Last ice age really did a number on eelgrass

This story was originally published by Hakai Magazine and appears here as part of the Climate Desk collaboration.

Over much of the past two million years, thick ice sheets blanketed the land and sea across vast swaths of the planet. Around 12,000 years ago, this frigid era began to lessen its icy grip. But the world is, to this day, recovering from the effects of the last glaciations. Relieved of its glacial burdens, the ground itself is gradually rebounding. Melting that began with the ice age’s wane is ongoing, making the oceans minutely fresher. And in the Atlantic Ocean, eelgrass is still trying to regain the genetic diversity it lost when ice isolated it from the Pacific.

Throughout the Northern Hemisphere, Zostera marina — commonly known as eelgrass — is the backbone of coastal ecosystems from the Mediterranean to the Arctic. Eelgrass meadows protect the coast, store carbon, and support a host of organisms, from economically important herring, sea bass and lobsters to vulnerable species such as sea turtles and dugongs. Zoom in closer, and you’ll find a metropolis of algae and tiny invertebrates clinging to each gently swaying blade. “Eelgrass is like the Serengeti of the sea,” says Emmett Duffy, an ecologist at the Smithsonian Institution.

“The plants are covered with what looks like black sand, but it’s these tiny little snails that are just all over the place,” Duffy says.

Eelgrass originated in the Pacific Ocean between five and 10 million years ago, descending from a rare group of flowering plants that colonized the sea. It only spread to the Atlantic Ocean starting around 3.5 million years ago, before the most recent ice age hit and ice sheets separated the two oceans. And now, eelgrass is in decline because of disease, environmental degradation, and other causes. Duffy and his colleagues wanted to know if eelgrass’s past is putting limits on its possible future.

For their new study, Duffy and an international team worked on 50 eelgrass sites throughout the Northern Hemisphere, surveying metre-square patches of seafloor, collecting samples, and measuring the biomass of the algae and invertebrates living on the eelgrass. Geneticists reconstructed parts of the eelgrass genome, looking for similarities and differences between and within the eelgrass living in the Pacific and the Atlantic Oceans.

Visually, eelgrass ecosystems look different in the Pacific and the Atlantic. The Pacific has sparse forests of metre-high eelgrass, while the Atlantic has shorter, denser meadows. Atlantic eelgrass meadows also support more invertebrates than the Pacific’s tall forests.

But the most striking differences aren’t visible to the naked eye. As their research shows, eelgrass ecosystems in the Atlantic have far less genetic diversity than in the Pacific. Duffy links that back to eelgrass’s initial migration from the Pacific to the Atlantic and the onset of Arctic ice not long afterward.

“Crossing the Arctic at that earlier, warmer time was probably a very difficult journey,” Duffy says. Few plants were successful, limiting the genetic diversity of Atlantic Ocean eelgrass from the get-go. The onset of Pleistocene ice effectively sealed Atlantic eelgrass off from the Pacific, and repeated glacial cycles shaped eelgrass from there.

“The biggest surprise, and the most important result for me, was that an event from tens to hundreds of thousands of years ago is still affecting this globally distributed plant,” Duffy says.

Atlantic eelgrass’s lack of genetic diversity might be bad news for its ability to survive climate change. Without an assortment of potential adaptations to lean on, eelgrass might fail. Given enough time, Atlantic eelgrass could eventually evolve to have the same breadth of diversity as its Pacific counterparts. Unfortunately, the swift pace of climate change may not give it that chance.

“It’s assumed that species will simply move,” Duffy says. “But sometimes, as we’ve seen with eelgrass, it’s only a small part of the population that makes it through, and the characteristics of those pioneers can be quite different from the larger population that they came from… You don’t just establish the same population at a different latitude.”

To Nicole Kollars, an ecologist at Northeastern University in Massachusetts who was not involved in the paper, the research “raises a lot of questions about what that [genetic diversity] means for the resilience of eelgrass to climate change.” She adds, “I think it opens up a lot of questions for us to tackle over the next 20, 30 years.”

People could lend eelgrass a helping hand, Kollars says, either by moving seeds and plants around to increase gene flow or by using eelgrass nurseries to support restoration efforts. For her, the study serves as an important reminder that the past can be a window to the future.