δ13C is a signature of light availability and photosynthesis in seagrass

We explored the role of light-saturated (carbon-limited) photosynthesis on delta C-13 of turtlegrass (Thalassia testudinum Banks ex Konig) populations from the clear, blue waters of the Great Bahama Bank and the turbid, green waters of Florida Bay using field observations and radiative transfer models. Consistent with numerous previous observations, leaf delta C-13 decreased significantly with water depth in both regions. However the delta C-13 for Bahamas turtlegrass was 3 parts per thousand heavier than that for Florida Bay turtlegrass at equivalent depths, and broadband irradiance explained even less of the delta C-13 variations than depth. Instead, leaf delta C-13 showed a stronger relationship to the fraction of the day that photosynthesis of the intact plant canopy was carbon-limited. When the Bahamas and Florida Bay delta C-13 values were related to the fraction of the day that photosynthesis was carbon-limited, the variations in leaf delta C-13 observed for Florida and Bahamas populations collapsed into a single relationship that explained 65% of the variation in leaf delta C-13. Consequently, turtlegrass from the Bahamas was isotopically heavier than Florida Bay populations growing at equivalent depths because they were more carbon-limited (= light-saturated) for a larger fraction of the day. The ability to predict turtlegrass delta C-13 from the daily period of carbon-limited photosynthesis provides a mechanistic link to fundamental relationships between light and photosynthesis that can transcend geographic differences in depth and water-column optical properties, and may permit leaf delta C-13 to provide a robust indicator of recent photosynthetic performance and plant survival in response to changing environmental conditions.