PHOTOSYNTHESIS AND ZONATION OF FRESHWATER MACROPHYTES

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Abstract
SUMMARY: The mean, range and standard deviation are given of the depths of water above the soil surface in which a number of Potamogeton species occur in Scottish lochs. Sun leaves of these species were produced in unscreened containers in a glasshouse and their rates of net O2 production were measured at irradiances of from 1.34 to 7.08 cal/cm2/hour in a Warburg apparatus, using Warburg buffer no. 11 as bathing solution. Using only leaves of species of which the rates appeared to be unaffected by buffer solution during the short experiments, it was shown that the shade tolerance of these leaves is correlated with the natural depth distribution of the species. This valid contrast in inherent photosynthetic response between some deep water species (e.g. P. praelongus, P. obtusifolius) and some of shallow water (e.g. P. polygotiifolius) indicates that light may be as important as substrate or competition in controlling the zonation of freshwater macrophytes.