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Carbon Assimilation in Posidonia oceanica: Biotic Determinants

Carbon assimilation in the Mediterranean seagrass Posidonia oceanica was measured by means of the C technique. Whole shoots were "harvested seasonally at 5 and 22 m depth at Lacco Ameno (Gulf of Naples) and incubated at constant saturating light in the laboratory. Carbon incorporation of different leaf sections as well as epiphytic assimilation were evaluated. Mean assimilation values of 1.42 ± 0.62 gC mg" DW h" (N = 60) and 1.43 ± 0.70 gC mg" DW h" (N = 48) were found in plants of P. oceanica collected at 5 and 22 m depth, respectively. Maximum carbon assimilation values, up to 2.98 gC mg" DW h", were recorded in February. The highest rates, as well as the lowest variability in carbon assimilation across seasons and depths, were generally recorded in mid-sections of leaves of rank II through IV, which pointed to a fundamental homogeneity in assimilative response of 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 tissue position along the leaf blade, was the major biotic factor affecting carbon assimilation capacity. On the other hand, depth seemed to have a minor influence on carbon production potential of P. oceanica leaves in comparison to the other factors. It is hypothesized that in seagrasses tissue age determines the capacity of carbon assimilation, while in situ light regimes regulates the plant adaptations to different depths.