Component-specific dynamics of riverine mangrove CO₂ efflux in the Florida coastal Everglades

Carbon cycling in mangrove forests represents a significant portion of the coastal wetland carbon (C) budget across the latitudes of the tropics and subtropics. Previous research suggests fluctuations in tidal inundation, temperature and salinity can influence forest metabolism and C cycling. Carbon dioxide (CO₂) from respiration that occurs from below the canopy is contributed from different components. In this study, we investigated variation in CO₂ flux among different below-canopy components (soil, leaf litter, course woody debris, soil including pneumatophores, prop roots, and surface water) in a riverine mangrove forest of Shark River Slough estuary, Everglades National Park (Florida, USA). The range in CO₂ flux from different components exceeded that measured among sites along the oligohaline-saline gradient. Black mangrove (Avicennia germinans) pneumatophores contributed the largest average CO₂ flux. Over a narrow range of estuarine salinity (25-35 practical salinity units (PSU)), increased salinity resulted in lower CO₂ flux to the atmosphere. Tidal inundation reduced soil CO₂ flux overall but increased the partial pressure of CO₂ (pCO₂) observed in the overlying surface water upon flooding. Higher pCO₂ in surface water is then subject to tidally driven export, largely as HCO₃. Integration and scaling of CO₂ flux rates to forest scale allowed for improved understanding of the relative contribution of different below-canopy components to mangrove forest ecosystem respiration (ER). Summing component CO₂ fluxes suggests a more significant contribution of below-canopy respiration to ER than previously considered. An understanding of below-canopy CO₂ component fluxes and their contributions to ER can help to elucidate how C cycling will change with discrete disturbance events (e.g., hurricanes) and long-term change, including sea-level rise, and potential impact mangrove forests. As such, key controls on below-canopy ER must be taken into consideration when developing and modeling mangrove forest C budgets.