Organic-rich mangrove soils release CH4, partially offsetting the climate benefits of high organic carbon sequestration. However, the local and global drivers and variability of mangrove CH4 emissions remain poorly understood. Here, we quantify water-atmosphere CH4 emissions over hourly, daily, and weekly time scales in an Australian mangrove ecosystem. We then combine our new observations with earlier data sets to link temperature and mangrove CH4 emissions on the global scale. The water-atmosphere CH4 emissions were partially controlled by temperature on both local and global scales. One degree warming increased mangrove water-atmosphere CH4 emissions by similar to 23% locally and similar to 13% globally. Globally scaled water-atmosphere CH4 emissions (0.07-0.10 Tg C yr-1) currently offset 6%-8% of mangrove carbon burial. CH4 emissions are predicted to increase by 10%-33% by 2100 under global warming scenarios and tropicalization. Therefore, mangrove CH4 emissions should be considered in blue carbon assessments in the context of global warming.

Estuaries are important sources and sinks of greenhouse gases (GHGs), yet assessments of their radiative balance often focus primarily on carbon dioxide (CO2), with less attention given to methane (CH4) and nitrous oxide (N2O). We combine measurements of CH4 and N2O air-water fluxes with concurrent CO2 data in a tropical estuarine delta (Para & iacute;ba do Sul River, Brazil), based on three surveys under contrasting hydrological conditions. CH4 concentrations were highest in freshwater-dominated waters and mangrove creeks and decreased seaward, with greater values during high river discharge. Surface waters were supersaturated in CH4, indicating a permanent atmospheric source driven by freshwater inputs and organic matter degradation. In contrast, N2O exhibited weaker variability and alternated between source and sink, with concentrations linked to ammonium (NH4 +) and apparent oxygen utilization, suggesting nitrification as a dominant control. In CO2-equivalent terms, CH4 emissions offset 30%-84% of the CO2 sink during dry conditions and enhanced CO2 emissions by 20%-44% during high discharge. N2O exerted a smaller but variable influence, either reinforcing or partially counterbalancing CO2 fluxes. Together, CH4 and N2O offset 10%-85% of the CO2 sink in dry periods and added 18%-42% to CO2 emissions during wet conditions. The relative influence of CH4 and N2O was greatest where CO2 fluxes were near equilibrium, particularly in the mixing and marine domains. Because many tropical deltas exhibit mixing-driven CO2 uptake of weak magnitude, CH4 and N2O fluxes can reshape the radiative balance, underscoring the need for combined GHG assessments where CO2 exchange is low or bidirectional.

We report hydrogeochemical and isotopic observations across the Baltic Sea from two research expeditions: (1) a ~5000 km cruise-track onboard the R/V Skagerak in 2023 and (2) a land-based sampling for terrestrial endmembers in 2024. The ship-based observations include continuous monitoring of hydrographic parameters, pH, and 222Rn in surface water. In addition, we collected 542 discrete samples from the water column, vertical profiles (n = 69 stations), and meteorological data. Land observations include discrete samples from beach groundwater (n = 77), nearshore surface water (n = 47), and rivers close to the coastline (n = 46). Discrete samples were analyzed for short-lived radium isotopes, nutrients, dissolved organic and inorganic carbon, total dissolved nitrogen, total alkalinity, methane, and stable isotopes (δ18OH2O, δ2HH2O, δ13CDIC, δ13CCO2, δ13CCH4). Data products include seven open-access files. This dataset forms the deposit for upcoming original research publications. This dataset will also be valuable to researchers interested in the hydrogeochemistry of coastal seas, like the Baltic Sea, and more generally interested in submarine groundwater discharge and estuarine biogeochemistry.

