An invasive seaweed settles in its Georgia niche

[USA] When University of Georgia ecology professor Jeb Byers first came to Georgia, he figured seaweed wasn’t on the list of species to study.

“They said you’re not gonna ever see seaweed again,” Byers recalled. “It makes sense because the muddy water here keeps the light low. And there’s lots of soft sediment here, but seaweed likes something hard to attach to.”

That’s why Georgia hosts little in the way of native seaweed.

Then Byers visited the Skidaway Institute of Oceanography.

“I remember going to a mudflat and seeing this alga called Gracilaria everywhere,” Byers said. “And people were like ‘Oh, it’s that.’”

The branching mud-colored seaweed is native to Japan, where it’s farmed for the gelling agent agar. It’s already a notorious invasive in Europe and the Mediterranean. Here, it just “snuck in here under everyone’s nose,” Byers said. Exactly how and when is unclear, though Byer’s best guess is about 25 years ago and likely from fragments of the seaweed in ballast water.

Shrimpers already knew about it and had complained to South Carolina Department of Natural Resources about it fouling their nets.

Byers wondered what else it was doing, so with other researchers he set about studying Gracilaria. One head scratcher was how the seaweed stayed on the mudflat instead of being washed away with the tide.

“How are they living on the mud?” Byers said. “That was one of the first things that intrigued us.”

The seaweed has claimed a foothold on study sites in the Wilmington River with a little help from a native tube worm that attached the seaweed to its shell-like tubes. The worms “for various reasons will decorate their tubes,” and “grab whatever they can,” Byers said.

Although invasive species are often noted for causing problems, the seaweed seems to benefit other natives, too, Byers and his colleagues report in the October issue of the journal Ecology.

That’s because Gracilaria vermiculophylla — it has no common name — turns out to be a great place to hide if you’re a tiny native crustacean called Gammarus. The flea-sized crustaceans typically live in the marsh grass. But when this seaweed covers a mudflat, they suddenly have new territory where they’re protected from predators and from drying out in the sun.

“They’re able to use huge areas of the estuary that would have been off limits,” said Byers who estimates their abundance is increased 100-fold in seaweed covered mudflats.

All that Gammarus could be a boon for seafood lovers because they provide an excellent food for shrimp, crab and fish.

The seaweed also absorbs excess nitrogen out of the water, like the nitrogen that can run into an estuary from fertilized land upstream. Too much nitrogen can result in an algal bloom and a resulting decrease in oxygen as bacteria feed on the algae. That’s less likely to happen with Gracillaria in the picture.

“It’s creating a sink for nitrogen,” Byers said. “Some areas of the world use Gracilaria purposefully for this problem.”

The researchers have found that the Gracilaria in Charleston harbor, which has a worse nitrogen problem than does Savannah, shows a higher level of nitrogen.

Byers isn’t ready to name the seaweed a welcome invader.

“It’s sucking away excess nitrogen and boosting primary and secondary productivity,” he said. “Those are fairly positive from a human perspective, but does that mean it’s an overall positive? I’m not sure.”

Neither is graduate student Lynsey Haram, who is working on her doctorate from UGA’s Odum School of Ecology. She’s been studying the invasive seaweed with Byers since 2011. Her ongoing study of shorebird foraging seems to indicate that the birds, including dunlins, semi-palmated plovers and sanderlings, peck more on Gracilaria-covered mudflats. It’s unclear yet if they’re getting more to eat or just making a bigger effort to sort through the seaweed.

“Some studies correlate pecking more with more results,” she said. “But it could be that they’re worse.”

And Gracilaria has been problematic elsewhere, including in Virginia where it’s associated with the oyster bacteria called Vibrio, she said.

“It’s too early to say whether it’s good or bad,” Haram said.


To answers questions about seaweed abundance, and how it fluctuates over time, researchers are asking for some help from citizen scientists in Savannah.

To participate, take a photo of the nearby mudflat from the Jay Wolf Nature Trail dock near the UGA Marine Extension Service Aquarium on Skidaway Island.

Signage there will soon provide further instruction, but all you will need to do is place your phone or camera in the bracket adjacent to the sign, take an unfiltered photo with your phone or camera, and upload it to Instagram or Twitter using the hashtag designated on the sign.


Photo courtesy Jeb Byers/UGA, Lynsey Haram does Gracilaria field work in Charleston.

View original article at: An invasive seaweed settles in its Georgia niche





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