The supply of oxygen to support aerobic metabolism within seagrass tissues derives from internal oxygen produced by photosynthesis and from passive diffusion of oxygen from water column or sediment, when oxygen partial pressures in the external media surpass plant oxygen partial pressures (Fig. 1). Photosynthesis of seagrasses mainly takes place in the epidermal cells with high chlorophyll contents assumed to represent an adaptation to the low light conditions often experienced by submerged macrophytes (Kuo and McComb, 1989; Larkum et al., 1989). Rates of photosynthesis on a dry weight basis are relatively low for seagrasses and other hydrophytes compared to terrestrial plants (Bowes, 1985; Nielsen and Sand-Jensen, 1989; Larkum et al., Chapter 14). However, high rates of oxygen evolution take place in individual leaves which is immediately apparent as formation of numerous gas bubbles during calm, sunny days (Zieman, 1974). In addition, leaf biomass of some seagrass beds may be very high and gross primary production can exceed 10 g O2 m-2 d-1 (Ziegler and Benner, 1998; Hemminga and Duarte, 2000).
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