Dissolved organic carbon (DOC) in coastal waters is integral to biogeochemical cycling, but global and regional drivers of DOC are still uncertain. In this study we explored spatial and temporal differences in DOC concentrations and stocks across the global coastal ocean, and how these relate to temperature and salinity. We estimated a global median coastal DOC stock of 3.15 Pg C (interquartile range (IQR) = 0.85 Pg C), with median DOC concentrations being 2.2 times higher than in open ocean surface waters. Globally and seasonally, salinity was the main driver of DOC with concentrations correlated negatively with salinity, without a clear relationship to temperature. DOC concentrations and stocks varied with region and season and this pattern is likely driven by riverine inputs of DOC and nutrients that stimulate coastal phytoplankton production. Temporally, high DOC concentrations occurred mainly in months with high freshwater input, with some exceptions such as in Eastern Boundary Current margins where peaks are related to primary production stimulated by nutrients upwelled from the adjacent ocean. No spatial trend between DOC and temperature was apparent, but many regions (19 out of 25) had aligned peaks of seasonal temperature and DOC, related to increased phytoplankton production and vertical stratification at high temperatures. Links of coastal DOC with salinity and temperature highlight the potential for anthropogenic impacts to alter coastal DOC concentration and composition, and thereby ecosystem status.

Mangroves store significant amounts of carbon in both sediment and water. Methane (CH4) is often produced in anoxic, organic-rich sediments during carbon degradation and released to overlying waters via porewater exchange. Yet, a portion of CH4 can be oxidized to CO2 before emission. Here, we investigate whether CH4 oxidation impacts its emissions using high-temporal resolution CH4 concentration and stable isotope (delta C-13-CH4) observations collected over 14 tidal cycles in 2 Brazilian mangrove creeks with no river inputs. We found higher CH4 concentrations (similar to 150 nM) more depleted in C-13 (-75 parts per thousand) during low tide than high tide at both creeks. Similar delta C-13-CH4 values between low tide surface waters and porewaters further suggest tidally driven porewater exchange as the main source of CH4. More C-13-enriched CH4 in surface waters and surface sediments than deep sediments indicate partial CH4 oxidation prior to exchange with the atmosphere. A stable isotope mass balance revealed that 17-58% of CH4 was oxidized at rates of 3-25 mu mol m(-2) d(-1) in the water column of tidal creeks. A larger portion of deep porewater CH4 (45-61%) was oxidized in sediments prior to porewater exchange with surface creek waters. The two mangrove creeks had average water-air CH4 fluxes of 51-109 mu mol m(-2) d(-1) over spring-neap tidal cycles. These aquatic CH4 emissions offset only < 3% of the mangroves' soil carbon sequestration. Overall, CH4 oxidation in both surface water and sediment attenuated CH4 emissions to the atmosphere.

The Jacarepagu ' a Lagoon System (JLS) receives industrial and domestic waste in an urban area with high population density and intense economic activity. The hydrography of the lagoons favours the sedimentation of particulate material transferred from the drainage basin. Water engineering, such as channel dredging and subsea outfall, did not satisfactorily mitigate pollution effects. Therefore, the environment is highly eutrophic, presents frequent blooms of algae and generates high emissions of greenhouse gases. There is no record in the literature on the analysis of organic compounds in the water compartment. The present work applies sterols as biomarkers to quantify the degree of pollution caused by biogenic compounds in riverine and lacustrine water of the JLS. n-Alkanes were applied to estimate the fractions of petrogenic contaminants. The sums of n-alkanes and sterols analysed had average concentrations of 21 +/- 20 mu g L-1 and 10 +/- 8 mu g L- 1, respectively, in the river samples and 235 +/- 156 mu g L-1 and 30 +/- 28 mu g L-1, respectively, in the lagoon samples. The work also showed that the organic compounds inside the lagoons are evenly distributed, and approximately 7% of them are transferred to the marine ecosystem. Biogenic biomarkers and the absolute concentrations of sterols showed that sewage contaminants transferred by the rivers are partially decomposed in the lagoons. The correlations between indices and physicochemical parameters indicated that the degradation of organic compounds in the lagoons occurs mainly in the sediment compartment under anoxic conditions. The indices for sewage indicate that the ecosystem has exceeded its carrying capacity. The indices based on n-alkanes reported strong contamination at all sampling stations and inferred that 75-100% of these compounds were derived from petrogenic sources. These indices did not show any difference between rivers and the lagoon, which demonstrates the resilience of these compounds in the ecosystem.

