A global database of dissolved organic matter (DOM) concentration measurements in coastal waters (CoastDOM v1)

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) con-centrations are used to characterize the dissolved organic matter (DOM) pool and are important components ofbiogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOMv1; available at https://doi.org/10.1594/PANGAEA.964012, L\uf8nborg et al., 2023) compiling previously pub-lished and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complementedby hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables(e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon andtotal alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However,most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the SouthernHemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC,20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequenceDOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focuson carbon cycling by the aquatic science community. The global database shows that the average DOC concen-tration in coastal waters (average \ub1 standard deviation (SD): 182 \ub1 314 μmol C L−1; median: 103 μmol C L−1) is13-fold higher than the average coastal DON concentration (13.6 \ub1 30.4 μmol N L−1; median: 8.0 μmol N L−1),which is itself 39-fold higher than the average coastal DOP concentration (0.34 \ub1 1.11 μmol P L−1; median:0.18 μmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and willhelp facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochem-ical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing abaseline for modelling future changes in coastal waters

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

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