Sea grasses vanish, marine life in peril

Scientists see it the world over: Where excess nutrients wash into coastal waters, the undersea landscape changes drastically and marine nurseries are destroyed.

Nitrogen's Deadly Harvest

September 26, 2000|By Tom Horton and Heather Dewar | Tom Horton and Heather Dewar,Sun Staff

PUCK BAY, POLAND — Between untangling gill nets and wrestling wooden boxes of flounder and turbot onto the dock, Derc Brunon tells his story -- more and more the story of fishermen all around Europe and the world.

His is the last of 15 boats that once fished from this harbor. He used to make a day's work in sight of his little village of Swarzewo, just down the Baltic Sea coast. But now he makes a four-hour round trip to his nets in open waters. If he can hang on a few more years, he'll retire.

As recently as 1985, fishermen in some of the small towns along Puck Bay could support a family for a month from one catch of yard-long pike, a prized sport and commercial species that inhabited the clear shallows in the bay's beds of eelgrass.

But now, the pike and grass beds are mostly gone, and mats of dense algae favored by sticklebacks, a worthless species, have replaced the eelgrass.

The reasons for this in Puck Bay are multiple and complex, but around the Baltic and the world one connection is strong: Because of an oversupply of nitrogen contaminating coastal waters, the underwater grasses and seaweed that populated an estimated 150 million acres of the planet's coastal shallows are disappearing on a massive scale.

And when the grasses go, so do clear water and fertile seafood nurseries -- an underwater world so bursting with life that scientists sometimes compare the sea grasses to rain forests.

From Cape Cod and the Chesapeake Bay to Scandinavia and the Netherlands, the rapid demise of these rich, submerged jungles has often been a classic case of out of sight, out of mind.

Imagine, by contrast, the worldwide alarm if huge swaths of the planet's most stately and productive forests -- habitats for astounding varieties of bird and animal life -- had lost half their trees, or in some cases disappeared entirely.

Overall sea grass losses are probably "higher than present rates of loss of tropical forests," which are down 20 percent in just the past few decades, says a 1996 survey by Spanish scientist Carlos Duarte and Carina Chiscano, a University of Maryland researcher now in private industry.

Like forests on land, the submerged grasses are habitat for a stunning variety of species. In Maryland, it's easy to see what's at stake.

Go out "scraping" through any of the Chesapeake Bay's remaining grass beds with a soft crabber, and watch the thigh-thick rolls of green eelgrass and widgeon grass come up in the mesh bags of his dredges.

These grasses literally quiver with the life they shelter -- the young of shrimp, striped bass, white perch, speckled trout, flounder, red drum, seahorses and dozens of other species, as well as the blue crabs seeking protection in the beds to mate and shed their shells. In winter, swans from Alaska, ducks from Saskatchewan and geese from Labrador all converge on the bay to feed on the grasses and their starchy roots.

Or a continent away, look down into waters along the seacoasts of Sweden at the branched forests of bladder wrack, a tree-like, yard-high seaweed. It reproduces each year only about 8 p.m. on the full and new moons in May and June, for reasons that remain a mystery.

Bladder wrack might live decades, or even a century, providing a rare, stable environment in open waters for a huge variety of sea life and a physical structure upon which spawning fish hang their eggs to develop. "Our rain forest," European ecologists sometimes call it.

But algae mats and blue mussels now often cover the hard sea bottom to which young bladder wrack needs to attach to grow. And in some places a small marine animal, once kept in check by predator fish that live in bladder wrack, now eats the seaweed faster than it can grow.

The long-lived bladder wrack might be like an impressive, ancient forest that has no young trees coming along, worries Swedish marine ecologist Lena Kautsky. "It might be decades before the bladder wrack dies," she says, "but when it does, there could be a crash."

Blocked light

Underwater vegetation not only provides critical habitats for sea life but also regulates the marine environment. It slows the flow of water, settling out sediment, and absorbs nutrients, making water clearer and cleaner. It also dampens wave energy, reducing shoreline erosion, and helps offset global warming by absorbing surprisingly large amounts of carbon from the atmosphere.

But these underwater forests have an Achilles' heel: To grow, they need light. And that light is reduced when blooms of algae cloud coastal waters.

Scientists estimate that such blooms, fueled by excess nitrogen and other nutrients, are the dominant reason sea grasses have disappeared in recent decades. Compounding the problem, nutrient-enriched waters also grow algal slimes that coat the underwater grasses, further blocking their light.

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