Rivers and estuaries are the main links between continents and oceans. The Paraíba do Sul River is among the most important rivers of the southeastern Brazilian region, carrying an average of 0.08 Tg of dissolved organic matter (DOM) to the ocean but has been facing significant changes in river discharge. In this study, we aimed to provide insights into the sources and transformations of chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) sources across a salinity gradient under changing river discharge scenarios. Three spatial surveys were performed covering the entire salinity gradient of the main estuarine channel and surrounding mangrove waters under contrasting river discharge (178 to 1240 m3 s-1), and diel sampling was conducted in the mangrove tidal creek. The characterization of DOM through the parallel factor analysis (PARAFAC) model identified six components across the river-ocean gradient and mangrove creek: terrestrial origin (C1 - fulvic acid and C2 and C3 - humic-like), protein-like (C4), tryptophan-like (C5), and tyrosine-like (C6). Our results showed a shift in DOM composition and contribution along the salinity gradient, from terrestrial (C3) to autochthonous (C5 and C6) signatures. The October-17 dry campaign was characterized by a higher proportion of microbial protein-like component C4 and a lower contribution of humic-like components compared to February-17 and March-18 across the salinity gradient with an increase in the mixing zone. The DOM compositions of the February 17 dry and March 18 wet campaigns were similar. Additionally, the March-18 wet campaign, marked by the highest river discharge, showed higher inputs of terrestrial DOM (C1-C3 components) compared to February-17 in the estuary, which allowed DOM to be transported rather than transformed. The mangrove diel study showed that tidal fluctuations are also an important driver of carbon input to the mangrove creek with a possible impact on DOM composition in estuarine waters.

This study unraveled the origin, fate and accumulation of organic matter (OM) of surface sediments, spread between the fresh and marine end members of Guaratuba Bay (SE- Brazil). Sedimentation rates were obtained through the 210Pb sediment dating method, biogenic element accumulation (C-organic carbon, N-nitrogen, P-phosphorus, and lignin phenols) and stable isotopes of organic carbon (δ13C) and nitrogen (δ15N) were measured along the well-defined salinity gradient (0–35). The higher C:N ratios and lower δ13C values signatures indicate major terrestrial contribution in regions dominated by fluvial inputs, while the contributions from autochthonous primary production and mangrove forests were greater in the middle and outer estuarine sectors. A stable isotope mixing model showed that the material contribution from rivers and mangroves in surface sediments ranged from 80 to 90% in the upstream sector, 50–70% in the central sector and 25–50% at the outer region, whereas this contribution decreased to 15–20% in the adjacent ocean. Ratios between phenolic groups indicated material deposited from predominantly woody angiosperms. Vertical CNP deposition, accumulation rates and lignin phenol concentrations along the sedimentary profiles indicated three phases: Phase I, the more recently deposited sediments, which contained the highest CNP sedimentary fluxes; Phase II, dated to be from 1960s to 1990s, was characterized by changes related with increasing deforestation associated to fires; Phase III from the 1920s–1960s, exhibited four-fold lower C, N and P concentrations compared the more recent rates. Along the estuarine gradient, the C accumulation was found to be related to specific sources as indicated by the lignin's, stable isotopes as well as C:N ratios. Furthermore, the vertical accumulation of OM in sediments concomitantly with geochronology showed the recent anthropogenic perturbations in the estuary. This study highlights the land-ocean sedimentary continuum of which are poorly studied with respect to the transport of C, N and P in estuarine systems, reinforcing the presence of a marked land-ocean gradient. Knowledge of isotopic fractions, as well as lignin phenols, for different areas along the estuarine gradient is important as sampling along coastal areas can represent a bias in understanding the transport and cycling of OM to the sea if gradients and specific regions are not taken into account.

