A Moveable Feast

The Ups and Downs of Coastal Upwelling

by on July 01, 2007

 

Sea lions swim above a reef populated with strawberry anemones in Monterey Bay.

Photo by Peter Liu.

 

 

Fog washed over San Francisco today, erasing tall buildings in a single gust. It stormed Ocean Beach, invaded city streets, and settled in for a sunshine-blocking siege.

Tourists on Market Street are wearing goose bumps beneath their Bermuda shorts; savvy locals bustle about in sweaters and long pants, clutching their Giants caps against the stiff breeze. Thus goes another summer day in the city.

Coastal residents in the Bay Area have come to expect endless weeks of fog from May through August. A few find it romantic; others mutter about how the apocryphal Mark Twain quotation (“the coldest winter I ever spent …”) is right on the money.

In truth, San Franciscans should be grateful for the blanket of mist around them. Its arrival marks the start of a great annual surge of life just offshore. Squadrons of Dungeness crabs, million-tentacled congregations of snow-white market squid, herds of barking sea lions—all are powered by the fog-generator known as upwelling, the churning of deep cold waters that recurs each summer off our shores. And though most people are unaware of its existence, upwelling touches our lives in many ways beyond murky summer weather. Fishery closures, salmon populations, mass die-offs of seabirds, and the impacts of global warming on our coastal ecosystems are all intimately tied to variations in this ocean phenomenon.

Upwelling begins, if it can be said to begin anywhere, far from California, in the close and sweaty air at the equator. Heated year-round by a relentless sun, tropical air expands and rises high into the atmosphere. Colder, denser air from the poles rushes south in the exchange.

Off western North America, this chilly air flows from the vicinity of the North Pacific and Alaska, pushing ocean waters before it. But instead of flowing directly south, coastal waters are deflected to the west by the west-to-east rotation of the earth (the Coriolis effect).

Cold waters from 300 to 600 feet deep percolate upward to take the place of the departing warmer water. Though rising just a few meters a day, upwelling can cause surface water temperatures to drop by as much as 20 degrees Fahrenheit within a week or two. The stronger the winds, the more deep water is pulled upward. When chilly upwelled water encounters warmer spring air at the surface, tiny droplets condense in the cooling air-making fog a fine barometer of upwelling.

However, fog—drawn inland through the Golden Gate and other breaks in the Coast Range—is just a minor byproduct of upwelling. “The single biggest thing about upwelling is that it continually resupplies nutrients” to the upper layers of the ocean, where photosynthesis can occur, says oceanographer John Largier of UC Davis’s Bodega Marine Laboratory.

Deep ocean waters carry the marine equivalent of Miracle-Gro—a heady cocktail of nitrogen, phosphorus, and silicates recycled from life at the surface. When they die, animals and plants sink down into a sprawling compost pile on the sea floor, where bacteria break down this detritus into a nutrient-rich mixture tailor-made for plants.

Upwelling pumps this marine fertilizer to the ocean’s surface. Its arrival in sunlit waters kicks off a weeks-long fiesta of feeding, breeding, and growth. Suddenly bathed in nutrients, clouds of microscopic algae crank their photosynthetic engines into overdrive, melding sunlight, carbon, and nutrients into living tissue. Diatoms and other tiny marine plants, known collectively as phytoplankton, can double their numbers twice in a day.

Grazers converge on the floating fields like cattle in a meadow. Zooplankton—krill, copepods, and larval fish—gorge themselves to repletion. Now awash in a blizzard of rice-grain-size zooplankton, the bloom attracts ever larger predators, until gangs of mature salmon and seabirds, sea lions and whales, and even great white sharks circle by to claim their share.

Ocean creatures have had plenty of time to adapt to upwelling—it’s an ancient fixture of coastal waters. The evidence comes from the fossils of organisms known as forams. Identified by their uniquely patterned, bead-size shells, each species of foram grows only within a narrow range of water temperatures. Cores pulled from Southern California seabeds indicate that cold-loving forams have lived along the coast for tens of thousands of years or more.

Upwelling occurs in many parts of the ocean. But thanks to a confluence of wind and geography, it is especially strong, sustained, and reliable in the waters off California and just a few other places. A glance at the globe helps explain why. All four of the world’s major upwelling centers—the California Current, the Humboldt Current off the coast of Chile and Peru, the Canary Current off northwest Africa and the Iberian Peninsula, and the Benguela Current off southern Africa—are located on the western edges of their continents at midlatitude, where the winds that drive upwelling blow. Though upwelling centers make up less than 2 percent of the surface area of the ocean, they have historically supplied half of the world’s fish catch.

Kelp crab
A northern kelp crab climbs a strand of giant kelp in Monterey Bay.Nutrients delivered by upwelling, plus sunlight penetrating the bay’sclear waters, allow giant kelp to grow up to a foot per day. Photo byTchell DePaepe.

