| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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feature Where Wind & Currents Meet Channel Islands Underwater Ninety feet below the ocean’s surface off Southern California’s San Miguel Island grows a deadly saltwater garden. Cylindrical blobs mat the rock walls in glowing reds, pinks, and oranges. These soft circles of waving fringe, or club-tipped anenomes (Corynactis californica), look like harmless blossoms, until an unwary fish larva happens to bump into one. Loaded with microscopic, poison-tipped, dartlike structures called nematocysts, the tentacles paralyze the intruder, then drag it slowly into the muscular central mouth. A large fish, lying in wait for a wandering crab, nestles within this deadly garden; brilliant dabs of blue along its flanks identify it as a kelp greenling (Hexagrammos decagrammus). Perhaps the sting from the nematocysts are just a weak jolt to the greenling, which must remain on the lookout for its own predators, such as the lingcod or one of the six species of pinnipeds that live here—California sea lion, Steller sea lion, northern elephant seal, harbor seal, northern fur seal, and Guadalupe fur seal. A chestnut cowry snail (Cypraea spadicea) climbs over the rocks, its shiny shell burled with gold and brown. The club-tipped anemones are dinner for the cowry, which may also take a white-spotted rose anemone (Urticina lofotensis), or an orange puffball sponge (Tethya aurantia). This is Wycoff Ledge, a popular dive site and only one of many underwater habitats pulsating with life in the waters around California’s Channel Islands. The islands are ringed by kelp forests and sandy plains, with pelagic, or “blue-water” zones a little further offshore. These diverse habitats help make this area one of the most biologically rich in California. The key to the profusion of life in this archipelago lies in the interaction between local winds, coastal currents, and a strange ocean phenomenon called upwelling. Powered by global forces that control both the mixing of the oceans and the circulation of the atmosphere, upwelling occurs in only a few places in the world, most of them midlatitude, western coasts of the continents. Upwelling in the Pacific Ocean, if it can be said to originate anywhere, begins beneath the blanket of sea ice that rings the Antarctic. As the brine begins to freeze into freshwater ice, the growing crystals squeeze the salts and minerals out from between the water molecules and back into the sea. Now thick with salts and denser than the surrounding seawater, these layers sink to the bottom of the Southern Ocean, pushing deep currents along a slow clockwise march around the Pacific Rim. Along this ocean-sized loop, the icy current collects and disperses marine snow, the detritus made up of diatoms, bits of algae, zooplankton, and fish larvae that rain down from the sunny world above. As they fall, these organic compounds are broken down by bacteria into DNA, protein, and sugars, and further still into carbon, nitrogen, and phosphates—the basic ingredients of any fertilizer. Meanwhile, on the surface, the Earth’s rotation and the resulting Coriolis effect, combined with hot air that rises from the equator, cools, and sinks in high latitudes, whips up powerful trade winds over the tropics and subtropics. These in turn, churn circular, ocean-sized currents called gyres that rotate clockwise in the northern hemisphere and counterclockwise in the southern. Off the west coast of North America, an arm of the North Pacific Gyre called the California Current drives an immense, cool, surface current south from Alaska. Winds blowing off the continent push this current out to sea near Point Conception, activating the oceanic escalator known as upwelling. Cold, bottom waters rise to replace these displaced surface waters, bringing with them nutrients concentrated by the deep, Antarctic-driven currents. Rich in marine snow, these waters spur surface-dwelling phytoplankton to bloom, creating the foundation of a vibrant food chain. Similar conditions driving upwelling occur off the western coasts of South America, Africa, and Australia. But one additional factor makes the underwater communities off the Channel Islands unique. Here the California Current, rich in nutrients from upwelling at Point Conception, collides with the warm, north-flowing California Countercurrent, mixing organisms and nutrients from offshore Baja with those of Alaska and Canada. The result is a biological dynamo: a mosaic of underwater habitats hosting a ewildering diversity of wildlife. The islands’ kelp forests are dominated by one of the world’s fastest-growing organisms: giant kelp (Macrocystis pyrifera). A type of brown algae, giant kelp can shoot up two feet in a day, covering the seafloor in dense forests from 20 to 100 feet deep. Sturdy holdfasts grip the bottom, while stemlike stipes sway with the hypnotic rhythm of the surge. Buoyant gas bladders keep leaflike blades reaching toward the sunshine that streams down from the surface. Schools of silver-blue blacksmith (Chromis punctipinnis) bustle through the waving stipes, blades, and bladders, feeding on zooplankton. Harbor seals (Phoca vitulina) speed by looking for blacksmith. On the bottom, a gorgonian coral stings plankton with its nematocysts, while nearby a pair of brown sea hares (Aplysia californica), slugs which feed on the kelp, mate in hermaphroditic fashion, each carrying a full complement of male and female sexual organs. Yellow-orange senoritas (Oxyjulis californica), their tails marked with black eyespots to confuse predators, pluck tiny crustaceans, worms, and parasites off larger fish that come to the kelp to be cleaned. At night, these members of the wrasse family seek safety by burying themselves in the sand with only their heads protruding. Bright orange garibaldi (Hypsypops rubicunda), the protected state fish of California, eat sponges, bryozoans, worms, and crabs found within the kelp forest and among the rocky reefs. In the deep blue-water zones, giant schools of silver sardines (Sardinops sagax caerulea) gulp down dense clouds of copepods and krill that, in turn, are busy devouring phytoplankton such as diatoms and dinoflagellates. California sea lions (Zalophus californianus) chase down sardine meals, while keeping a wary eye out for sharks. Above the water, brown pelicans (Pelecanus occidentalis), their numbers strong again after DDT-caused die-offs in the 1950s and 1960s, ride wave updrafts in search of fish. Should they find a school of sardines or anchovies, they plunge into the shoals of silvery fish, their expandable bills swelling to prodigious size to engulf their prey. On the underwater sandy plains that lie between the kelp forests and rocky reefs, bottom-dwellers such as the shovelnose guitarfish (Rhinobatos productus) and the round sting ray dine (Urolophus halleri) on burrowing worms, crabs, and shrimps. Here also California halibut (Paralichthys californicus) lie in ambush for an unwary passing sargo (Anisotremus davidsoni) or kelp bass (Paralabrax clathratus). Opal, or market squid (Loligo opalescens) form one of the basic building blocks of the Channel Islands’ marine food web. These voracious cephalopods gobble up anything they can wrap their beaks around, including crustaceans, fish larvae, and smaller squid. By the same token, bigger fish, marine mammals, and sea birds rely on the squid for food. Market squid swarm together by the millions every autumn and winter to perform the reproductive act that will end their year-long lives. At night, over the sandy plains, a male unfurls a specialized tentacle and deposits his sperm packet, or spermatophore, near a female’s egg chamber. She then anchors the four- to ten-inch egg capsule into the sand, where it will remain through development. After the most important act in their short lives, both squid perish. Although graceful, theirs is hardly a peaceful romance. Predators knife through the water to devour the distracted squid. Blue sharks, bat rays, and sea lions, sculpin, rockfish, and sheephead all take advantage of the unexpected bounty of protein. By the time this grand spectacle is over, exhausted male and female squid missed by the hungry hunters perish on their own accord. Their pearl white bodies litter the sand floor among the clumps of massed egg sacks as though left by a marine snowstorm. A few weeks later, planktonic baby squid hatch in clouds. They will find plenty of microscopic prey in these waters, a gift from the upwelling and currents that fuel life here in the Channel Islands. Glenn Vanstrum is a nature photographer and science journalist based in San Diego. His photography has appeared in Audubon, Discover, and Sierra magazines.
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Summer 2001
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