Variation in δ13C values for the seagrass Thalassia testudinum and its relations to mangrove carbon

Abstract

Carbon isotope ratios ( 13C/ 12C) were measured for the leaves of the seagrass Thalassia testudinum Banks ex König and carbonates of shells collected at the seagrass beds from seven sites along the coast of southern Florida, U.S.A. The δ 13C values of seagrass leaves ranged from −7.3 to −16.3‰ among different study sites, with a significantly lower mean value for seagrass leaves from those sites near mangrove forests ( −12.8 ± 1.1‰) than those far from mangrove forests ( −8.3 ± 0.9‰; P < 0.05). Furthermore, seagrass leaves from a shallow water area had significantly lower δ 13C values than those found in a deep water area ( P < 0.01). There was no significant variation in δ 13C 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 δ 13C value in the mangrove areas ( −2.3 ± 0.6‰) than in the non-mangrove areas ( 0.6 ± 0.3‰; P <0.025). In addition, the δ 13C values of seagrass leaves were significantly correlated with those of shell carbonates ( δ 13C seagrass leaf = −9.1 + 1.3 δ 13C shell carbonate ( R 2 = 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.