Local geography influences upwelling on a smaller scale. It’s easy to pick out the areas of strongest upwelling—they literally stick out on a map. Water upwells most strongly at promontories, creating plumes that trail offshore and to the south.

In California, local upwelling centers power the extraordinary abundance of marine life at sites such as the Farallones, Monterey Bay, and the Channel Islands. Nutrient-rich waters that hit the surface at Point Arena, in Mendocino County, direct a river of diatoms toward the Farallones, about 115 miles to the southwest. The three to five days it takes nutrient-filled waters to reach this biological hot spot coincides perfectly with the time needed for plankton populations to boom. Great swarms of shrimplike krill and antenna-waving copepods devour the drifting banquet, only to be gulped down by slender sardines and gargantuan humpback and blue whales alike. Surrounded by waters bursting with life, the jagged rock piles of the Farallones are transformed into raucous maternity wards each spring. Tens of thousands of tufted puffins, pigeon guillemots, cormorants, western gulls, and other birds shriek and battle over island nesting spots, the better to supply their broods with beakfuls of fish plucked from the surrounding waters.

Some 70 miles to the south, an upwelling cell at Davenport feeds the spectacular kelp forests of Monterey Bay. Though far weaker than the powerhouse at Point Arena, the nutrients this cell delivers remain concentrated inside the bay. This, plus the sunlight that penetrates deep into the bay’s relatively clear waters, make it possible for giant kelp to grow up to a foot per day. Each kelp plant is a universe unto itself, sheltering thousands of bryozoans and snails, crabs and juvenile fish, along with kelp-nibbling sea urchins that are themselves the favorite food of southern sea otters.

In a typical year, the winds that produce upwelling off California wax in spring and summer and wane in late fall and winter. They originate in the North Pacific High, a weather system that looms over much of the Pacific Ocean. In spring, the system strengthens and migrates north toward the Aleutians, blasting upwelling winds toward California as early as February. Those bouts escalate in power and frequency as summer reaches its zenith. But as days shorten, and autumn sets in, the system weakens and drifts south again. Its departure calms coastal winds, all but halting upwelling off California.

The cycle is so reliable that ocean creatures stake their lives on it. Sardines spawn in early spring, just as meadows of phytoplankton bloom. Strong winds in February also mean copepods and krill will be plentiful at the height of spring. Then, says oceanographer Bill Peterson of NOAA Fisheries, “when the salmon get there in mid-May, the ocean is ready to receive them with bountiful resources.” Meanwhile, fish-eaters such as gulls might hold out until midsummer, when salmon and other smolts have reached meal size.

However, says Largier, “It’s not so simple as ‘it’s blowing hard, and you get more production.’” For example, the pace of upwelling bouts is critical. Upwelling winds tend to linger for a week or two, then pause or reverse for a few days. The respite keeps nutrients in one place, fueling an even stronger plankton bloom. Calm weather corrals plankton in clumps profitable for foraging birds and whales to visit.

As with most complex systems, “typical” circumstances are only part of the story. Other cycles that affect the entire Pacific basin alter sea surface temperatures and help determine whether upwelling will inject extra nutrients into coastal ecosystems—or leave animals hungry.

The most famous of these cycles is the El Nino Southern Oscillation. Known for lashing the West Coast with epic winter storms, El Nino’s power lies in its tropical energy. Every three to seven years, a blanket of anomalously warm water spreads along the Pacific coast, creating a thicker layer of warm water at the surface and stranding nutrients in deeper layers. “During El Ninos, you can actually have a lot of wind and upwelling,” says marine ecologist Bill Sydeman of PRBO Conservation Science in Petaluma, “but it is totally ineffective; it’s just turning over warm, low-nutrient water”—leaving marine larders virtually bare. An El Nino year is often followed by a La Nina event, when typical spring and summer upwelling are intensified and species can recover from the previous year’s losses.

Another, more recently discovered player in the system is called the Pacific Decadal Oscillation, or PDO. It doesn’t appear to amount to much; it raises or lowers the average temperature of the Pacific by just a degree or two. But what it lacks in strength it makes up in staying power. Each warm or cold regime typically lingers for two to three decades at a time, with profound effects on marine ecosystems. Oceanographers Francisco Chavez and John Ryan of the Monterey Bay Aquarium Research Institute found that sardines and anchovies boom and bust in synchrony with the PDO. The pattern explains the overnight disappearance of Monterey’s sardine fishery in the 1940s, and it’s visible in centuries-old layers of fish scales dredged from the sea floor off Santa Barbara. However, because one go-round of the PDO takes roughly half a century to complete, scientists haven’t had much chance to understand how it works.

