Seagrass seedlings are key to meadow recovery under global change, as they enable recolonization of degraded areas and provide genetic variability needed for adaptation. While invasive macroalgae increasingly threaten seagrass communities, elevated CO2 has been proposed to enhance seagrass performance and potentially buffer other stressors. Here, we conducted a mesocosm experiment to test the combined effects of two invasive macroalgae (Lophocladia trichoclados and Caulerpa cylindracea) and elevated CO2 on Posidonia oceanica seedlings. CO2 enrichment increased carbohydrate reserves in rhizomes and induced subtle shifts in root-associated microbiomes. In contrast, invasive macroalgae had consistently negative effects on seedling development and physiology and strongly altered both above- and belowground microbial communities. Despite its potential to stimulate seagrass productivity, elevated CO2 did not mitigate the detrimental impacts of invasive macroalgae. These findings indicate that future CO2 conditions may not offset invasion-driven stress at the recruitment stage, highlighting the need for targeted management efforts to limit macroalgal proliferation and support seagrass meadow regeneration.

Abstract. Methane (CH4) accumulates in bottom waters of lakes, however, the extent and drivers of inter-lake variation in bottom-water CH4 concentrations are poorly understood and have been studied mainly in northern lakes. This limits predictions of how bottom-water CH4 concentrations respond to warming and eutrophication in lakes, and how these changes might influence surface-water CH4 concentrations and, consequently, CH4 emissions. We report 168 measurements of paired bottom- and surface-water CH4 concentrations from 46 African lakes spanning a wide range of surface area (SA; 0.02–67,075 km²) and maximum depth (2–180 m). Bottom-water CH4 concentrations ranged from 7 to 5,608,382 nmol L⁻¹, spanning six orders of magnitude, and increased with increasing stratification, quantified from vertical density profiles using potential energy anomaly (PEA) and mixed layer depth (MLD), and inferred from NH₄⁺ concentrations or vertical conductivity gradients. Surface-water CH4 concentrations ranged from 7 to 168,114 nmol L⁻¹ and increased with both bottom-water CH4 concentrations and vertical stratification (positively related to PEA and negatively to MLD). The most strongly stratified lakes exhibited high bottom-water CH4 concentrations, resulting in enhanced vertical transfer of CH4 to surface waters despite lower vertical diffusion coefficients. In addition, these lakes had shallower mixed layers and therefore thinner oxygenated surface layers, likely reducing CH4 removal via methane oxidation. The positive relationship between both bottom- and surface-water CH4 concentrations and chlorophyll-a (Chl-a) has previously been interpreted as reflecting enhanced methanogenesis driven by phytoplankton-derived organic matter delivered to sediments. However, such relationships may be indirect and should be interpreted cautiously, as Chl-a was negatively related to MLD in our dataset, and both bottom- and surface-water CH4 concentrations were also negatively related to MLD. The ratio of surface to bottom CH4 concentrations (surface:bottom CH4 ratio) may indicate the relative increase in surface CH4 in response to increases in bottom CH4 driven by warming and eutrophication. This ratio was negatively related to bottom depth, PEA, and MLD, and positively related to bottom-water O2, indicating that the relative increase in surface-water CH4 with increasing bottom-water CH4 is greater in shallower, less stratified systems than in deeper, more stratified systems. Diffusive CH4 emission rates were highest in shallower, less stratified systems, where the response of surface-water CH4 to increases in bottom-water CH4 is expected to be greatest, as indicated by high surface:bottom CH4 ratios. We further tested whether surface-water CH4 concentrations scale with simple metrics in a dataset including highly stratified, small, and deep crater lakes with elevated hypolimnetic CH4. A multiple linear regression using SA and Chl-a explained ~51 % of the variance and appears suitable for upscaling dissolved CH4 concentrations. This approach could enable large-scale extrapolation of diffusive CH4 emissions using spatial datasets for SA and remotely sensed Chl-a.

