Ocean Watch

By Susan Scott

Friday, July 27, 2001

Oh, what a tangled web
seaweed has weaved

Stories about seaweed in the local news lately have not been good.

Kaneohe Bay is being threatened by the sudden, vigorous growth of a species from the Philippines, and Maui is suffering from an unexplained superbloom of a native species.

Both of these have had such growth spurts here before, but they died down on their own before researchers could determine the cause. Now, workers are once again looking for the culprits producing this imbalance between these plants and their environment.

When it comes to marine organisms, seaweed ranks low on people's favorites lists. That's because few of these plants have common English names, making them difficult to discuss. Many have slimy or prickly textures, and when they wash up on beaches, they stink.

Besides that, most of us don't like the spooky feeling of swimming into masses of loose seaweed or standing in a bed of mushy or spiky plants.

But seaweed is our friend. It may bug us at times with its foul odors and tendency to muck up the water, but the fact is, we wouldn't be here without it.

In their simplest form, marine plants called phytoplankton influence all life on Earth. By converting water, carbon dioxide and minerals to carbohydrates, these microscopic plants are the first link between the living (organic) and the nonliving (inorganic).

Land plants get their inorganic nutrients through leaves, branches and roots. Aquatic plants, however, don't need such elaborate structures because they absorb their nutrients directly from the water surrounding them.

It's easy to see the sequence in the marine food chain from plants to animals to humans. But phytoplankton, as well as the larger seaweed, has a crucial role in the earth's atmosphere. Marine plants give off a gas called dimethyl sulfide (that's where the smell comes from). In the atmosphere, this gas regulates Earth's temperatures.

Seaweed has other interesting features. Here are a few that might make it seem a little more lovable:

>> Unlike nearly all other living organisms, seaweed is classified according to its color. The four major divisions are green, blue-green, brown and red.

>> Land plants evolved from green seaweed.

>> No seaweed has leaves. The parts that wave in the water are called sheets, blades or filaments.

>> Ancient Hawaiians viewed seaweed as food and called all types limu. Men in canoes picked limu growing on rocks offshore. Women collected limu that broke loose and drifted near shore.

>> Seaweed doesn't have roots but attaches to the ocean bottom with holdfasts, structures that often resemble roots.

>> Coralline algae, a type of red seaweed, absorbs large amounts of calcium carbonate (seashell material) from the water and turns hard, like cement. This seaweed cement spreads out in crusts, gluing coral rubble, shells and sand into tough reef flats.

>> Algae (another name for aquatic plants) is found in nearly every habitat in the world: snow fields, hot springs, soil, rocks, trees, inside other organisms, streams, lakes and oceans. In Antarctica a few months ago, I saw huge fields of snow turned pink by algae.

>> Seaweed rarely lives alone. Once a species gets established, other seaweed grows on and around the first until the area is a tangled, intertwined seaweed neighborhood. For unknown reasons, the troublesome Maui and Kaneohe Bay seaweed is currently outgrowing its neighbors in leaps and bounds.

Marine science writer Susan Scott's Ocean Watch column
appears weekly in the Star-Bulletin. Contact her at

E-mail to City Desk

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