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.
Enclosed tracts of water are as a rule still waters. But there may be an inflowing and outflowing stream, regulated frequently by a sluice. The disturbance of such waters is effected as a rule solely by the wind, or by springs from beneath or at the sides. Such tracts of water are normally sweet, though stagnant, but may be rendered foul by the percolation of sewage, by cattle, by waterfowl, or by the decomposition of leaves in the case of lakes, etc, in wooded areas.
Ponds when stagnant may have a floating association of Lesser Duckweed only, with marginal wet-soil types, Rushes, Sedges. A clearer pool may have either Stonewort or Nitella on the bottom, Canadian Waterweed, Pondweeds, Water Lily, Water Buttercup, Manna Grass, Horned Pondweed, Celery-leaved Crowfoot, Amphibious Knotgrass, and be lined with Sedges, Rushes, Skullcap, etc.
The zonation typical of the aquatic vegetation is in a lake, pond, etc., cyclic or elliptic. A highland lake as mentioned contains few types, whilst a lowland lake richer in mineral constituents contains, in addition to the above, such plants as Hippuris (Mare's-tail), Horn-wort, Milfoil, Bladderwort, Water Soldier. Starwort, etc, submerged or nearly submerged; whilst floating forms include also Frogbit, Arrowhead, etc.; and in the marginal reed swamp also, Purple Loosestrife, Great Hairy Willow Herb, Creeping Jenny, Marsh Woundwort, Great Water Dock, Reed Mace, Bulrush, and Reeds.
Reservoirs are artificial or natural, and are usually of lowland type with little or no peat, but occasionally along the margin, Shore-weed, Pillwort, etc, may be found. Hippuris, or Mare's-tail, is common in such aquatic habitats.
The aquatic vegetation of rivers differs little from that of streams, except in the greater width of the former, and the usually greater depth of the water. There are quickly-flowing rivers poor in mineral salts, and slow-flowing rivers richer in mineral salts in solution. The former are upland and the water hard as a rule, the latter lowland and the water soft.
Some peculiar types are more at home in slow-flowing water than in still waters; but they exhibit different adaptations, having ribbon-shaped leaves in this case, as in the Arrowhead; and so does the common Pond-weed of the pond in running water. Here also grow Ranunculus circinatus, Water Dropwort, Canadian Waterweed, Bur Reed, Shining Pondweed, Perfoliate Pondweed, Bulrush, and at the margin Brooklime, Water Cress, Scorpion Grass, Marshwort, Great Hairy Willow Herb, Starwort, etc. Few floating-leaved types occur except where the marginal vegetation deflects the course and force of the current, as Water Lily, Duckweed, Amphibious Knotgrass. In the reed swamp many plants occur, as Meadow Rue, Meadow Sweet, Water Betony, Sweet Flag, Bur Reed, etc. Such plants occur also in canals.
In swift-flowing streams with no lime-salts in solution occur Ranunculus Lenormandi and R. hederaceus, with Manna Grass, Starwort, etc.; and in those with lime salts in solution other types occur.