ity proceeded. If it occurs uniformly overthe sea to a depth of only
one metre it leads to a production of about 6 tons of carbohydrate
per sq. km. of sea.
Following the great spring production of plant substance there is, therefore, a summer outburst of animal life. Following that again is a less well-marked maximum of phyto-plankton in the autumn, oc- curring just after the period of highest sea temperature. The temperature then falls rapidly and there is a gradual slackening in the production of organic substance and a general lethargy of life. The plankton, both animal and vegetable, attains its minimal values and many of the larger forms of animal life pass into a kind of con- dition of hibernation.
The Transport of Essential Food Substances. First of all we con- sider inorganically combined nitrogen (as nitrates and nitrites chief- ly), since upon this depends all the life of the ocean. The concen- tration of these substances is least in the warm equatorial seas and greatest near the poles. The temperature is, however, only an in- direct cause of this variation and the direct cause is now known to be the activity of the nitrogen-bacteria. The nitrogen-bacteria that concern us here are of two main categories: (i) those that assimilate elementary nitrogen from its solution in sea-water, building it up into combination with carbohydrate as proteid; and (2) those that break down nitrate into nitrite, nitrite into ammonia and ammonia into elementary nitrogen. Two antagonistic processes proceed simultaneously, the fixation of atmospheric nitrogen and the reverse change, and either process is accelerated by an increase and retarded by a decrease in temperature. It is maintained by Brandt and others belonging to the Kiel school of marine biologists that the process of denitrification is, on the whole, more significant in the sea than that of nitrogen-fixation.
If this is admitted the poverty of tropical sea-water in mineral nitrogen compounds is explained by the higher temperature, which accelerates the activity of denitrifying bacteria. Since there is less of the indispensable food material in the warmer seas there is, therefore, less phyto-plankton. This is really the case, for all obser- vations show that the Antarctic and Arctic ice-bound seas are enor- mously rich in diatom life when compared with temperate and tropi- cal regions: the great Antarctic zone of sea-bottom deposit, in which the skeletons of diatoms predominate, covers some ten millions of square miles. The relative abundance of nitrates and nitrites at the bottom of deep oceans as compared with the surface can be explained in the same way, for at the bottom the temperature is about zero Centigrade and the activities of the denitrifying bacteria are practically suspended. The dead bodies of organisms fall down from the surface and are slowly resolved into products of putrefac- tion, which gradually pass into the mineral forms, nitrates, carbonic acid and ash. The bottom water is relatively rich in these substances as well as in decaying organic matter, and would become progres- sively richer but for the slow drift towards the equator and the well- ing-up of bottom water to the surface in these latitudes.
It would seem that, on the whole, nitrogen compounds in the ocean (whether existing in the organic or inorganic forms) remain constant in amount. Nitrogen is always being synthesized from the atmosphere (by plants, and by electrical discharges which combine nitrogen and oxygen), and this combined nitrogen is either utilized by land organisms or is washed down into the sea in the water of the rivers. In the end much inorganic nitrogen salts must be added to the sea both in the above way and as the result of the putrefaction of the dead substance of terrestrial animals and plants.
As a general rule the sands in the immediate vicinity of the shore contain organic matter resulting from land drainage (particularly near great centres of human population) and from the remains of dead plant and animal organisms. At the same time the denudation of rocks sets free iron compounds which dissolve in the sea to a slight extent and permeate the littoral sands which contain organic matter. The putrefaction of the latter sets free sulphuretted hydrogen, which then acts on the iron compounds, precipitating ferrous sulphide. The latter discolours the sand and so one finds, round the coast and towards the upper margin of the zone between high- and low-water marks, an under layer of black sand formed in this way. On the surface, where the sand is bathed by the tidal water, the ferrous sulphide becomes oxidized and the sand is bleached, but underneath it is dense black or grey, as the case may be.
A considerable degree of denitrification must, therefore, take place in the ocean, for the concentration of combined nitrogen is always excessively small. The regional differences, as we have seen, can oe explained by the regional difference of temperature.
The quantities of oxygen and carbonic acid in the sea are nearly constant so far as we can determine. The former gas is continually being evolved by the plants and absorbed by the animals, and pre- cisely the reverse actions occur in the case of carbonic acid. Fur- ther, the ocean and the atmosphere stand in equilibrium with each other; if there is excess of carbonic acid anywhere in the sea it is absorbed by the atmosphere and vice versa, and so also with the oxygen. Differences of temperature and atmospheric pressure must disturb this equilibrium, but the movements of both ocean and atmos- phere lead to a high degree of uniformity in both envelopes as regards their gaseous constitutions.
