by Sir John Murray (1908)
During the past half-century the science of Oceanography has made rapid strides. The depth of the ocean, the temperature, the composition and the circulation of oceanic waters, the nature and distribution of marine organisms at the surface and in deep water, the origin and distribution of marine deposits over the floor of the ocean, are now all known in their broad general outlines. In recent years there has also been a great development of the science of Limnology from the chemical, physical, and biological points of view, although a detailed examination of some tropical lakes is still a great desideratum.
This being so, the time seems to have arrived for a more systematic comparative study of the conditions which prevail in the great oceans, in partially-enclosed seas, in estuaries, in rivers, and in fresh- and salt-water lakes. Such will be the most certain method of gaining a fuller knowledge of the physical and biological conditions, and of the causes which have brought about the present distribution of organisms, in the waters of the globe. To all studies of the kind here indicated this new Journal should give a great impetus. A few instances may be cited, showing how physical and biological appearances in different regions may be compared with great advantage.
After spending many years in the "Challenger" and other deep-sea expeditions for the exploration of the great ocean basins, I fitted out and employed for some years a small steam yacht--the "Medusa"--for the purpose of exploring in detail some of the narrow salt- and fresh-water lochs about the coasts of Scotland. Nothing could be more striking [[p. 11]] than the differences to be observed in passing from the examination of a sea-loch to that of a fresh-water loch. The climatic conditions were precisely similar in the two kinds of lochs; similar also were the depths and the temperature of the water. In salt water the maximum density point is situated below the freezing point, while in fresh water the maximum density point is, as is well known, about 4 degrees above the zero of the centigrade scale. This physical fact very largely determines the distribution of temperature, and the circulation of the water, in the two kinds of lochs. A comparison of the effects of wind on the surface of a salt- and a fresh-water lake gave important hints for the explanation of many current movements in the great oceans, which cannot be examined in such detail.
In the sea-lochs thousands of animals belonging to nearly all the invertebrate marine groups were brought to the surface in dredgings and trawlings in depths of 500 and 600 feet, and these usually presented magnificent displays of phosphorescent light. In fresh-water lochs like operations yielded not more than five or six small dwarfed species from similar depths, and the phenomenon of phosphorescent light has never been observed in any organisms from the fresh-water lochs.
Again, the decomposition of the sulphates of sea-water under the influence of decaying organic matter soon renders the deposits on the bottom of the sea-loch very foul, while the deposits in the fresh-water lochs are but slightly affected in this way, although analysis shows that they contain even more organic matter. It is believed that two species of bacteria--Microspira desulphuricans in fresh water, and Microspira estuarii in salt water--are chiefly concerned in the reduction of the sulphates in solution.
Observations like the foregoing led me to devote the past five or six years to a systematic survey of the fresh-water lochs of Scotland, in the hope that some new light might be thrown on the causes which have brought about the present distribution of marine and fresh-water organisms. The results of this survey will soon be published in completed form, and they may possibly indicate why salt water is the more congenial medium for the development of organisms, as well as the lines along which new investigations may be profitably undertaken.
In the great oceans, which are freely open to the Southern Ocean, the surface waters, on being cooled towards the Antarctic, sink to the bottom and carry down with them an abundant supply of oxygen. These Antarctic waters are very slowly drawn to the north to supply the place of surface waters driven towards the poles, and they supply with oxygen the marine organisms which live on the floor of the ocean at all depths. The same favourable conditions do not obtain in the arms of the ocean [[p. 12]] which penetrate the continental masses of land. In these partially-enclosed seas circulation is to some extent cut off by submarine barriers, and vertical circulation is much restricted by other physical conditions. In consequence, there is a relatively poor supply of oxygen in the greater depths, and organisms are much less abundant than at similar depths on the floor of the great oceans. In the Black Sea, which is very far removed from direct circulation with the great ocean basins, the deeper waters are more or less saturated with sulphuretted hydrogen, and living organisms (except bacteria) are probably wholly absent in depths beyond one hundred fathoms. Here it is the quantity of absorbed atmospheric gases which determines the greater or less abundance of life, while in the instance just cited above it is the absence of dissolved sea-salts which appears to limit the vigorous development of life in the fresh-water lakes. In two Scottish sea-lochs--Loch Etive and Upper Loch Fyne--Arctic Crustacea, such as Nyctiphanes norvegica, Euchæta norvegica, and Conchcia elegans, seem to have been shut in ever since the glacial period, and are now abundant in the deep water of these basins.
