The Sargasso is a place forgotten by the winds, undisturbed by the strong flow of waters that girdle it as with a river. Under the seldom-clouded skies, its waters are warm and heavy with salt. Separated widely from coastal rivers and from polar ice, there is no inflow of fresh water to dilute its saltiness; the only influx is of saline water from the adjacent currents, especially from the Gulf Stream or North Atlantic Current as it crosses from America to Europe. And with the little, inflowing streams of surface water come the plants and animals that for months or years have drifted in the Gulf Stream.
The sargassum weeds are brown algae belonging to several species. Quantities of the weeds live attached to reefs or rocky outcroppings off the coasts of the West Indies and Florida. Many of the plants are torn away by storms, especially during the hurricane season. They are picked up by the Gulf Stream and are drifted northward. With the weeds go, as involuntary passengers, many small fishes, crabs, shrimps, and innumerable larvae of assorted species of marine creatures, whose home had been the coastal banks of sargassum weed.
Curious things happen to the animals that have ridden on the sargassum weed into a new home. Once they lived near the sea’s edge, a few feet or a few fathoms below the surface, but never far above a firm bottom. They knew the rhythmic movements of waves and tides. They could leave the shelter of the weeds at will and creep or swim about over the bottom in search of food. Now, in the middle of the ocean, they are in a new world. The bottom lies two or three miles below them. Those who are poor swimmers must cling to the weed, which now represents a life raft, supporting them above the abyss. Over the ages since their ancestors came here, some species have developed special organs of attachment, either for themselves or for their eggs, so that they may not sink into the cold, dark water far below. The flying fish make nests of the weed to contain their eggs, which bear an amazing resemblance to the sargassum floats or ‘berries.’
Indeed, many of the little marine beasts of the weedy jungle seem to be playing an elaborate game of disguise in which each is camouflaged to hide it from the others. The Sargasso sea slug—a snail without a shell—has a soft, shapeless brown body spotted with dark-edged circles and fringed with flaps and folds of skin, so that as it creeps over the weed in search of prey it can scarcely be distinguished from the vegetation. One of the fiercest carnivores of the place, the sargassum fish Pterophryne, has copied with utmost fidelity the branching fronds of the weed, its golden berries, its rich brown color, and even the white dots of encrusting worm tubes. All these elaborate bits of mimicry are indications of the fierce internecine wars of the Sargasso jungles, which go on without quarter and without mercy for the weak or the unwary.
In the science of the sea there has been a long-standing controversy about the origin of the drifting weeds of the Sargasso Sea. Some have held that the supply is maintained by weeds recently torn away from coastal beds; others say that the rather limited sargassum fields of the West Indies and Florida cannot possibly supply the immense area of the Sargasso. They believe that we find here a self-perpetuating community of plants that have become adapted to life in the open sea, needing no roots or holdfasts for attachment, and able to propagate vegetatively. Probably there is truth in both ideas. New plants do come in each year in small numbers, and now cover an immense area because of their very long life once they have reached this quiet central region of the Atlantic.
It takes about half a year for the plant’s torn from West Indian shores to reach the northern border of the Sargasso, perhaps several years for them to be carried into the inner parts of this area. Meanwhile, some have been swept onto the shores of North America by storms, others have been killed by cold during the passage from offshore New England across the Atlantic, where the Gulf Stream comes into contact with waters from the Arctic. For the plants that reach the calm of the Sargasso, there is virtual immortality. A. E. Parr of the American Museum has recently suggested that the individual plants may live, some for decades, others for centuries, according to their species. It might well be that some of the very weeds you would see if you visited the place today were seen by Columbus and his men. Here, in the heart of the Atlantic, the weed drifts endlessly, growing, reproducing vegetatively by a process of fragmentation. Apparently almost the only plants that die are the ones that drift into unfavorable conditions around the edges of the Sargasso or are picked up by outward-moving currents.
Such losses are balanced, or possibly a little more than balanced, by the annual addition of weeds from distant coasts. It must have taken eons of time to accumulate the present enormous quantities of weed, which Parr estimates as about 10 million tons. But this, of course, is distributed over so large an area that most of the Sargasso is open water. The dense fields of weeds waiting to entrap a vessel never existed except in the imaginations of sailors, and the gloomy hulks of vessels doomed to endless drifting in the clinging weed are only the ghosts of things that never were.
The Changing Year
Thus with the year seasons return.
FOR THE SEA AS a whole, the alternation of day and night, the passage of the seasons, the procession of the years, are lost in its vastness, obliterated in its own changeless eternity. But the surface waters are different. The face of the sea is always changing. Crossed by colors, lights, and moving shadows, sparkling in the sun, mysterious in the twilight, its aspects and its moods vary hour by hour. The surface waters move with the tides, stir to the breath of the winds, and rise and fall to the endless, hurrying forms of the waves. Most of all, they change with the advance of the seasons. Spring moves over the temperate lands of our Northern Hemisphere in a tide of new life, of pushing green shoots and unfolding buds, all its mysteries and meanings symbolized in the northward migration of the birds, the awakening of sluggish amphibian life as the chorus of frogs rises again from the wet lands, the different sound of the wind stirs the young leaves where a month ago it rattled the bare branches. These things we associate with the land, and it is easy to suppose that at sea there could be no such feeling of advancing spring. But the signs are there, and seen with understanding eye, they bring the same magical sense of awakening.
In the sea, as on land, spring is a time for the renewal of life. During the long months of winter in the temperate zones the surface waters have been absorbing the cold. Now the heavy water begins to sink, slipping down and displacing the warmer layers below. Rich stores of minerals have been accumulating on the floor of the continental shelf—some freighted down the rivers from the lands; some derived from sea creatures that have died and whose remains have drifted down to the bottom; some from the shells that once encased a diatom, the streaming protoplasm of a radiolarian, or the transparent tissues of a pteropod. Nothing is wasted in the sea; every particle of material is used over and over again, first by one creature, then by another. And when in spring the waters are deeply stirred, the warm bottom water brings to the surface a rich supply of minerals, ready for use by new forms of life.
Just as land plants depend on minerals in the soil for their growth, every marine plant, even the smallest, is dependent upon the nutrient salts or minerals in the sea water. Diatoms must have silica, the element of which their fragile shells are fashioned. For these and all other microplants, phosphorus is an indispensable mineral. Some of these elements are in short supply and in winter may be reduced below the minimum necessary for growth. The diatom population must tide itself over this season as best it can. It faces a stark problem of survival, with no opportunity to increase, a problem of keeping alive the spark of life by forming tough protective spores against the stringency of winter, a matter of existing in a dormant state in which no demands shall be made on an environment that already withholds all but the most meager necessities of life. So the diatoms hold their place in the winter sea, like seeds of wheat in a field under snow and ice, the seeds from which the spring growth will come.