Centred on Loch Druidibeg, macrophyte vegetation was surveyed in and around 30 lochs in northern South Uist. Alkalinity ranges from 0·014 to 2·33 m.-equiv −1, conductivity (25°C) from 100 to 33,900 μs cm−1, chlorinity from 0·66 m.-equiv −1 upwards. Lochs are classed broadly as non-calcareous (gneissic), calcareous (machair) and brackish. This is an example of a Hebridean range not found elsewhere in the British Isles. Fen vegetation is highly modified by grazing, whilst tall reedswamp and floating-leaved plants are scarce on the mainly exposed and rocky shores, factors which contribute to the predominance of Fucoids in brackish water, of the open Littorella-Lobelia association in non-calcareous water. The Potamogeton filiformis-Chara association is confined to sand in calcareous machair lochs. Deep freshwater vegetation is typified by Isoetes, Potamogeton perfoliatus and P. praelongus.
The very wide ranges in alkalinity and conductivity are reflected in some unusual species' distributions. For example, in freshwater lochs (conductivity up to 660 μs cm−1) several species like Isoetes lacustris, confined elsewhere to oligotrophic water, also occur in South Uist at moderately high alkalinities (to 1·8 m. -equiv 1−1). Conductivity varies with chloride concentration in all but the calcareous (machair) lochs; brackish lochs range from 2500 μs cm −1), which floristically, apart from Fucus ceranoides, is fresh water, to Fucoid-dominated rocks, and Ruppia on silt, in conductivities up to 33,900 μs cm−1. Of other angiosperms, Potamogeton gramineus appears to tolerate conductivities of 22,000 μs cm −1. Causal distribution in general is discussed.