Surface to 660 feet (200 m)
Perhaps nothing evokes the visceral transition from air to water more than the abrupt change in the soundscape. As humans take the plunge from air to water, our ears seem to shut down as the water presses in—we are rendered mostly deaf by this sudden transition. But many animals that live underwater have adapted another way of hearing known as bone conduction, whereby the bones of the animal’s skull pick up the pressure waves of sound traveling through the water and relay them to the inner ears.
As the iMonstercam’s hydrophone switches on, picking up acoustic pressure waves in the water, we become aware of another world. The first 200 feet (60 m) below the surface sounds staticky, like the white noise of some 20th-century experimental music. Then we hear faint rumbles and occasional whistles and cries. Could it be whales, dolphins, strange fish? And then the rumbling grows louder.
The hydrophone starts to pick up a background whining sound that increases steadily, eventually obliterating the whistles and cries. This is the noise of ship traffic. In Leonardo da Vinci’s day, you could put a tube into the water and hear the sound of a sailing ship moving through the water miles away. Today, ocean traffic comprises some 50,000 container ships, as well as ferries, cruise ships and the navies of the world. The faster they go, the noisier they are. Only submarines and sailboats are “quiet.” Given the extraordinary level of sound generated in the surface waters in some areas of the world, you might as well be in downtown Manhattan, Manila or Mexico City at rush hour.
Sound travels nearly 4½ times faster and roughly 100 times farther underwater than it does in the air, depending on water conditions, depth and the loudness and frequency of the sound. Low sounds travel farthest, while high-pitched sounds don’t travel much distance before dissipating. The physics of sound underwater is a matter of great interest to whales, dolphins and other marine mammals that use sound to find their food and mates and to communicate with one another. Visibility underwater, even in the clearest topmost layer, is limited to a few dozen feet. For fast-moving, wide-ranging marine mammals, sound is the tool of choice.
The abrupt air-water transition is also made manifest by the absence of waves and the rocking of the sea. Just 10 to 14 feet (3–4 m) below the surface, all is calm. In fact, there is some movement—massive surface-water currents—but we are being carried along at the same slow, steady speed as everything else. The surface currents typically travel at three to five miles per hour (5–8 km/h). The classic schoolchild example of an ocean surface current is the Gulf Stream, which ranges from 50 miles (80 km) wide off Miami, Florida, to 300 miles (480 km) wide off New York City, with a depth of 2,100 feet (640 m).
The Gulf Stream has been described as a river in the sea, and the volume of moving water represents more than all the world’s rivers combined. It transports warm water from the subtropics of the Gulf of Mexico northeast across the North Atlantic, where part of the Gulf Stream becomes the North Atlantic Current, which angles up toward Great Britain and western Europe. Some of it branches off toward Norway, while the rest turns clockwise back toward the equator. In the late 18th century, Benjamin Franklin traveled regularly by ship across the Atlantic Ocean on diplomatic business and became intrigued by the phenomenon of the Gulf Stream. Even then, European ship captains knew that when crossing the Atlantic to the New World, they first had to sail south toward the equator but could take the more direct route home across the North Atlantic—a fast-lane transport that cut days off the return voyage.
The surface water is where the world ocean and the atmosphere alternately clash and couple to drive the Earth’s weather. It can carry great warmth, the soup of life, as well as the makings of fierce storms.
Surface currents, found throughout the world ocean, are constantly on the move. In the North Pacific, the Kuroshio (“black tide”) travels from the southwest to the northeast, moving from Japan to the Pacific Coast of Canada and Alaska before turning south along the coast of North America and continuing around clockwise along the equator. In the southern hemisphere, however, the large surface-water transports move in a counterclockwise gyre across the broad expanse of the South Pacific, South Atlantic and Indian oceans. These motions are the result of Earth spinning on its axis, producing the so-called Coriolis effect, which causes both wind and water currents to move in a clockwise direction in the northern hemisphere, gradually flowing out and away from the equator, and in a counterclockwise direction in the southern hemisphere. The effect can be crudely demonstrated by observing the way water flows off a wet spinning top.
