HIGHLIGHTS: Sedimentation of fine sediment particles onto seagrass leaves severely hampers the plants' performance in both light and darkness, due to inadequate internal plant aeration and intrusion of phytotoxic H₂S. Anthropogenic activities leading to sediment re-suspension can have adverse effects on adjacent seagrass meadows, owing to reduced light availability and the settling of suspended particles onto seagrass leaves potentially impeding gas exchange with the surrounding water. We used microsensors to determine O₂ fluxes and diffusive boundary layer (DBL) thickness on leaves of the seagrass Zostera muelleri with and without fine sediment particles, and combined these laboratory measurements with in situ microsensor measurements of tissue O₂ and H₂S concentrations. Net photosynthesis rates in leaves with fine sediment particles were down to ~20% of controls without particles, and the compensation photon irradiance increased from a span of 20-53 to 109-145 μmol photons m^-2 s^-1. An ~2.5-fold thicker DBL around leaves with fine sediment particles impeded O₂ influx into the leaves during darkness. In situ leaf meristematic O₂ concentrations of plants exposed to fine sediment particles were lower than in control plants and exhibited long time periods of complete meristematic anoxia during night-time. Insufficient internal aeration resulted in H₂S intrusion into the leaf meristematic tissues when exposed to sediment resuspension even at relatively high night-time water-column O₂ concentrations. Fine sediment particles that settle on seagrass leaves thus negatively affect internal tissue aeration and thereby the plants' resilience against H₂S intrusion.

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