Effects of light on the stable carbon isotope composition of the seagrass Thalassia testudinum

Abstract

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