Ponds are a large source of atmospheric methane (CH4), a potent greenhouse gas, resulting from the net balance between input from sedimentary methanogenesis and removal by CH4 oxidation (MOX). Here, we test whether methanogenesis pathways (acetoclastic or hydrogenotrophic) and MOX might differ between clear-water (macrophyte-dominated) and turbid-water (phytoplankton-dominated) ponds. We measured the 13C/12C ratio of CH4 (δ13C-CH4) from gas trapped in bubble traps, from bubbles deliberately released by the perturbing sediments, and in dissolved CH4 in the water column in four urban ponds in Brussels, Belgium (Leybeek, Pêcheries, Tenreuken, Silex). In summer, the δ13C-CH4 values of sediment bubbles indicated that the hydrogenotrophic methanogenesis pathway appeared to be more important in clear-water (macrophyte-dominated) ponds (Leybeek and Pêcheries), whereas the acetoclastic methanogenesis pathway appeared to be more important in turbid-water (phytoplankton-dominated) ponds (Tenreuken and Silex). The δ13C-CH4 values from bubble traps indicated a seasonal shift from acetoclastic methanogenesis pathway in spring–summer to hydrogenotrophic methanogenesis in fall. The δ13C-CH4 of dissolved CH4 indicated higher rates of MOX in turbid-water ponds (Leybeek and Pêcheries) compared to clear-water ponds (Tenreuken and Silex), with an overall positive relation with total suspended matter and chlorophyll-a concentrations. The presence of suspended particles likely enhanced MOX by reducing light inhibition of MOX and/or by serving as substrates in the water column for attached methanotrophic bacteria. MOX represented 80

Rivers are large natural sources of methane (CH4) resulting from the net balance of inputs from methanogenesis and removal by methane oxidation (MOX). Here, we use an extensive dataset collected from African and European rivers to investigate spatial patterns and causes of variability of MOX. The MOX rates were highest in African streams draining flooded forests and increased with river catchment size. In large rivers, MOX represented the more important pathway of dissolved in-stream CH4 removal compared to diffusive degassing to the atmosphere; but this relative share was minimal in small rivers and was, on average, higher in African streams (37%) than European streams (9%). The relationships between MOX rates and potential drivers such as total suspended matter, pH, or dissolved nutrient concentrations did not follow the patterns expected from controlled experiments reported in the literature and reflected the broad spatial patterns across and within the studied river networks driven by stream size and wetland connectivity.

(Sub)tropical inland waters are important greenhouse gas (GHG) sources, yet limited observations have long hindered broad analyses of GHG variability across this diverse region. Here, through a meta-analysis, we have examined the rates and drivers of GHG emissions from flowing and standing (sub)tropical inland waters. We find considerable spatial variation in fluxes, largely related to differences in hydroclimate, geomorphology, land cover and human disturbance. Flowing waters emit more carbon dioxide (3,3872,1215,702 TgCO2 yr-1, expressing medianfirst quartilethird quartile), methane (10.60.128.8 TgCH4 yr-1) and nitrous oxide (0.620.351.10 TgN2O yr-1) than standing waters (11473219 TgCO2 yr-1, 5.42.19.1 TgCH4 yr-1 and 0.030.020.05 TgN2O yr-1, respectively). (Sub)tropical inland waters release 4,23824737375 TgCO2-equivalents annually, with first- to third-order streams contributing 75% of riverine emissions and lakes larger than 100 km2 contributing 59% of standing water emissions. Our results suggest emissions from (sub)tropical waters are 29-72% lower than earlier estimates, a downward revision with important implications for global GHG budgets.

Sub-daily variations might significantly impact the estimates of GHG emissions from lakes and ponds. The objectives of this study are (i) to quantify sub-daily variations of the emissions of CO2, CH4, and N2O from two urban ponds (Silex and Pecheries) in the city of Brussels, (ii) to quantify if the sub-daily variations of GHG emissions were significant compared to their seasonal variations and to inter-pond variations among 20 other ponds in the city of Brussels. The partial pressure of CO2 (pCO2), CH4 concentration, and N2O saturation level (% N2O) were measured hourly from dawn to dusk in the Pecheries turbid-water pond and in the Silex clear-water pond during the four seasons in 2023-2024. pCO2 followed the day-night cycle of photosynthesis in spring and summer but was more erratic in winter and fall. The variations of CH4 concentration and %N2O were on most occasions erratic and difficult to attribute systematically to specific biogeochemical processes. The sub-daily variations of computed GHG emissions were mostly driven by variability in wind speed that usually peaked around mid-day. The comparison with previously acquired seasonal and inter-pond data (n = 22) showed that sub-daily variations of GHG fluxes were lower than seasonal variations, which were in turn lower than inter-pond variations. Consequently, to design sampling strategies to reduce the uncertainty on the estimate of CO2, CH4, and N2O emissions a priority should be given to describe inter-system variability, followed by seasonal variability, and lastly sub-daily variability, in the context of the environmental management of inland waters, including urban ponds.

