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

Abstract

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, short-term 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.