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

This paper attempts to analyse the factors controlling the distribution of freshwater aquatic vegetation, using as its factual basis the results of a study of the macrophytic vegetation of Scottish lochs made by the present author (Spence 1964). Since little work on this topic had previously been written, that account presented many original data on the distribution and zonation of aquatic plant communities in Scotland, a study of possible controlling factors and their relevance to the physiology of hydrophytes. Many of these findings are of a general nature; also, some ideas first expressed there have been developed further, and experimental work has been proceeding at Makerere for some time on a number of factors such as water chemistry, substratum and light. It therefore seemed appropriate to provide a review of results and conclusions based on the field study carried out in Scotland. Aquatic macrophytes include vascular plants, bryophytes and members of the Characeae. They comprise hemicryptophytes and helophytes or marsh plants and, mainly, hydrophytes with perennating buds entirely or chiefly below the surface of standing water. Hydrophytes comprise five forms: (1) species with emergent leafy, and flowering, shoots (total or partial hydrophytes); (2) species with floating leaves; species with submerged leaves only (3) linear-leaved, (4) rosette and (5) broad-leaved. These definitions of aquatic macrophytes are combined with habitat factors to distinguish fen, swamp, floating-leaved and submerged vegetation. Fen comprises vegetation stands dominated by partial hydrophytes or helophytes in which the water table lies below the soil surface in summer, above it in winter. Fens may be classed by soil or vegetational criteria as rich or poor. Swamp comprises vegetation stands dominated by total or partial hydrophytes with linear emergent leaves in which the water table lies permanently or seasonally above the soil surface. Floating-leaved and submergedleaved stands as a rule occupy permanently submerged soils. The present paper mainly studies the factors controlling the distribution of swamp, floating-leaved and submerged vegetation.