Mangrove outwelling is a significant source of oceanic exchangeable organic carbon

Abstract

Exchangeable dissolved organic carbon (EDOC) makes up a significant proportion of the oceanic dissolved organic carbon (DOC) pool, yet EDOC sources to the coastal ocean are poorly constrained. We measured the exchange of EDOC and concentrations of EDOC and DOC in mangrove waters over a 268 latitudinal gradient. A clear latitudinal trend was observed, with the highest EDOC concentrations in the tropics. EDOC exports to the coastal ocean were 4.7 6 1.9 mmol m 22 d 21 , equivalent to 11% of DOC exports (42.1 6 6.7 mmol m 22 d 21). Pore-water and groundwater exchange were minor sources of EDOC. EDOC concentrations were equal to 13% 6 4% of DOC concentrations. Based on previous global DOC export estimates, and our EDOC : DOC ratios, mangroves outwell 3.1 Tg C yr 21 as EDOC, equivalent to 60% of the global EDOC flux from the ocean to the atmosphere. However, seasonality of mangrove EDOC cycling requires further research.Exchangeable dissolved organic carbon (EDOC) consists of volatile organic compounds (VOCs) and semi-volatile organic compounds (SOCs) which are ubiquitous carbon species that play a major role in global tropospheric chemistry and the global carbon cycle (Fehsenfeld et al. 1992;Fuentes et al. 2000). Sources of VOCs and SOCs can be both anthropogenic and biogenic. Anthropogenic VOC and SOC production occurs during fossil fuel combustion, industrial chemical production, and waste processing (Fuentes et al. 2000). Biogenic VOC and SOC emissions from terrestrial vegetation accounts for the vast majority of the inputs to the atmosphere (Guenther et al. 1995;Laothawornkitkul et al. 2009). The atmospheric exchange of marine biogenic VOCs and SOCs has also been identified as both an important sink *Correspondence: j.sippo.11@student.scu.edu.au Author Contribution Statement: IS, DM, and SR-H came up with the research question and designed the study approach. DT led the field survey for five of the six sites and DM and SR-H led the field survey for one site. All authors helped with data analysis and writing of the manuscript. JS helped to collect the data, analyzed the data, and wrote the manuscript.Data Availability Statement: All data from this study are attached in a supplementary file and are separately loaded in the Dryad database at doi: 10.5061/dryad.cq91s.This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Scientific Significance StatementExchangeable dissolved organic carbon (EDOC) consists of volatile and semi volatile organic compounds which are globally ubiquitous carbon species and play an important role in atmospheric chemistry. However, no studies have quantified EDOC outwelling from mangrove forests, in spite of these systems contributing > 10% of the terrestrially derived dissolved organic carbon (DOC) to the ocean. By measuring EDOC and DOC outwelling from a mangrove tidal creek and conc...

Controls on Carbon, Nutrient, and Sediment Cycling in a Large, Semiarid Estuarine System; Princess Charlotte Bay, Australia

“…For comparison with these estimates, we assumed that the dissolved material that accumulated in the water column during dry season was exported to the PCB at the onset of wet season. The DIC, DOC, and TA export from the upper Normanby and Kennedy fell within the range reported in six mangrove creeks by Sippo et al (, ) but were an order of magnitude lower than other empirical and synthesis studies (Table ). Most empirical studies have focused on a single mangrove creek, which may explain the high variability if exports are balanced by sinks in other areas along the estuarine continuum.…”

Impacts of land management practices on blue carbon stocks and greenhouse gas fluxes in coastal ecosystems—A meta‐analysis

“…Overall, GHG fluxes in BCEs need more attention given that they are not often assessed and are difficult to quantify, as lateral exchange in these open systems can remove large amounts of GHGs in dissolved form during high tide and via porewater exchange (Fuentes & Barr, ; Maher, Santos, Golsby‐Smith, Gleeson, & Eyre, ; Santos, Maher, Larkin, Webb, & Sanders, ; Sippo et al, ). Studies concentrating on surface‐to‐atmosphere GHG exchange will therefore not be able to accurately capture sediment carbon cycling, a factor which is as of yet largely unaccounted for in BCE monitoring (but see Maher, Call, Santos, & Sanders, ).…”

“…Overall, GHG fluxes in BCEs need more attention given that they are not often assessed and are difficult to quantify, as lateral exchange in these open systems can remove large amounts of GHGs in dissolved form during high tide and via porewater exchange (Fuentes & Barr, 2015;Maher, Santos, Golsby-Smith, Gleeson, & Eyre, 2013;Santos, Maher, Larkin, Webb, & Sanders, 2019;Sippo et al, 2017).…”

Coastal carbon cycle changes following mangrove loss

“…Other carbon loss pathways such as oceanic Particulate Organic Carbon (POC) and Exchangeable Dissolved Organic Carbon (EDOC) outwelling were not considered in this study. POC and EDOC can account for approximately 10% (Bouillon et al 2008) and 2% (Sippo et al 2017) of mangrove carbon budgets, respectively.…”