Next week might be Shark Week, but the California Department of Fish and Wildllife has declared August 2-10 California Invasive Species Action Week, an event intended to boost public awareness of the problems some non-native species of plants, animals, and even microorganisms can cause when they're imported into the state. We're celebrating, if that's the right word, by featuring five of California's most prevalent invasive species every day this week. We've already covered blue gum eucalyptus, Argentine ants, bullfrogs, and wild pigs.

Which invasive species poses the biggest threat to California? The answer depends on who you ask. A desert ecologist might point to the Sahara mustard working its way north to fill the Colorado and Mojave deserts with tinder. Her counterpart in Sonoma County might say Scotch broom, or gorse. Urban birders might point at starlings or outdoor cats, anglers at giant reeds, foresters at the sudden oak death pathogen. A jaded environmental activist might say humans since we brought all the other stuff with us, ignoring the minor detail that humans are native to California.

It's a subjective question, and there are just too many candidates. But we thought we'd wrap up Invasive Species Week with a species that's certainly in the Top Ten Worst In California. Its effect on the landscape and native wildlife is truly frightening. And ironically, it's only here because people who love nature brought it into California.

Water hyacinth (Eichhornia crassipes) is pretty, there's no denying it. An aquatic plant with deep glossy leaves and showy purple flowers unrelated to the florist's hyacinth, the plant was brought into the United States in the late 19th Century as an ornamental plant for lakes and ponds. Introduced into Louisiana to decorate the 1884 Cotton States Exposition in New Orleans, the plant got to California by 1904, introduced to the Sacramento River by aquatic plant fanciers. It has been growing rapidly ever since.

And I mean rapidly. In the course of one California growing season, one water hyacinth plant small enough to fit in the palm of your hand can grow to become a mat of floating vegetation covering almost 6,500 square feet of open water. In fact, water hyacinth is commonly called the fastest growing plant in the world.

The plant doesn't just reproduce by growing vegetatively. It also produces seeds -- lots of them -- that can survive for many years without germinating, making eradication of the plant a near-impossibility. The seeds are sticky, which may help the plant spread: sticky seeds can catch a free ride on birds' feet and feathers, for instance.

Eichhornia stays on top of the water by using Personal Floatation Devices. The small stems that connect water hyacinth leaves to the rest of the plant, known to botanists as petioles, are essentially little air bladders filled with a spongy tissue. They hold the plant's leaves and flowers up and out of the water, and allow a single plant broken off from a mat to float downstream for miles to start a mat of its own.

Those floating mats of water hyacinth, by the way? Up to six feet thick. The plants effectively block sunlight from what had been open water, drastically reducing the amount of native algae and plankton in the water, which reduces the food supply for native fish. Water hyacinths are constantly generating dead plant material as well, especially if a cold snap hits. When bacteria and fungi start to break down those dead leaves, stems, and PFDs, they consume most of the oxygen in the water, leaving none for fish or other aquatic animals.

In growing so aggressively, water hyacinth also displaces native floating plants like pennywort (Hydrocotyle umbellata), whose sparser mats are important habitat for juvenile fish. As it deprives juvenile fish of habitat with one metaphorical hand, Eichhornia provides habitat for mosquitos with the other: the larvae do very well in the tiny protected pockets of water atop hyacinth mats. And the above-mentioned dead organic matter in hyacinth mats can lower the local water's pH, making it too acidic for native wildlife's tastes.

That's potentially a lot of dead biomass, by the way. An acre of water hyacinth mat can grow 36 dry weight tons of new tissue in 90 days. That much compost in the water supply can render reservoir water undrinkable without expensive treatment.

The plants can lower the amount of water in the reservoir, to boot. Cover a body of water with Eichhornia, let the plants get to work sucking up water from their roots and transpiring it through their leaves, and you can lose eight times as much water as would have just evaporated from the bare surface.

In a state that depends on a constructed network of canals and aqueducts to move water around, Eichhornia is a problem plant indeed. One particularly thick infestation in the Central Valley caused an 80 percent reduction in water flow through local irrigation canals and pumps. In 1981 and 1982, one California irrigation district removed a reported 20,000 truckloads of water hyacinth from its infrastructure.

California's experience with water hyacinth hasn't come without an upside: when Hydrilla, a similarly invasive aquatic plant, was found in the state in 1976, state agencies mounted an all-out war on the species, banning its possession in the state and mounting efforts to clear it out of our waterways. Hydrilla still grows in California, but it's under increasing control. There remains a reasonable chance that we'll eradicate it successfully.

No such eradication campaign exists for Eichhornia; state agencies have resigned themselves to spending millions of dollars a year to manage it. It's not even illegal to possess in the state, and you can still buy the stuff at aquarium shops. The state is experimenting with biological control, using two weevils and a fungus, and also sprays problem patches of water hyacinth with the herbicides Roundup and 2,4-D. But there's no hope of ever getting rid of the stuff.

With growth as fast as water hyacinth engages in, you'd think there would have to be some kind of beneficial use for the stuff. The plant shows promise in sewage treatment: with its proclivity for sucking nutrients out of ponds, water hyacinth turns out to be very efficient at cleaning up organic effluents. There's also been some talk of using the plant as a source of carbon-neutral biomass energy, or as livestock feed. The plants can also remove contaminants such as hexavalent chromium from the water in which they grow, which continues to be a topic of study among environmental scientists.

The problem is that all those beneficial uses presumably involve moving Eichhornia from one part of the state to another, and that raises the possibility of new outbreaks of the plant in lakes and streams that hadn't seen it before.

It's kind of ironic. All the people who introduced the water hyacinth wanted to do was to appreciate its natural beauty. The main reason for new infestations in California is pretty much the same. Even after a century in which we've all come to know better, people are still deciding the plant looks pretty in the Delta and taking cuttings home to grow in their backyard pond. From which it escapes to the local creek, and then the lake downstream, brought there by someone who truly loved nature and green, growing things.