Interactions of aquatic microphytes and epiphytes and epiphytic algae in freshwater ecosystems

Abstract

The distribution, population dynamics, photosynthesis and nutrition of epiphytic algae were investigated in natural and artificial freshwater ecosystems, with special reference to interactions between the algae and their macrophytic substrata. The distribution of epiphyton in the littoral zone of Windermere was related to light shading effects of dense emergent vegetation. The distribution of epiphyton in the phyllosphere of Lemna minor in a small pond and in culture media was related to the rate of multiplication, age, topography, and exudates of the fronds, and to tactic responses of motile algal propagules. The population dynamics of epiphytic algae on Lemna minor in a small pond were correlated with seasonal changes in light and temperature, the growth cycle of Lemna minor, and with depletions of dissolved substances associated with primary production. Standing crops of epiphyton which accumulated on Lemna minor in culture media in the laboratory were related to the topography and rate of multiplication of the fronds, and to the inorganic nutrient concentrations in the media. Calothrix brevissima reduced the multiplication rate of fronds. The accumulation of Calothrix was correlated with an increase in the pH of the media to values which inhibited multiplication and photosynthesis in Lemna. Photosynthetic rates and yields of epiphytic algae were reduced in nitrogen deficient culture media. Photosynthesis and yields of Lemna minor were not severely reduced under nitrogen deficient conditions. Competition for nitrogen between Lemna and algae was proposed. Lemna minor excrete 14C labelled substances into axenic culture media. Epiphytic algal and bacterial populations assimilated the substances excreted by Lemna minor. Evidence for the excretion of antibiotic substances by Lemna minor was not conclusive.