In Situ Oxygen Variability in Seagrass

By use of microelectrodes, diel changes in the internal oxygen partial pressure of both Zostera

Fig. 10. Internal gas pressure in rhizome internodes of Cymod-ocea nodosa during a dark-light-dark transition experiment. In the light, gas pressures above atmospheric pressure were build up and a steady state pressure gradient occurred between the young rhizome internode #1 and the older internode #5. In the dark, an inverse gradient was formed at sub-atmospheric pressures (Redrawn from Terrados et al., 1999).

Time (min)

Fig. 10. Internal gas pressure in rhizome internodes of Cymod-ocea nodosa during a dark-light-dark transition experiment. In the light, gas pressures above atmospheric pressure were build up and a steady state pressure gradient occurred between the young rhizome internode #1 and the older internode #5. In the dark, an inverse gradient was formed at sub-atmospheric pressures (Redrawn from Terrados et al., 1999).

marina (Fig. 11; Greve et al., 2003) and Thalassia testudinum (Borum et al., 2005) have been assessed in situ under different environmental conditions. The oxygen content of eelgrass meristems followed similar temporal patterns and varied substantially over a diel cycle (Fig. 11). Internal oxygen partial pressures were above water column oxygen partial pressures and above atmospheric equilibrium in the afternoon at high surface irradiances and fluctuated systematically with changes in irradiance the following morning. In the dark, internal oxygen partial pressures declined steadily to low levels of about 15% of atmospheric equilibrium around sunrise. Similar patterns in plant oxygen contents have been recorded during other diel measurements on stands of Zostera marina (Borum, Pedersen and Binzer, unpublished) and of Thalassia testudinum (Borum et al., 2005).

As suggested from Fig. 11, oxygen partial pressures within the plants seem primarily dependent on changes in surface irradiance in the light and controlled by changes in water column oxygen concentrations at night. This suggestion is confirmed when internal oxygen partial pressures are plotted vs. surface irradiance in the light (Fig. 12A) and water column oxygen in the dark (Fig. 12B). In the light, the relationship resembles a typical photosynthesis-irradiance curve with increasing internal oxygen contents at low light reaching saturation at high light, while in the dark, plant oxygen contents are linearly related to the oxygen concentration in the water column. The oxygen content at high light is determined by the balance between the light-saturated oxygen evolution in leaves and the oxygen losses due to plant respiration and the oxygen efflux to the water

Fig. 11. Diel changes in surface irradiance and oxygen partial pressures of the water column and meristematic tissues of three eelgrass shoots measured in situ. During daylight, the fluctuating internal oxygen contents are intimately coupled to surface irradiance, while at night, changes in water column oxygen concentration seem to be the most important forcing factor controlling internal oxygen partial pressures (Borum, Pedersen and Binzer, unpublished).

Time of day

Fig. 11. Diel changes in surface irradiance and oxygen partial pressures of the water column and meristematic tissues of three eelgrass shoots measured in situ. During daylight, the fluctuating internal oxygen contents are intimately coupled to surface irradiance, while at night, changes in water column oxygen concentration seem to be the most important forcing factor controlling internal oxygen partial pressures (Borum, Pedersen and Binzer, unpublished).

Fig. 12. Plant oxygen partial pressures from Fig. 11 plotted (A) vs. surface irradiance (data from afternoon), and (B) versus water column oxygen partial pressure for the dark period (Borum, Pedersen and Binzer, unpublished).

column and sediment. The baseline oxygen partial pressure at zero irradiance (Fig. 12A) and the linear relationship between plant and water column oxygen in the dark (Fig. 12B) are determined by the balance between oxygen supply from the water column and oxygen losses due to plant respiration and the oxygen efflux to the sediment.

In the tropical turtle grass in Florida Bay, diel changes in internal oxygen content, similar to those shown in Fig. 11, have been recorded at several sites. Here, internal plant content of oxygen varied not only with surface irradiance and water column oxygen but also with sediment composition and plant density (Borum et al., 2005). In a sparsely vegetated bed with a moderate content of organic matter in the sediment, the oxygen partial pressure in the meristematic tissues remained relatively high throughout the diel cycle, while in a dense stand with organically rich sediments, the meristematic tissues turned anoxic during darkness, and rhizome and root metabolism had to rely on anaerobic metabolism for several hours during the night. These observations suggest that the oxygen partial pressure inside these tropical seagrasses is significantly influenced by reduced oxygen supply from the water due to lower water flow velocity in dense seagrass stands and/or by higher oxygen losses due to higher respiratory oxygen demands of more organically rich sediments, at higher temperatures compared to temperate sea-grasses.

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