Gas exchange and isotopic signature of mangrove species in Southern Brazil

• We investigated gas exchange and isotopic signature in subtropical mangrove species.
• Salinity, organic matter and pH differentiate mangrove soil and forest structure.
• Leaf δ13C and WUEi indicate different salinity tolerance among species.
• Fringe mangroves may use a marine source of N.
• Species shows distinctive ecophysiological responses to salinity variations.

The study measured leaf gas exchange parameters and abundance of stable isotopes of C and N of three mangrove species (Rhizophora mangle L., Avicennia schaueriana Stapf & Leechm. ex Moldenke. and Laguncularia racemosa (L.) Gaertn) to investigate how photosynthetic capacity and water use efficiency characterize salt tolerance along an environmental gradient, in the Guaratuba Bay Estuary, Brazil, around 25°S. Along the estuary, parallel to the river, nine 50 m2 plots were marked: three plots at the fringe, three plots at the intermediary zone (around 150 m from the fringe), and three plots at the interior zone (around 350 m from the fringe), to represent the zonation pattern expressed by mangrove species. We used gas exchange parameters to calculate intrinsic water use efficiency (Amax/gs, WUEi). Long-term water use efficiency was calculated based on δ13C values, and δ15N values were related to source of N. Avicennia schaueriana showed the capacity to maintain high WUEi, even with high gs under higher salinities. The foliar δ13C of L. racemosa, was lower than the other species and negatively correlated to pore-water salinity. Rhizophora mangle gas exchange parameters did not correlate to the measured soil variables. Although the difference was not significant in R. mangle, δ15N values indicate that fringe mangroves may use a marine source of N. How species respond to changes in pore-water salinity is expressed in both gas exchange parameters and isotopic signature through the floodplain gradient.