Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

Abstract

A majority of the global net primary production of mangroves is unaccounted for by current carbon budgets. It has been hypothesized that this ''missing carbon'' is exported as dissolved inorganic carbon (DIC) from subsurface respiration and groundwater (or pore-water) exchange driven by tidal pumping. We tested this hypothesis by measuring concentrations and d 13 C values of DIC, dissolved organic carbon (DOC), and particulate organic carbon (POC), along with radon ( 222 Rn, a natural submarine groundwater discharge tracer), in a tidal creek in Moreton Bay, Australia. Concentrations and d 13 C values displayed consistent tidal variations, and mirrored the trend in 222 Rn in summer and winter. DIC and DOC were exported from, and POC was imported to, the mangroves during all tidal cycles. The exported DOC had a similar d 13 C value in summer and winter (, 230%). The exported d 13 C-DIC showed no difference between summer and winter and had a d 13 C value slightly more enriched (, 222.5%) than the exported DOC. The imported POC had differing values in summer (, 216%) and winter (, 222%), reflecting a combination of seagrass and estuarine particulate organic matter (POM) in summer and most likely a dominance of estuarine POM in winter. A coupled 222 Rn and carbon model showed that 93-99% of the DIC and 89-92% of the DOC exports were driven by groundwater advection. DIC export averaged 3 g C m 22 d 21 and was an order of magnitude higher than DOC export, and similar to global estimates of the mangrove missing carbon (i.e., , 1.9-2.7 g C m 22 d 21 ).

Small-scale variability in geomorphological settings influences mangrove-derived organic matter export in a tropical bay

“…Indeed, marked gradients were observed, with the most depleted values in the landward station close to mangrove forest, especially in the Kinondo mouth, and the most enriched ones in the seaward station. A similar enrichment was already observed in Gazi Bay and other tropical areas (Hemminga et al, 1994;Lugendo et al, 2007) and mirrors changes in δ 13 C DIC (Alongi, 2014;Maher et al, 2013). δ 13 C DIC is typically more negative close to mangroves as a result of the intense localised mineralisation of mangrove detritus (Bouillon et al, 2007) and increases seaward due to the higher contribution of oceanic DIC, whose δ 13 C is typically around 0 ‰ .…”

Carbon Budgets for Caribbean Mangrove Forests of Varying Structure and with Phosphorus Enrichment

“…This is similar in magnitude to our estimates of NEP for fringing mangroves in our study (70 [11,16] and globally, 118 mol C m −2 ·year −1 [10]. Maher et al [81] bserved that DIC export from mangroves in subterranean groundwater flow was driven by tidal pumping. Despite the very low tidal range at Twin Cays (<50 cm), photographic evidence and observed δ 13 C of adjacent seagrass suggest that export of DIC from porewater is a plausible fate for the unaccounted for carbon at Twin Cays.…”

Seasonal Variations of Carbon Dioxide, Water Vapor and Energy Fluxes in Tropical Indian Mangroves

“…The study does not, however, account for other important elements of aquatic pathways within which factors such as tidal and groundwater CO 2 export and dissolved inorganic and organic carbon (DIC) exports can influence the NEP to a considerable degree ( [17,53]). Information on such holistic factors which drive the carbon fluxes can add value to our current estimates, which the authors envisage in future studies.…”