SEASONAL AND SPATIAL PATTERNS OF OXYGEN PRODUCTION, RESPIRATION AND ROOT RHIZOME RELEASE IN POTAMOGETON-PERFOLIATUS L AND ZOSTERA-MARINA L

Oxygen release to the rhizosphere was measured in situ with O2 microelectrodes and in hydroponic, split-compartment chambers. Light-dark experiments revealed that O2 release from the rhizomes of Potamogeton perfoliatus L. was directly dependent on photosynthesis and that O2 concentrations in sediments near the rhizomes started to decrease within 2 min after plants were darkened. Using Fick's first law of diffusion, the calculated net O2 flux in P. perfoliatus was approximately 120-mu-mol g-1 dry weight h-1, similar to fluxes calculated from hydroponic measurements. Root-rhizome O2 release, photosynthesis and respiration were measured for P. perfoliatus and Zostera marina L., with split-compartment hydroponic chambers in spring, summer and fall. For Z. marina, variability of root-rhizome O2 release, photosynthesis and respiration were observed between different sites within an eelgrass bed. The highest rates of O2 release from roots and rhizomes occurred in spring for both species and declined during the summer. For P. perfoliatus, the shortest plants (< 10 cm stem length) had the highest O2 release, about 43-mu-mol g-1 dry weight h-1. Root-rhizome O2 release was significantly related to photosynthesis only for P. perfoliatus. Oxidation of the rhizosphere depended on root-rhizome O2 release and biomass, which changed seasonally. Although weight-specific release rates were higher for P. perfoliatus than Z. marina, the potential oxidation of the rhizosphere was similar for both species (approximately 4-6 mmol O2 m-2 h-1) because of relatively high Z. marina root and rhizome biomass.