THE DISTRIBUTION OF ANIMALS IN INLAND WATERS While in many respects conditions of life in the ocean are equa ble, owing to the general intermingling of the waters, those in inland waters show great differences in the various regions. This is due to the splitting up of these waters into many larger or smaller sections, and is reflected in the variety of their populations.
The chemical composition of inland waters is very changeable, particularly as regards lime salts. In granite and porphyry regions it may be as little as 24mg. per litre, while in limestone regions it may rise to 42omg. per litre. Some animals, such as the water-flea Holopedium gibberum, and the pearl mussel (Margaritana mar garitifera) dislike them. Others require them, e.g., most molluscs. The salinity also is very varied. In the Rhine, the water contains o• 14g. NaC1 per litre. When the salinity rises above o•3g. we speak of "salt" water. In the salt marshes of Lorraine, the water contains between 5o and i log. NaC1 per litre; in the Dead sea, as much as 237.5g. No living organism can exist in such a high salinity. In bog-water, the large quantity of humus is unfavoura ble to life.
The amount of oxygen in inland waters varies with time and place. It is highest in the eddying waters of mountain torrents, and in shallow ponds with dense plant growth when exposed to the sun, as this causes the plants to give off much oxygen. On the other hand, in late summer, the oxygen at the bottom of some lakes is completely used up, so that animals particularly requiring it, such as some fishes (Coregonus), cannot live there.
The temperature of inland waters undergoes much greater variations than that of the sea. Inland waters are usually shallow ; they are seldom more than 30o metres deep, while the majority are not deeper than i o metres. In ponds and pools, the depth is considerably less. There is, therefore, a high ratio of surface to volume ; heating and cooling take place rapidly. In temperate regions, a constant temperature is found only in springs welling up from great depths, in deep parts of lakes and in waters in caves; cold-stenothermal animals are found only in such places.
Light does not penetrate so deeply in inland waters as in the sea, on account of their more turbid condition. Usually, it pene trates only 3o-4o metres; the lower limit of plant life is often at only 7 metres or less. In general, shallow waters have more plant life and therefore more animal life than deep ones.
The movements of the water are particularly important to aquatic animals. Flowing water and standing water make quite different demands on their inhabitants, and have therefore differ ent types of population. They differ in chemical composition, thermal conditions, depth and extent. Rivers are almost always fresh, the flow preventing accumulation of matter in solution. Flowing water takes longer to warm, and cools more rapidly than standing water, and the difference of temperature between the surface and the depths is less on account of intermingling. There are, however, intermediate conditions. The rapidity of the cur rent depends on the fall of the land. The Rhine, at its source, has a fall of 2.5% (2.5 metres in ioo) ; the Upper Rhine from Basle to Bingen about o.o5%, the Lower Rhine, o•o12%. The Volga, in its whole course, has a fall of only o.007%, and the Lower Amazon only o.0019%; they show conditions resembling those of standing water.
The velocity of river currents may be classed as "below average," "average" and "above average," according to the degree of the fall. In sluggish rivers (below aver age) erosion is minimal, and deposition of sediment maximal. Fine mud sinks down and forms a nutritious ooze on the bottom.
This supports many detritus-eaters, such as worms, molluscs and insect larvae. At the average rate of flow, erosion and sedi mentation maintain a balance, and the bottom is covered with gravel. In rapid streams (above average) erosion prevails, and deposition of sediment is minimal ; the bottom is formed of large stones, which, by their movements, would crush to pieces any living organisms among them. The inhabitants must be adapted to this movement of the water. The dif ferent stretches of rivers harbour dif ferent animal populations. As regards fishes, rivers, from mouth to source, have been divided into the region of bream, of barbel, of grayling and of trout. Sharp delimitation, however, is not possible. In slowly flowing waters various kinds of fishes may be present, good swimmers and poor, with rounded bodies or with flat, provided the temperature and amount of oxygen are suitable. The stronger the current, the greater the swimming powers a fish must possess to resist it. In such places, we find fishes predominating which are round in trans verse section, and so are not spun round on their axis by the whirling of the waters (fig. 7). For this reason, the number of species of fish decreases as we approach the source; the continually increasing demands made by currents and falling temperature have a selective influence. Fishes of 49 species are found in the Rhine; of these 41 are found in Holland, 33 in the Upper Rhine below the Falls, 25 above the Falls; at 70o metres above sea-level 11 species are found, at 190o metres three species, the trout (Salmo fario), minnow (Pjioxin1cs laevis), and loach (Cobitis barbatula). Above 190o metres the shallow alpine streams contain no fish.
