Vanishing zooplankton.

Fish and some birds have become less abundant off the Southern California coast in recent decades. Now, some researchers think they know why: an 80 percent drop in the population of zooplankton -- tiny, drifting creatures that form a crucial link in the ocean food chain.

The decline appears to stem from a warming sea surface. The result is "a dramatic change in the abundance of open-ocean sea life," says marine biologist John McGowan of Scripps Institution of Oceanography in La Jolla, Calif. Pacific mackerel, anchovies, and many other fish feed on zooplankton.

McGowan and oceanographer Dean Roemmich analyzed data on zooplankton density and ocean temperatures gathered between 1951 and 1993 during 222 cruises in a 130,000-square-kilometer area off the California coast from San Diego to Point Conception. They found a dramatic drop in zooplankton, along with a rise in ocean surface temperatures of 1.2o to 1.6oC, they report in the March 3 Science. Waters below 200 meters had not warmed.

The result, a steeper density gradient, has slowed the upwelling of nitrates, phosphates, and other nutrients used by phytoplankton to the surface, the researchers suggest. Zooplankton feed on phytoplankton.

The decline in zooplankton may help explain why California fishing tonnages have dropped 35 percent since the 1950s, McGowan says. In addition, the population of the sooty shearwater, a seabird that eats zooplankton, has plunged 90 percent over the last 8 years, he notes. But "there are mixed signals," he adds. For example, squid and whales, which eat fish, seem to be as abundant as ever.

The warming ocean off California may reflect a natural trend. But it could also result from the greenhouse effect, the trapping of heat by certain gases, the scientists say. If ocean surfaces warm globally over the next 40 years, they report, "the biological impacts could be devastating."