Methane oxidation in surface water reduces emissions to the atmosphere in mangroves

peer reviewedMangroves store significant amounts of organic carbon in sediments. During carbon burial, methane (CH4) is produced in anoxic, organic-rich sediments and released to the surface waters via porewater exchange and ebullition. Yet, highly variable CH4 emissions have been reported in mangroves because of high uncertainty of methane production and oxidation rates. Combining the stable isotopic composition of methane (δ13C-CH4) in porewater and surface water can reveal the fraction of CH4 oxidized or emitted to the atmosphere. Here, we report high-temporal resolution CH4 concentrations from creek waters and porewater at two mangrove creeks in Brazil along with measurements of δ13C of CH4. Enriched δ13C-CH4 in top surface layer sediments indicates CH4 oxidation in the surface sediment before porewaters reach the water column. Tidal pumping in mangrove facilitates CH4 oxidation in the water column. Surface waters at low tide exhibited a lighter δ13C-CH4 than at high tide. A similar δ13C-CH4 signature between low tide surface water (-70 ± 2 ‰) and porewater (-74 ± 4 ‰) imply that sediment is the source of CH4. A stable isotope mass balance showed that 30 – 66% of was oxidized within water column, with the rate of 19-121 µmol m-2 d-1. Air-sea CH4 emissions were estimated at both mangroves (68 ± 65 in a pristine and 179 ± 205 µmol m-2 d-1 in an urbanized mangrove), on the same order of magnitude as the CH4 oxidation rate. Overall, our results suggested that CH4 oxidation in mangrove surface water and sediments partially reduce CH4 emissions to the atmosphere

Coastal acidification and carbon sequestration driven by inorganic carbon export from tidal wetlands

peer reviewedCoastal ecosystems are under threat from ocean acidification. Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and biogeochemical processes altering carbonate chemistry. Mangroves and saltmarshes are global biogeochemical hotspots sequestering large amounts of carbon in sediments and in the ocean following lateral carbon export (outwelling). Here, we investigate whether mangroves and saltmarshes drive or buffer coastal waters against acidification and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of tidal wetlands export more DIC than alkalinity, enhancing pH declines of coastal waters. Porewater-derived DIC outwelling (81 ± 47 mmol/m2/d in mangroves and 57 ± 104 mmol/m2/d in saltmarshes) was the major fate of plant production. However, substantial amounts of fixed carbon remain unaccounted for in budgets. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant carbon sequestration mechanism enhancing the overall value of tidal wetlands as a nature-based solution to climate change

Carbonate chemistry and carbon sequestration driven by inorganic carbon outwelling from mangroves and saltmarshes

\ua9 2023, The Author(s).Mangroves and saltmarshes are biogeochemical hotspots storing carbon in sediments and in the ocean following lateral carbon export (outwelling). Coastal seawater pH is modified by both uptake of anthropogenic carbon dioxide and natural biogeochemical processes, e.g., wetland inputs. Here, we investigate how mangroves and saltmarshes influence coastal carbonate chemistry and quantify the contribution of alkalinity and dissolved inorganic carbon (DIC) outwelling to blue carbon budgets. Observations from 45 mangroves and 16 saltmarshes worldwide revealed that >70% of intertidal wetlands export more DIC than alkalinity, potentially decreasing the pH of coastal waters. Porewater-derived DIC outwelling (81 \ub1 47 mmol m−2 d−1 in mangroves and 57 \ub1 104 mmol m−2 d−1 in saltmarshes) was the major term in blue carbon budgets. However, substantial amounts of fixed carbon remain unaccounted for. Concurrently, alkalinity outwelling was similar or higher than sediment carbon burial and is therefore a significant but often overlooked carbon sequestration mechanism

The western south atlantic ocean in a high-CO2 world: current measurement capabilities and perspectives

An international multi-disciplinary group of 24 researchers met to discuss ocean acidification (OA) during the Brazilian OA Network/Surface Ocean-Lower Atmosphere Study (BrOA/SOLAS) Workshop. Fifteen members of the BrOA Network (www. broa. furg. br) authored this review. The group concluded that identifying and evaluating the regional effects of OA is impossible without understanding the natural variability of seawater carbonate systems in marine ecosystems through a series of long-term observations. Here, we show that the western South Atlantic Ocean (WSAO) lacks appropriate observations for determining regional OA effects, including the effects of OA on key sensitive Brazilian ecosystems in this area. The impacts of OA likely affect marine life in coastal and oceanic ecosystems, with further social and economic consequences for Brazil and neighboring countries. Thus, we present (i) the diversity of coastal and open ocean ecosystems in the WSAO and emphasize their roles in the marine carbon cycle and biodiversity and their vulnerabilities to OA effects; (ii) ongoing observational, experimental, and modeling efforts that investigate OA in the WSAO; and (iii) highlights of the knowledge gaps, infrastructure deficiencies, and OA-related issues in the WSAO. Finally, this review outlines long-term actions that should be taken to manage marine ecosystems in this vast and unexplored ocean region

Spatio-temporal variability and controls on methane and nitrous oxide in the Guadalquivir Estuary, Southwestern Europe

Estuaries are significant methane (CH4) and nitrous oxide (N2O) emitters, although dynamics of both greenhouse gases in these ecosystems are regulated by complex processes. In this work, we aimed at characterizing the spatio-temporal distribution of CH4 and N2O in the Guadalquivir river estuary (SW Spain), the southernmost European estuary. During eight sampling cruises conducted between 2016 and 2017, surface water CH4 and N2O concentrations were measured along the salinity gradient of the estuary by using static-head space equilibration gas chromatography. The CH4 and N2O saturation ranges over the estuarine transect were 520–30,800% (average 2285%) and 40–390% (average 183%), respectively and air–water fluxes ranged from 13 to 1000 µmol m− 2 day− 1(average 66.2 µmol m− 2 day− 1) for CH4 and from − 7 to 35 µmol m− 2 day− 1 (average 8.5 µmol m− 2 day− 1) for N2O. A slight increase in the emissions was detected upstream and no seasonal trends were observed. Mixing between freshwater and oceanic waters influenced biogeochemistry of estuarine waters, affecting CH4 and N2O fluxes. In order to identify potential sources of CH4 and N2O, biogeochemical parameters involved in the formation pathways of both gases, such as salinity, dissolved oxygen, nutrients and organic matter were analyzed. Results suggested that sulfate inhibition and microbial oxidation played a relevant role in dissolved CH4 accumulation in the water column whereas associations found between N2O, nitrate and oxygen indicated that nitrification was a major source of this gas. Therefore, the influence of the tidal-fluvial interaction on ecosystem metabolism regulates trace gas dynamics in the Guadalquivir estuary.This research was funded by the project 1539/2015 from the Spanish Ministry for Agriculture, Food and Environment.Peer reviewe