Factors Controlling the Distribution of Freshwater Macrophytes with Particular Reference to the Lochs of Scotland

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Abstract
The commonest Scottish fresh water has low concentrations of dissolved HCO3 and CO_3 (alkalinity less than 0.3 m-eq HCO3 per litre), is brown, and overlies the shores of glacial rock basins. These facts and the principles of morphometry and of thermal stratification explain why the most widespread substrata in the photic zones are skeletal soils and brown muds. In these nutrient-poor habitats, and without accumulation of inorganic sediment, a slow rate of vegetation change may be expected; from photographic evidence cited, in most localities no succession of emergent and floating-leaved vegetation has occurred in 50 years. The hydrosere has usually remained static. The few known examples of vegetation change seem to be correlated with the less common accumulation of inorganic sediments. It is suggested that the water chemistry of a lake determines whether a species grows in it, if propagules of the species, and space, are available; two plant associations with some mutually exclusive species are noted respectively in lochs of high, and low, alkalinity. Variations in [ Cl-], conductivity, concentrations of metal ions, and alkalinity of waters are discussed. The distribution in Scotland of named submerged macrophytes is partly explained by the ability of some species, in vitro, to photosynthesize in bicarbonate or in free CO2 solutions, of others to photosynthesize only in free CO2. Within individual lochs, the distribution of macrophytes can be controlled directly by turbulence, or indirectly through its effects on substratum, or directly by substratum. In low-alkaline waters a brown mud may, through contact exchange, provide nutrients for rooted macrophytes but at present no proof exists that the distribution of any species is causally connected with this or any other mud. Concerning zonation, gradients of increasing depth of water in different lochs are correlated with recurrent series of fen and swamp sociations and with the sequence emergent, floating-leaved and submerged broad-leaved, dominants. Although aspect and competition may affect the depth at which a species grows in a particular loch, its overall depth range is probably controlled by light.

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