December 5th, 2009; Vol.176 #12 (p. 22) Science News
OK, OK, a plant can’t really look a person in the eye and share its thoughts. But after a strange couple of days, I’m almost ready to commune with vegetable matter. A string of wet, pinkie-tip–sized green leaves sits on a paper plate in front of me, and I begin to think that this little sprig and I are both wondering, “You? What in the world are you doing here?”
I’m a terrestrial vertebrate rocking slightly from side to side on a research ship more than 100 kilometers west of the tip of Florida, near the Tortugas Ecological Reserve. I’m tagging along with marine biologists on the National Oceanic and Atmospheric Administration’s research vessel the Nancy Foster. Though my first three days with nothing but water in all directions have been thrilling, I haven’t shaken some bone-deep sense that I don’t belong here and that air-breathing land creatures visit the seas on sufferance.
In one sense the little sprig does belong to this world. Diver Abigail K. Poray of California State University, Northridge plucked this sample of Halophila decipiens seagrass from the pale sediment on the seafloor where the species thrives.
But in another sense, seagrasses are interlopers too. These aren’t marine algae like kelp. Seagrasses are true flowering plants, and the world’s 50 or so species come from marsh and freshwater plant families. Seagrasses’ ancestors lived on land, where seeds and flowers first evolved, but more than 100 million years ago some went off the deep end into the oceans. And today’s species still grow flowers, release pollen and form seeds, all underneath meters of saltwater.
Bizarre as this venture to the sea seems, other famous lineages have taken the same path, going oceanic but never quite losing all traces of landlubber biology. I’m communing with a botanical whale.
Seeing one seagrass sprig makes me want to meet whole plants, whole meadows even. That’s not going to be easy for a terrestrial journalist with no dive training who has been plunked into the middle of a high-speed, data-gathering mission with stern safety restrictions that prevent aquatic sightseeing.
Seagrasses will be worth finagling for, though. Among the marvels of the Foster’s voyage, these bits of greenery stand out as surprise survivors in an alien world. Recent work is providing clues to how seagrasses have adapted to survive in their saltwater ecosystem. But they’re not just survivors. Other studies are showing how some marine icons and even whole coral reef ecosystems wouldn’t be what they are today without the energy input and nearby refuges of seagrass meadows.
But like reefs, seagrass expanses are shrinking under the human bootprint. An appreciation of the seagrasses and the perils they face is growing in the research community, but public attention in the form of T-shirt wearing, TV-special viewing and postcard-sending hasn’t caught up yet. Meadows of seagrasses “aren’t vacation destinations,” acknowledges seagrass ecologist Mark Fonseca of the NOAA National Ocean Service’s research center in Beaufort, N.C. But without seagrasses, he says, you wouldn’t have so many vacation destinations.
In a just world, seagrasses would rank as a sought-after tourist spectacle. West of the Florida coast, a low, green Halophila carpet sprouts anew from the sediment each spring over some million acres. “Half the size of Yellowstone,” Fonseca says.
This spectacle’s future, and that of the other seagrass beds in shallow coastal waters around the world, looks iffy. Long-term data on the extent of seagrass meadows aren’t great, but combined there’s enough to sketch trends along populous coasts, says Jud Kenworthy, also of NOAA’s Beaufort research station. Along the coasts of North America, Australia and Europe, nearly a third of the known seagrass landscape has disappeared since the 1870s, an international research team including Kenworthy reports July 28 in the Proceedings of the National Academy of Sciences.
Losses look as if they’re speeding up along these coasts, the paper warns. Before 1940, records indicate a median decline of some 0.9 percent per year. Since 1990, though, total seagrass meadowland has been shrinking about 7 percent a year.
Development along coasts is muddying and polluting offshore waters, and seagrasses falter without clear water allowing light to filter down, Kenworthy and colleagues say. With heavy ship traffic, boats ground in shallow meadows and leave long scars that can take decades to heal. (Fonseca can point out the scrapes and bald spots visible in Google Maps’ satellite images.) As for climate change, researchers are just beginning to try to figure out what rising water temperatures and pH changes might do.
At least a billion people live within 50 kilometers of seagrass meadows, Kenworthy and colleagues note. Coastal residents may not give much thought to their green underwater neighbors but could still feel the pinch of seafloor erosion or declining fisheries productivity, among other woes of lost ecosystem services.
Seagrass losses have reached the magnitude of declines seen in celebrity ecosystems such as tropical rain forests and coral reefs. Kenworthy and colleagues say that the new calculations put seagrass meadows among the planet’s most threatened ecosystems.
At home or away
Nose-to-node with a Halophila, though, what intrigues me is how something with such terrestrial-looking little green leaves could adapt to so much water, and salty at that.
The ocean rocks, in both the good sense and the bad. When I walked up the Foster’s gangplank and stepped onto the deck, the ship was as steady as the land I had just left. Not until she was well away from Key West did I notice my computer screen — and chair, and, once I thought about it, everything else —swaying from side to side.
Tippy typing was the least of my problems. I had wondered how long it would take me to master a seaworthy stagger along a swinging deck. What I hadn’t realized was how tricky a gentle motion, which was almost but not perfectly predictable, would be for walking. Getting up from a chair and stepping across a floor went as usual for a few steps. Then some outer force redirected my next step, often into a door, wall, table, cabinet or innocent bystander. Fortunately a research ship in the tropics is a long-sleeves, long-pants environment, so I never had to explain the bruises.