It is a well-known fact that the clayey matter which is carried to the ocean by rivers is nearly all precipitated to the bottom on contact with the sea-water, but a small quantity of siliceous matter is apparently still retained in salt water of relatively low salinity. Diatoms and Radiolaria, the shells and skeletons of which are chiefly composed of silica, flourish in the surface waters of the ocean where there is reason to believe clayey matter is more abundant than elsewhere. For instance, north of the Antarctic continent, where detrital matter is thrown into the sea from melting land-ice; in the tropical Indian and Pacific Oceans, where the greatest rainfall occurs, and where much detrital matter is carried into the ocean from the land; and off the mouths of great rivers both in the tropics and elsewhere, there is a great development of silica-secreting organisms in the surface waters. Further, it is just in these positions that the dead frustules and skeletons of siliceous organisms accumulate on the bottom, and form the diatom oozes of the Antarctic and Arctic Oceans and the radiolarian oozes of the Indian and Pacific Oceans. On the other hand, wherever the salinity of the ocean-water is high, as, for instance, in the trade-wind regions, the Red Sea, and the Mediterranean, the number of silica-secreting organisms is relatively small. The agricultural principle of "minimum" supply may here be applicable to the silica and to the marine flora, but one cannot see how it can be applied to carbonate of lime secreted from sea-water. In this connection the abundance and large size of the radiolarian skeletons in some palæozoic schists may be recalled. It seems probable that in primeval seas there were much more detrital matter and silicate of alumina in suspension than in the present ocean, [[p. 13]] and probably the water of the primeval seas was much less saline and of a somewhat different chemical composition.
Many of the fresh-water lakes in the Scottish Highlands are surrounded by extensive peat deposits. The humic acids, which reach the lakes from these deposits, form the dominating factor in their biology, and appear to prevent many forms of animals from living in these waters; on the other hand desmids are specially abundant. The palæozoic schists on which these lakes are situated yield little lime, and consequently, all organisms which secrete lime have a dwarfed appearance. In an adjoining island, Lismore, where the geological formation is limestone, and where there are no peat deposits, instead of humic acids, an appreciable supply of ammonia salts finds access to the lakes, and a marked difference is observable in the biology. The photic zone of vegetation is much less restricted than in the peaty lakes, many plants are heavily coated with an incrustation of calcium carbonate, and the mollusc shells and calcareous sponges are large and massive.
The water from these various fresh-water lakes is poured into the adjoining salt-water lochs, and the effect on the various marine organisms can be well observed on the shores, at the bottom, and in the surface water. The effect of this mixture of salt- and fresh-water is gradually lost as the observer passes a few miles further to the westward, where the blue waters of the Eastern Atlantic break on the shores. In this region I have contemplated the foundation of a marine laboratory, having for its object the comparative physiological study of organisms under different natural and artificial conditions.
The waters of the ocean are to be regarded as the real Hydrosphere; the outlying portions of the hydrosphere, such as lakes, rivers, the water-vapour of the atmosphere, and the interstitial water of the rocks, are merely transitory. These were evaporated from the ocean, and after a temporary sojourn in these various positions are now being constantly returned to the ocean.
In early ages the land-surfaces were almost certainly less extensive than in these days, and so also were the basins holding fresh water. Continental land and bodies of fresh water have gradually increased during the slow evolution of the existing surface features of the planet. It is generally accepted that life originated in the ocean, and that in the course of time some species, with more or less difficulty, adapted themselves to a fresh-water and terrestrial habitat. From the most fundamental point of view an organism may be regarded as essentially a simple marine aquarium, in which the cells of the body continue to live in a circulating medium or serum, having a composition similar to that of ocean-water in palæozoic times. In the gradual dispersal of organisms over the earth, [[p. 14]] those organisms have been able to flourish best in fresh-water and on land which have been able to maintain for their circulating fluids a composition nearly approaching that of palæozoic sea-water.* [*Quinton, "L'eau de Mer Milieu organique", p. 425, Paris 1904.] Some years ago I suggested that there were many reasons for supposing that chemical composition was just as likely to be handed down by heredity as morphological structure, and consequently that with a fuller knowledge of the soft tissues and circulating fluids of organisms we might be able to reach the past history of the ocean, just as the palæontologist now dimly reads the past history of rocks from the morphological structures of organisms.
