THE FOOD RESOURCES OF THE SEA By GEORGE W. MARTIN ASSISTANT PROFESSOR OF BOTANY, RUTGERS COLLEGE, RIMITIVE man got his food as his competitors did that is PRI to say, he picked it up or killed it where he could find it. Very early in his civilized career he ceased to be a hunter and began to cultivate the land; in fact, the beginnings of civilization and of agriculture were contemporaneous. Since that time the pressure of increasing populations demanding to be fed has been a prolific source of human strife. There are not lacking economists who would maintain that the recent catastrophic war in Europe was the direct result of an increasing demand for food on the part of the rapidly multiplying German nation. East, in THE SCIENTIFIC MONTHLY for December, 1921, points out that the agricultural resources of the United States can in all probability not support in reasonable comfort more than two hundred million people and that the present indications are that our population will reach that figure within the next century. Furthermore, we can not count on importing food indefinitely, since by the time our own population reaches its limit, the now scantily peopled parts of the world will produce little or no food in excess of the needs of their own greatly augmented populations. His is but one of many voices warning us that there is a limit to the number of human beings whom the earth can support, and however we may disagree with the various estimates as to what the limit may be, we can not doubt that it exists, and that, historically speaking, we are rapidly approaching it. The purpose of this article is, however, not to consider this question in detail, but merely to point out one source of food of which the possibilities are still largely unrealized. If we were to-day still depending upon the chase as the main source of our food, most of us would be dead, or, rather, we should never have been born. Yet so far as the oceans, which cover three fourths of the surface of the earth, are concerned, we have made little essential advance over the methods of the primitive fishermen. The flocks and herds of the sea still roam freely in their native haunts, and we cast our lines and nets over their feeding grounds, and catch what we can. Our operations are on a larger scale, it is true, than those of our predecessors, our tackle is superior, our nets larger and stronger, and, by equipping our fishing vessels with steam or gasoline power, we have enlarged the area we can cover. Once the fish are landed, we have an elaborate system of distribution and marketing, so that cities a thousand miles inland can have fish a day out of the ocean shipped to them in fast refrigerated cars. There is also a little direct utilization of the plants of the sea. In certain parts of the world, notably in China and Japan, and to a less extent in Europe, a few of the algæ are eaten by man or his domestic animals, or gathered to be utilized in some minor industry. But so far as the actual cultivation of the sea's resources as distinguished from their mere exploitation is concerned, we have made only the feeblest beginning. The reasons for this are, of course, the uncontrollability of the sea as compared with the land; its instability; the vastness of the oceans and the relative inaccessibility of much of their area; and especially the difficulty of attempting to control living organisms in the sea, out of man's natural element, as they may be controlled on the land where he is at home. Yet if the demand becomes insistent enough we can not doubt that methods will be devised which will give us the desired results. To question that would be to admit that man has neared the culmination of his evolutionary career and is preparing to bequeath the mastery of the earth to his successor, whoever that may be. The bulk of the food supply which we have come to expect the ocean to furnish us is animal. Animals that live in the sea are, however, no less dependent upon plant life for their food than are land animals. We all know that the beef we eat is built up from the grass and grain upon which the cattle have fed; and, while there are many animals that feed upon other animals only, sooner or later the cycle goes back to the green plants. This is because the green coloring matter contained in plants is the only substance known that can so combine carbon and water as to form the carbohydrates that are the fundamental materials of which all living beings, whether plants or animals, are constructed. The plants living in the sea are then the equivalents, so far as the life of the sea is concerned, of the land plants. Like land plants, they not only need carbon and water, but certain mineral salts. Of these, those that furnish them with nitrogen and phosphorus are most important, since they are most apt to be present in insufficient quantity and thus to be limiting factors. The plants in the sea are continually using these substances and are continually dying or being eaten by animals. Sooner or later there comes a time when by the death and decay of the plant or animal most of these materials are returned to solution. Part, however, have been transformed into insoluble compounds and are lying inert in the depths of the ocean. Thus there is a constant loss of nutrient salts. This loss is replaced by drainage from the land, the great rivers carrying constantly into the ocean an almost incredible amount of dissolved minerals in addition to suspended matter. Another important requirement of plants is light. Plants on land receive the full benefit of the sun's rays as we know them. Plants living under water receive only a portion of the rays that reach the land. Part of the light that strikes the water is reflected, and the part that penetrates the water is gradually absorbed in passing through that medium, the red and yellow rays first, the blue and violet last. This differential absorption is reflected in the curious and well-known vertical distribution of marine algæ according to color-the green kinds growing in shallow water, the browns in an intermediate zone and the reds in the deepest water, although there are, of course, numerous exceptions to this general rule of distribution. Another property of light is that it is refracted by water, and the greater the angle at which the rays strike the