Effects of experimental reduction of light availability on the seagrass Amphibolis griffithii

The response of the meadow-forming seagrass Amphibolis griffithii (Black) den Hartog to light reduction was examined over a 3 mo period and a subsequent 1 mo recovery period. Morphological and physiological variables were measured in meadows subjected to an average reduction in photosynthetic photon flux density (PPFD) of 88% relative to unshaded controls. Leaf biomass, leaf cluster density and the number of leaves per cluster all declined in shaded plots, and after 3 mo were about 30, 50 and 60% of the controls, respectively. Leaf extension was one-third that of the control plots. Epiphyte biomass in shaded plots was 44% of the controls after 6 wk of shading and 18% after 3 mo of shading. Leaf chlorophyll concentration was affected by shading, but only in the upper canopy: shaded leaves had 55% more chlorophyll than control leaves. Shading reduced the carbohydrate stored in the rhizomes of shaded plants: sugars declined rapidly and continuously and, after 3 mo, were <20% of the control values; a decline in starch concentrations lagged behind that of sugars. All variables showed a significant shift towards the values in control plots 42 d after removal of shading, indicating a capacity for recovery, though in many cases these variables remained significantly lower than those in the controls. A. griffithii and its epiphytes respond rapidly to severe, shortterm reductions in light availability, but responses at the scale of shoots and whole meadows also allow the plants to respond rapidly to improved light conditions.