The Diversity Within
The diversity within a species of bay grass could play a key role in its restoration and survival
Wild celery shoots float in the Potomac River. These were just a few of the plants that Maile Neel and her colleagues saw during their strenuous, four-day bike ride. Photograph by Maile Neel.
THE SCIENTISTS LEFT CUMBERLAND, MARYLAND on their bikes on a steamy summer morning. Their destination was Washington, D.C., nearly 185 miles away and down a looping trail, the C&O Canal, that follows the Potomac River. The ride wasn't a pleasure jaunt: along the way, the five researchers would stop about every six miles to wade into the river to collect aquatic grasses.
This survey-by-bike, which took four days to finish, was the brainchild of Maile Neel. She works as a plant scientist at the University of Maryland, College Park. The scientist, whose time outdoors shows on her tanned skin, competes in "ultra-distance" rides, crossing nearly 750 miles of terrain in only 90 hours. So the team's pace of about 45 miles a day was downright leisurely for her. It was less easy for the four students she brought along, some of whom hadn't ridden a bike in years.
The sore hamstrings, however, were worth it. On their trip, the five riders were able to collect shoots representing the full range of diversity in the upper Potomac's wild celery (Vallisneria americana).
Before beginning their trip, Maile Neel and colleagues pose in front of a sign (top) for Cumberland, Maryland, 184.5 miles away from Washington, D.C., on the C&O Canal. From left: Brittany West Marsden, Hayley Tumas, Paul Widmeyer, and Maile Neel. Wild celery (middle) grows during its peak in mid-summer. More recently, Neel, West Marsden, and Tumas gather in the greenhouse (bottom) to plant wild celery “tubers,” small structures that spend the winter buried under sediment. Photographs by Maile Neel (top and middle) and Daniel Strain (bottom).
This green plant, which has thin leaves that roll with the current, is one of the most common and recognizable species of underwater vegetation in the Chesapeake Bay watershed. And it plays an important role in the ecosystem, helping to trap the floating, excess nutrients and sediments that make the Bay's water murky. These aquatic grasses also provide shade and shelter to dozens of Bay animals.
But just like people, wild celery plants aren't all alike, Neel explains. Look closely, and you'll see subtle differences in the lengths of their roots and the widths of their leaves. Scientists have uncovered other, less-obvious differences, too, such as how well the plants tolerate saltiness in the water. This diversity of characteristics comes from the unique combination of genes — called a genotype — that each of these plants carries. The differences help the plants to survive in the unique habitats where they grow.
Such genetic variety within a single species is a form of biodiversity. But how important this genetic diversity can be to the survival of a species is still poorly understood. It was a question that Neel and her team wanted to answer for wild celery in the Bay. "Ultimately, the question is, does the genetic diversity that's present have some contribution...to how these populations are going to respond to climate change or to environmental assault," she says.
In many cases, the answer seems to be yes. New scientific tools have revealed that genetic diversity within a single species may be important for the long-term survival of a species. Understanding this diversity could change how scientists approach restoring vulnerable bay grasses to the Chesapeake watershed, where their numbers have dwindled because of deteriorating water quality and other factors.
"You can't just treat a species as a monolithic entity," says Randall Hughes, a marine ecologist at Northeastern University in Massachusetts. "There's a lot of diversity within that species that is important."
Such research suggests that biodiversity benefits ecosystems at many different levels — not only when multiple species are working together (see Forest Biodiversity) but also when there is a lot of variety within a single species.
Mountains to the Bay
In a greenhouse in Frostburg, Maryland, plant scientist Katia Engelhardt can see that genetic diversity beginning to emerge. She points to a corner of the greenhouse in the sun where several rows of plastic buckets have been lined up on a table. They're filled with water and a few inches of sediment. And wild celery sprouts.
"There's some that are just popping up now," says Engelhardt of the Appalachian Laboratory, part of the University of Maryland Center for Environmental Science. Sure enough, if you lean over and peer into the buckets, you can see a few poofs of wild celery starting to poke out of the mud.
While its name may call up thoughts of supermarket veggies, wild celery is a bay grass known for its green leaves that can grow several feet long. In the summer, you can find wild celery swaying in freshwater portions of the Bay and its tributaries, mostly from the Potomac north to the Susquehanna River.
