Oxygen dynamics in the rhizosphere of Zostera marina: A two-dimensional planar optode study

The oxygen dynamics in the rhizosphere of Zostera marina was studied by use of planar optodes. Oxygen leakage to the rhizosphere was restricted to the root tip and extended only up to ∼8 mm up along the root. The oxic sediment volume around the roots increased linearly with irradiance in the interval of 0-250 μmol photons m-2 s-1, but the leakage rate saturated at the maximum irradiance of 500 μmol photons m-2 s -1. Oxygen leakage decreased by ∼60% from light to darkness and Z. marina was able to maintain an oxic zone around the root tip even in darkness as long as oxygen in the overlying water was at 100% air saturation (280 μmol L-1). O2 leakage from the root tips stopped at 25% air saturation (70 μmol L-1) and the oxic microniche rapidly disappeared. Increasing the oxygen concentration above 100% air saturation induced oxygen leakage from zones that otherwise appeared impermeable to oxygen. The roots on average grew by 8.7 mm d-1, and a series of O 2 images documented the high spatial and temporal dynamics of the oxic microniches around the root tips. The estimated total oxygen release to the rhizosphere of Z. marina beds was 2.3 mmol m-2 d-1, which only corresponded to 12% of the diffusive oxygen uptake at the primary sediment-water interface. Rhizospheres of seagrass are thus probably of minor importance for total benthic O2 uptake rates.