ABSTRACT: Zostera marina and Ruppia maritima often share the same habitat, but R. maritima appears more resistant to environmental stress. We investigated the impact of light intensity and water column O₂ concentrations on radial oxygen loss (ROL), in young specimens of Z. marina and R. maritima. Planar optode imaging revealed that ROL of Z. marina was localized to the root tip, while R. maritima showed ROL along extensive root sections. The total root biomass of the 2 species was similar, but, while R. maritima had only 1 root, of which 33% of its length showed ROL, Z. marina had 2 to 5 individual roots, where only 2 to 3 exhibited O₂ leakage, but then only at root tips. ROL resulted in an oxic volume of 4.26 ± 0.51 mm³ plant^-1 for Z. marina and 5.39 ± 0.47 mm³ plant^-1 for R. maritima (n = 3). ROL per plant at light saturation was 2.32 ± 0.30 and 2.89 ± 0.38 nmol h^-1 for Z. marina and R. maritima, respectively. These values declined by 71 and 60% in darkness. However, both species were able to maintain ROL as long as ambient O₂ levels remained >50% air saturation. The calculated ROL integrated over a 24 h cycle was 48.8 ± 10.6 nmol O₂ plant^-1 d^-1 (n = 3) for R. maritima and 30% less for Z. marina. The ability of R. maritima to maintain higher ROL than Z. marina could be an important feature defining its potential for colonizing and maintaining growth in eutrophic sediments.