Each small or large fresh-water lake has a peculiarity or individuality of its own, dependent on its position with reference to the adjacent land; and this may be contrasted with the nearly uniform conditions which prevail over wide areas in the great ocean basins far from land. Even in these last-mentioned positions the influence of waters from different sources, and with different amounts of absorbed gases and dissolved salts, can sometimes be distinctly traced on the distribution of organisms; still, the factor which chiefly forces itself on the attention of the observer when studying the distribution of organic life in the open oceans is temperature.
It has been shown by thousands of observations in the open ocean far from land that the daily fluctuations of temperature in the surface-waters do not exceed one degree Fahrenheit. Hence the atmosphere over this area--one half of the surface of the globe--rests on a surface the temperature of which is practically uniform during all hours of the day. This is in striking contrast to what obtains on the land-surfaces, where solar and terrestrial radiation produces a very wide daily range of temperature. In the Sahara and the American deserts I have registered between 3 p.m. and 3 a.m. a range of about 100° Fahrenheit. We thus come to one of the prime factors in meteorology. Air with a large quantity of water-vapour absorbs more of the sun's rays, and becomes, in consequence, more heated and specifically lighter than dry air; hence the light moist air ascends in cyclonic, and the heavy dry air descends in anticyclonic, areas. These conclusions, due to careful meteorological observations at sea, go a long way towards a rational interpretation of many atmospheric phenomena, such as the distribution of the mass of the earth's atmosphere, the prevailing winds and consequent oceanic currents of the globe, and the diversified climates in similar latitudes.
At the surface of the earth, both on sea and on land, bands of equal temperature run more or less parallel to the equator. This is true notwithstanding the fact that oceanic currents cause wide deflections, as in the case of the Gulf Stream. The extreme range of temperature in the [[p. 15]] surface-waters of the ocean may be said to be from 28° to 95° F., and 84 per cent of the surface-waters has a temperature exceeding 40° F. There is a circumtropical zone with a high temperature throughout the year and an annual range not exceeding 10° F. This is the home of coral reefs and of all organisms which secrete calcium carbonate abundantly for their shells and skeletons. In this warm area the calcium carbonate appears to be mostly laid down in the form of aragonite; all metabolic changes take place with great rapidity, and pelagic larvæ are exceedingly abundant. Again, there are two circumpolar zones where there is a low temperature throughout the year and an annual range not exceeding 10° F. In these cold areas the calcium carbonate in organic structures is feebly developed, and is laid down in the form of calcite; all metabolic changes take place at a relatively slow rate, and pelagic larvæ are almost absent.
Between these two circumpolar zones and the circumtropical zone are two intermediate zones within which there are very wide ranges of temperature during the year, sometimes as much as 45° F. at the same place at different seasons, warm currents and cold currents alternating. There is a great destruction of marine life in these positions due to rapid changes of temperature, as, for instance, off the Atlantic coasts of the United States; off the Cape of Good Hope; and off the eastern coasts of Australia. It is in these places that there are now forming in the marine deposits at the bottom large quantities of phosphatic and glauconitic nodules. It is evident that these deposits are directly correlated with the changes of temperature and with the destruction of life which takes place at the surface of the ocean as above indicated; and as these deposits occur in the stratified rocks of all geological periods, we receive some hints as to how they were laid down in ancient oceans.
