THE CALIFORNIA COAST: Tidepools in Trouble.

AN HOUR PAST DAWN, THE CENTRAL CALIFORNIA SUN IS BURNING THROUGH THE MORNING HAZE and the waters of Monterey Bay are rising after a 6:21 a.m. low tide. Rafe Sagarin, curly hair tousled by the unruly wind, crosses the exposed granite tidepools like a teenager negotiating a cluttered bedroom floor. But this intertidal tangle is a library to Sagarin, who knows precisely where reefs of tube snails set their mucous nets, striped sunburst anemones open their tentacles to the tides, and barnacle-like limpets farm algae for their supper.

In 1993, Sagarin counted every critter along a 108-yard line crossing the rocky intertidal triangle off Hopkins Marine Station on the Monterey Peninsula, precisely reproducing a 1930s survey by a Stanford student named Willis Hewatt. Sagarin and fellow Stanford junior Sarah Gilman heard that counting every crab, sea star and urchin along the line marked by brass bolts Hewatt pounded into the rocks might uncover something interesting. What they found was substantial species shifts in the intervening 60 years that were best explained as migration away from warming water--exactly the kind of response predicted by models of human-caused climate change.

When Hewatt first looked at the tidepools, there were no tube snails or sunburst anemones at all. When Gilman and Sagarin did the same they found reefs of the tube snails and hundreds of southern anemones. In all, 10 of 11 species previously identified as southerners increased significantly, while six of eight northern species decreased significantly. Those changes showed up regardless of what the animals ate, how they reproduced or where they sat in the taxonomic hierarchy. Meanwhile, daily temperature records show waters there warmed about 1.8 degrees F since Hewatt squatted in the surf.

While Sagarin's study can't definitely tie the shifts to warming water, the research is among the hardest evidence yet of long-predicted effects of climate change. Also predicted is that the sea's surface layer of warm water will thicken, becoming more resistant to mixing with the cold, nutrient-rich waters below--which means less food for everything that lives in or off the sea.

The prestigious journal Science first published Sagarin's findings, as well as a study by John McGowan, a professor at Scripps Institution of Oceanography in San Diego, showing what McGowan calls "the largest change ever measured in plankton productivity in the ocean."

McGowan took 42 years of data from the waters off southern California and found a 70 percent drop in the amount of tiny animals, or zooplankton, that form the second-lowest rung of the marine food chain. That drop coincided with an overall sea-surface warming of two to three degrees F. What McGowan observed between San Diego and Point Conception near Lompoc may affect a much larger region: Although there are no long-term numbers further up the West Coast, plankton and temperature shifts tend to be parallel from British Columbia down to Baja California.

A huge hit to the base of the food chain should ripple up through fish and seabirds. But long-term fisheries data are difficult to interpret because declines can come from overfishing as much as underfeeding. Nevertheless, catches of key commercial species like sardines, anchovies, squid and mackerel declined substantially during McGowan's study period. So did populations of sooty shearwaters, a seabird that eats young fish and large plankton, which plummeted 90 percent. "Looks like it fits in with the plankton picture, doesn't it?" McGowan asks. Even if the changes don't come from full-on climate change, the results show one likely consequence of a warmer world.

Further up the coast, another bird potentially faces a dire threat from climate change. The secretive California clapper rail, one of the original birds on the federal endangered species list with a current population of only 600, relies on salt marshes for its habitat.

From the hill above the visitor center of San Francisco Bay National Wildlife Refuge, the marshes look like geometric farm plots outlined by sandy dirt roads. Here, however, the roads are earthen levees and the fields are ponds used by Cargill to evaporate water and harvest salt. Those ponds once hosted vegetation like cordgrass, gumplant and salty, bitter pickleweed, which in turn provide homes for rails and the salt marsh harvest mouse, also listed as endangered.

Just east of the visitor center's hill is a breached salt pond that was restored to marshland in 1985. It doesn't have the complexity of the more mature salt marsh nearby, but Marge Kolar, the refuge's manager, says rails and harvest mice are in it again already.

But behind the recovering marsh Thornton Road is filled with commuter cars, and just beyond it is Sun Microsystems, sitting on land that cost $477,000 an acre. That's bad news for rail and mouse, since they can't offer competitive bids and the salt marsh can't grow over pavement if sea levels rise as predicted. In fact, Kolar says, the levees enclosing the salt ponds could become critical to control flooding of human habitat, while that of the rail and mouse slowly drowns. CONTACT: Hopkins Marine Station, [phone], www-marine.stanford.edu; Scripps Institution of Oceanography, [phone], www.sio.ucsd.edu.

ORNA IZAKSON is a former environmental reporter for the Bangor Daily News and an active member of the Society of Environmental Journalists.