As these surface waters are on the move, some of the warm, salty surface water flowing from the equatorial region to the cold temperate or polar regions becomes dense or heavy and sinks, at times moving so rapidly to the bottom off Antarctica, Greenland and Labrador that vertical currents can be measured in the water. This is how the deep waters of the world ocean are formed. The deep water flows slowly compared with the movement of the surface water and sometimes travels in the opposite direction, but after hundreds of years, the deep water eventually reaches the North Pacific—the “end of the world ocean”—where it rises again to the surface. By the time the water reaches its starting point in the system, something on the order of 1,000 years has elapsed. The world ocean is thus one system, and all the water flows through it. This is referred to as thermohaline circulation, because the water currents are driven largely by changes in the temperature and salinity, or salt content, of the water.
The surface water is where the world ocean and the atmosphere alternately clash and couple to drive the Earth’s weather. It can carry great warmth, the soup of life, as well as the makings of fierce storms. It is the meeting place and feeding ground for millions of seabirds, fish, whales, dolphins, seals and sea lions. It’s the skin of the world ocean—a sort of upside-down Serengeti Plain. It is the epipelagic zone, a who’s who of sea life familiar to all. These are the world’s high-profile ocean organisms.
A tour of the surface waters reveals an extraordinary diversity of fish and invertebrates that accompany the better-known “sea monsters.” Many sharks, including the white, oceanic whitetip, blue, hammerhead and tiger, feed mainly in this zone, although other shark species live deeper or are capable of dives far below the epipelagic zone. Big rays, such as the manta ray, spend considerable time in this surface zone. Complicated jellyfish and siphonophores with their colonial lifestyles, including the dreaded Portuguese man-of-war, also reside at the surface, as they float and wait for whatever might drift by. Of course, the whales, dolphins, porpoises, seals and sea lions spend most of their time in these topmost surface waters. Some of them hunt for fish and squid in deeper waters, but most travel through surface waters their whole lives.
Killer whales (Orcinus orca) on the prowl sometimes use the surface of the sea as a wall against which to trap their prey. Hunting in packs, transient-type killer whales quietly station themselves in the Monterey Submarine Canyon, a few miles offshore of Point Pinos, California. Unsuspecting young gray whales (Eschrichtius robustus) as well as various dolphins, elephant seals and sea lions have nowhere to hide when crossing this deep-water open area. Every April, gray whale mothers with their recently born calves, measuring up to 20 feet (6 m) long, migrate north along the coast of Baja California to Alaska. The calves may still be nursing, but there will be no solid food for mother or baby until they reach far northern waters.
After a comfortable journey nudging north along the shallows of the southern California coast, the grays approach Monterey. There, they must make a decision: to cross the deep open waters or take the longer route close to shore through the kelp beds. Each year, a few gray whale mothers and their calves elect to cross this dangerous passage, taking the shortcut that risks a coordinated attack by killer whales. Swimming up from below, the killer whales charge the calf, which is roughly their individual size. They attempt to corral the young gray, keeping him from his mother and preventing him from diving deep to escape. The killer whales do not attack the much larger mother, though she may try to defend her calf. Some calves escape, but more often on a typical late-spring afternoon, the waters across the surface of the Monterey Submarine Canyon begin to turn red. Within a few minutes or hours, it is all over.
The reason so much life congregates here is because the sunlight that penetrates the uppermost layers drives the photosynthesis of plant plankton, which in turn provides the basis for most life in the sea. The surface waters are also known as the euphotic zone, from the Greek meaning “well lit.” As the skin of the sea, this zone—the top 660 feet (200 m)—represents less than 5 percent of the world-ocean volume, but it is crucial to life down below. Many deep-sea animals spend their larval lives feeding in surface waters, and even as adults, some steal to the surface at night on food raids. The sinking carcasses of ravaged animals and other detritus nourish the animals of the midwaters and deep ocean below, helping to make life possible in these regions.
