Stable isotopes (δ13C, δ15N) ratios as tracers of salinity stress in natural and afforested mangroves in the Arabian Gulf

Abstract The time-integrated expression of δ 13 C and δ 15 N in components (leaves, stems, and roots) and associated biota in native and planted mangrove forests in the Central Arabian Gulf (CAG) was assessed to evaluate the forests’ functional responses to environmental extremes. Both mangroves and associated biota at the planted mangroves exhibited isotopic signatures that differed markedly from those of native stands. The native mangrove forests had lower δ 13 C (− 27.6 ± 0.4) and higher δ 15 N (0.8 ± 0.7) ratios than those that had been planted (− 24.5 ± 0.6 and − 3.8 ± 0.7, respectively). The leaves, stems, and roots at the planted sites showed about ∼3‰ higher δ13C and lower δ 15 N values compared to native plants. Similarly, macroalgae exhibited a comparable trend, with δ 13 C about 1.5–3‰ and δ 15 N values ∼ 3–6‰ lower at the planted site compared to native stands, with the strongest δ15N depletion observed at extreme salinities (55). This pronounced δ 15 N depletion suggests nitrogen limitation and potential alteration in nitrogen cycling processes under elevated salinity. The effects of extreme climatic conditions limit resource acquisition and overall plant performance in the Gulf. A high sediment δ 13 C (− 16.6 ± 1.0) and low carbon:nitrogen ratio (7.7 ± 3.7) in natural forests reflect enriched organic matter from a diverse variety of sources, including benthic microphytes. Our findings suggest that the ecological services provided by the mangrove planted four decades ago are likely inferior to those of natural forest. Policymakers and restoration programs concerned with the conservation of mangrove forests should consider the relative trade-offs of deforesting of ‘old’ growth forests for urban development versus the limited potential of afforestation on barren beaches.