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 O 2 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 O 2 leakage, but then only at root tips. ROL resulted in an oxic volume of 4.26 ± 0.51 mm 3 plant −1 for Z. marina and 5.39 ± 0.47 mm 3 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 O 2 levels remained > 50% air saturation. The calculated ROL integrated over a 24 h cycle was 48.8 ± 10.6 nmol O 2 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.

The polychaete Marenzelleria viridis is an invasive species and often replaces the native Nereis diversicolor. This shift leads to more reduced conditions and changes in the biogeochemical function of the sediments. By combining imaging techniques for O 2 (planar optodes) and irrigation patterns (rhodamine WT and brilliant blue), we investigated the relationship between irrigation and O 2 dynamics in burrows of M. viridis. The investigated animals shifted between 2 modes of ventilation: ciliary pumping for 77% of the time and muscular pumping for 23% of the time. On average, muscular pumping was induced every 0.4 h. During ciliary pumping, oxic water was pumped into blind-ended burrows and into the surrounding sediment, inducing an upward porewater transport of O 2 -depleted water. This pattern was reversed during muscular pumping. The 2 pumping modes induced oscillating O 2 penetration along the burrow wall and along the primary sediment-water interface. The average net downward irrigation rate, including both pumping modes, amounted to 11.1 × 10 −3 ± 2.4 × 10 −3 ml min −1. The estimated average oxic sediment volume was 2.1 ± 0.5 cm 3 per burrow, and the burrow-specific O 2 consumption was 45.6 ± 18.1 nmol min −1 . M. viridis burrows and the ambient sediment are relatively O 2 depleted, with intensified rates of O 2 consumption, compared to similar-sized native N. diversicolor. The complex O 2 dynamics induced a unique microenvironment that must favor meiofauna and microbial communities that are tolerant to oxic-anoxic oscillations or that have the ability to migrate along with the pulsing oxic-anoxic interface.