MEPS 163:267-278 (1998)  -  doi:10.3354/meps163267

Three interconnected light gradients and their influence on structure and function of seagrass leaves were investigated in both a shallow (-3 m depth) and a deep (-10 m) stand of Posidonia oceanica (L.) Delile in Calví (Corse, France). Light attenuation in the water column amounted to 51.3, 63.5, and 79.3% irradiance of the subsurface value at -2.5 m and 70.4, 67.3, and 98.1% at -9.5 m depth for blue, green and red wavelengths, respectively. Horizontally lying fronds of bent P. oceanica leaves (between 30 and 50 cm above the bottom) induced a selfshading effect on the whole stand at both sampling sites, further reducing the already dimmed incident light of the blue, green and red wavelengths by 57-58, 61-63, and 51-53%, respectively. Nevertheless, within the P. oceanica stand (at 0 and 30 cm above the bottom) similar light conditions were found at both sampling sites for blue and green wavelengths. Changes in the meadow structure of the deep stand make this possible: shoot density is reduced by 72%, leaves are generally 5 to 10 cm shorter, maximal leaf length is reduced by 23.8%, and dead tips are shorter compared to the shallow seagrass stand. The number of leaves per shoot and leaf blade width increase photosynthetically active surfaces at the deep stand. Pigment concentrations decreased in the order: chlorophyll a, chlorophyll b, and carotenoids. Along the length of a P. oceanica leaf (0-10, 30-40, and the top live 10 cm) highest pigment concentrations were found in the tips of juvenile leaves and the midsection of long leaves, restricting the highest photosynthetic rates to a layer between 20 and 40 cm above the bottom. The bases of short and juvenile leaves contain significantly higher concentrations of pigment at the deep site than at the shallow site, with generally higher concentrations in the basal sections of short than of long leaves. Epiphytes were responsible for the third light gradient, directly shading the photosynthetically active surface of P. oceanica,especially of long leaves, where they shaded up to almost 50% of the surface. The main epiphyte, the red crustose coralline alga Fosliella farinosa, shows a functional adaptation in its distribution: in the shaded deep stand it grows at the light-exposed convex surface of the P. oceanica leaf, whereas in the more highly irradiated shallow stand it can be found mainly in the shaded concave surface of the leaf, indicating a functional photoadaptation.

Sea grass · Light attenuation · Selfshading · Leaf morphometry · Epiphytes · Chlorophyll · Pigment concentration