Inland waters in Arctic landscapes act as conduits of terrestrial organic material, transporting and processing organic material into the greenhouse gases (GHGs) carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O), and subsequently exchanging these gases with the atmosphere. To assess the role of inland water emissions in the Arctic GHG budget, it is necessary to quantify their emissions in relation to the terrestrial sink capacity. We present measurements of dissolved CO 2 , CH 4 , and N 2 O from lake, pond, and low‐order fluvial systems across two summers (2016–2017) in the Arctic Siberian Indigirka River tundra lowlands. During May–July 2017, the region experienced large‐scale flooding, of which we captured the tail end. Using remote sensing images to upscale inland water emissions to an area of approximately 18 km 2 , we calculated combined carbon (C) emissions, CO 2 ‐C, and diffusive CH 4 ‐C under nonflood and flooded scenarios. These ranged from 7.03 ± 1.30 Mg C d −1 (nonflood; mean ± SD) to 9.63 ± 1.24 Mg C d −1 (flooded). Integrating these values into the total C landscape exchange offset the terrestrial C sink by ∼9–∼13%. When N 2 O emissions were calculated as CO 2 equivalents, these emissions were negligible relative to CO 2 and CH 4 . Our study shows that in the northeast Siberian Arctic tundra, summertime CO 2 and CH 4 emissions from inland waters are a potentially important component of landscape C exchange with the atmosphere, offsetting the terrestrial sink capacity, and this may be an important consideration for constraining future Arctic responses to climate warming.

Africa is recognized for its high levels of endemism across many groups of organisms, including Cladocera. Several studies contributed to our understanding of the diversity and geographic distribution of some groups of Chydoridae on this continent. The literature, however, points to species presumed to occur naturally on other continents, suggesting that both diversity and endemism in Africa continue to be underestimated. Despite the absence of more comprehensive knowledge about the morphology of Kurzia longirostris (Daday, 1898) from the terra typica (Oriental region), our findings revealed small morphological differences between populations of the Congo River when compared with literature reports. Looking at the high morphological variability along the range of its geographic distribution, it becomes clear that K. longirostris might be indicated as a species complex. Thus, the idea of continental endemism should be tested in a future revision of the group.

Shallow ponds can occur either in a clear-water state dominated by macrophytes or a turbid-water state dominated by phytoplankton, but it is unclear if and how these two alternative states affect the emission of greenhouse gases (GHGs) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) to the atmosphere. We measured the dissolved concentration of CO2, CH4, and N2O from which the diffusive air–water fluxes were computed, in four urban ponds in the city of Brussels (Belgium): two clear-water macrophyte-dominated ponds (Silex and Tenreuken), and two turbid-water phytoplankton-dominated ponds (Leybeek and Pêcheries) on 46 occasions over 2.5 years (between June 2021 and December 2023). Ebullitive CH4 fluxes were measured with bubble traps in the four ponds during deployments in spring, summer, and autumn, totalling 48 d of measurements. Measured ancillary variables included water temperature, oxygen saturation level ( %O2), concentrations of chlorophyll-a (Chl-a), total suspended matter (TSM), soluble reactive phosphorus (SRP), nitrite (NO2-), nitrate (NO3-), and ammonium (NH4+). The turbid-water and clear-water ponds did not differ significantly in terms of diffusive emissions of CO2 and N2O. Clear-water ponds exhibited higher values of ebullitive CH4 emissions compared to turbid-water ponds, most probably in relation to the delivery of organic matter from macrophytes to sediments, but the diffusive CH4 emissions were not significantly different between clear- and turbid-water ponds. Across seasons, CH4 emissions increased with water temperature in all four ponds, with ebullitive CH4 fluxes having a stronger dependence on water temperature (Q10) than diffusive CH4 fluxes. The temperature sensitivity of ebullitive CH4 fluxes decreased with increasing water depth, implying that shallow sediments would respond more strongly to warming (e.g. heat waves). Total annual CH4 emissions (diffusive + ebullitive) in CO2 equivalents equalled those of CO2 in turbid-water ponds and exceeded those of CO2 in clear-water ponds, while N2O emissions were negligible compared to the other two GHGs. Total annual GHG emissions in CO2 equivalents from all four ponds increased from 2022 to 2023 due to higher CO2 diffusive fluxes, likely driven by higher annual precipitation in 2023 compared to 2022 (leading putatively to higher inputs for organic or inorganic carbon from run-off), possibly in response to the intense El Niño event of 2023. The findings of this work suggest that it might be necessary to account for the presence of submerged macrophytes when extrapolating ebullitive CH4 fluxes in ponds at a larger scale (regional or global) (particularly if Chl-a is used as a descriptor), although it might be less critical for the extrapolation of diffusive CH4, CO2, and N2O fluxes.

