low in Zostera (1%) and among stream macrophytes (2-4%), but quite high in the isoetids (28-100%). Among the species tested, Lobelia dortmanna had the highest rate of release from roots during illumination (3.12 9g 02 mg-' h-1) which represented 100% of the total oxygen release. The ready exchange of oxygen across the roots of isoetids may play a role for the uptake of sediment carbon dioxide. Dissolved oxygen measurements of overlying water cannot be used as a measure of photosynthesis or respiration in isoetids, but the technique is valid for other growth forms.