water the greater will be the refraction. In the tropics, where the rays are practically vertical, the amount of refraction is insignificant, but in high latitudes, where the rays strike the water at a sharp angle, the refraction is marked, as a result of which the rays are bent into a more nearly vertical direction, thus increasing their penetration in depth and partly compensating for the unfavorable angle at which they strike the water. The penetration is also markedly affected by the amount of suspended matter and the number of microorganisms present in the water. HellandHansen was able to show that in the Atlantic Ocean south of the Azores, on a bright summer's day, light is abundant at a depth of 100 meters, still including at that depth a few red rays. At 500 meters the red rays have completely disappeared, but blue and ultra-violet rays are still plentiful, and may be detected at 1,000 meters, but have completely disappeared at 1,700 meters. It is not probable, however, that under the most favorable conditions photosynthesis may be carried on at depths greater than 200 meters. 71 Temperature is less directly important in the sea than on land since there is no great danger of injurious extremes being reached. Indirectly, its importance lies in the fact that carbondioxide is much more soluble in cold water than in warm, (Fig. 1) and it is probably this, rather than the direct influence of temperature which accounts for the fact that the most luxuriant development of plant life is in the colder waters of the earth. FIG. 1. CUBIC CENTIMETERS OF CO2 SOLUBILITY OF CARBON DIOXIDE IN WATER. SOLUBLE IN 1 C. C. OF WATER, REDUCED TO NORMAL TEMPERATURE AND PRESSURE On the basis of habitat, marine plants may be divided into two great groups. On all coasts, and in shallow waters near the coasts, there is usually a conspicuous zone of "seaweed." This is known. as the benthos, from a Greek word denoting bottom, because it occurs attached to the bottom on the shelf of shallow water surrounding the coasts. The plants of the benthos are extremely varied and range in size from microscopic to very large, the giant kelps of the northern Pacific sometimes reaching a length of nearly four hundred feet. We are accustomed to think of these plants as algæ, and most of them belong to one or another of the great algal groups, but the most important of all the plants of the benthos is not an alga but a flowering plant, belonging to the pondweed family, and not very distantly related to the grass family, which is the most important economic family of land plants. This is Zostera marina, the common eel-grass. Zostera marina or some of its near relatives occurs on almost all the ocean shores of the globe, being absent only in the extremely hot and extremely cold portions. It grows in water varying from a little over a meter in depth at low tide, to depths of fifteen or twenty meters in certain clear Mediterranean waters; and in waters varying in salinity from that of the sea water of the open ocean to that of brackish bays and estuaries of less than half full salinity. It prefers mud, but will grow nearly as well in sand. It will not grow, however, where the bottom is composed of loose stones nor where the wave action is severe, and these localities are inhabited by the true algæ. The windrows of dead "seaweed" commonly found cast up on our Atlantic shores are very largely composed of Zostera. A few other flowering plants occur in salt or brackish water, but they are relatively unimportant. The algae of the benthos occur, as previously stated, in three rather indefinitely limited zones. In the quiet waters of shallow bays we find a great many green and blue-green alga. Such forms as Ulva lactuca and Enteromorpha intestinalis, both often called sea lettuce, are typically found in such situations. The branching, feathery masses of Cladophora and the thick green felt of Vaucheria are also often prominent. The blue-green alga are entirely microscopic, but often occur in great abundance. A species of Lyngbya frequently forms felted mats two or three inches thick and many feet across. Masses of Spirulina as large as a dinner plate are common. The plants which grow between tide levels or just below low tide, at places where there is considerable wave action, belong mostly to the brown algæ. Various rockweeds, belonging to such genera as Fucus and Ascophyllum, are common in northern waters, and the gulfweed, Sargassum, is typical of the forms growing in the warmer waters. Below low tide level the great kelps the Laminarias and their allies-are the largest plants of the ocean, the larger species occurring, however, only in the cooler waters. The algae of the deeper waters are mainly the reds, and the farther south we go the greater becomes the preponderance of the red algae. It must be remembered, however, that red, brown, green and blue-green alga are more or less intermingled at all depths and in all latitudes. All the plants we have been considering are alike, however, in that they occur only near the shores, except in cases where they have been torn loose from their fastenings and carried by currents into the open sea. The Sargasso Sea of the Atlantic Ocean is merely an area outside of the track of the great oceanic currents and therefore constituting a huge eddy in which such material accumulates, growing vegetatively to a certain extent and finally dying and sinking. If the only marine plants were in the benthos, in spite of the local luxuriance of its growth, the great mass of the ocean would be a desert, incapable of supporting anything like the amount of life which actually exists in it. There is, however, another great group of living organisms, the plankton. The plankton comprises those plants and animals that are neither attached to the bottom nor able to swim against a current, but normally live floating in the water and carried by it from place to place. Some of the animals of the plankton are of rather large size-such, for example, as the Portuguese man o'war and some of the larger jellyfish. Most of them, and all the plants, are microscopic. The plants, in fact, are all unicellular, although the cells are often united into filaments or colonies of various shapes. Two groups of plankton organisms are of special importance: the diatoms and the peridines. |