For tourists, coastal North Carolina is a paradise. And it can feel that way for tiny little Eastern Mud Snails, too. For them, though, it’s perhaps a different part of the coast: instead of white sandy beaches, these tiny gastropods love the brackish water and muck in the estuaries.
“They are little omnivorous detritivores. They will eat anything that can fit in their little mouths," says Dr. Byron Toothman, a coastal reserve researcher in the Cape Fear Region. He’s brought WHQR out to one of his primary research sites: Zeke’s Island Reserve. It’s beautiful: coastal grasses sit high over the water during low tide.
Dr. Mariko Polk is the coastal process specialist for North Carolina Sea Grant, and says there’s an idealistic view of what the estuary here would look like without humans: smooth, yet wavy lines where the water meets the grass.
“Naturally our shorelines, we describe it as sinuous, so it wibble-wobbles up and down,” Polk said. “There's no necessarily straight, perfect line, right?”
But on Zeke’s Island, some sections of the grass have been torn away, thanks to mats of invasive macro algae: called Gracilaria vermiculophylla. It looks quite a lot like a black tangle of witch’s hair: long strands that branch off each other, Toothman said.
"The macro algae will wash up against the shore and get entangled in the shoots of the grasses," he explained. "And then that process will continue to happen, until this ball of algae accretes, and you have this large mat that stays in place. And they can exist for some time, months, sometimes years.”
Once that happens, the algae sits there, agitating the sediment, taking it from the bottom of the estuary bed and suspending it in the water. Polk said that can weaken the structure of the sediment under the grass, basically taking a big bite out of the sea grass after a big wave or storm washes that mat away.
“Those bite marks that you're talking about are areas where you have, like, aggravated instances of erosion," she said.
That leaves behind extra mucky mud where the mat used to be, and those, too, can fill with invasive algae. Toothman discovered something else unusual, though: those mats appear to have a significant impact on the estuary’s mercury cycle.
That's because there are tiny microbes under the surface of the sediment that thrive in low-oxygen conditions.
“Some of those bacteria, they carry a gene that allows for the methylation of mercury or adding a methyl group to it, and that's the form of mercury that really bioaccumulates up through the food chain,” Toothman said.
When that happens under the sediment surface, it creates a mercury sink: a place where mercury will sit without getting into the food chain, where it could end up eventually consumed by humans.
But Toothman hypothesized that these mats of algae created a similar low-oxygen environment which would let those microbes thrive above the sediment. He decided to investigate: using those little mud snails that love eating everything.
“They can travel around, but they don't really travel far," he explained. "And so my idea was that if these guys are all over the place, and some I can find on algae mats, and some I can find elsewhere on the mud flat, maybe I can look for mercury in these and if they're accumulating the mercury from their environment, maybe they will tell me whether there's more mercury associated with these algae mats. And in fact, they told me just that.”
Essentially, the food these snails scavenged near or on these mats of algae was much higher in mercury than the areas without the dense invasive algae.
It’s because the interiors of these mats are without oxygen, Toothman said: “So the interior of these mats, I had this idea that they were maybe replicating the environment that we find in the sediment.”
When the methylmercury stays in the sediment, it doesn’t as readily end up in the food chain. But when it’s closer to the surface, like with those algae mats, Toothman saw significant differences.
“I found that the snails that were growing within a meter or so of the mats contained, sometimes as much as 10 times the mercury as those that were, you know, 20-30, meters away, on average, it's about two to three times as much mercury. And we found similar patterns in other organisms like shrimp and oysters,” he said.
Toothman said that might have an impact on sea food humans consume.
“This is a very important place in terms of our fisheries and our food supply," he said. "So if you're increasing the areas where this mercury cycling goes on in the water column, then you're also exposing more of our food supply to Mercury. And so I have to think, I don't know, but I have to think that if it's happening in acute areas around a basin, then the overall concentration of mercury.”
It’s worth noting — there’s no safe level of exposure to methylmercury. While the FDA has exposure limits, the metal bioaccumulates in the body much faster than you lose it.
