Mangroves store a total of 2.6 Pg of carbon in their sediments and, together with wetlands, contribute to more than 50% of global carbon (C) export to the coastal ocean through lateral outwelling. However, understanding mangrove outwelling is difficult due to the heterogeneity and complexity of these ecosystems and their carbon reserves, as well as their seasonal processes.
Mangroves are our most carbon rich forests and most productive blue carbon ecosystems. They globally store 2.6 Pg C in their sediments and contribute, together with wetlands, more than 50% of global carbon (C) export to the coastal ocean through lateral fluxes (i.e., outwelling). Organic C provides about 20-30% of this outwelling, but a large part is unaccounted for in current C budgets. The outwelling of dissolved inorganic carbon (DIC) and alkalinity (TA) could account for this missing C. TA furthermore controls the capacity of water to sequester anthropogenic CO2. The ratio between DIC and TA determines the buffering capacity to changes in pH of mangroves and its neighbouring coastal ocean. To close the global C budget, focus needs to be set on coupling terrestrial and oceanic C cycles and it is thus of urgent matter to better understand outwelling of mangrove ecosystems.
The inclusion of C outwelling of mangroves in the global C budget is difficult due to the heterogeneity and intricacy of these ecosystems and their C pools, and their changing processes seasonally. The scarcity of measurements, both spatially and temporarily, is an additional obstacle. Moreover, sampling in those ecosystems is quite challenging because of the harsh conditions (limited accessibility, high salinity and/or a large tidal range). Sample analysis is difficult due to the large salinity range and fluctuating detection signal of δ13C at low DIC concentrations. As a consequence, despite their known importance on the global C budget, there are huge gaps of knowledge that make the calculation of mangroves C budget quite uncertain. It is essential to gather new data in order to understand how seasonal fluctuations might impact the role of mangrove ecosystems on C outwelling and therefore, on coastal ocean acidification.
In DOICOM, a collaboration between ENS, LMD and LSCE (France), CEAB-CSIC (Spain), in the frame of LMI TAPIOCA IRD-MARBEC (France) and UFPE, UFRPE (Brazil), and financed by IPSL, we tackle some of the identified knowledge gaps. We will analyze water samples for DOC, DIC, TA and δ13C from two different mangrove ecosystems in The Gambia (dry vs. wet season) and in Brazil (monthly), to better understand C outwelling from different mangrove systems in a seasonal context. The first samples have arrived from The Gambia thanks to SALBIA, a project of the Spanish Ministry of Science and are being analyzed.
A 36h field campaign, sampling each 2h, will be held mid-november in the Canal of Santa Cruz, Brazil, together with monthly sampling which will last until 2026 in the Frame of the project CHEMTRAIL (LEFE, founded by IREMER).
A. Stegehuis