EFFECTS OF ULTRAVIOLET AND PHOTOSYNTHETICALLY ACTIVE RADIATION ON 5 SEAGRASS SPECIES

Five seagrass species [Halophila ovalis (R.Br) Hook. f., Halodule unin ervis (Forsk.) Aschers., Zostera capricorni Aschers., Cymodocea serrul ata (R.Br) Aschers. (ed.) and Syringodium isoetifolium (Aschers.) Dand y] from Moreton Bay, Australia, were grown under increased (+25%) and ambient levels of ultraviolet (UV) radiation and photosynthetically ac tive radiation (PAR), and various morphological and physiological resp onses were examined. Leaf fluorescence ratio (variable:maximum fluores cence) in conjunction with xanthophyll pigment content (violaxanthin, antheraxanthin and zeaxanthin) were used as a measure of photosyntheti c efficiency. In addition, absorbance in the UV spectrum, chlorophyll content and chloroplast density were used as indicators of photosynthe tic capacity. The seagrass species examined had varying degrees of sen sitivity to UV radiation. Halophila ovalis and Halodule uninervis were the most sensitive species, exhibiting the largest decrease in photos ynthetic efficiency and chloroplast density and the smallest increase in UV-blocking pigments in response to UV radiation. The more UV-toler ant species, Z. capricorni, C. serrulata and S. isoetifolium, were onl y significantly affected by increased levels of UV radiation, showing a gradual decline in photosynthetic efficiency and chloroplast density and the largest increases in UV-blocking pigment. UV sensitivity corr esponded with leaf morphology, with thicker leaves (as in Z. capricorn i, C. serrulata and S. isoetifolium) providing greater morphological p rotection for UV-sensitive organelles. Not al species were significant ly affected by increasing PAR, with decreases in fluorescence ratio an d increases in zeaxanthin content observed only in C. serrulata and S. isoetifolium. Sensitivity to PAR corresponded with morphological plas ticity; species exhibiting a wide range of growth forms (e.g. Halophil a ovalis, Halodule uninervis vis and Z. capricorni) were the least sen sitive to increases in PAR. Seagrass depth-distributions in Moreton Ba y appear to be influenced by species sensitivity to UV radiation and P AR, with other factors such as epiphytes, shading and nutrients also a ffecting species' tolerance. All species were affected to some degree by UV radiation, thus future changes in UV intensity may have repercus sions on the distribution of seagrasses.