EFFECTS OF LIGHT AND TEMPERATURE ON THE PHOTOSYNTHESIS OF TEMPERATE MEADOW-FORMING SEAGRASSES IN WESTERN-AUSTRALIA

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 and Kuo & Posidonia australis Hook.f. The optimum temperature range for net photosynthesis of Posidonia sinuosa was 18-23 degrees 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.