SANTA CLARITA / ANTELOPE VALLEY : Algae Dwindles at Castaic Lake Reservoir : Water: The improved situation avoids further closures to the popular recreation area for chemical treatment.

Algae that bloomed in record levels at Castaic Lake’s main reservoir this summer has dwindled dramatically, avoiding further closures to the popular recreation area for chemical treatment.

State water officials shut down the lake for half a day last month and poured 16,000 pounds of copper sulfate pellets into the reservoir by helicopter. The closure came during a heat wave and spoiled plans for the 2,500 people who use the upper lake area daily for fishing, boating, water-skiing and jet-skiing.

Geosmin, an algae form that produces a musty taste and odor but is not harmful to drink, was discovered at 2,800 parts per trillion on July 5--an abnormally high level, according to Bill Taylor, principal microbiologist for the Metropolitan Water District.

High temperatures and abundant nutrients in the water are thought to have spurred the growth of the algae.

“We just kept watching it grow and grow and grow,” Taylor said.

Sensitive palates will notice the bacteria at 20 p.p.t., so it was no surprise when dozens of customers complained daily about the odor and taste of the water.

Millions of Southern California residents receive a portion of their drinking water from Castaic Lake. The Metropolitan Water District draws water from the Castaic reservoir and the Colorado River, in turn providing it to 27 water agencies that include city, county and municipal water districts in Los Angeles and Ventura counties.

The natural cycle of the geosmin combined with the copper sulfate has dramatically reduced its concentration since last month. Geosmin levels on the surface of the lake were measured Monday at 11 to 13 p.p.t., Taylor said.

Despite the algae’s current low levels, water officials say they cannot guarantee future lake closures for treatment. Copper sulfate treatments were last used in 1973.

Castaic Lake officials have, however, asked for more advance notice.

Water officials originally sought to close the area July 1--on the busiest weekend of the year--with less than 24 hours’ notice.

“We just need a little more coordination and communication,” said Brian Roney, acting superintendent of the Castaic Lake Recreation Area.