Sometimes the sea bottom slopes steeply downwards just outside the littoral zone : this is the case often where the coast line is bold and there are high cliffs, and it is particularly the case round oceanic islands and coral reefs. (In the latter regions the sea bottom is sometimes said to descend "precipitously," though this description exaggerates the slope.) In many parts of the world, however, there are extensive flat and shallow sea bottoms in the neighbourhood of the land, and here the depths may vary only between about io and 20 fathoms. The North sea is such a region. This shallow-water zone includes Forbes' Laminarian region and part of his Coralline one. The physical conditions must be noted : the water being shallow, sun light may penetrate to the bottom; rivers and streams spread over the zone; there are usually rapid tidal streams and currents which mix the water and also distribute planktonic organisms (see PLANKTON) ; there is a certain amount of shifting of the bottom deposits and there are greater annual temperature variations than in the deeper water far from the land.
All these conditions are favourable for marine life; the pene tration of sunlight to the sea bottom enables the Algae to live and reproduce there while being also advantageous to the Diatoms and other unicellular plants that exist in the plankton at all levels down to the bottom; the fresh water that enters the sea contains organic matter coming from land that is covered with vegetation and it also contains silica and phosphates, which are materials that are indispensable for plant life in the sea ; the dilution is important, for a degree of salinity that is rather less than that of open sea water is the most favourable condition for marine life ; the rapid tidal streams distribute these indispensable nutritive substances over the whole area and prevent stagnation, and they also carry the spores, eggs and larvae of fixed plants and animals to regions where they have opportunities of settling down in suitable en vironments; a certain amount of disturbance of the bottom de posits of sand and mud is favourable to life, for decomposed organic materials are removed and distributed in the sea, where they become nutritive substances for the plants; finally, the greater range of temperature variation seems to be a stimulus to reproduction and growth of most kinds of organisms.
It is on the shallow-water region, with its limited depths and its flat and smooth, sandy and muddy bottom deposits that the sea fisheries proceed. All kinds of fishing operations are easier on these shallow grounds and the fish-life is always more abundant here than it is offshore. But, above all, the sea bottom and the overlying water from low tide mark out to a depth of io to 20 fathoms is the supreme region of organic production. Much more life comes into existence per unit area here than anywhere else in the seas and oceans.
The very shallow sea bottoms—from low tide level out to about five fathoms are the nurseries of the marine faunas. The great majority of fishes and invertebrates are reared up to the later juvenescent stages here. Most of the common fishes spawn at some distance from the land and over deeper water than five fathoms, but the eggs and the resulting larvae nearly always drift. in towards the shallowest parts of the sea and it is, indeed, prob able that the spawning grounds have been determined (by the process of natural selection) so that they may be placed in such positions that the eggs and larvae produced may be carried by the resultant tidal streams and the prevailing wind-drifts on to the shallow water zone near the land. Spawning nearly always occurs in the spring and early summer, so that the fish larvae arrive on the nursery grounds at the time of increasing and maximal sea temperature and this accelerates their further development. The
strong illumination of the bottom and the presence of nutritive substances coming down from the land encourage the reproduction of the Algae so that vast numbers of spores are liberated into the sea. All unicellular Algae, the Diatoms and the Peridinians are favoured in the same way. These spores and unicellular organ isms may be eaten by the larval fishes, or they may be eaten by small crustaceans and molluscs, which are in their turn the foods of the little fishes. At the same time, and for the same reasons, a great number of species of marine invertebrates—crustaceans, molluscs, worms and echinoderms chiefly—abound on the shallow water zone.
It is here that the evolution of most species of marine animals (and all marine plants) has taken place and it is probable that from the shallow water zone, all other regions of the seas and oceans and lands have been populated during the geological period.
It is usual to distinguish between marine plants and animals but the best division is into "producers" and "consumers." Consider the nutritive processes of such animals as fishes, Crustacea, Mollusca, etc.—they are usually carnivorous, eating other smaller animals, but many are herbivorous, living on Diatoms, unicellular Algae, or even the larger marine plants. The fleshy parts of these food-organisms are digested and assimilated and the animal thus obtains the energy necessary for its existence. When the proteins, carbohydrates and fats, of which the food con sists, have been utilized in the production of energy the elements of these substances are excreted as nitrogenous residues (such as urea and uric acid), carbonic acid and water. These excreted sub stances cannot be utilized as food by animals. Obviously, then, animals can only subsist on the fleshy materials of other animals and plants. If there were only animals in the sea (or on the land) they would eat each other until there was only one individual left —which would then die of starvation ! This is why animals are called "consumers." The plants or plant-like organisms can, however, utilize as food just those nitrogenous residues and carbonic acid that are ex creted by animals. Perhaps the micro-organisms called Bacteria may have to transform the nitrogenous residues into ammonia salts, or salts of nitric acid, before the plants can utilize them, but this is not always necessary. From water, carbonic acid, simple inorganic nitrogenous substances, and traces of other salts the plants can build up in their tissues protein, carbohydrates and fat, which substances can then be utilized as food by herbivorous ani mals. The carnivores feed upon other carnivores, or upon the herbivores. The latter feed on the plants. All animal life therefore depends, in the long run, upon plant life : "All flesh is grass." This is why the plants are called "producers." But for plant production light is necessary, for it is only by utilizing the energy of solar radiation that the marine plants are able to convert simple inorganic chemical substances into proteins, carbohydrates and fats. This is why the shallow water zone is the great region of organic production—because it is lit up to a suffi cient degree at all levels even to the bottom. It is true that the superficial layers of the ocean are also well-illuminated but, far from the land, there is not the same abundant supply of mineral food stuffs for the pelagic plants, therefore the degree of produc tion is far less there than it is in the shallow water zone. The sum of the processes by which the producers form carbohydrate ma terials from carbon dioxide and water is called photosynthesis. See PLANTS : Photosynthesis.