The mountain stream affords the best instance of a charac teristic fauna. The animals here take on a certain general stamp, since none can live except those able to accommodate themselves to the severe conditions of temperature and current. All have adaptations which prevent them being swept away by the current. Some are flat, and creep under stones (Gammarus, insect larvae), others adhere firmly by a broad sole or sucker (turbellarian worms, gastropods), or with special kinds of suckers (larvae of the gnat Blepharocera, fishes and tadpoles of tropical mountain streams), others spin threads which form a strong attachment to the bottom (larvae of the sandfly Melusina, pupae of mayflies). They are often flat and depressed to offer the least possible hold to the current. Since they are not strong swimmers, their power of movement is limited compared with that of related forms; e.g., the water-mites of mountain streams do not swim and have limbs without swimming-bristles (fig. 8). The inhabitants of mountain brooks are generally eurythermal, or cold-stenothermal. Animal communities which require a constant low temperature are found chiefly in springs (the turbellarian Planaria alpina, the gastropod Bythinella dunkeri) .
In shallow basins circulation of the water takes place through winds, and thus the lower layers are aerated; as in Lake Balaton in Hungary. In deeper lakes, where no such complete mingling of the water is possible, the lower layers are aerated by convection currents set up by the cooling of the sur face water in the cold season. In summer in lakes rich in organic life much oxygen is used up in the disintegration of the dead organisms which sink to the bottom, so that the amount of oxygen in the deep layers may be scanty, or absent. This ex cludes many animals from such depths, and gives a definite character to the com position of the fauna.
Larger basins of such depth that the greater part of the bottom is free from vegetation are described as lakes ; in con trast to these are the ponds, pools and puddles, termed collectively small water basins. In lakes we distinguish a shore region (littoral), a deep water zone and a region of open water.
The outer portion of the littoral, which in summer is occasionally left dry, is poor in life. On the other hand, the deeper littoral region where there are plants for food and hiding places is the richest in living organisms.
The open water is populated by plankton, and by fishes which feed on it, e.g., Alburnus, Coregonus. The bottom fauna is varied. Three types of lakes are distinguished, eutrophic, oligotrophic and dystrophic. The eutrophic type, with flat shores overgrown with vegetation, and with a rich plankton, has its deep layers filled up with petrifying ooze composed of the disintegrating bodies of plankton organisms or detritus. Only animals able to make use of the smallest quantities of oxygen are able to live there, e.g., oligo chaetes (Tubificidae), larvae of the harlequin-fly Chironomus. Both these have haemoglobin in their blood, apparently enabling them to use fully whatever oxygen is present. In oligotrophic lakes, with steep banks and little vegetation, the plankton is scanty, and the bottom therefore has less ooze; other insect larvae are found here (Tanytarsus larvae). In spite of the greater sup ply of oxygen, the number of organisms is less because of the smaller amount of food. Dystrophic lakes are those with bog water, in which the acidity caused by humus is unfavourable, and the deficiency in lime also excludes many animals. The plankton here is chiefly animal, and consists mainly of rotifers and small crustaceans, which feed on colloidal matter in the humus.
In ponds and pools the fauna is similar to that of the over grown littoral zone of eutrophic lakes. Decaying vegetable matter is present in sufficient quantity to provide food and oxygen for the growth of plants and only the great variations in tempera ture are unfavourable. In pools and puddles there is often a rich fauna of small rotifers, crusta ceans and insect larvae. In waters liable to dry up periodi cally a particularly characteristic fauna is found. In temperate regions these waters are small, but in subtropical steppe areas (South African pans) they are sometimes of much greater extent. In such places animals must pass through stages of development quickly, and therefore must be small ; and they need some protection against desiccation. Many produce hard shelled resting ova (Hydra, rotifers, Cladocera) or spores (gem mules of sponges, statoblasts of Bryozoa), others are able to sur round themselves by a capsule formed by a glandular secretion (the small annelid Aeolosoma, some copepods), and some burrow into the ooze and form a capsule of mud around themselves (phyllopod crustaceans, fishes such as Protopterus). Lastly, there are animals which can dry up into a cyst without losing their power of living, such as many rotifers (Philodina) and Nematoda (thread worms). These groups can live also in the mossy growth on rocks and tree trunks.
In salt lakes and pools the number of species decreases with increasing salinity. Those able to endure the greatest salinity are the brine-shrimp (Artemia salina), and a number of fly larvae, but even the brine-shrimp suffers changes with increasing salinity, the bristles become stunted, the size of the animal decreases, it be comes enfeebled, and finally disappears. In the Dead sea, in Palestine, the salinity is so high that all life is absent.