Silica is continually being added to the ocean. Land masses are denuded and minerals containing silicates are carried down to the
sea as sediments. The coarser particles of the sediments are depos- ited near the shore as gravels, sand and muds, but the very fine parti- cles remain in suspension in the colloidal form, and some of this may be acted upon by marine bacteria or (it is surmised) even utilized by diatoms as a source of silica. The silica, in the form of diatom or radiolarian skeletons, is eventually deposited on the ocean floor after the death of the organisms. Most of the fine colloidal clay is, how- ever, deposited as river-sludges when the fresh water carrying it mixes with denser sea-water. The colloidal particles are electrically charged and become discharged by the ions of sodium, magnesium and calcium present in the sea-water. This "coagulation" leads to the formation of the river-sludges that form deltas.
Lime is transported in solution as sulphate and bicarbonate, both of which salts are soluble to some extent in water. The water of the ocean is usually nearly saturated with calcium salts, which must con- tinually be removed since they are always being added in the water brought down from the land. Lime is, in fact, absorbed to an enor- mous extent by fishes, molluscs, Crustacea, calcareous algse and sponges, starfishes, sea-urchins and feather stars, many polyzoa and a multitude of protozoa (mainly the foraminifera). All these ani- mals have calcareous skeletons or shells of some form and they secrete the calcium from its solution as sulphate, converting it into carbonate. Some unicellular organisms are said to segregate salts of strontium from sea-water.
Coral Formations. Coral reefs remove calcium from solution in the sea on a vast scale. During recent years the controversies with regard to the modes of formation of these structures have entered on a new phase. The theories of Darwin, Agassiz, Dana, Semper, Murray and others had led to apparently interminable discussion, and the great boring experiments at Funafuti atoll, which were expected to be crucial, gave results that backed both the rival theories of Darwin and Murray. On the other hand, Wayland Vaughan (see Annual Report of the Smithsonian Institution, 1917) has shown clearly that the problem is essen- tially a biochemical one and may finally be solved by the methods of the latter science.
It is not at all certain that the masses on which coral reefs are built consist entirely of the remains of the skeletons of reef-forming organisms and it is probable that chemically precipitated carbonate of lime predominates. The water in shallow seas, off the shores of is- lands or in lagoons, is saturated with calcium bicarbonate and if the amount of carbonic acid in solution be reduced by any means, normal carbonate must be precipitated. Therefore a reduction in the partial pressure of the gas in the atmosphere, or a rise in the temperature of the water, or a violent agitation of the sea itself, will lead to precipi- tation of calcium carbonate. Evaporation of the water and anything that lowers the hydrogen-ion concentration have the same effect.
Therefore an increase in photosynthesis caused by the multipli- cation of plant microorganisms will lead to the precipitation of cal- cium carbonate, for carbonic acid will be withdrawn from solution to take part in carbohydrate synthesis by the plants. Denitrifying bacteria will raise the alkalinity (or reduce the H-ion concentration) by forming ammonia, which will combine with the carbonic acid in solution and so throw down normal carbonate of lime. Drew found as many as 160 millions of denitrifying bacteria per c.c. of sea- water on the W. side of Andros I. in the Bahamas. There are, therefore, a number of agencies, all of which operate in shoal waters on the lee side of islands, or in shallow lagoons in such regions as the Bahamas, and the result of all these is to throw down calcium carbonate from solution in sea-water as minute needle-shaped crystals or little balls of aragonite. Such material, it is suspected, may form the massive bases on which barrier or fringing or atoll reefs are built up.
The "Glacial Control" Theory. Interesting speculations as to the periods of origin of great coral reefs have been made by Wayland Vaughan, Andrews and Daly and Humphreys. (The causes or con- ditions of glaciation, it may be noted here, are no better known than in 1910. It has been suggested, however, that a prolonged period of volcanic activity may reduce the air temperature to a marked degree by throwing large quantities of dust into the atmosphere: this will act by preventing the penetration of solar radiation.) During a period of prolonged glaciation water becomes withdrawn from the ocean, for rainfall goes to form solid ice-caps that accumulate upon polar and continental land areas. Daly estimates that the maximum lowering of ocean level due to this cause would only amount to 36 fathoms, but even that would be the cause of very marked geological effects. In Pleistocene times, then, when there were prolonged gla- cial ages, the sea-level was lowered and at the same time there was a reduction in sea temperature, so that the rate of reproduction of the coral polypes, and so the growth of reefs, was diminished. The protection of the shore may therefore have been decreased, with the result of increased land erosion and the formation of extensive shallow submarine plateaux. When the warmer interglacial periods recurred the polar and continental ice-caps melted and the sea-level became raised again that is, there was submergence of the eroded plateaux formed as indicated above. Corals would now grow luxuriantly in these shallow coastal waters of increasing temperature, forming reefs
