EFFECTS OF LIGHT ON THE STABLE CARBON ISOTOPE COMPOSITION OF THE SEAGRASS THALASSIA-TESTUDINUM

Changes in the stable carbon isotope composition of Thalassia testudinum (turtle grass) leaves were measured in response to in situ light-reduction treatments in Tampa Bay, Florida, USA. Leaf deltaC-13 values of shaded T. testudinum were significantly lower than those of unshaded controls in both shallow (0.75 m below MLW) and deep (2 m below MLW) sites. Changes in leaf deltaC-13 were correlated with differences in the relative amount of light reaching the experimental treatments, and the magnitude of the responses increased between 1 and 3 mo after initiation of the shade treatments. Because of the close proximity of the experimental and control sites, the decrease in deltaC-13 in response to shading probably reflects a process (i.e. isotopic fractionation) effect rather than a source (i.e. dissolved inorganic carbon) effect. Greater isotopic fractionation in shaded T. testudinum may reflect reduced carbon demand and, hence, greater relative availability of carbon at lower irradiances. Thus, as light is reduced to levels that limit photosynthetic rates in T. testudinum, carbon appears to become non-limiting.