Flow velocity affects internal oxygen conditions in the seagrass Cymodocea nodosa

The internal oxygen status of seagrass tissues, which is believed to play an important role in events of seagrass die-off, is partly determined by the rates of gas exchange between leaves and water column. In this study, we examined whether water column flow velocity has an effect on gas exchange, and hence on internal oxygen partial pressures (pO2) in the Mediterranean seagrass, Cymodocea nodosa. We measured the internal pO2 in the horizontal rhizomes of C. nodosa in darkness at different mainstream flow velocities, combined with different levels of water column oxygen pO2 using an experimental flume in the laboratory. Flow velocity clearly had an effect on the internal oxygen status. In stagnant, but fully aerated water the mean internal pO2 was 6.9 kPa, corresponding to about 30% of air saturation. The internal pO2 increased with increasing flow velocity reaching saturation of around 12.2 kPa (60% of air saturation) at flow velocities ≥7 cm s−1. Flow had a relatively larger influence on internal pO2 at lower water column oxygen concentrations. By extrapolating linear relationships between internal and water column pO2 in this experimental setup, rhizomes would become anoxic at a water column oxygen pO2 of 4-4.5 kPa (∼20% of air saturation) in flowing water, but already at 6.4 kPa (∼30% of air saturation) in stagnant water. Water flow may play an important role for seagrass performance and survival in areas with poor water column oxygen conditions and may, in general, be of importance for the distribution of submerged rooted plants.