Carbon storage in natural and restored macroalgae forests in Mediterranean shallow water bays.
Supervisors: Oscar Serrano, Inés Mazarrasa (CEAB-CSIC)
They usually grow in rocky substrates and their C org is mostly stored beyond their habitat. Here, we compared the sedimentary C org storage within and beyond natural and restored macroalgae forests dominated by G ongolaria barbata in Mediterranean shallow coastal settings to adjacent bare and s eagrass Cymodocea nodosa mixed with Caulerpa prolifera algae habitats . We characterized the biogeochemistry of top 30 cm sedimentary deposits , including sediment grain size, organic matter and C org contents, and C org burial rates and their provenance throughout stable carbon isotop es and pyrol y sis (Py GC MS and THM GC MS) analyses . We found that sediment C org stocks (in top 30 cm) and burial rates (since 1950) in natural (1.3 0.3 kg C org m 2 accumulated at 12.9 ± 1. 6 g C org m 2 y 1 ) and restored macroalgae forests (1.6 0.1 kg C org m 2 accumulated at 18. 1 ± 2.7 g C org m 2 y 1 ) fall within the range of those reported for traditional blue carbon ecosystems. Although the main contributor to sediment C org stocks in the three vegetated habitats examined was C. nodosa (36 2%), we estimated that G. barbata contributed 19 ± 2 % and C. prolifera 30 3% based on stable carbon isotopes. Pyrolysis confirmed the presence of specific macroalgae derived compounds in the sediments of all the vegetated habitats surveyed , which includ e 2 acetylfuran and alpha Tocopherol methyl ether linked to G. barbata and C. prolifera , respectively. The sediment C org burial rate linked to macroalgae in the natural and restored forests ranged from 5.4 to 9.5 g C org m 2 y 1 average 7.5 ± 2.6 g C org m 2 y 1 )), which is 50 higher than previous estimates for shelf sediments and thereby , suggest s that the global contribution of macroalgae to sediment C org sequestration within the habitat is likely underestimated. Moreover, the sediment C org burial rates in the restored forest increased 4 fold after restoration in 2010 suggesting that the recovery of G. barbata enhanced sediment C org burial. This study provides empirical evidence for the long term (~70 years) sequestration of macroalgae derived C org within and beyond seaweed forests in shallow coastal embayments and therefore , supports the inclusion of macroalgae in blue carbon frameworks.