Oxygen Loss to the Water Column

Oxygen is lost from the leaves to the water column during the day when the oxygen partial pressure within the leaves, produced by photosynthesis, exceeds the oxygen partial pressure of the water column surrounding the leaves. The oxygen may either be lost by passive diffusion through the DBL or by bubble formation on the leaves. Pressurization of fully submerged macrophytes has been described for Egeria densa attaining lacunal gas pressures up to 25 kPa above atmospheric pressure (Angelstein, 1910; Sorrell and Dromgoole, 1988). The pressurization can result in bubble formation at leaf tips, but bubbles can also be formed on leaf surfaces as a result of oxygen loss by diffusion across the leaf epidermis. Under calm, warm conditions the release of oxygen into the diffusive boundary layer around leaves may increase local oxygen concentrations above the solubility in water. Bubble formation is, however, less important when the water flow around leaves is high enough to reduce the thickness of the boundary layer and lower the pressurization within the leaves (Sorrell and Dromgoole, 1988).

The relative importance for the overall oxygen balance of seagrasses, of the oxygen lost from leaves to the water column, probably varies substantially with plant morphology (biomass/area of leaves vs.

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Distance behind root tip (cm)

Fig. 6. Radial oxygen loss from roots of Halophila ovalis as a function of distance from the root tip (n = 5 ± SE). Relatively low permeability of older parts of the roots prevents oxygen from being lost to the sediment before reaching the young and actively growing root tips (Redrawn from Connell et al., 1999).

below-ground tissues) and with strength of respiratory sinks within the plants and sediment. Split chamber experiments with Zostera marina have suggested that most of the oxygen produced by photosynthesis escapes to the water column (Sand-Jensen etal., 1982; Kemp and Murray, 1986;Caffrey and Kemp, 1991), in agreement with theoretical predictions (Larkum et al., 1989). However, the absolute rates of oxygen loss to the water column may be substantially biased by experimental conditions (Sorrell and Armstrong, 1994) and are highly dependent on plant species and the permeability of leaves (Sand-Jensen etal., 1982).

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