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A Moveable Feast
The Ups and Downs of Coastal Upwelling
by Kathleen M. Wong on July 01, 2007
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Fog washed over San Francisco today, erasing tall buildings in a single gust. Tourists 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. San Francisco residents have come to expect endless weeks of fog from May through August. A few find it
romantic; others mutter about how the Mark Twain quotation, “the coldest winter I ever spent was a summer in San Francisco,” is right on
the money.
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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.
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Upwelling begins far from California, in the close and sweaty air at the equator. Heated year-round by a relentless sun, tropical air rises
high into the atmosphere. Colder, denser air from the poles rushes in to take its place. This creates strong winds which blow from the North
Pacific to the California coast, pushing away warm surface waters in front of them. 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.
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However, fog 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. Deep ocean waters carry the marine
equivalent of Miracle-Gro—a concentrated mixture 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.
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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.
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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 are located on the western edges of their continents at mid-latitude, 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.
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The upwelling 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, “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 fish have reached meal size.
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As with most complex systems, “typical” circumstances are only part of the story. Other cycles 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 El Nino. 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 unusually warm water spreads along the Pacific coast. This results in a thicker layer of warm water at the surface and weaker
ocean winds. This weather pattern strands nutrients in deeper ocean layers and leaves marine larders virtually bare. Usually, 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.
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Changes in upwelling have been noted in the last several decades. One explanation may be the natural variation in climate systems.
Another is global warming: temperature changes caused by increasing greenhouse gases could be changing the age-old ballet between
wind and sea. Increasing atmospheric and sea temperatures may strengthen upwelling in some areas, but weaken them in others. The
location, timing and duration of upwelling are also beginning to change. Some California seabird populations that depend on zooplankton
as a major food source have begun to falter, while other seabirds and marine mammals along the California coast are still going strong.
California’s coastal 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. 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.
Upwelling Text Dependent Questions
1.
What is upwelling as described in the text?
2.
Explain the relationship between ocean currents and the atmosphere that affects upwelling. Support your answer using evidence from the
text.
3.
Explain how the author uses words and imagery in paragraphs 2 and 5 to describe the effect of upwelling on ocean organisms and
ecosystems. Use evidence from the text to support your answer.
4.
Paragraphs 4 and 5 use the words “Miracle Gro” and “fertilizer” to describe upwelling. Explain how these terms apply to upwelling using
evidence from the text.
5.
The author use the phrase “A Moveable Feast” as a title to the article. Using evidence from the text, explain why this is appropriate when
describing upwelling.
Winds blowing from the north move surface waters offshore, driven by the earth’s west-to-east rotation (the Coriolis effect).
As warmer waters move west, nutrient-rich colder waters are drawn to the surface, feeding tiny zooplankton and giant humpback whales alike.
Illustration by Fiona Morris
Modified from:
“A Moveable Feast. The Ups and Downs of Coastal Upwelling”, Kathleen M. Wong, July, 1 2007, BayNature Magazine, JulyAugust 2007 https://baynature.org/articles/a-moveable-feast/