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 seagr asses to different light intensities and water temperatures in the lab oratory. The observed depth distribution of the seagrass species can b e related to their light requirements and metabolic rates. The photosy nthetic efficiency and maximum photosynthetic rates of Amphibolis grif fithii (Black) den Hartog were significantly higher than those of Posi donia sinuosa Cambridge and Kuo & Posidonia australis Hook.f. The opti mum temperature range for net photosynthesis of Posidonia sinuosa was 18-23 degrees C and photoinhibition occurred at relatively low tempera ture 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 re spiration rates of the leaves were four to seven times higher than tho se of the root/rhizome component in the Posidonia species. There was n o evidence for physiological. adaptation to low light environments, us ing P. sinuosa from different depths and with different epiphyte loadi ngs.