Scientists are even more baffled by the most recent disruption in the upwelling cycle, which doesn’t seem attributable to either El Nino or the PDO. In 2005, strong, sustained upwelling arrived in July instead of late spring. Its absence proved catastrophic to marine animals up and down the Pacific coast.

“The Cassin’s auklets on the Farallones did something we’d never seen before—they completely abandoned their nests. In mid-May, in the middle of egg laying, they just disappeared from the colony,” Sydeman says. Auklet rookeries on Tatoosh Island in Washington north to Triangle Island off British Columbia met the same fate.

As the year progressed, the effects of delayed upwelling reverberated through the food web. In every case, animals expecting the usual surge in food supplies went hungry. “Plankton eaters like Cassin’s auklets were the first to be affected. Then the birds that eat plankton and fish, like the common murre, and then the birds that eat fish, like Brandt’s cormorants,” Sydeman says. Murres washed up dead in record numbers on Monterey beaches. Gulls on Protection Island near Seattle fledged just 88 chicks, compared to the usual 8,000.

Birds were only the most conspicuous sign of widespread marine famine. Researchers trawling for juvenile salmon off Oregon caught a few dozen smolts compared to about 1,000 in a typical year. Meanwhile, the number of young rockfish caught off Central California was the lowest in nearly three decades.

Last year’s delay in upwelling has left scientists scratching their heads—and researchers reported only weak upwelling through May of this year. One explanation, of course, is natural variation—the sweep and play so typical of climate systems. Another is global warming: temperature changes caused by increasing greenhouse gases could be changing the age-old ballet between wind and sea.

 

Upwelling diagram
Winds blowing from the north move surface waters offshore, driven bythe earth’s west-to-east rotation (the Coriolis effect). As warmerwaters move west, nutrient-rich colder waters are drawn to the surface,feeding tiny zooplankton and giant humpback whales alike. Illustrationby Fiona Morris.

 

In fact, the chain of events in 2005 matches scientists’ predictions for a warming world. Lisa Sloan of the University of California, Santa Cruz, and colleagues modeled how elevated levels of carbon dioxide expected from 2080 to 2099 might affect upwelling off California. They reported in 2003 that upwelling along the Northern California coast could arrive up to a month later and extend well into fall. The model predicts that the continents will warm more rapidly than the oceans, strengthening the atmospheric pressure gradient behind upwelling winds and pushing more water offshore. This temperature difference will be greatest later in summer, keeping upwelling strong through October.

One would think that stronger upwelling would benefit sea life. Instead, there will likely be a net decrease in cold water reaching the surface. Any additional upwelling “will be more than offset by the increase in temperature projected for future global warming,” says oceanographer Frank Schwing of NOAA Fisheries in Pacific Grove.

Indeed, though average ocean temperatures have increased worldwide in recent decades, the effect is particularly pronounced in California. The warming of local waters has even accelerated since the mid-1970s. Chavez says this may mean the California coast is particularly sensitive to this trend.

Scientists are concerned that birds and other animals will be unable to adapt to rapid shifts in the upwelling cycle. “The impacts of global warming may overcome the natural decadal cycles that animals have adapted to,” says Schwing. Species with the most rigid breeding and migration schedules—including Cassin’s auklets and gray whales—will be hurt the most, and could become rarities off state beaches.

California’s coastal ecosystems may already be feeling early effects of global warming. Since 1951, scientists have tracked zooplankton populations off Southern California as part of the California Cooperative Oceanic Fisheries Investigations (CalCOFI) study. In 1995, in the journal Science, Dean Roemmich and John McGowan of Scripps Institution of Oceanography reported a sobering 80 percent decrease in the biomass of larger zooplankton between Point Conception and San Diego. They ascribe the losses to ocean warming; during the same time period, surface water temperatures in the region increased by about 3 degrees Fahrenheit. With cold, nutrient-rich waters too deep to upwell, they write, “shallower layers bearing fewer nutrients are exposed to light, leading to less new production and ultimately to decreases in zooplankton.”

Declines in other animal populations seem to be occurring in lockstep. Cassin’s auklets again offer a stark example. While roughly 100,000 of the squat black-and-white seabirds nested on the Farallones in the 1970s, about 30,000 birds return today. Cassin’s auklets eat only zooplankton—making warming and a falloff in upwelling prime suspects in their decline.

Though auklet numbers have faltered for now, other seabirds and marine mammals on the Farallones are still going strong. These islands remain perhaps the best place in California to experience the astonishing abundance and variety of the state’s upwelling-nourished marine wildlife. Rocky sentinels of the Golden Gate, they disappear behind a curtain of mist each summer like the fabled isle of Avalon. Smart sailors brave the foggy voyage swathed in jackets and hats against the cold.

But those who yearn to wear short sleeves during the summer months might want to reconsider. Given upwelling’s importance to our shores, keeping a polar fleece handy through autumn seems a very small price to pay.

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