Stable carbon isotope variability in the seagrass Posidonia oceanica: evidence for light intensity effects

Stable carbon isotope ratios were determined for leaves of the seagrass Posidonia oceanica (L ) Delile collected over a 38m depth gradient at Bay of Calvi, Corsica Although variability in 6 values among individual leaves at specific depths was as high as 6%0, there was an overall trend towards less discnmination against "C at the shallowest water levels. The mean 613C value of leaves at 5 m was -11 0 %O and declined to a minimum mean of -16 4 %O at 35 m. Isotopic variability in individual leaves divided into thuds was consistent with this trend, except in those leaf sections nearest to the menstem, whose 613C values did not vary with depth A variety of explanations for these observations were considered, including the effects of light intensity, temperature, pressure, relative water motion, seasonal variability in biochemical fractionation, isotopic variability in the dissolved inorganic carbon pool, and differential use of bicarbonate and dissolved carbon dioxide on the carbon isotopic composition of P oceanica Differences in light intensity and consequently photosynthetic rate are most likely to influence discrimination against "C and thus account for the observations reported here