Carbon outwelling and greenhouse gas exchange across mangrove seascapes

Abstract

Mangroves, renowned for their high primary productivity and carbon burial rates, play a crucial role in the ocean carbon cycle. However, uncertainties persist regarding the fate of mangrove-derived carbon across seascapes. This doctoral thesis addresses these uncertainties by quantifying mangrove vertical water-atmosphere and horizontal mangrove-ocean exchange, an often-overlooked pathway for carbon sequestration and greenhouse gas exchange. I combine new regional-scale observations with comprehensive global-scale reviews to better understand mangrove carbon fluxes. I used radium isotopes to quantify carbon outwelling across several spatial scales, from porewater to the continental shelf. My results demonstrated that the mangrove-derived carbon is rapidly exported to the continental shelf mostly as bicarbonate, increasing the perceived carbon sequestration capacity of mangroves when compared to soil carbon burial alone. My findings also revealed that mangroves not only act as sources of carbon dioxide (CO2) and methane (CH4) but also as sinks for nitrous oxide (N2O). A global mangrove N2O sink can offset 18% of global mangrove CH4 emissions over a 20-year time horizon. Furthermore, neglecting lateral exports could underestimate CH4 emissions by up to 50% in relation to water-atmosphere fluxes. Global porewater-derived CO2 emissions represents 25% of the mangrove net primary production and are twice the estimated global sediment carbon burial rates. Overall, my thesis revels complex carbon flux pathways in diverse mangrove seascapes and emphasizes the importance of incorporating lateral transport and multiple greenhouse gases to resolve the net climate change mitigation potential of blue carbon ecosystems. The significant role of outwelling, particularly as bicarbonate, highlights the need for further research on the long-term fate of mangrove-derived carbon in the ocean. Moreover, my findings shift our perception of mangroves as N2O sources to N2O sinks that partially offset their methane emissions. Integrating multi-scale observations and diverse geochemical tracers was essential to build a more comprehensive understanding of mangrove carbon cycling and its implications for climate change mitigation strategies.