Contrasting organic matter composition in pristine and eutrophicated mangroves revealed by fatty acids and stable isotopes (Rio de Janeiro, Brazil)

Mathias Chynel, Sofia Rockomanovic, Gwenaël Abril, Glenda Barroso, Humberto Marotta, Wilson Machado, Christian J. Sanders, Najet Thiney, Tarik Meziane. Contrasting organic matter composition in pristine and eutrophicated mangroves revealed by fatty acids and stable isotopes (Rio de Janeiro, Brazil). Estuarine, Coastal and Shelf Science 277, 108061, 2022. https://doi.org/10.1016/j.ecss.2022.108061

Mangrove sediments have a high carbon storage capacity, due to the greater input of organic matter (OM) by mangroves trees (litterfall and fine root production) than microbial degradation of OM and its export to coastal waters. Mangrove sediments also act as traps for suspended solids and particulate OM from surrounding water masses (rivers/ocean). Inputs of easily biodegradable OM (sewage, algae...) could lead to microbial degradation of refractory carbon present in the sediments and thus limit the capacity of mangroves to store carbon. This phenomenon is called "priming effect". The objective of this study is to characterize the composition of OM in three mangroves located in three coastal bays of the state of Rio de Janeiro (Brazil) with increasing urbanization from a pristine mangrove (M1) to a moderately impacted mangrove (M2) and a heavily impacted mangrove (M3). In the pristine mangrove, the predominant input of OM comes from mangroves, especially in the inland zone where a high contribution of poorly biodegradable OM and a high concentration of carbon are observed. In the sediments of the moderately impacted mangrove, the high bacterial contribution and the high proportion of poorly biodegradable OM suggest that bacteria preferentially degrade the most biodegradable OM fraction. At the most impacted site, our results suggest that the deposition of biodegradable OM (microalgae) induced an increase in fungal biomass on the sediment, reinforcing the microbial loop, and potentially leading to the degradation of poorly biodegradable OM and the lower carbon concentrations observed at this site via a priming effect.

BOREA contact : Mathias Chynel, mathias.chynel@edu.mnhn.fr