Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

Abstract

Mangroves and salt marshes are among the most productive ecosystems in the global coastal ocean. Mangroves store more carbon (739 Mg CORG ha-1) than salt marshes (334 Mg CORG ha-1), but the latter sequester proportionally more (24%) net primary production (NPP) than mangroves (12%). Mangroves exhibit greater rates of gross primary production (GPP), above-ground net primary production (NPP) and plant respiration (RC) with higher PGPP/RC ratios, but salt marshes exhibit greater rates of below-ground NPP. Mangroves have greater rates of subsurface DIC production and, unlike salt marshes, exhibit significant microbial decomposition to a soil depth of 1 m. Salt marshes release more soil CH4 and export more dissolved CH4 , but mangroves release more CO2 from tidal waters and export greater amounts of POC, DOC and DIC to adjacent waters. Both ecosystems contribute only a small proportion of GPP, RE (ecosystem respiration) and NEP (net ecosystem production) to the global coastal ocean due to their small global area, but contribute 72% of air-sea CO2 exchange from the world’s wetlands and estuaries and contribute 34% of DIC export and 17% of DOC + POC export to the world’s coastal ocean. Thus, both wetland ecosystems contribute disproportionately to carbon flow of the global coastal ocean.

Surface Water and Groundwater Interactions in Salt Marshes and Their Impact on Plant Ecology and Coastal Biogeochemistry

“…Groundwater‐derived CH 4 fluxes in salt marshes were quantified only on three occasions and range from 0.2 to 1.2 mmol m −2 d −1 (Porubsky et al., 2014; Santos et al., 2019; Schutte et al., 2020). These values are comparable to the global average of 0.24 mmol m −2 d −1 emitted from salt marsh surface water (Alongi, 2020).…”

“…Salt marshes are primary production hotspots that accumulate sediment and carbon at high rates (Alongi, 2020; D. Wang et al., 2021). Part of the carbon accumulated in salt marsh sediments can be flushed out via shallow porewater exchange and deep groundwater flow, eventually reaching the coastal sea (Figure 13).…”

“…The average global CH 4 water‐air flux in salt marshes is 0.24 mmol m −2 d −1 (Alongi, 2020) and can range from −0.09 to 94.4 mmol m −2 d −1 when including both water and sediment fluxes (Rosentreter et al., 2021). Methane is generated during microbial decomposition of organic matter usually under anaerobic conditions.…”

“…The hydrological background was modified from Santos, Chen, et al. (2021), carbon fluxes upscaled to global salt marshes in units of Tg C y −1 are obtained from Alongi (2020), and simplified biogeochemical reactions are obtained from Burdige (2007). Globally, the gross primary production of salt marshes is 97 Tg C y −1 .…”

Temperature acclimation of leaf respiration differs between marsh and mangrove vegetation in a coastal wetland ecotone

“…These ecosystems have high rates of C accumulation and burial, slow decomposition rates, and a relatively high fraction of biomass belowground. Autotrophic R (a large proportion of which is leaf R ) is a major component of the coastal wetland C cycle, accounting for an estimated ~50% of GPP in marsh and mangrove ecosystems (Alongi, 2020). However, direct measures of leaf R or its temperature sensitivity over space and time are relatively rare for marsh and mangrove species.…”