In situ oxygen dynamics in rhizomes of the seagrass Posidonia sinuosa: impact of light, water column oxygen, current speed and wave velocity

The presence of oxygen in seagrass tissues, which plays a role in preventing seagrass die-off, is partly regulated by environmental conditions. Here, we examined the relationship between oxygen (O-2) in the rhizomes of Posidonia sinuosa and key environmental variables at Garden Island, Western Australia. We made in situ measurements of internal oxygen partial pressure (pO(2)) of rhizomes using fibre optic optodes and compared these to pO(2) in the water column, photosynthetically active radiation (PAR) and wave and current velocities within the seagrass canopy. During daytime, tissue pO(2) was regulated by PAR, whereas in darkness, both near-bed mean current and wave orbital velocities were important in influencing pO(2). Tissue pO(2) was positively correlated with current speed at night up to a threshold of similar to 0.045 m s(-1), likely because of a reduction in the thickness of the diffusive boundary layer surrounding the plant, allowing for more rapid exchange of O-2 with the surrounding water. The flow velocities in the meadow were generally low and at times near-stagnant, and pO(2) in rhizomes declined to critical levels at night. This may explain the lack of recovery of seagrasses in the area despite management efforts that have improved water quality. Our observations of tissue pO(2) in P. sinuosa show remarkable similarities to previous laboratory and field studies across a range of seagrass species, suggesting that the relationships to hydrodynamic conditions and light levels that are described here are general across taxa.