A technique is described for measuring the metabolic response of seagrasses to different light
intensities and water temperatures in the laboratory. The observed depth distribution of the seagrass
species can be related to their light requirements and metabolic rates. The photosynthetic efficiency
and maximum photosynthetic rates of Amphibolis griffithii (Black) den Hartog were significantly
higher than those of Posidonia sinuosa Cambridge & Kuo and Posidonia australis Hook.f.
The optimum temperature range for net photosynthesis of Posidonia sinuosa was 18-23°C and
photoinhibition occurred at relatively low temperature and high light intensity. Maximum gross
photosynthetic rates of P. sinuosa in an attenuated light regime simulating that experienced by plants
in the field were higher than in a uniform light field. The respiration rates of the leaves were four to
seven times higher than those of the root/rhizome component in the Posidonia species. There was no
evidence for physiological adaptation to low light environments, using P. sinuosa from different
depths and with different epiphyte loadings.