Lateral Export of Dissolved Inorganic and Organic Carbon from a Small Mangrove Estuary with Tidal Fluctuation

Abstract

The significance of aquatic lateral carbon (C) export in mangrove ecosystems highlights the extensive contribution of aquatic pathways to the net ecosystem carbon budget. However, few studies have investigated lateral fluxes of dissolved organic carbon (DOC) and inorganic carbon (DIC), partly due to methodological difficulty. Therefore, we evaluated area-based lateral C fluxes in a small mangrove estuary that only had one exit for water exchange to the coast. We sampled water from the mouth of the creek and integrated discharge and consecutive concentration of mangrove-derived C (ΔC). Then, we estimated the area-normalized C fluxes based on the inundated mangrove area. DIC and DOC concentrations at the river mouth increased during ebb tide during both summer and winter. We quantified the ΔC in the estuary using a two-component conservative mixing model of freshwater and seawater. DIC and DOC proportions of ΔC concentrations at the river mouth during ebb tide was between 34% and 56% in the winter and 26% and 42% in the summer, respectively. DIC and DOC fluxes from the estuary were estimated to be 1.36 g C m−2 d−1 and 0.20 g C m−2 d−1 in the winter and 3.35 g C m−2 d−1 and 0.86 g C m−2 d−1 in the summer, respectively. Based on our method, daily fluxes are mangrove area-based DIC and DOC lateral exports that can be directly incorporated into the mangrove carbon budget.

Dissolved and particulate carbon export from a tropical mangrove‐dominated riverine system

“…In the case of exported OC, DOC remains the major form with DOC : POC of 3.2 ± 0.75, which is higher than the ratios observed globally in mangrove waters (> 1.5; Bouillon et al 2007). In addition, high water discharge due to seasonal typhoons could enhance organic carbon export from mangrove forest (Ohtsuka et al 2020) such was observed in Panay Island where reduction of 40–50% organic matter in eroded mangrove soil was reported after successive severe typhoons in the past (Salmo 2021).…”

“…Comparison of carbon export/import fluxes as DOC, POC, and DIC between this study and selected mangroves around the world (unit: mmol m −2 d −1 ).Uncertainties in mean fluxes are linked to seasonal variability in our study site. Data source: Amazon and Brazil: Dittmar and Lara (2001), Dittmar et al (2006); Florida: Romigh et al (2006), Ho et al (2017); Micronesia: Call et al (2019); India: Ray et al (2018); East Australia: Maher et al (2013); NE Australia: Sippo et al (2016); Fly delta, PNG: Robertson and Alongi (1995); Vietnam: Taillardat et al (2018); Japan: Ohtsuka et al (2020); Global mean: Alongi (2020). …”

“…Unlike most of the previous flux estimates, we have considered internally produced or mangrove-derived carbon (ΔC, Eq. 2) rather than the absolute concentrations to determine fate of the "actual amount of carbon" delivered from the mangrove waters (similar to works by Ho et al 2017;Ray et al 2018Ray et al , 2020Ohtsuka et al 2020;Reithmaier et al 2020). Likewise, stable isotope mixing models should be critically evaluated.…”

“…Different methods for lateral carbon flux estimates in mangrove‐dominated coasts have been reported. These are hydrodynamic modeling and estuarine geometry (Sánchez‐Carrillo et al 2009; Ray et al 2020), cross‐sectional monitoring of creeks (Dittmar and Lara 2001; Ohtsuka et al 2020), flume constructed across creeks (Romigh et al 2006; Ray et al 2018), digital elevation modeling (Sippo et al 2016; Ray et al 2020), and mass balance approach (Akhand et al 2021). Estimates can be highly variable at different spatial scales depending on various factors such as seasonal rainfall (wet vs. dry), mangrove settings (riverine, fringing, or basin), tidal regime (i.e., tidal amplitude, symmetry), and terrestrial connectivity via freshwater inputs (Kristensen et al 2017; Taillardat et al 2018; Ray et al 2020).…”

Distribution, Sources, and Biogeochemistry of Carbon Pools (DIC, DOC, and POC) in the Mangrove-Fringed Zhangjiang Estuary, China

“…This may be due to the input of pore water via the tidal pump. In mangrove creeks, many studies have shown that the DOC is controlled by the pore water (Bouillon et al, 2007a;Bouillon et al, 2007b;Maher et al, 2013;Pierre et al, 2018;Call et al, 2019;Ohtsuka et al, 2020). During ebb tides, the water that retreats from the tidal creek has a composition similar to that of the pore water, which explains >60% of the DOC and >90% of the DIC (Bouillon et al, 2007a;Maher et al, 2013;Pierre et al, 2018;Call et al, 2019;Ohtsuka et al, 2020).…”