a closed food cycle that shuts out fish, shrimp and other more substantial species.

The cycle traps the food web in a primitive state, one that some researchers compare to the ocean that existed more than 550 million years ago, before more complicated animals evolved.

Cool temperatures, stormy weather and other environmental influences eventually break the jellyfish cycle, allowing the ocean to return to its normal state.

However, several studies in the past decade have cautioned that nutrient pollution, dead zones, overfishing, artificial seawall structures and warmer oceans are all making life easier for jellyfish at the expense of other fish.

"All these things individually can potentially lead to more jellyfish, and then we add them all together," said Monty Graham, one of several co-authors of the study and senior marine scientist at Dauphin Island Sea Lab. "We’re very good at messing up the sea and we don’t just do in one dimension — we mess the sea up multi-dimensionally."

Science is inconclusive about whether blooms of jellyfish are increasing globally, Graham said, but people are altering the environment in ways that would support more blooms in the future.

Life in the dead zone

Jellyfish aren’t actually fish. They are simple marine creatures that live roughly half their lives in a floating medusa stage. The other half is spent as polyps on the sea bottom, usually on hard surfaces. Jellyfish often bear barbed tentacles that inject a stinging poison into whatever they contact.

Unlike most other animals, jellyfish can survive, and reproduce, in oxygen-starved waters, such as those that form each year along the northern Gulf coast, giving them a big advantage as so-called dead zones expand globally.

More than 400 dead zones form yearly around the world, with the one in the Gulf the second-largest. Since 1985, the Gulf dead zone has spanned an average of about 5,200 square miles each July, according to a report issued last week. Scientists with Louisiana Universities Marine Consortium predicted that it would grow to more than 9,000 square miles this year, the largest ever.

Most animals cannot survive in such low-oxygen conditions. They swim away or perish. Jellyfish, however, don’t mind the hostile environment.

Jellyfish taking over a dead zone is probably beneficial, to a point, "by being able to take excess nutrients out of the system," Graham said.

What happens in the northern Gulf affects fishermen here, said Glen Brooks, who runs a commercial fishing fleet out of Cortez. He was preparing to send a ship to Texas last Thursday and another off Alabama’s shore, uncomfortably close to the dead zone.

"In the years past we’ve been OK because we’re in deep enough water. I don’t know what it’s going to be like up there this year," Brooks said.

Although the Gulf of Mexico is a large body of water, damage to parts of the system can also affect the whole.

Ocean currents and winds occasionally transport water and nutrients, as well as red tide and jellyfish, from the northern Gulf to the waters off Southwest Florida. That was the likely scenario when an unusual bloom of sea nettles appeared just offshore here last October, Graham said at the time, noting a conspicuous absence of nettles that year in the northern Gulf.

Frequent jellyfish blooms are a sentinel of a degraded ecosystem, Graham said. And Condon’s report suggests that tipping the scales toward jellyfish would harm not only the tourism along affected beaches, but also the commercial and recreational fishing industries.

"It’s not the jellyfish’s fault. It’s our fault," Graham said. "We might be actually doing things to the ocean that allow the jellyfish to propagate."