Saturday, February 8, 2003; B1

Plant imperils Laguna

Prolifertion of water primrose invites flooding, explosion of mosquitoes

By CAROL BENFELL
THE PRESS DEMOCRAT

    The Laguna de Santa Rosa is being overrun by a fast-growing water weed that threatens to destroy its flood control capability, stop kayakers and fishermen and could pose a serious health hazard.

The yellow-flowered water primrose creates a dense mat on the water surface, providing safe haven for disease-carrying mosquitoes, including those that can carry the West Nile virus.

The problem surfaced last year, after an eruption in the water primrose population produced an explosion in the mosquito population.

"The Laguna is a treasure in our minds, not just because it's pretty to look at, but because it provides an important part of our community's health," said Dan Schurman, executive director of the Laguna de Santa Rosa Foundation.

He joined scientists from a half dozen regulatory agencies in a meeting Friday in Santa Rosa to discuss what could be done about the weed and the mosquitoes.

The Laguna de Santa Rosa and its tributaries drain rain and runoff from 240 square miles of land stretching from Cotati and Rohnert Park to the Russian River.

The water primrose has been seen in the Laguna for several years. Scientists aren't sure what caused the massive surge in growth last year -- they aren't even sure which of the several kinds of water primrose are involved.

Other invasive water plants have plagued area waters in past years, including the hydrilla, which choked Clear Lake last year, and the arundo, which formed dense thickets along the Russian River in 1999.

Like the others, the water primrose in the Laguna may also be an exotic plant -- an invasive newcomer from another country. But scientists believe gradual environmental degradation of the Laguna has left it vulnerable to any invasive water primrose, homegrown or foreign.

In the past 10 years, the Laguna has experienced:

* An increase in run-off from farms and towns, so that more water and more fertilizer-type chemicals enter the Laguna for more months of the year than under natural conditions.

* The lack of a canopy of trees to shade the stream. The water primrose doesn't grow in shade.

* An increase in sediment, through erosion and run-off, which is raising the Laguna's water level and drowning shoreline trees and shrubs.

Historically, the Laguna and its network of streams and wetlands have acted as a flood control reservoir by channeling and slowing the pelting rain from winter storms and easing it into the Russian River.

But the Laguna's holding capacity has decreased because of the increased sediment and because new development has paved over wetlands.

The water primrose exacerbates the sediment problem because its roots encourage soil particles and nutrients to fall into the Laguna instead of flowing on to the Russian River.

"If it weren't for the sedimentation, the Laguna would go back to a natural channel. Now it spreads farther and flatter and the ground is boggy," said Chuck Krause, who specializes in the Laguna area for the Marin/Sonoma Mosquito and Vector Control District, based in Cotati.

A lone oak tree is reflected in the Laguna de Santa Rosa north of Sebastopol on Friday. The water primrose is threatening to overrun the Laguna, hindering its flood control capabilities and allowing mosquitoes to flourish.

And the flatter the Laguna becomes, the less flood water it can hold.

"If the Laguna ever loses its flood control capacity, Guerneville will be under water every year," said Andrew Jensen, an environmental scientist with the regional Water Quality Control Board in Santa Rosa.

The Laguna is a popular recreation spot for birdwatching, kayaking, fishing and duck hunting. It's also a corridor through which salmon and steelhead move to the Russian River.

The water primrose blocked some areas of the Laguna so thickly in the summer that boats couldn't get through, ducks couldn't swim and any fish hook would have landed on plants, not in the water.

The dense mat also created a barrier so that neither fish, nor mosquito control teams, could get to the mosquito larvae to kill them.

Crews could go in and cut out the water primrose and carry it away, but the plant grows back very quickly and the cost is high -- an estimated $10,000 an acre.

Moreover, it's difficult to get into affected areas because of overgrown shrubbery, dead and dying trees and the boggy soil, Krause said.

As a result, crews may have to use an airborne mosquito-killing spray periodically throughout the summer and perhaps for several summers to come, until a better solution is found.

"We don't like to broadcast pesticide into the air, but we're between a rock and a hard place," said Jim Wanderscheid, manager of the mosquito control district.

Mosquito control has gained new urgency since the introduction of the West Nile virus, which reached Southern California last year and is expected to reach Northern California this year.

The Culex tarsalis mosquito, which flourished amid the water primrose last year, is a known carrier of the West Nile virus.

The virus can cause flu-like symptoms and in rare instances, death. The risk is considered small, however. Fewer than 1 percent of those bitten by an infected mosquito will develop serious symptoms. About 50 percent will show no symptoms at all, according to the U.S. Centers for Disease Control.

"You're more likely to be killed in an accident driving to the drug store to buy mosquito repellent than by the West Nile virus," said Chris Canterbury, a spokesman for the mosquito control district.