Microprofiles of oxygen were measured by 2- to 10-μm diameter microelectrodes within 0.2- to 2.0-mm thick epiphyte layers on different marine and freshwater macrophytes during artificial light-dark cycles. The epiphyte community created large fluctuations in oxygen concentrations at the macrophyte surface by increasing the thickness of the layer around the macrophyte in which the transport of solutes occurred by molecular diffusion and by contributing to the combined epiphyte-macrophyte activity. When epiphytes were removed, the diffusive boundary layer was reduced to 100 to 200 μm, and oxygen fluctuations declined. The oxygen concentration at the epiphyte-coverted macrophyte surface varied the most with the photosynthetically very active Scytosiphon lomentaria (Lyngb.), i.e. from 3 times atmospheric saturation of oxygen in the light to 0 in the dark. Low photosynthetic activity and extensive air lacunae in leaves of Littorella uniflora (L.) Aschers reduced oxygen fluctuations and extended the period necessary to reach steady state oxygen profiles. Detailed analysis of the epiphyte community on Potamogeton crispus L. showed that oxygen exchange followed typical light-photosynthesis saturation curves without exhibiting photoinhibition. The light compensation point of the association, (20–37 μE m-2s-1), and the saturation point, Ik (105–135 μE m-2s-1), were both low despite extensive self-shading. With increasing age under nutrient limitation, oxygen release of the epiphyte-P. crispus association under saturating illumination dropped from 10.0 to 5.0 nmol O2 cm-2 min-1, whereas the dark consumption remained about the same (2.4 to 2.9 nmol O2 cm-2 min-1). The contributions of the epiphyte community and the macrophyte to these oxygen exchanges were about equal. Epiphyte communities attenuate light and increase the transfer resistance for fluxes of dissolved substances between the bulk phase and the plant surface. Dense epiphyte communities which develop in eutrophicated waters may therefore be a severe stress to macrophyte metabolism by shading and by generating anoxic conditions at the macrophyte surface in the dark and high O2 and low CO2 concentrations in the light.