Jellyfish blooms might be more than just a nuisance to beach-goers. These explosions of stinging swarms might also be doing some major disruption to marine food webs, according to a study published online June 6 in Proceedings of the National Academy of Sciences.
Warty comb jellies (Mnemiopsis leidyi) and Atlantic stinging sea nettles (Chrysaora quinquecirrha) have been emerging by the thousands each summer in brackish Chesapeake Bay tributaries. "Jellyfish are voracious predators," researcher Robert Condon, of the Virginia Institute of Marine Science, said in a prepared statement; and they feast on some of the same plankton that fish eat. That means less food for fish and other higher-level animals, potentially shifting the distribution of predators in the food web.
These jellyfish blooms also seem to have an outsized impact on the less visible—but also important—nutrient balance in the waters. Jellyfish aren’t a popular meal for most predators, so they are generally considered a "dead end" for carbon in a food system, the authors of the new paper explained. But even if the jellyfish themselves don't become a source of nutrients, they do contribute co-called "colloidal and dissolved organic matter" (read: mucus and poo) after processing their food, transforming the available carbon from plankton to fuel that's favored by bacteria.
These excretions are apparently welcome fodder for otherwise rare local bacteria populations, which gobbled up at least half of—and sometimes all of—the gunk within eight hours of its production, the researchers found. "Our findings suggest major shifts in microbial structure and function associated with jellyfish blooms," they reported.
But the bacteria populations weren't blooming in response as much as scientists might expect. Why not? The jellyfish waste turns out to be much richer in carbon than nitrogen, and "it just doesn't provide an efficient food source for marine bacteria," Deborah Steinberg, also of the Virginia Institute of Marine Science and co-author of the new paper, said in a prepared statement. So bacteria used the jellies' fuel more for respiration than for reproduction—which means they were creating more carbon dioxide rather than more usable biomass via increased numbers.
And, what extra bacterial reproduction does occur thanks to these blooms is likely boosting populations of their predators, such as flagellates, and their predators, such as copepods. And copepods just happen to be a popular food for jellyfish.
This cycle might just be a recipe for a "positive feedback 'jelly loop,'" the researchers wrote. And lots of jellyfish also leads inevitably to lots of dead jellyfish, which could further feed the bacterial populations.
The cause of the jellyfish blooms in the Chesapeake Bay and elsewhere is still up for debate, with some scientists suggesting agricultural runoff, climate change and/or overfishing as possible variables. And beyond the bay, the authors point out, "We know very little about the causes, magnitude, and extent of jellyfish blooms in the open ocean."
But, concluded Condon and his team, "Given current and projected global increases in ocean temperature, combined with anthropogenic influences, these trends are likely to continue into the near future with unknown consequences at the ecosystem level."
Image of sea nettle jellyfish courtesy of Wikimedia Commons/Omegacentrix