Dr. Coker estimated the annual consumption of fish by the guano-producing birds of Peru as equal to a fourth of the total production of all United States fisheries. Because of this diet, which links the birds with all the minerals of the sea, their excrement is the most valuable and efficient fertilizer in the world.
Leaving the coast of South America at about the latitude of Cape Blanco, the Humboldt Current turns westward into the Pacific, carrying its cool waters almost to the equator. About the Galapagos Islands it gives rise to a strange mixture of waters—the cool green of the Humboldt and the blue equatorial waters meeting in rips and foam lines, suggesting hidden movements and conflicts deep in the sea.
The conflict between opposing water masses may, in places, be one of the most dramatic of the ocean’s phenomena. Superficial hissings and sighings, the striping of the surface waters with lines of froth, a confused turbulence and boiling, and even sounds like distant breakers accompany the displacement of the surface layers by deep water. As visible evidence of the upward movement of the water masses, some of the creatures that inhabit the deeper places of the sea may be carried up bodily into the surface, there to set off orgies of devouring and being devoured such as Robert Cushman Murphy witnessed one night off the coast of Colombia from the schooner Askoy. The night had been still and dark, but the behavior of the surface made it clear that deep water was rising and that some sort of conflict was in progress among opposing water masses far below the ship. All about the schooner small, steep waves leaped into being and dissolved in foaming whitecaps, pricked with the blue fire of luminescent organisms. Suddenly,
On either side, and at a bafflingly uncertain distance from the ship, a dark line, like a wall of advancing water, seemed to be closing in upon us … We could hear the splash and murmur of a troubled surface close by … Presently we could see a gleam of foam sprinkled with points of luminescence on the slowly approaching swell or head to the left. Vague and unfounded thoughts of marine earthquake bores occurred to Fallon and me together, and we felt peculiarly helpless with a dismantled engine and no breeze to make the craft answer her helm. The dreamlike slowness of all that was going on, moreover, gave me a feeling that I had not yet fully shaken off the bonds of three hours’ slumber.
However, when the dark, white-outlined menace, reached us, it proved to be nothing more than a field of the dancing water, tossing its little peaks a mere foot or so into the air and beating a tattoo on the steel flanks of ‘Askoy’ …
Presently a sharp hissing sound, different in character from the bursting of small waves, came out of the darkness to starboard, and this was followed by strange sighings and puffings … The puffers were blackfish, many scores, or perhaps hundreds of them, rolling and lumbering along and diving to pass beneath ‘Askoy’ shortly before they reached her bilge … We could hear the bacchanalian clamor of their rumblings and belchings. In the long beam of the searchlight, the hissing proved to come from the jumping of small fishes. In all directions as far as the light carried, they were shooting into the air and pouring down like hail …
The surface was seething, boiling with life, much of which was de profundis. Larvae of clawless lobsters, tinted jellyfish, nurse chains of salpa, small herringlike fishes, a silvery hatchetfish with its face bitten off, rudder fishes, hanging head downward, luminous lantern-fishes with shining light pores, red and purple swimming crabs, other creatures which we could not name at sight and much that was too small even to see distinctly …
A general holocaust was in progress. The little fishes were eating invertebrates or straining out the plankton; the squids were pursuing and capturing fish of various sizes; and the blackfish were no doubt enjoying the squids …
As the night wore on, the amazing manifestations of abundance and devouring gradually, almost imperceptibly, died away. Eventually, ‘Askoy’ lay once more in water that seemed as still and dead as oil, and the lap-lap of skipping waves drew off farther and farther into the distance until it was lost.*
Although such exciting displays of upwelling are seen and recognized by comparatively few people, the process takes place regularly off a number of coasts and at many places in the open ocean. Wherever it occurs, it is responsible for a profusion of life. Some of the world’s largest fisheries are dependent on upwelling. The coast of Algeria is famous for its sardine fisheries; the sardines are abundant here because upward streams of deep, cold water provide the minerals to support astronomical numbers of diatoms. The west coast of Morocco, the area opposite the Canary and Cape Verde islands, and the southwest coast of Africa are other sites of extensive upwelling and consequent richness of marine life. There is an amazingly abundant fish fauna in the Arabian Sea near Oman and on the Somali Coast near Cape Hafun, both occurring in areas of cold water rising from the depths. In the South Equatorial Current north of Ascension Island is a ‘tongue of cold’ produced by the rise of sea water from the bottom. It is extraordinarily rich in plankton. Upwelling around the island of South Georgia, east of Cape Horn, makes this one of the world’s centers of whaling. On the west coast of the United States the catch of sardines is sometimes as much as a billion pounds in a year, supporting one of the largest fisheries in the world. The fishery could not exist except for upwelling, which sets off the old, familiar biological chain: salts, diatoms, copepods, herring. Down along the west coast of South America, the astonishing profusion of life in the Humboldt is maintained by upwelling, which not only keeps the waters of the current cold in all its 2500-mile course to the Galapagos Islands but brings up the nutrient salts from the deeper layers.