Warm-blooded and air-breathing animals preserve a nearly uniform temperature of their bodies, whether they live in tropical or polar regions, but it is otherwise with cold-blooded marine organisms. In the case of some fish a temperature of the body has been recorded some 8° F. above that of the water in which they were living, but we may take it that as a rule the temperature of the body is very slightly above the temperature of the water in which the fish and marine invertebrates live. Indeed, in the case of Copepods and Amphipods, for example, the temperature of the body is not likely to be more than a fraction of a degree above that of the water. We may then have two animals of quite similar zoological structure, belonging, indeed, to the same genus or family, one living all its life in tropical waters at a temperature of about 80° F., the other living all its life in polar waters at a temperature of 30° F., i.e. below the freezing point of fresh water. What is likely to be the life-history of these two animals? Apparently all metabolic changes, such as digestion, [[p. 16]] assimilation, reproduction, must take place extremely slowly in the above-mentioned cold water, and with relatively great rapidity in the warm water. In the tropics an organism may pass through its whole life-cycle in a few hours, days, or weeks, whereas a quite similar organism in the polar waters may take years to pass through a similar life-cycle. May we not in this way account for the great abundance of individuals belonging to few genera and species in polar seas, and for the relatively few individuals belonging to numerous genera and species, as well as for the many pelagic larvæ, in tropical seas?
It is a well-known fact that the velocity of reactions in chemistry is affected by a change in the temperature to which the reagents are exposed. It has been found that a rise of 10° C. in the temperature increases the velocity of most reactions from two to three fold, which means that a reaction taking place in a few minutes at 80° F. may last a whole year when it takes place at the freezing-point. The activity of enzymes, or catalytic agents, in the organism is controlled in this way by temperature; the optimum temperature of most of them is about the blood heat. There is no enzyme known having an optimum temperature so low as the freezing-point, at which temperature most polar invertebrates live and carry on their functions; possibly such enzymes may exist. My former assistant, Mr. James Murray, who is now on the Antarctic Continent with Lieutenant Shackleton's expedition, hopes to carry on some experiments with marine invertebrates, with the view of showing their total metabolism over a considerable length of time during the Antarctic winter. Should these experiments be successful, they will be valuable for comparison with similar observations in warmer seas.
It is sometimes stated that the pelagic life of the polar or colder oceans has been found by some recent expeditions to be everywhere more abundant than in the tropical seas. Such statements must be received with caution. Tow-nets are, it is admitted, likely to capture more vegetable plankton in colder waters, because of their large size and their filamentous processes, while, on the other hand, the small calcareous algæ, like the coccospheres and rhabdospheres, may wholly escape capture in the tropical seas. The whole surface of the ocean within the photic zone appears to be one vast floating meadow, where myriads of minute siliceous, calcareous, and other algæ are ever busy converting--under the influence of chlorophyll and of the sun's rays--inorganic into organic compounds. This is the primary source of food for all animals living on the surface, in the intermediate waters, and on the floor of the ocean. Deep-sea animals live largely by eating the surface layers of the marine deposits, containing organic matter which has fallen from the surface waters. Probably most stratified deposits have in this way passed through the intestines of marine [[p. 17]] animals before they were consolidated into rock. It is, I think, admitted that the enormous coral-reefs and huge deposits of globigerina and pteropod oozes in the very warmest oceans suggest a very lively metabolic circulation of matter (Stoffwechsel). It remains for those who maintain that the marine fauna and flora increase in abundance as we pass from equator to poles to explain the abundance of carbonate of lime structures in the tropics.
Preliminary experiments seem to indicate that water from the coral-reef atolls of the Louisiade Archipelago contain nearly twice as much ammoniacal salts as water from the North Atlantic, and nearly three times as much as water from the German Ocean, and that, on the other hand, albuminoid ammonia is more abundant in cold than in warm water. The inference from this would be that cold currents bear foodstuffs for marine algæ to the tropics.
It would appear that the carbonate of ammonia arising from the decomposition of animal-products in presence of sulphate of lime in sea-water becomes carbonate of lime and sulphate of ammonia. The sulphate of ammonia is in turn absorbed by the marine flora which forms the food of the marine fauna, and is resolved into nitrates and free nitrogen, possibly under the action of denitrifying bacteria. In questions of this kind it seems necessary to take into account the velocity of physiological processes, as well as the quantity of plankton captured in tow-nets. It is to be hoped that this new Journal will contribute greatly towards the elucidation of the many unsolved oceanographical problems.