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.
There are distinct zones of altitude of the different types of aquatic vegetation. The maritime vegetation or marine plants grow at sea-level. It is important to remember that inland water finds its own level and flows, unless enclosed as in a pond, lake, loch, lough, or tarn, regularly by a series of stages to the sea from the highest points to lower levels.
This gives a division into highland and lowland aquatic vegetation. In the highland loch are found such plants as Awlwort, Alternateflowered Water Milfoil, Starwort, Lobelia, Bladderwort, Shoreweed, various types of Pondweeds, Spike Rush, Floating Bulrush, Pillwort, etc, and White Water Lily, Floating Marshwort, Amphibious Knot Grass, and other Pondweeds are floating types; whilst in the Reed swamp grow Bog Bean, Floating Bur Reed, Common Spike Rush, Bulrush, Prickly Twig Rush, Sedges, Reed, Manna Grass, etc.
There are the upland quickly-flowing rivers, which again are poor as a rule in mineral salts, further differentiated by relative altitude. The lowland rivers are more or less stagnant or slow-flowing, and are richer in mineral salts. They form in the first case tracts like the Norfolk broads, very little above sea-level. The slow-flowing rivers are intermediate in altitude, and their vegetation differs from the last, being much richer in the forms of plant life.
Aquatic vegetation is distinguished by the immersion of the plants entirely, or nearly so, in water. The land vegetation forms the opposite extreme, for water there, except in low-lying areas, does not lie near the surface except on clay soils. Thus the relative lie of the ground, and the porous or non-porous character of the soil, determines largely the gradation from a dry to a wet meadow. Between these two types - aquatic and land plants - lies an intermediate series, the Hygrophiles or marsh plants and wet meadow types, into which the former may merge.
Thus an aquatic formation may, through the marginal reed swamp, become a marsh formation laterally; the latter may also, where peat is formed to a considerable depth, and the lime salts are gradually lost, become a bog or fen. The aquatic and fen or bog formations on a large scale are largely transitional in East Anglia, and the preference of each for alkaline water renders such transitions easy. Highland bogs, however, are poor in lime and richer in humous acids.
Temporarily a dry season or drought, especially in the case of pools or ponds, has a great effect upon the vegetation. Pools may dry up, as they have done almost everywhere, and a relic of a bog flora, with Sundew or Butterwort, disappear for ever. The felling of trees may artificially cause desiccation over a wide area, and conduce to the disappearance of marsh or aquatic plants in much the same way; whilst on the contrary, the destruction of a forest naturally and the water-logging of the area may give rise, as has occurred over and over again in upland areas, to a bog. But by far the most potent factor in disturbing the stability of aquatic formations is the drainage of land. It may, of course, incidentally produce fresh aquatic formations, as ditches and drains (and artificial canals are of the same type). But in very low-lying areas the formation of dikes or drains, and the withdrawal of large sheets of water and their conduction into definite and restricted areas, destroys aquatic vegetation. This is what has occurred in the Fens, causing the disappearance of wide areas of aquatic, marsh, and fen vegetation.