When upwelling takes place along coastlines, it is the result of the interplay of several forces—the winds, the surface currents, the rotation of the earth, and the shape of the hidden slopes of the continent’s foundations. When the winds, combined with the deflecting effect of rotation, blow the surface waters offshore, deep water must rise to replace it.
Upwelling may occur in the open sea as well, but from entirely different causes. Wherever two strongly moving currents diverge, water must rise from below to fill the place where the streams separate. One such place lies at the westernmost bounds of the Equatorial Current in the Pacific, where the powerfully moving stream turns and pours part of its waters back into the counter-current, and part northward toward Japan. These are confused and turbulent waters. There is the strong pull to the north by which the main stream, sensitive to the force of the rotating earth, turns to the right. There are the swirls and eddies by which the lesser stream turns again upon itself and flows back into the eastern Pacific. There is the rushing up from below to fill the otherwise deepening groove between the streams. In the resulting disquietude of the ocean waters, chilled and enriched from below, the smaller organisms of the plankton thrive. As they multiply, they provide food for the larger plankton creatures, which, in turn, provide food for squid and fish. These waters are prodigiously rich in life, and there is evidence that they may have been so for many thousands of years. Swedish oceanographers recently found that under these areas of divergence the sediment layer is exceptionally thick—the layer composed of all that remains of the billions upon billions of minute creatures that have lived and died in this place.
The downward movement of surface water into the depths is an occurrence as dramatic as upwelling, and perhaps it fills the human mind with an even greater sense of awe and mystery, because it cannot be seen but can only be imagined. At several known places the downward flow of enormous quantities of water takes place regularly. This water feeds the deep currents of whose courses we have only the dimmest knowledge. We do know that it is all part of the ocean’s system of balances, by which she pays back to one part of her waters what she had latterly borrowed for distribution to another.
The North Atlantic, for example, receives quantities of surface water (some 6 million cubic meters a second) from the South Atlantic via the Equatorial Current. The return payment is made at deep levels, partly in very cold arctic water, and partly in som
e of the saltiest, warmest water in the world, that of the Mediterranean. There are two places for the down-flow of arctic water. One is in the Labrador Sea. Another is southeast of Greenland. At each the quantity of sinking water is prodigious—some 2 million cubic meters a second. The deep Mediterranean water flows out over the sill that separates the basin of the Mediterranean from the open Atlantic. This sill lies about 150 fathoms beneath the surface of the sea. The water that spills over its rocky edge does so because of the unusual conditions that prevail in the Mediterranean. The hot sun beating down on its nearly enclosed water creates an extraordinarily high rate of evaporation, drawing off into the atmosphere more water than is added by the inflow of rivers. The water becomes ever saltier and more dense; as evaporation continues the surface of the Mediterranean falls below that of the Atlantic. To correct the inequality, lighter water from the Atlantic pours past Gibraltar in surface streams of great strength.
