Within canopy growth strategies of the two seagrass species Amphibolis griffithii (J. Black) den Hartog and Amphibolis antarctica (Labillardiere) Sonder & Ascherson ex Ascherson

Responses in leaf production to variation in light climate throughout Amphibolis griffithii and A.antarctica canopies were studied at Rottnest Island, Western Australia. To test the importance of small scale changes in the physical environment (e.g. light, temperature and water movement) within the canopy, leaf production rates at different heights within the canopies and Radiation Use Efficiency (RUE), for the entire canopy of each species, were calculated and compared. Comparative (radiation) Use Efficiency (CUE) was also measured from the top to the base of the canopies of both species. Mean leaf production rate was 0.038±0.004 leaves cluster-1 day-1, for A. griffithii, and 0.086±0.007, for A. antarctica. Neither species showed differences in leaf production rates at different heights within the canopy. There was a positive correlation between leaf size and height above the sediment in both species. CUE was higher closer to the sediment and RuE of the entire canopy was similar for both species (0.30±0.04 g mol quanta-1 , for A. griffithii and 0.26±0.02 for A. antarctica). Although there were differences in canopy structure between the species, there was no difference in total canopy RUE. This was probably due in part to the location of the study site in a high water movement environment in the middle of the distributions of the two species. Smaller leaf size at the base of the canopy, constant leaf production rates through the canopy and variation in CUE suggest physiological integration of leaf clusters rather than control of leaf turnover by changes in the physical environment through the canopy. Physiological support of small leaf clusters close to the sediment may be a mechanism to promote rapid stem growth, and hence maximise leaf area in the high light region at the top of the canopy.

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