VARIATION IN DELTA-13-C VALUES FOR THE SEAGRASS THALASSIA-TESTUDINUM AND ITS RELATIONS TO MANGROVE CARBON

Carbon isotope ratios (C-13/C-12) were measured for the leaves of the seagrass Thalassia testudinum Banks ex Konig and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The delta-C-13 values of seagrass leaves ranged from - 7.3 to - 16.3 parts per thousand among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests (-12.8 +/- 1.1 parts per thousand) than those far from mangrove forests (-8.3 +/- 0.9 parts per thousand; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower delta-C-13 values than those found in a deep water area (P < 0.01). There was no significant variation in delta-C-13 values between young and mature leaves (P = 0.59) or between the tip and base of a leaf blade (P = 0.46). Carbonates of shells also showed a significantly lower mean delta-C-13 value in the mangrove areas (-2.3 +/- 0.6 parts per thousand) than in the non-mangrove areas (0.6 +/- 0.3 parts per thousand; P < 0.025). In addition, the delta-C-13 values of seagrass leaves were significantly correlated with those of shell carbonates (delta-C-13 seagrass leaf = -9.1 + 1.3 delta-C-13 shell carbonate (R2 = 0.83, P < 0.01)). These results indicated that the input of carbon dioxide from the mineralization of mangrove detritus caused the variation in carbon isotope ratios of seagrass leaves among different sites in this study