Now we give the matter little thought, but in the days of sail, passage out into the Atlantic was a difficult problem because of this surface current. An old ship’s log of the year 1855 has this to say of the current and its practical effect:
Weather fine; made 1¼ pt. leeway. At noon, stood in to Almira Bay, and anchored off the village of Roguetas. Found a great number of vessels waiting for a chance to get to the westward, and learned from them that at least a thousand sail are weather-bound between this and Gibraltar. Some of them have been so for six weeks, and have even got so far as Malaga, only to be swept back by the current. Indeed, no vessel has been able to get out into the Atlantic for three months past.
Later measurements show that these surface currents flow into the Mediterranean with an average velocity of about three knots. The bottom current, moving out into the Atlantic, is even stronger. Its outward flow is so vigorous that it has been known to wreck oceanographic instruments sent down to measure it, apparently pounding them against stones on the bottom; and once the wire of the Falmouth cable near Gibraltar ‘was ground like the edge of a razor, so that it had to be abandoned and a new one laid well inshore.’
The water that sinks in the arctic regions of the Atlantic, as well as that spilling over the Gibraltar sill, spreads out widely into the deeper parts of the ocean basins. Traversing the North Atlantic, it crosses the equator and continues to the south, there passing between two layers of water that are moving northward from the Antarctic Sea. Some of this antarctic water mingles with the Atlantic water—that from Greenland and Labrador and the Mediterranean—and with it returns south. But other antarctic water moves northward across the equator and has been traced as far as the latitude of Cape Hatteras.
The flow of these deep waters is hardly a ‘flow’ at all; its pace is ponderously slow, the measured creep of icy, heavy water. But the volumes involved are prodigious, and the areas covered world-wide. It may even be that the deep ocean water, on such global wanderings, acts to distribute some of the marine fauna—not the surface forms but the dwellers in deep, dark layers. From our knowledge of the source of the currents, it seems significant that some of the same species of deep-water invertebrates and fishes have been collected off the coast of South Africa and off Greenland. And about Bermuda, where a greater variety of deep-water forms has been found than anywhere else, there is a mingling of deep water from the Antarctic, the Arctic, and the Mediterranean. Perhaps in these sunless streams the weird inhabitants of deep waters drift, generation after generation, surviving and multiplying because of the almost changeless character of these slowly moving currents.
There is, then, no water that is wholly of the Pacific, or wholly of the Atlantic, or of the Indian or the Antarctic. The surf that we find exhilarating at Virginia Beach or at La Jolla today may have lapped at the base of antarctic icebergs or sparkled in the Mediterranean sun, years ago, before it moved through dark and unseen waterways to the place we find it now. It is by the deep, hidden currents that the oceans are made one.
* From Am. Phil. Soc. Trans., vol. 2, 1786.
* It is now the fashion among oceanographers to speak of the Gulf Stream System, reflecting the discovery that east of Cape Hatteras there is no longer a continuous river of warm water but a “series of overlapping currents arranged somewhat like the shingles on a roof.” Not only do the streams “overlap” but they are narrow and swift. The main branches of the stream that have long been recognized east of the Grand Banks are now known to originate far to westward of the Banks, developing not as branches in the ordinary sense but as a series of new currents, each to the north of the next older one.
As oceanographers study more about the dynamics of circulation in the sea, they are more and more struck by parallels between the ocean of water and the ocean of air. One of the leading students of the Gulf Stream, Columbus Iselin, has commented on the branching of the Stream in terms of a fascinating analogy: “Much the same phenomena seem to be present in the jet streams found at high elevations in the great belts of prevailing westerly winds of mid-latitudes,” he says, “although each atmospheric jet has greater dimensions than the overlapping subdivision of the Gulf Stream System.”
