Halophila stipulacea is the dominant seagrass in the Gulf of Aqaba (northern Red Sea), where it grows from the intertidal to depths exceeding 50 m. Its successful growth under such a broad irradiance gradient shows either a high plasticity or is caused by longer-term adaptations to the various depths, possibly resulting in the formation of ecotypes. In April 2008 we transplanted shoots of this seagrass between the extreme depths of its distribution at the study site (8 and 33 m) in order to evaluate its acclimation potential to various irradiances. We compared photosynthetic parameters derived from light response curves generated by PAM fluorometry (so-called rapid light curves, RLC) and measured chlorophyll a and b concentrations. RLCs from the shallow (∼400 μmol photons m-2 s-1 at midday) and deep (∼35 μmol photons m-2 s-1 at midday) sites were characteristic for high-and low-light growing plants, respectively, and the transplanted seagrasses acclimated to their new environments within 6 d, at which time their RLCs resembled those of the original plants growing at the depths to which they had been transplanted. Concentrations of both chlorophyll a and b decreased or increased when the plants were transferred to high vs. low-light environments, respectively, but the chlorophyll a:b ratios remained constant. These fast changes in photosynthetic responses and light absorption characteristics in response to changing light environments points to Halophila stipulacea as being a highly plastic seagrass with regard to irradiance, which may partly explain its abundance across a wide range of irradiances along the depth gradient that it occupies.
- Depth gradient
- Halophila stipulacea