It was a sad day down on the river. Our spring-like February had abruptly ended with a blast of polar air and a lot of overeager plants got fooled. Now they’re paying for it.
The patches of Virginia bluebells down by the river were among the victims. Early in the spring they thrust their leaves through the lifeless soil, and in no time their lush foliage transforms the dreary forest floor. They presently send forth stalks of bell-like flowers, first shades of purple and lavender as they open, then turning a delicate blue. Coming upon a patch of bluebells is like stepping into an impressionist painting.
Today was different. The cold and several inches of wet snow had struck down many of the plants. Beaten to the ground, their still-lovely flowers lay limp and defeated. We’ll see if the coming warmer weather can revive at least some of them.
A gaudy bug. On a similarly pessimistic note, I was struck by the number of trees in the woods whose swaths of tawny bark stood out from the dark bark of their neighbors. These are dead or dying ash trees that had been attacked by the emerald ash borer. Woodpeckers had chiseled off the darker outer layer of bark to get at the grubs, which accounted for the light color I was seeing. Some of the dead trunks were already riven by horizontal cracks and ready to fall to the ground.
A native of Asia, the ash borer first appeared in Maryland in 2006. In Virginia, an initial infestation in 2003 was eradicated, but the beetle returned for good in 2008.The insect’s striking metallic color—more emerald even than an emerald—contrasts with the dreary succession of events that take place after laying its eggs in the cracks in the tree bark. The newly hatched larvae bore into the trunk, where their feeding disrupts the tree’s ability to transport water and nutrients. The larvae later chew their way out to complete their life cycle.
I’m normally not a hater when it comes to introduced species. “Nature continually changes,” I say. It’s just that I don’t like this change.
The ash is a fine tree. The white ash (Fraxinus americana) in particular is much esteemed for its tough wood, which was the material of choice for baseball bats before the era of aluminum. When I was a boy I learned to cook on kitchen counters made of ash.
The ash borer can be controlled by insecticides, but treatments are only feasible for high value trees, for example in urban parks. Biological controls are being developed but have yet to prove their effectiveness.
What’s happening to the ash reenacts the chestnut blight that removed this similarly magnificent tree from our forests. I have never seen a mature chestnut tree. My grandchild might never see an ash tree. I still have an old ash baseball bat, and some day I’ll give it to him.
Seeing the bottom of the Potomac River through a facemask is a little like looking through a microscope. It’s a new world down there, and you never know what you might find.
I was looking sponges. The previous year they were right here, at the head of Watkins Island, near Pennyfield Lock. It was apparently the first sighting of sponges in the river’s main stem. What happened to them? Would they come back?
I examined the bottom minutely, admiring the shimmering pebbles and the green patina on the mussel shells. I poked at anything that looked even the slightest bit spongy.
Then I saw something that made me come to a full stop. Wedged among the pebbles was a pair of tiny white tubes, then another, and another. Hundreds of them. I had found a colony of freshwater clams.
This was interesting, but not because clams are unusual in the Potomac. Far from it. The shells of this little bivalves literally pave parts of the river in Montgomery County. Along some stretches of shoreline, their shells crunch under every footstep.
But I had never seen these little creatures actually at work. And here they were, drawing water through one tentacle-lined tube, retaining nutrients other matter, and then expelling the same water through the other tube.
It struck me that they might be a little like the Higgs boson of the Potomac. Even though most people don’t know anything about them, their vast numbers could give them a decisive role in the river’s ecology. Are they another of Charles Darwin’s humble creatures that rule the earth, such as the barnacles that were the subject of his first book and earthworms which he lovingly saved for his last?
Oysters of the Upper Potomac. It seems so, at least according to biologist Harriette Phelps, a professor emeritus at the University of the District of Colombia. Phelps is widely known for her work with this clam species in the tidal Potomac.
She told me the clam’s scientific name is Cobicula fluminea. Its common name will have to wait, so as not to give away the second part of the story.
“Hard working Corbicula,” she calls them. Put a layer of them on the bottom of a pail of murky water, and an hour and a half later the water will be clear. Multiply this effect by the countless millions of clams in the river, and you have what Phelps calls a “key species.”
Forget the feisty bass, the graceful heron, the majestic eagle. One of the real drivers of life in the river is this humble mollusk, most of them no bigger than a quarter. Darwin would have understood.
Corbicula almost seems designed with the express purpose of purifying water. The particles it extracts from the river are not simply cast back again. Instead, they are combined with mucus to build up the sediment in which they live.
Does this sound familiar? Elsewhere on this site I wrote how oysters were the cheapest and most efficient water purifier system in the Chesapeake Bay, until they were nearly wiped out.
Clearing up the water kicks off a chain of happy events. More sunlight penetrating the water fuels the growth of aquatic plants. With oxygen-producing plants come fish and birds, and pretty soon, the river environment becomes whole again.
Phelps saw this happen in the tidal Potomac. There’s “absolutely every reason” to assume that Corbicula is providing the same service in our upstream portion as well, she says.
Certainly something good is going on out there. In some places in the river, plants are so thick that you can barely plow your kayak through them. In the channels through the vegetation you see darting shoals of minnows and bass slipping back into the cover. Big catfish chug along the bottom like monster tadpoles.
A dark secret. But these sunlit waters contain a dark secret. Corbicula is not a native of the Potomac region or anywhere else in North America. It originally came from eastern Asia, and is know by the popular name Asian clam. It arrived first on the West Coast around 1930, then showed up in the Potomac in 1977.