The African Cladocera fauna is recognized by high endemism. Several studies have helped to understand the diversity and geographic distribution of some groups of Chydoridae on the continent. However, the literature indicates the presence of species whose natural distribution is presumed to be in other continents, suggesting that the diversity and endemism in Africa are still underestimated. Despite the absence of more comprehensive knowledge about morphology of Kurzia longirostris to terra typica (Oriental region), our findings reveled slightly morphological differences between Congo River populations when compared with literature data. Looking at the high morphological variability along the range of geographic distribution, it is increasingly clear that K. longirostris might be indicated as a species complex. Thus, the idea of continental endemism should be tested in a future revision of the group.

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.

We report, in 15 Ecuadorian mountainous lakes, dissolved concentrations of CO2, CH4, and N2O and a suite of ancillary biogeochemical variables (dissolved inorganic nutrients, oxygen, major cations, dissolved and particulate organic carbon, and the stable isotope composition of dissolved inorganic carbon and water). The sampled lakes were located in the páramos of Salve Facha and Antisana y Mojanda (northern region of Ecuadorian Andes), along an elevational gradient from 2213 to 4361 m above sea level, as well as a gradient of lake surface area (0.003–6.1 km2) and depth (1–74 m). Most lakes were characterized by lower values of the partial pressure of CO2 (pCO2) (644–2152 ppm) than usually attributed to tropical lakes ( 1900 ppm). Three lakes influenced by volcanic inputs were characterized by the highest pCO2 values (3269–10,069 ppm), while two lakes bordered by large cities were characterized by the lowest pCO2 values (208–254 ppm). Dissolved CH4 concentrations ranged between 170 and 24,908 nmol/L and were negatively correlated to lake area and depth. N2O saturation levels ranged between 64

A number of species of Chydorus Leach, 1816 (Crustacea: Cladocera) need improvements in their taxonomy much more than any other genus within the family Chydoridae Dybowsky & Grochowski, 1894 emend. Frey, 1967, which makes the systematics of the genus still a puzzle that lacks several pieces. Here, we redescribe the African species Chydorus tilhoi Rey & Saint-Jeans, 1969 and compare its morphology with that of Chydorus sphaericus (O.F. Müller, 1776). The two taxa might be easily differentiated because C. tilhoi has a single and relatively large major head pore with a wide rim, labral keel elongated with a large spine, and postabdomen with postanal part elongated, narrowing distally and with denticles near its anal margin, organized in groups. These morphological traits are absent in C. sphaericus. Chydorus tilhoi and C. sphaericus also differ in the morphology of the first (Inner Distal Lobe setae), third (exopodite proportion), and fifth (exopodite shape) limbs. Based on the literature and our observations, the limb morphology of C. tilhoi has important similarities with that of C. breviceps, C. nitidulus and C. dentifer, and their translocation to a new genus seems to be a fundamental piece in the puzzle of Chydorus.

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by 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 and total 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 Southern Hemisphere. 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 sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±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±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 will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

We report estimates of community gross primary production (GPP), community respiration (CR), and net community production (NCP) based on the change of dissolved O-2 during incubations over epilithic turf-forming macroalgae (Halopteris scoparia, Padina pavonica, and Dictyota dichotoma) on 7 occasions and in accumulations of Posidonia oceanica macrophytodetritus (i.e. litter) on 8 occasions in the Bay of Revellata (Corsica) from March 2009 to May 2011. In the epilithic macroalgae community, GPP ranged between 7.8 and 82.2 mmol O-2 m(-2) d(-1), CR ranged between -108.5 and -13.6 mmol O-2 m(-2) d(-1), and NCP ranged between -53.2 and -5.7 mmol O-2 m(-2) d(-1). In the P. oceanica macrophytodetritus accumulation, GPP ranged between 5.7 and 91.6 mmol O-2 m(-2) d(-1), CR ranged between -112.8 and -27.2 mmol O-2 m(-2) d(-1), and NCP ranged between -46.8 and -9.9 mmol O-2 m(-2) d(-1). GPP in both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation peaked in summer and was lowest in fall, following the seasonal variation of incoming light. GPP correlated to macroalgal biomass but was unrelated to the biomass of living macroscopic plant material in the P. oceanica macrophytodetritus accumulation. The annual average of GPP was equivalent in the epilithic macroalgae and P. oceanica macrophytodetritus accumulation communities (17.6 and 19.4mol O-2 m(-2) yr(-1)). Both the epilithic macroalgae community and the P. oceanica macrophytodetritus accumulation were net heterotrophic with an annual average NCP of -6.1 and -8.8mol O-2 m(-2) yr(-1), respectively. The NCP of the adjacent P. oceanica meadow at 10 m depth based on simultaneous measurements based on the open water O-2 mass balance from moored O-2 probes (optodes) was 28.9mol O-2 m(-2) yr(-1). The potential export of dissolved organic carbon from the P. oceanica meadow could quantitatively meet the carbon demand to sustain the net heterotrophy of the adjacent epilithic macroalgae community in the Bay of Revellata. We also show the limitation and possibly over-estimation of extrapolating decay rates based on litter bag experiments with small quantities of material to "real" macrophytodetritus biomass densities.