The plants spread throughout the Bay in two ways, Engelhardt explains: by scattering their seeds, like many flowering plants, and by reproducing asexually, like aspen trees. They do that by sending out underground shoots that sprout into genetically identical plants — called clones.
Engelhardt, who's grown a lot of aquatic plants during her career, likes to spend time out here in this greenhouse with the grasses. She calls them "graceful."
"They seem to have a personality," she says. By which she means that each one in each bucket is a little bit different.
These plants were all plucked from different locales, including the northern stretches of the Potomac where Neel began her trek.
Engelhardt's favorites are a group she's studying from the Sassafras River, a small tributary that empties into the northernmost regions of the Bay far from the Potomac. These plants have unusually long and lush leaves. But their roots are surprisingly short and dinky — perhaps, she suspects, because the river has a sandy bottom in which plants can anchor without needing long roots.
Visible features like these that help organisms to survive are what biologists call adaptations. They're programmed by a plant or animal's DNA, or genetic code. Differences like these can be seen in the genetics of wild celery all across the Bay.
Through genetic analyses, Maile Neel and colleagues discovered that the Bay’s wild celery (Vallisneria americana) populations are split into four distinct zones: what they called the upper Potomac (green markers), the lower Potomac (orange), the central Bay (red), and the northern Bay (blue). Plants from one zone rarely mix with plants from another zone. Map Source: Lloyd, Burnett, Engelhardt, and Neel, 2011, Conservation Genetics 12:1269-1285, used with the permission of the authors.
In 2007, several years before Neel began her cycling trip, colleagues Neel and Engelhardt took on a different survey to explore this variation. In research funded by Maryland Sea Grant, they pulled up shoots from rivers up and down the Bay. Then a team decoded, or "sequenced," the DNA of each plant. Such sequencing is cheaper than ever, giving researchers an unprecedented ability to explore the genetics of Bay species.
Engelhardt and Neel uncovered a surprising diversity: the team collected 675 wild celery shoots, discovering 427 separate genotypes. That suggests that the Bay is dominated by a lot of unique individuals rather than a few, widespread clones. Their work also indicated that the personalities that Engelhardt had spotted among the plants in her buckets had likely arisen from the grasses' diverse genetics.
The team hadn't expected to find that much diversity. That's because it's an axiom in biology that as the range of a species shrinks, so does its genetic diversity. And populations of bay grasses within the Chesapeake watershed have shrunk a lot.
For centuries, wild celery had helped to keep the Bay clear by trapping excess nutrients and sediments in the water column. But as long as a century ago, human activities began to overload the estuary with those same nutrients, such as nitrogen and phosphorus. Those fed huge blooms of algae that cut underwater plants off from sunlight, stunting or killing them. Add in copious sediments floating in the water, and by the 1960s and 1970s, the natural ability of aquatic plants to clear the water was overwhelmed.
The Bay's grasses received a further death blow when record quantities of sediments were washed into the estuary by Hurricane Agnes in 1972. Today, plants like wild celery and eelgrass (Zostera marina) cover around 10 percent or less of their historic area, according to ongoing monitoring work by scientists at the Virginia Institute of Marine Science.
How wild celery has remained so genetically diverse despite this great decline remains unclear — but the surviving diversity may offer hope for the future recovery of the species in the Bay. That's because a growing body of research indicates that populations of genetically diverse plants are better able to ride out environmental disasters.
Randall Hughes of Northeastern University was one of the first scientists to show how genetic diversity can help underwater plants to recover from environmental disturbances. As a graduate student in California, Hughes planted a few dozen square meters of eelgrass, creating plots carrying different levels of genetic diversity. In some plots, the grasses were very different on the genetic level. In others, they were more alike.
For a while, however, she didn't see any difference in how they grew. Then, months into her research, flocks of geese descended on Hughes's study site and devoured most of her plants. That's when she noticed something interesting: the eelgrass plots with higher levels of genetic diversity survived the attacks in better shape than did other plots. The scientist and her colleagues published their results in 2004 in the journal Proceedings of the National Academies of Science.
As Hughes puts it, "When there is disturbance or stress, that's when diversity tends to really shine."
To understand why, think of a Swiss army knife. These camping gadgets have tools for opening a can of beans or uncorking a bottle of Zinfandel.
