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Low light levels are a major factor determining the depth distribution of submerged macrophytes. In the Indo-Pacific tropical region, different seagrass species have specific zonation patterns from intertidal to subtidal areas along depth and light gradients. Cymodocea serrulata gen- erally occurs in deeper water than Thalassia hemprichii and C. rotundata, suggesting that C. serru- lata probably has the lowest minimum photon requirement of the 3 species. We compared photosyn- thesis and respiration balances of these 3 tropical seagrasses by examining photosynthesis-irradiance (P-E) curves for leaves and for the whole plant bodies. Sample collection and measurement were conducted on 2 occasions (July and February) on Ishigaki Island, southwestern Japan. Leaf photo- synthetic traits were not concordant with the observed zonation pattern of the species. When respira- tion rates of the root and rhizome were taken into account, T. hemprichii had a significantly higher light compensation point (158.1 µmol photons m -2 s -1 in July, 187.2 in February) than C. rotundata (47.9 in July, 41.0 in February) and C. serrulata (85.1 in July, 40.0 in February). The large amount of belowground respiration in T. hemprichii was a major cause of the variation in the compensation irra- diance. This result is in concordance with the realized distribution pattern of T. hemprichii, which is sparse under low light conditions caused by depth or heavy siltation. Conversely, C. serrulata had less belowground biomass and a smaller respiration demand, enabling the species to maintain a better photosynthetic/respiration balance under lower light. C. rotundata had a lower saturating irra- diance, a lower compensation point and a lower respiration rate than the other species. Thus, C. rotundata has the potential to inhabit the deepest water, although its realized distribution range is shallower than that of C. serrulata, suggesting that other factors limit the depth range of C. rotundata.

Interspecific variation in photosynthesis and respiration balance of three seagrasses in relation to light availability