Coral reefs are ecosystems highly vulnerable to changes in seawater carbonate chemistry, including those related to the ocean acidification and global warming. Brazilian coral reefs contains the major area of reefs coverage in the Southwestern (SW) Atlantic Ocean, however, studies aimed at investigating the controls of seawater carbonate chemistry in coral reefs are still overlooked in Brazil. This study comprehends the first investigation of complete seawater carbonate chemistry parameters in a section of the equatorial continental shelf dominated by coral reefs in the SW Atlantic Ocean. The sampling included spatial continuous underway measurements of sea surface CO2 fugacity (fCO(2)sw), temperature (SST), salinity (SSS), and discrete investigations of total alkalinity (TA), dissolved inorganic carbon (DIC), bicarbonate (HCO3-), carbonate (CO32-), and saturation state of aragonite (Omega(ara)). The study was conducted during a dry period (July-2019) in the Marine State Park of Pedra da Risca do Meio (PRM), a marine protected area dominated by coral reef communities. Overall, the coral-reef dominated waters presented higher values of fCO(2)sw (475 +/- 28 mu atm), and lower values of pH(T) (7.98 +/- 0.008), CO32- (217 +/- 5 mu mol kg(-1)) and Omega(ara) (3.49 +/- 0.07), compared to nearshore regions without the influence of coral reef waters, where the averages of fCO(2)sw, pH(T), CO32-, and Omega(ara) were, respectively, 458 +/- 21 mu atm, 8.00 +/- 0.007, 224 +/- 4 mu mol kg(-1) , and 3.58 +/- 0.05. The relationship between salinity-normalized TA (nTA) and salinity-normalized DIC (nDIC) showed a slope higher than 1 (1.26) in the coral reef, evidencing the occurrence of calcium carbonate (CaCO3) precipitation and prevalence of inorganic carbon metabolism. The CaCO3 precipitation involves the consumption of TA and DIC in a ratio 2:1, with production of CO2. This mechanism explains the higher values of fCO(2)sw in the coral reef-dominated waters. The values of fCO(2)sw were always higher than the atmospheric values (fCO(2)air), indicating a permanent source of CO2 in the study area during the sampled period. The calculated fluxes of CO2 at the air-sea interface averaged 8.4 +/- 6.5 mmolC m(-2) d(-1) in the coral reef-dominated waters, and these data are higher than those verified in nearshore and offshore locations. These higher emissions of CO2 in coral reef-dominated waters evidence that the carbon budgets calculated for North and Northeastern continental shelf of Brazil must include these environments taking into account the widespread coral reef coverage in the region. This study also confirms that biogeochemical processes occurring in coral reefs are modifying the seawater carbonate chemistry, with implication in the context of the current process of ocean acidification.

During land-aquatic transfer, carbon (C) and inorganic nutrients (IN) are transformed in soils, groundwater, and at the groundwater-surface water interface as well as in stream channels and stream sediments. However, processes and factors controlling these transfers and transformations are not well constrained, particularly with respect to land use effect. We compared C and IN concentrations in shallow groundwater and first-order streams of a sandy lowland catchment dominated by two types of land use: pine forest and maize cropland. Contrary to forest groundwater, crop groundwater exhibited oxic conditions all-year round as a result of higher evapotranspiration and better lateral drainage that decreased the water table below the organic-rich soil horizon, prevented the leaching of soil-generated dissolved organic carbon (DOC) in groundwater, and thus limited consumption of dissolved oxygen (O2). In crop groundwater, oxic conditions inhibited denitrification and methanogenesis resulting in high nitrate (NO3-; on average 1140 ± 485 μmol L-1) and low methane (CH4; 40 ± 25 nmol L-1) concentrations. Conversely, anoxic conditions in forest groundwater led to lower NO3- (25 ± 40 μmol L-1) and higher CH4 (1770 ± 1830 nmol L-1) concentrations. The partial pressure of carbon dioxide (pCO2; 30,650 ± 11,590 ppmv) in crop groundwater was significantly lower than in forest groundwater (50,630 ± 26,070 ppmv), and was apparently caused by the deeper water table delaying downward diffusion of soil CO2 to the water table. In contrast, pCO2 was not significantly different in crop (4480 ± 2680 ppmv) and forest (4900 ± 4500 ppmv) streams, suggesting faster degassing in forest streams resulting from greater water turbulence. Although NO3-concentrations indicated that denitrification occurred in riparian-forest groundwater, crop streams nevertheless exhibited important signs of spring and summer eutrophication such as the development of macrophytes. Stream eutrophication favored development of anaerobic conditions in crop stream sediments, as evidenced by increased ammonia (NH4+) and CH4 in stream waters and concomitant decreased in NO3- concentrations as a result of sediment denitrification. In crop streams, dredging and erosion of streambed sediments during winter sustained high concentration of particulate organic C, NH4+ and CH4. In forest streams, dissolved iron (Fe2+), NH4+ and CH4 were negatively correlated with O2 reflecting the gradual oxygenation of stream water and associated oxidations of Fe2+, NH4+ and CH4. The results overall showed that forest groundwater behaved as source of CO2 and CH4 to streams, the intensity depending on the hydrological connectivity among soils, groundwater, and streams. CH4 production was prevented in cropland in soils and groundwater, however crop groundwater acted as a source of CO2 to streams (but less so than forest groundwater). Conversely, in streams, pCO2 was not significantly affected by land use while CH4 production was enhanced by cropland. At the catchment scale, this study found substantial biogeochemical heterogeneity in C and IN concentrations between forest and crop waters, demonstrating the importance of including the full vegetation-groundwater-stream continuum when estimating land-water fluxes of C (and nitrogen) and attempting to understand their spatial and temporal dynamics.