Individual organisms in a population are also like tools, each one carrying its own genetic code that allows it to tackle certain problems. Like the Swiss army knife, however, more diverse populations have a broader toolkit to draw from than less diverse ones. As a result, they can respond to crises in many different ways. Some plants may be good at fending off geese, while others can grow well in especially muddy water. When the environment changes, "if one genotype doesn't do well, another genotype will do well," Engelhardt says.
Populations with low genetic diversity, however, don't have that flexibility. They're like the lone can opener or corkscrew, useful in some situations but not others.
Engelhardt hasn't completed similar research using wild celery, but she suspects that what's true for eelgrasses should be true for them. In other words, the high levels of genetic diversity in the Bay's wild celery should provide the population with the raw materials it will need to recover and survive many future disasters — including, perhaps, warming waters due to climate change.
That survival advantage could also translate into expansive and thick grass beds that can better do what they do well: namely, to make the Bay's waters clearer. To top it off, large grass beds can also slow down waves in the Bay, helping to save the local shorelines from rapid erosion.
Or as Neel notes, high levels of diversity in the Bay's wild celery means that "we're very lucky."
Neel has spent much of her career studying endangered plants, such as silvery-white milkvetch (Astragalus albens) from California. So, tongue in cheek, she says it's nice to focus on a species "that's got some hope."
But luck and hope can only get you so far. Bay grasses around the estuary still face significant problems, including low light levels in many habitats. Overcoming that may require research studies with yet another focus: how can you use genetic diversity to your advantage?
Neel collected some data years ago that may help answer that question. During their sweaty, muddy bike ride along the Potomac, she and her colleagues noticed something curious about the river: compared to other patches of wild celery scattered around the Bay, the Potomac carried a lot of widespread clones. In fact, grasses belonging to just one genetically distinct line stretched down the waterway for nearly 100 miles. It was a feat that Neel and Engelhardt hadn't seen in any other Bay tributary.
Dipping her hands into a bucket filled with water and a layer of sediment, Katia Engelhardt pulls out a thin wild celery shoot. “I like to get my fingernails dirty and wet,” she says. Photograph by Daniel Strain.
What the scientists couldn't be sure of, however, was why this line of clones was so prolific. "Are they widespread because they're the best performers in all these environmental conditions, or are they there by chance?" Neel asks. In other words, were the plants lucky, or are a few genotypes in a diverse population simply better at growing than others?
To further test that, one of Neel's students, University of Maryland undergraduate Hayley Tumas, conducted a simple experiment in the greenhouse using light. She grew diverse kinds of wild celery, collected from all around the Bay, under different light levels. Some had access to a lot of sunlight while others were shaded. And, sure enough, certain individuals naturally grew better than their counterparts in the shade. They seemed to carry genes that gave them an advantage when light was low.
Such insights could benefit aquatic plant managers, says Lee Karrh, a biologist at the Maryland Department of Natural Resources. He's overseen many bay-grass restoration projects locally, including replanting two acres of wild celery in Baltimore County's Back River. But the success rates of such ventures are low. Few of the plants that his team sows ever grow into adult plants. One of the most persistent problems, he suspects, is the turbidity of the surrounding water — floating sediment has made much of the Bay too opaque to allow little plants to grow.
Experiments similar to Tumas's, however, might be able to identify a particular set of plants that could grow well even under those tough conditions. If Karrh planted just those plants, he suspects that his restoration projects would have a better shot at succeeding.
It's a way to take advantage of the diversity that the Bay's wild celery offers — like forming an all-star baseball team. If conditions are rough, you plant only those grasses that have the best odds of surviving the growing season. Neel and Engelhardt's research is "allowing us to fill in the pieces that we were missing," Karrh says. "What genotypes do we need to put where for what growing conditions?"
Still, a lot more research will have to be done before the scientists know which genotypes do best under which conditions. And managers like Karrh will have to balance efforts to build a wild celery team using a few star players with the need to keep genetic diversity high — equivalent to maintaining a deep bench. The environment could change, making today's all-stars tomorrow's wash-ups.
But regardless of the strategies that managers take, genetic diversity can only do so much, especially if the Bay's water quality stays poor. The best way to ensure the future of wild celery is to reduce the excess nutrients and sediment in the estuary that have slammed its populations so drastically, Neel says. She's standing in her own greenhouse and looking over a few sprouts of wild celery beginning to grow inside their buckets.
At least in this peaceful setting, one thing is true: "If you have clear water, these things are hardy," she says. "They'll grow."