Once in the Potomac the clams rapidly increased their numbers, carpeting many parts of the river bottom. This would seem to qualify Corbicula for membership in that increasingly familiar “genus” of the world of advocacy ecology called “invasive.” Here we have another foreign creature that has invaded our own natural communities, like a shiploads of Vikings burning monasteries, slaughtering townspeople, and carrying off treasure.
But not according to Phelps. “Corbicula were not invaders,” she said emphatically. “They occupied areas that were previously unoccupied.” And like many immigrants, they got their start by doing jobs that natives wouldn’t or couldn’t do themselves.
In the tidal Potomac, Corbicula inhabits the upper layer of the river bottom, she explained. The estuary’s native clam lives deeper down. “Even with Corbicula, the native clams are all over,” said Phelps.
Paradoxically, Phelps worries that this “invasive” species might disappear. It already has in her former study areas in the tidal Potomac. The same could happen in the upper river as well.
If Corbicula disappears, the river’s ecological clock could start to go in reverse. With no more clams filtering out so much stuff that gets washed into the river, the water could again lose its clarity, and with it, many of its plants, fish, birds, and other life forms.
The clams’ disappearance would also be felt by one group of local people whose ties to Corbicula extend to long ago and far away. You can sometimes see them, the men bent over the water, raking out clams with fan covers, the women sitting in the river, plastic baskets resting on their outstretched legs, picking the clams out from the stones and empty shells.
These people are also immigrants, harvesting the same clams in the same way they they and their families have done for many generations in their southeast Asian homelands. Following age-old traditions they will cook the clams in a wok with ginger, chile peppers, basil, and soy sauce.
Evidently it’s an acquired taste. “They’re good,” says Phelps, “but not really clam-like to me. I like clams that are saltier.”
It was a blustery February afternoon, but I wanted out. I’d go down to the river to see what I could find.
I threw my waders into the back of my car and headed to Carderock, in Bethesda. A short hike down the Billy Goat Trail I found a likely place and slid down the riverbank, using exposed tree roots as handholds. My boots hit the mud.
A few feet out from shore I plunged my arm into the icy water to grab a large, flat rock. I tipped it up on its end and inspected every nook and cranny. In the summer on such a rock I would find all manner of tiny crawling things, mostly the larval forms of insects. With their multi-spiked tales, armored bodies, and fearsome jaws, they look like monsters in a low-budget horror movie.
Several stones later my hands began to grow numb and I gave up.
Citizen science. I would find my monsters at the nearby Audubon Naturalist Society (ANS) in a class with a fittingly awe-inspiring name: “Benthic Macroinvertebrate Identification.”
The classroom was filled with people of all ages, all eager to learn how these little creatures lived and how to identify them. After the course ends many of the students will go on to collect specimens in streams and record their findings. Different species have different tolerances to pollution, so their presence or absence says something about the quality of the water.
Probably no group of such “citizen scientists” except birdwatchers makes a greater contribution to our knowledge of the environment than these stream monitors. Many schools in the Potomac region and across the country make macroinvertebrates a part of science education. It’s a way to experience nature in the raw and do some real-life data collection.
Bottom dwellers. I leaned over my microscope to examine the tiny creatures while inhaling the alcohol fumes in which they took their last swim.
Cathy Wiss, ANS Water Quality Monitoring Program Coordinator, dissected the name of the course. The term “macro,” means anything you can see with a naked eye. “Invertebrate” refers to anything without a backbone, in this case, mostly insect larvae. The initial word in the course name was “benthic.” It simply means that these particular insect larvae live on the bottom of our rivers and streams, under rocks, in the mud, or hidden among the roots of weeds.
Conducting the class this week was ANS stream monitor Gretchen Schwartz. Her subject was one particular group of macroinvertebrates called Trichoptera, the caddis flies.
Enlarged by the lens, many of them looked a little like tiny, elongated shrimp with curved bodies and six legs poking out of their thorax. But actually they are most closely related to moths and butterflies. They live their larval lives in the water. After they hatch into flying adults they live on nectar, at least for the few days until they mate and lay their eggs. Then they die.
Nets of silk.Schwartz told us other things about Trichoptera that once again showed nature’s infinite variety and inexhaustible ingenuity.
Different species make their living in different ways. Some scrape algae off of rocks. Others shred up leaves and eat them. Aggressive predators seize and consume other macroinvertebrates.
Some species even make silk to fashion delicate nets. Some set the nets on the stream bottom to catch any edibles coming their way. Others use silk strands as kind of lifeline to hold them to the bottom in fast currents.
Still other Trichoptera species use this same silk to make dwellings, wrapping it around grains of sand and then tying grains together to form a tube that they wear as a kind of a flour sack. Some enthusiasts give colored grains to captive Trichoptera. The insects use them to make their tubes, which people fashion into earrings. It could be a conversation starter or stopper, depending on the company.
Beyond useful and clever, these macrointertebrates are also extremely important. They and countless billions of other tiny creatures are what renowned Harvard biologist E.O. Wilson calls “the little creepy-crawlies that run the earth.” If Pandas or elephants—or us—suddenly disappeared, life on earth would pretty much go on. But remove bees and ants—and benthic macroinvertebrates—and our ecosystems would collapse in a hurry.
A search for fact and fable along the Potomac River