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 sltu light-reduction treatments in Tampa Bay, Florida, USA. Leaf SI3C values of shaded T. testudinum were significantly lower than those of unshaded con- trols in both shallow (0.75 m below MLW) and deep (2 m below MLW) sites. Changes in leaf 6I3C 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 6I3C in response to shading probably reflects a process (i.e. isotopic fractionation) effect rather than a source (i.e. dissolved inorganic carbon) effect.