CARBON ASSIMILATION IN POSIDONIA-OCEANICA - BIOTIC DETERMINANTS

Carbon assimilation in the Mediterranean seagrass Posidonia oceanica w as measured by means of the C-14 technique. Whole shoots were harveste d seasonally at 5 and 22 m depth at Lacco Ameno (Gulf of Naples) and i ncubated at constant saturating light in the laboratory. Carbon incorp oration of different leaf sections as well as epiphytic assimilation w ere evaluated. Mean assimilation values of 1.42 +/- 0.62 mu gC mg(-1) DW h(-1) (N = 60) and 1.43 +/- 0.70 mu gC mg(-1) DW h(-1) (N = 48) wer e found in plants of P. oceanica collected at 5 and 22 m depth, respec tively. Maximum carbon assimilation values, up to 2.98 mu gC mg(-1) DW h(-1), were recorded in February. The highest rates, as well as the l owest variability in carbon assimilation across seasons and depths, we re generally recorded in mid-sections of leaves of rank II through IV, which pointed to a fundamental homogeneity in assimilative response o f the bulk of the photosynthetic tissue. Epiphytic carbon assimilation was estimated to range between 33 and 52% of total P. oceanica shoot production, except for February when the epiphytic coverage was minor, in particular at 22 m. Tissue age, as expressed by leaf rank and tiss ue position along the leaf blade, was the major biotic factor affectin g carbon assimilation capacity. On the other hand, depth seemed to hav e a minor influence on carbon production potential of P. oceanica leav es in comparison to the other factors. It is hypothesized that in seag rasses tissue age determines the capacity of carbon assimilation, whil e in situ light regimes regulates the plant adaptations to different d epths.