The dynamics of the aragonite saturation state (Ωarag) were investigated in the eutrophic coastal waters of Guanabara Bay (RJ-Brazil). Large phytoplankton blooms stimulated by a high nutrient enrichment promoted the production of organic matter with strong uptake of dissolved inorganic carbon (DIC) in surface waters, lowering the concentrations of dissolved carbon dioxide (CO2aq), and increasing the pH, Ωarag and carbonate ion (CO32-), especially during summer. The increase of Ωarag related to biological activity was also evident comparing the negative relationship between the Ωarag and the apparent utilization of oxygen (AOU), with a very close behavior between the slopes of the linear regression and the Redfield ratio. The lowest values of Ωarag were found at low-buffered waters in regions that receive direct discharges from domestic effluents and polluted rivers, with episodic evidences of corrosive waters (Ωarag<1). This study showed that the eutrophication controlled the variations of Ωarag in Guanabara Bay.

We investigate the carbon dynamics in Guanabara Bay, an eutrophic tropical coastal embayment surrounded by the megacity of Rio de Janeiro (southeast coast of Brazil). Nine sampling campaigns were conducted for dissolved, particulate and total organic carbon (DOC, POC and TOC), dissolved inorganic carbon (DIC), partial pressure of CO 2 ( p CO 2 ), chlorophyll a (Chl a ), pheo-pigments and ancillary parameters. Highest DOC, POC and Chl a concentrations were found in confined-shallow regions of the bay during the summer period with strong p CO 2 undersaturation, and DOC reached 82 mg L −1 , POC 152 mg L −1 , and Chl a 800 μg L −1 . Spatially and temporally, POC and DOC concentrations varied positively with total pigments, and negatively with DIC. Strong linear correlations between these parameters indicate that the production of TOC translates to an equivalent uptake in DIC, with 85% of the POC and about 50% of the DOC being of phytoplanktonic origin. Despite the shallow depths of the bay, surface waters were enriched in POC and DOC relative to bottom waters in periods of high thermohaline stratification. The seasonal accumulation of phytoplankton-derived TOC in the surface waters reached about 105 g C m −2 year −1 , representing between 8 and 40% of the net primary production. The calculated turnover time of organic carbon was 117 and 34 days during winter and summer, respectively. Our results indicate that eutrophication of coastal bays in the tropics can generate large stocks of planktonic biomass and detrital organic carbon which are permanently being produced and partially degraded and buried in sediments.

