Irradiance-induced changes in the photobiology of Halophila johnsonii

The endangered seagrass Halophila johnsonii Eiseman, exhibits high-light adapted photophysiology consistent with its distribution in intertidal and shallow subtidal (0–3 m) coastal-lagoon habitats along 200 km of southeastern Florida. To examine the short-term responses of this seagrass to three controlled-irradiance treatments (PAR + UVA + UVB [full spectrum], PAR + UVA, and PAR only), greenhouse-acclimated plants were transferred to outdoor mesocosms during July–August 2002. Chlorophyll fluorescence, UV fluorescence, and samples for pigment extraction were collected in the greenhouse, prior to moving the plants outside and on days 1, 2, 3, 4, 6, 10, and 21 of the 24-day experiment. Typical of sun-adapted plants, effective quantum yields measured by pulse-amplitude modulated (PAM) fluorometry were relatively low in all treatments, ranging from 0.46 ± 0.09 (PAR only) to 0.58 ± 0.08 (PAR + UVA + UVB). In the PAR only treatments, there were strong effects on days 1 and 4, presumably because the irradiance in the greenhouse not only lacked all λ