Seasonal and spatial patterns of oxygen production, respiration and root-rhizome release in Potamogeton perfoliatus L. and Zostera marina L.

Abstract

Oxygen release to the rhizosphere was measured in situ with O 2 microelectrodes and in hydroponic, split-compartment chambers. Light-dark experiments revealed that O 2 release from the rhizomes of Potamogeton perfoliatus L. was directly dependent on photosynthesis and that O 2 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 O 2 flux in P. perfoliatus was approximately 120 μmol g −1 dry weight h −1, similar to fluxes calculated from hydroponic measurements. Root-rhizome O 2 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 O 2 release, photosynthesis and respiration were observed between different sites within an eelgrass bed. The highest rates of O 2 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 O 2 release, about 43 μmol g −1 dry weight h −1. Root-rhizome O 2 release was significantly related to photosynthesis only for P. perfoliatus. Oxidation of the rhizosphere depended on root-rhizome O 2 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 O 2 m −2 h −1) because of relatively high Z. marina root and rhizome biomass.