Plumes of Sediment Darken the Ocean in Flooding’s Wake

Scientists studying the effect of runoff on the Santa Barbara Channel got an eyeful in recent aerial and satellite photos: a vast swath of mud darkening the sea after heavy El Nino rains.

“We were astonished at how much sediment there was in the ocean,” dirt washed out of the hills of Ventura and Santa Barbara counties, said UC Santa Barbara geography professor Leal Mertes. She is one of a team of about 20 scientists working with the Channel Islands Marine Sanctuary to study the effects of runoff on marine life.

“What I saw was sediment reaching all the way out to the Channel Islands,” Mertes said, describing a flight over the Santa Barbara Channel in February.

The plumes of sediment, obvious to the naked eye, have significantly dispersed in recent weeks, she added. The scientists believe the runoff produced a greater effect this year than during the 1983 El Nino rains, but don’t yet know why.

The scientists expect to publish the results of their research into the effect of sediment on marine life this spring.

The single largest source of sediment along the Santa Barbara Channel, which borders more than 100 miles of coastline in the two counties, is the Santa Clara River north of Oxnard.

“It’s a 4,000-square-kilometer watershed. And because it’s so close to the Ventura River, together they create quite an effect,” Mertes said.

From a boat beyond the mouth of the Santa Clara River on Feb. 12 and 13, scientists measured and studied sediment, water chemistry, light and the biological effects of the sediment at various depths, Mertes said.

“One of the big impacts is, if you have muddy water on the surface, it will affect anything below that needs light,” said the geographer.

Thus, the scientists’ name for their project: Plumes and Blooms. “Plumes” is for the way the muddy water sweeps out from a river and “blooms” is for how plankton, a basic form of marine life, is affected by it.

“We’re asking: How much dirt is out there? Where does it go and how does it get there? What is the effect on the life below it in the sea? Where there’s mud, there’s less light,” Mertes said.

The runoff is obvious in satellite pictures, she said.

“But there’s a conflict here,” said Mertes. “This sediment carries nutrients--some would say effluents--and some ocean species do well with less light and more nutrients, while others don’t.”

Algae may grow better with the additional nutrients, for example, while kelp may do poorly with less light.

“We’ve got our measurements now. We have an idea about how thick the sediment is,” she said. “Eventually, we’ll know the whole story.”