Carbon dioxide (CO2) fluxes from aquatic systems are generally derived from the gradient in the partial pressure of CO2 (pCO(2)) between air and surface waters. In this study, we compare real-time measurements of water pCO(2) using an equilibrator and non-dispersive infrared gas detector, with calculations based on pH and total alkalinity (TA) in two contrasting Brazilian estuaries: Guanabara Bay (Rio de Janeiro) and the Sao Francisco River Estuary (Alagoas). In Guanabara Bay, the measured and calculated values showed an excellent agreement (R-2 = 0.95, p < 0.0001), without significant statistical differences between the two methods. In the Sao Francisco River Estuary, where the entire gradient from freshwaters to seawater could be sampled, important overestimates were found for the calculated pCO(2). The overestimation was on average 71%, and reached up to 737%. This large bias in pCO(2) calculation was verified at low pH and TA concentrations in freshwaters (pH < 7.5; TA < 700 mu mol kg(-1)) possibly due to the contribution of organic alkalinity, lowering the buffer capacity of the carbonate system. As such, direct measurements of pCO(2) should be considered as a priority for CO2 studies conducted in estuarine systems, particularly tropical systems where physical and biological processes are prone to significant spatial and temporal variability.

The increasing concentrations of methane (CH 4 ) in the atmosphere stress the importance of monitoring and quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014, we measured the spatial CH 4 concentrations, identified major sources and calculated the fluxes at the air‐water interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presented high spatial variability of CH 4 concentrations, without a significant trend with salinity, but observed the influence of the urban areas at its watershed. Although the more polluted sector of the bay accounts for about 10% of the sampled surface area, it contributed to one half of the bay's total CH 4 emissions. In most cases, high CH 4 concentrations seemed be sustained by allochtonous sources such as the sewage network and polluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at the inner and centre of the Bay, CH 4 concentrations were not significantly higher in bottom hypoxic waters than in surface waters, suggesting that CH 4 diffusion from these sediments was modest, due to the prevalence of sulphate reduction over methanogenesis. Our calculated annual air‐sea fluxes (565–980 μmol m −2 d −1 ) are well above those of most estuaries worldwide, showing that urban pollution can be an important source of CH 4 to the coastal waters and even more significant than the presence of organic‐rich environments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO 2 ‐equivalent, CH 4 emissions reduced the net CO 2 sink in Guanabara Bay by 16%.

In contrast to its small surface area, the coastal zone plays a disproportionate role in the global carbon cycle. Carbon production, transformation, emission and burial rates at the land–ocean interface are significant at the global scale but still poorly known, especially in tropical regions. Surface water pCO2 and ancillary parameters were monitored during nine field campaigns between April 2013 and April 2014 in Guanabara Bay, a tropical eutrophic to hypertrophic semi-enclosed estuarine embayment surrounded by the city of Rio de Janeiro, southeast Brazil. Water pCO2 varied between 22 and 3715 ppmv in the bay, showing spatial, diurnal and seasonal trends that mirrored those of dissolved oxygen (DO) and chlorophyll a (Chl a). Marked pCO2 undersaturation was prevalent in the shallow, confined and thermally stratified waters of the upper bay, whereas pCO2 oversaturation was restricted to sites close to the small river mouths and small sewage channels, which covered only 10 % of the bay's area. Substantial daily variations in pCO2 (up to 395 ppmv between dawn and dusk) were also registered and could be integrated temporally and spatially for the establishment of net diurnal, seasonal and annual CO2 fluxes. In contrast to other estuaries worldwide, Guanabara Bay behaved as a net sink of atmospheric CO2, a property enhanced by the concomitant effects of strong radiation intensity, thermal stratification, and high availability of nutrients, which promotes phytoplankton development and net autotrophy. The calculated CO2 fluxes for Guanabara Bay ranged between −9.6 and −18.3 mol C m−2 yr−1, of the same order of magnitude as the organic carbon burial and organic carbon inputs from the watershed. The positive and high net community production (52.1 mol C m−2 yr−1) confirms the high carbon production in the bay. This autotrophic metabolism is apparently enhanced by eutrophication. Our results show that global CO2 budgetary assertions still lack information on tropical, marine-dominated estuarine systems, which are affected by thermal stratification and eutrophication and behave specifically with respect to atmospheric CO2.