* One of the most exciting recent events in oceanography was the discovery of a powerful current running under the South Equatorial current but in the opposite direction. The core of the counter current lies about 300 feet below the surface (although shallower near its eastern terminal in the vicinity of the Galapagos Islands). This subsurface current is about 250 miles wide and it flows at least 3500 miles eastward along the equator at a speed of about 3 knots. (The speed of the surface current is only about one knot.) The existence of the current was discovered in 1952 by Townsend Cromwell in the course of a U.S. Fish and Wildlife Service investigation of methods of tuna fishing. Cromwell observed that long lines set for tuna at the equator did not move westward with the surface current, as would be expected, but drifted rapidly in the opposite direction. It was not until 1958, however, that an extensive survey of the current was made by the Scripps Institution of Oceanography and its impressive dimensions measured. This same survey gave further proof that the deep circulation of the ocean is far more complicated than has generally been realized, for beneath the swift-flowing eastward current was still another, flowing to the west. In only the uppermost half mile of Pacific equatorial waters, therefore, there are three great rivers of water, one above the other, each flowing on its own course independent of the other. When such surveys can be extended all the way to the floor of the ocean an even more complex picture will undoubtedly be revealed.
Only a year before the detailed charting of this Pacific current, British and American oceanographers discovered a south-flowing counter current running from the North to the South Atlantic under the Gulf Stream and the Brazil Current. The techniques that make such discoveries possible have only very recently become available to oceanographers. As their use becomes more widespread our almost complete ignorance of the deep circulation of the ocean will be dispelled.
*From Bulletin, U.S. Bureau of Fisheries, vol. XXVIII, part 1, 1908, p. 338.
*From Natural History, vol. LIII, no. 8, 1944, p. 356. ?
The Moving Tides
In every country the Moon keeps ever the rule of alliance
with the Sea which it once for all has agreed upon.
THE VENERABLE BEDE
THERE IS NO DROP of water in the ocean, not even in the deepest parts of the abyss, that does not know and respond to the mysterious forces that create the tide. No other force that affects the sea is so strong. Compared with the tide the wind-created waves are surface movements felt, at most, no more than a hundred fathoms below the surface. So, despite their impressive sweep, are the planetary currents, which seldom involve more than the upper several hundred fathoms. The masses of water affected by the tidal movement are enormous, as will be clear from one example. Into one small bay on the east coast of North America—Passamaquoddy—2 billion tons of water are carried by the tidal currents twice each day; into the whole Bay of Fundy,
100 billion tons.
Here and there we find dramatic illustration of the fact that the tides affect the whole ocean, from its surface to its floor. The meeting of opposing tidal currents in the Strait of Messina creates whirlpools (one of them is Charybdis of classical fame) which so deeply stir the waters of the strait that fish bearing all the marks of abyssal existence, their eyes atrophied or abnormally large, their bodies studded with phosphorescent organs, frequently are cast up on the lighthouse beach, and the whole area yields a rich collection of deep-sea fauna for the Institute of Marine Biology at Messina.
The tides are a response of the mobile waters of the ocean to the pull of the moon and the more distant sun. In theory, there is a gravitational attraction between every drop of sea water and even the outermost star of the universe. In practice, however, the pull of the remote stars is so slight as to be obliterated in the vaster movements by which the ocean yields to the moon and the sun. Anyone who has lived near tidewater knows that the moon, far more than the sun, controls the tides. He has noticed that, just as the moon rises later each day by fifty minutes, on the average, than the day before, so, in most places, the time of high tide is correspondingly later each day. And as the moon waxes and wanes in its monthly cycle, so the height of the tide varies. Twice each month, when the moon is a mere thread of silver in the sky, and again when it is full, we have the strongest tidal movements—the highest flood tides and the lower ebb tides of the lunar month. These are called the spring tides. At these times sun, moon, and earth are directly in line and the pull of the two heavenly bodies is added together to bring the water high on the beaches, and send its surf leaping upward against the sea cliffs, and draw a brimming tide into the harbors so that the boats float high beside their wharfs. And twice each month, at the quarters of the moon, when sun, moon, and earth lie at the apexes of a triangle, and the pull of sun and moon are opposed, we have the moderate tidal movements called the neap tides. Then the difference between high and lower water is less than at any other time during the month.
The Sea Around Us Page 17