Freshwaters have been recognized as important sources of greenhouse gases (GHG) to the atmosphere. However, urban ponds have received little attention even though their number is increasing due to expanding urbanisation globally. Ponds are frequently associated to urban green spaces that provide several ecosystemic services such as cooling local climate, regulating the water cycle, and acting as small carbon sinks This study aims to identify and understand the processes producing GHGs (CO2, CH4, and N2O) in the urban ponds of the temperate European city of Brussels in Belgium. 22 relatively small ponds (0.1–4.6 ha) surrounded by contrasted landscape (strictly urban, bordered by cropland or by forest), were sampled during four seasons in 2021–2022. The mean ± standard deviation was 3,667 ± 2,904 ppm for the partial pressure of CO2 (pCO2), 2,833 ± 4,178 nmol L−1 for CH4, and 273 ± 662% for N2O saturation level (%N2O). Relationships of GHGs with oxygen and water temperature suggest that biological processes controlled pCO2, CH4 concentration and%N2O. However, pCO2 was also controlled by external inputs as indicated by the higher values of pCO2 in the smaller ponds, more subject to external inputs than larger ones. The opposite was observed for CH4 concentration that was higher in larger ponds, closer to the forest in the city periphery, and with higher macrophyte cover. N2O concentrations, as well as dissolved inorganic nitrogen, were higher closer to the city center, where atmospheric nitrogen deposition was potentially higher. The total GHG emissions from the Brussels ponds were estimated to 1kT CO2-eq per year and were equivalent to the carbon sink of urban green spaces.

The rate of ocean uptake of anthropogenic CO2 has declined over the past decade, so a critical question for science and policy is whether the ocean will continue to act as a sink. Large areas of the ocean remain without observations for carbonate system variables, and oceanic CO2 observations have declined since 2017. The Mediterranean Sea is one such an area, especially its eastern part, where there is a paucity of carbonate system data, with large areas not sampled or only sampled by ship-based discrete measurements as opposed to high frequency, sensor-equipped time-series fixed stations. The aim of this study was to analyze a multi-year time-series of high-frequency (hourly) partial pressure CO2 (pCO2) and pH measurements in the Eastern Mediterranean, along with low-frequency (monthly) measurements of total dissolved inorganic carbon and total alkalinity. The pCO2 time-series was the first obtained in the Eastern Mediterranean. The study was conducted at a fixed platform of the POSEIDON system (Heraklion Coastal Buoy) located near Crete Island. Temperature was the dominant factor controlling the temporal variability of pCO2 and pH, while the remaining non-thermal variability appeared to be related to evaporation, water mixing, and biological remineralization-production. The air-sea CO2 fluxes indicated a transition from a winter-spring sink period to a summer-autumn source period. The annual air-sea CO2 flux was too low (-0.16 ± 0.02 mol m-2 yr-1) and variable to conclusively characterize the area as a net source or sink of CO2, highlighting the need for additional high frequency observation sites. Algorithms were developed using temperature, chlorophyll and salinity data to estimate pCO2 and total alkalinity, in an effort to provide tools for estimates in poorly observed areas/periods from remotely sensed products. The applicability of the algorithms was tested using Surface Ocean CO2 Atlas (SOCAT) data from the Eastern Mediterranean Sea (1999 to 2020) which showed that the algorithm pCO2 estimates were generally within ±20 μatm of the pCO2 values reported by SOCAT. Finally, the integration and analysis of the data provided directions on how to optimize the observing strategy, by readapting sensor location and using estimation algorithms with remote sensing data.

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 ± 47 mmol m −2 d −1 in mangroves and 57 ± 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.