In the world ocean’s surface waters, the diversity of species and the density of life are patchy yet extraordinary. The abundance depends on phytoplankton concentrations that support almost all life in the sea. These concentrations vary considerably throughout the ocean, depending on latitude and time of year. In certain tropical or subtropical regions, such as the Sargasso Sea and parts of the central Pacific, concentrations are low. In general, the phytoplankton is densest in summer toward the poles and especially near continental shelves and in areas of upwelling currents.
This overview presents the picture with broad brushstrokes; much work must still be done before we understand the patchiness of phytoplankton at smaller scales. Without an understanding of these basic life-forms, we can never fully grasp the abundance, diversity and movements of the larger animal life-forms, including the so-called sea monsters.
From the time of the first humans, there has been considerable curiosity about the top layer of the ocean. Our forebears may have made their initial forays into the sea, in part, to evade large nonswimming predators. Just as likely, they may have waded in simply to exploit a ready source of food—fish, crabs and other accessible sea life—having exhausted the intertidal supply of clams, oysters, mussels and other delicacies.
Using such breathing mixtures, experienced scuba divers can reach depths of about 500 feet (150 m). To venture to the edge of the epipelagic zone and truly glimpse the dark blue world below 660 feet (200 m), however, it is necessary to use submersibles.
Primitive underwater vehicles were first launched in 1620 and, for most of three centuries, spent all their time in the epipelagic zone, the downward limits of which were forbidding to humans and machinery. With the invention of the self-contained underwater breathing apparatus (scuba) by Jacques-Yves Cousteau and Émile Gagnan in 1943, untethered divers began to penetrate to depths of 150 feet (45 m) or more. Pearl and sponge divers, who carry no underwater breathing devices, have been reported to reach 100 feet (30 m), but normally, they descend no deeper than 40 feet (12 m). The usual mix of compressed air and oxygen limits scuba divers to a maximum depth of about 250 feet (75 m), although any time spent at such a depth requires a lengthy decompression as the diver returns to the surface. Any deeper and the nitrogen in the breathing mixture, which is part of the compressed air, dissolves in the blood, producing intoxication by obstructing the blood’s ability to transport oxygen to the brain. So-called nitrogen narcosis often has fatal consequences. To replace nitrogen in the breathing mixture, oxygen can be combined with helium or hydrogen, both of which are less soluble in human tissues. Using such breathing mixtures, experienced scuba divers can reach depths of about 500 feet (150 m). To venture to the edge of the epipelagic zone and truly glimpse the dark blue world below 660 feet (200 m), however, it is necessary to use submersibles.
As we lower the iMonstercam, the pressure steadily mounts. At 33 feet (10 m), the pressure is 29.4 pounds per square inch (psi), twice that at the surface, but at 330 feet (100 m), it reaches an intense 147 psi (10 times the surface pressure, or 10 atmospheres). At this relatively modest depth, the 147-pound (67 kg) weight of the water column presses down on every square inch of a diver’s body.
Three hundred feet (90 m) is only halfway through the surface layer, but it marks the usual limit of the wind’s effect on the sea. Below 330 feet (100 m), we are no longer carried along on the surface currents. The water below, however, retains its own unique and identifiable character, or flavor, as oceanographers put it. It has a different temperature and salinity and can move at a different speed, often slower, and even in a different direction. But it is usually the calmer, more stable part of the epipelagic zone.
For every additional 33 feet (10 m), we add another atmosphere of pressure. At 660 feet (200 m), where the surface layer gives way to the mesopelagic zone, the pressure is an uncompromising 294 psi (20 times the surface pressure, or 20 atmospheres), which would easily crush our iMonstercam if we had not fitted it with a special housing.
As the iMonstercam reaches the lower limit of the surface waters, it encounters an extraordinary sight: “ocean snow.” The camera light captures a blizzard of nutrients, waste products, dead plant and animal parts and even the odd carcass. Everything that doesn’t get swallowed en route as it heads through the middle layers of the sea is destined for the very bottom. This constant snow becomes most pronounced at certain times of the year, especially following the production of phytoplankton in cold-temperate waters. And in order to see it, you must look up toward the light directly overhead or depend on the illumination of artificial lights.
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