Errors in measurements of aquatic macrophyte gas exchange due to oxygen storage in internal airspaces

Abstract

Dissolved oxygen measurements of respiratory and photosynthetic gas exchanges of the aquatic angiosperm Egeria densa Planch. and vesicles of the macroalga Carpophyllum maschalocarpum (Turn.) Grev. were made using a Clark-type oxygen electrode in a well-stirred closed chamber. Comparisons were made of dark—light and light—dark transients and light and dark oxygen exchanges for intact material and the same material with the internal airspaces flooded. For Egeria, there was an apparently greater gas exchange (up to 17% for photosynthesis and 50% for respiration) for infiltrated material. The duration of transient lags following light and dark treatments remained unchanged by infiltration, indicating that the storage error could not be detected from lag duration. The storage error during photosynthesis agreed closely with theoretical predictions based on oxygen solubility, but additional factors contribute to the dark error. Carpophyllum results were unpredictable from solubility calculations as the thick vesicle walls restricted exchange between the internal and external phases, allowing internal oxygen storage only at high external partial pressures. The widespread assumption that short lag phases indicate negligible internal oxygen storage in the gas spaces is questioned on the basis of these results. Solutions to the storage error are proposed.