Net carbon accumulation in mangroves forests strongly depends on the quantity and quality of organic matter (OM) supplied to sediments, including also OM transported by coastal waters such as the macroalgae Sargassum. Mesocosms that simulate the tides were used during 60 days to assess the effect of eutrophication by Sargassum thallus spp. of mangrove sediments. The concentration and d¹³C signature of fatty acids (FAs) and organic carbon as well as the sediments-air CO2 fluxes were used to follow the evolution of the quantity and the quality of sedimentary OM (SOM) in the top 10 cm layer. Sargassum beaching led to a preferential degradation of the labile fraction of OM from both Sargassum (d¹³C = -17,7‰ and high concentration of essential FAs) and mangrove leaves (d¹³C: -28,9‰ and high concentrations of 18:2w6 and 18:3w3), better preserving the refractory OM from leaves (Long chain FAs). Inputs of macroalgae induced a negative priming effect and enhanced the preservation of blue carbon in the sediments. This effect led to a 2 to 3 times higher OM accumulation in subsurface sediments, compared to when no thallus were added. The FAs composition of crabs hepatopancreas reared in some of the mesocosms revealed they preferentially fed on OM from Sargassum, and that their activity enhanced the negative priming effect by 30%.

Primary Presenter: Tarik Meziane, Muséum National d'Histoire Naturelle (tarik.meziane@mnhn.fr)

Authors:

Mathias Chynel, Muséum National d'Histoire Naturelle  (mathias.chynel@mnhn.fr)

Gwenaël Abril, CNRS  (gwenael.abril@mnhn.fr)

Mélissa Narayaninsamy, Muséum National d'Histoire Naturelle  (melissa.Narayaninsamy@mnhn.fr)

Tarik Meziane, Muséum National d'Histoire Naturelle  (tarik.meziane@mnhn.fr)