Land plants setting up home in the rolling water have their own issues, and seagrasses adjusted much more gracefully than I did.
Fonseca and his colleagues reported a dramatic adaptation in 2007 in the Journal of Experimental Marine Biology and Ecology. In places where water flows consistently in one direction, Fonseca and other seagrass specialists had noticed that seagrasses sprout from the sediment in an orderly manner. Straight lines show up in meadows around the world, from San Francisco Bay to Two Peoples Bay in Western Australia. “Straight as rows of corn,” he says.
To see what might be driving this spookily farmlike arrangement in the wild, Fonseca and colleagues set out seagrass shoots in a controlled-flow channel at the NOAA laboratory. Researchers planted in both random patterns and rows perpendicular to the direction of the water movement. Plants in the rows experienced less force than did the random shoots overall. And shoots in rows also caught slightly more light filtering through the water than the random plants.
Plants in many families have evolved to get a little bit wet, but only the small seagrass group in the subclass Alismatidae have managed to go all the way to gather their light and pass on their genes completely under saltwater. One of the hardest problems the group cracked was underwater pollination, says Donald Les of the University of Connecticut in Storrs. One genus releases its male flowers to float to the water’s surface “like breaking open a bag of pingpong balls,” he says. On the surface, the flowers pop open and eventually float like boats keeping their pollen dry for transfer to female flowers on stalks. Other seagrasses have evolved the physiology for reproductive parts to stay viable drowned in saltwater. Their pollen often grows in long filaments, notes Joe Ackerman of the University of Guelph in Canada. His biomechanical analysis shows that approach to work much better than little spheres for tumbling in current flows to snag on female flowers.
Seagrasses may be newcomers to the ocean but have become what Fonseca calls “the breadbasket of the reef.”
Thousands of fish, invertebrate and bird species rely on seagrass habitats, according to the Kenworthy team’s analysis. These animals feed there or seek shelter as youngsters before gaining enough size to graduate to the bigger, badder world of coral reefs or open oceans. Charismatic species, including manatees, green turtles and dugongs, need seagrasses. The meadows account for an estimated $1.9 trillion in nutrient cycling a year.
The Nancy Foster’s science team recognizes this connection. The ship’s researchers are gathering data on the aftereffects of a much-debated decision to create the ecological reserve in 2001, closing the area to fishing and other human activities. The team of diver scientists is counting fish and other populations at 30 spots both in and out of the no-fishing zone. At the time of establishment, this reserve represented the largest block of no-fishing zones protected by the U.S. government (the vast majority of sanctuaries and parks maintained by the government do allow fishing as well as other marine enterprises). Fish counting has been occurring at these 30 points since a year before the reserve’s creation, and the divers swim along transects that include reef as well as soft sediment (which usually means seagrasses).
Reefs have plenty of commuter fish, says NOAA behavioral ecologist John S. Burke, also of Beaufort. Species such as the abundant grunts stay close to the craggy coral during the day, where plenty of crannies can provide shelter from bigger predators. Darkness offers a better chance of evading a predator, and a commuter class of fish gathers at the reef edge. When the crowd gets large enough, Burke says, he and other divers can watch the fish spill out into the seagrass meadow to feed. Grunts poke into the canopy (a grand term for a low fur of Halophila) in search of littler things such as shrimp clinging to the blades.
Sated fish swim back to the reef, and the seagrass-captured carbon enters the reef ecosystem. Bigger fish eat the commuters, and bigger things eat them. Commuter excrement enters the nutrient cycle, too. Kenworthy and colleagues have estimated that seagrasses can boost reef productivity, in terms of fish, tenfold.
Fonseca and Carolyn Currin, also of Beaufort, are working on quantifying this migration of nutrients by comparing carbon isotope ratios in the Halophila with ratios in various fish. Much of what ecologists know about nutrient cycling comes from studies of upwelling systems rich in tiny floating phytoplankton and not from waters with wide seagrass meadows such as the West Florida shelf.
“What you hear is that phytoplankton drive the oceans,” Fonseca says. Working around big seagrass meadows, though, the researchers suspect that the meadows’ plant life plays an important role as a primary producer too. Data so far show a contribution from the seagrass itself.
If the idea is right, it will add more evidence to the connection between coral reefs and seagrass meadows. So efforts to save reefs, which certainly do inspire T-shirt wearing and TV-special watching, will need to include a save-the-seagrass component as well.
My own see-the-seagrass mission finally succeeds. At the end of the excursion, some of the divers visit a small island almost entirely occupied by the ruins of the massive brick Fort Jefferson, built in the mid-19th century. It’s a national park, and so if I drown in three feet of water or suffer a serious table collision, it’s not NOAA’s problem.
I take a snorkel into the clear, blood-temperature water off the postcardy sand beach. At last, I can see for myself. In a patch about the size of a picnic table, straps of a seagrass called Thalassia cluster in a little depression. Small speckled fish poke around the entrance to a burrow. Around the corner of a wall, I’m suddenly face-to-fin with two stainless steel–colored tarpon, about a meter long. After an initial gasp and subsequent snorkel issue, I realize they’re classic browsers in seagrass instead of classic eaters of snorkelers. So I take another look at the green leaves thriving without drowning, and I pass along greetings from seagrass relatives back on land.