This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Aquatic vegetation shows a well-marked zona-tion or arrangement of different types in zones or parallel bands. This applies whatever be the character of the aquatic habitat - lake, river, etc. - though there may be an absence of one or more types, and the one may take the place of the other, showing the manner in which aquatic vegetation may invade the land, or vice versa. Of freshwater types of formation there are four main divisions. These are based upon the relative force of the current, or the richness or otherwise of the water in mineral salts.
Thus there are slow-flowing rivers which are rich in mineral salts (alkaline), and these are usually lowland. Then there are stagnant or foul waters, which are usually devoid of flowering plants, and are colonized by Blue-green Alga;, as sewage waters. Some nearly stagnant waters may be rich in lime, and contain a characteristic phanerogamic flora. Where the water is poor in mineral salts, as in lakes and tarns in upland regions, usually on siliceous rocks, certain rare and local types, as Quillwort, Lobelia, Shore Weed, etc., grow. These upland lakes are usually moorland highland lakes. There are also quickly-flowing streams on hill slopes, which may either be rich or poor in lime, and each has its own particular flora.
In all these cases the plants show a zonal arrangement. In the middle grow the plants that float or have floating leaves, as Duckweed, Water Buttercup. Around the central zone grow half-submerged or totally submerged types, as Pondweed, Hornwort, etc.; whilst in what is called the third or Reed Swamp association grow the Reeds, Rushes, Sedges, Purple Loosestrife, etc, which are only half-submerged.
Water preserves a more or less uniform temperature, and is less modified than land by the alternation of summer and winter, so that there are no aquatic Tropophvtes, in the same sense at any rate as with land plants. The conditions of light are more or less uniform in freshwater formations, but are influenced by the weather in the shape of clouds, fog, and extremes of sunshine.
Water may vary in respect of its constituent salts, and be either in general salt or fresh water. There is also an intermediate type which is neither saline nor fresh, but brackish, as along the coast in salt marshes or estuari<s.
Fresh water may be hard or soft, poor or rich in mineral salts. The water of upland lakes is usually poor in mineral salts, but peat bogs are usually charged with humous acids and acid in reaction. Other upland waters situated upon siliceous sandy rocks are poor in mineral salts, and clear or pure. It is in such pools or lakes that Desmids are especially abundant. Diatoms, which require water with a good deal of mineral salts (alkaline), are found in more lowland situations.
Flowering plants are equally susceptible to differences in the water. The Stoneworts or Charas luxuriate in water highly charged with carbonate of lime, and help to precipitate it, as do some mosses.
Water may also vary in temperature, but in the British Isles there is little variation in this respect. Sea-water, however, is more constant in temperature than other waters, and is seldom frozen. The water of lowland ponds and lakes is not so readily frozen as that of highland lakes, etc. The depth of the water naturally influences the temperature, hence the zonal arrangements of plants, which are connected with the individual thermal constant.
The state of motion of the water again is important. When motionless it may be actually foul, or merely stagnant, and in the first case few flowering plants will live in it. But some plants require stagnant, motionless water, as Bladderwort. Naturally such waters are relatively rich in mineral salts, and so are those that move but slowly, in which the pondweeds luxuriate. The quickly-moving waters are far less rich in salts, and so are the upland lakes and tarns.
Much depends upon whether water is of aerial origin, or telluric, or underground. The aquatic element, beneficial as it is in many respects, however renders transpiration difficult, and the supply of oxygen is small except in running water; hence the distinctions between aquatic associations due to the aerating power of water.
Then, again, light is impeded, and no light penetrates at the greater depths. The character of the light is also different, for red and yellow rays are absorbed, and the light alters from white at the surface to a green colour below, hence the apparent colour of clear water is green, or, as in the case of the still deeper sea, blue. Hence the colour types of marine algae, red, brown, and green, the green colour in the first two being masked by colouring matter, anthocyan, etc.