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Coastal ecosystems release or absorb carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), but the net effects of these ecosystems on the radiative balance remain unknown. We compiled a dataset of observations from 738 sites from studies published between 1975 and 2020 to quantify CO2, CH4 and N2O fluxes in estuaries and coastal vegetation in ten global regions. We show that the CO2-equivalent (CO(2)e) uptake by coastal vegetation is decreased by 23-27% due to estuarine CO(2)e outgassing, resulting in a global median net sink of 391 or 444 TgCO(2)e yr(-1) using the 20- or 100-year global warming potentials, respectively. Globally, total coastal CH4 and N2O emissions decrease the coastal CO2 sink by 9-20%. Southeast Asia, North America and Africa are critical regional hotspots of GHG sinks. Understanding these hotspots can guide our efforts to strengthen coastal CO2 uptake while effectively reducing CH4 and N2O emissions. The authors show that estuarine and coastal vegetation are collectively a greenhouse gas (GHG) sink for the atmosphere, but methane and nitrous oxide emissions counteract the carbon dioxide uptake. Critical coastal GHG sink hotspots are identified in Southeast Asia, North America and Africa.

Estuaries are dominant conduits of mercury (Hg) to the coastal ocean and the salt marshes within play an important role in coastal Hg cycling. While Hg cycling in upland terrestrial systems has been well studied, processes in salt marsh ecosystems are poorly characterized. We investigated Hg dynamics in vegetation and soils in the Plum Island Sound estuary in Massachusetts, USA and specifically assessed the role of marsh vegetation for Hg deposition and turnover. Monthly quantitative harvesting of aboveground biomass showed strong linear seasonal increases in plant Hg, with a four-fold increase in Hg concentration and an eight-fold increase in standing Hg mass between June (3.9±0.2 µg kg-1 and 0.7±0.4 µg m-2, respectively) and November (16.2±2.0 µg kg-1 and 5.7±2.1 µg m-2, respectively). Hg ceased to increase in aboveground biomass after plant senescence, indicating physiological controls of vegetation Hg uptake in salt marsh plants. Hg concentrations in live roots and live rhizomes were 11 times and two times higher than concentrations in aboveground live biomass, respectively. Furthermore, live belowground biomass Hg pools (roots and rhizomes, 108.1±83.4 μg m-2) is more than ten times larger than peak standing aboveground Hg pools (9.0±3.3 μg m-2). A ternary mixing model suggests Hg sources in marsh aboveground tissues originates from a mix of root uptake (~35 %), precipitation uptake (~33 %), and atmospheric gaseous elemental mercury (GEM) uptake (~32 %). The results suggest a more important role of Hg transport from belowground (i.e., roots) to aboveground tissues in salt marsh vegetation compared to upland vegetation, where GEM uptake is generally the dominant Hg source. GEM deposition via uptake and subsequent senescence (5.9 µg m-2 yr-1) and throughfall (1.0 µg m-2 yr-1) hence is lower in this salt marsh ecosystem compared to upland vegetation and is similar to open field wet and dry deposition (6.2 µg m-2 yr-1). Hg contained in salt marsh aboveground tissues leads to direct Hg export to tidal water and oceans via wrack (tidal flushing of vegetation), which accounts for ~1.6 µg m-2 yr-1. Hg consumption by herbivory ranges between 0.5 and 2.4 µg Hg m-2 yr-1. The similarity in isotopic signatures between roots and soils suggest that belowground plant tissues mostly take up Hg directly from soils. Annual root turnover results in large internal Hg recycling between soils and plants accounting for 58.6 µg m-2 yr-1. An initial mass balance of Hg in this whole estuarine salt marsh ecosystem considering atmospheric inputs (atmospheric GEM and precipitation Hg(II), throughfall, including plants) and losses (wrack export and lateral exchange of dissolved and particulate Hg) shows that the salt marsh presently serves as a small net Hg sink for environmental Hg of 5.2 µg m-2 yr-1.

Seagrass meadows worldwide provide valuable ecosystem services but have experienced sharp declines in recent decades. This rapid loss has prompted numerous restoration efforts with variable levels of success, often depending on the suitability of the restoration sites. The selection of sites can be guided by simple habitat suitability models driven with environmental variables deemed critical to the successful growth of new transplants. Habitat suitability models typically consider the influence of bathymetry, sediment type, salinity, wave exposure, and water quality. However, they typically do not explicitly include benthic exposure to ultraviolet (UV) and commonly use depth as a coarse proxy for photosynthetically active radiation (PAR). Benthic exposure to UV and PAR are both key parameters for habitat suitability but can be challenging to determine, especially in coastal environments influenced by rivers and tides where they are extremely variable. Here, we demonstrate the development of a simple but effective model of spectrally-resolved benthic solar irradiance for a dynamic marsh-influenced mesotidal estuary in Massachusetts. In-situ measurements were used to develop and validate an empirical model predicting the UV–visible vertical diffuse attenuation coefficient spectra of downwelling irradiance, Kd(λ), from simple physical parameters about tides, river discharge and location. Spectral benthic solar irradiances (280–700 nm) were calculated hourly for 3 years (2017–2019) using modeled and validated cloud-corrected surface downwelling irradiances, estimates of water depth, and the modeled Kd(λ) spectra. The mapped irradiances were used to provide improved seagrass habitat suitability maps that will guide future restoration efforts in the estuary. We expect the approach presented here can be adapted to other dynamic coastal environments influenced by tides and rivers and/or applied to other light-dependent organisms and biogeochemical processes.

Large datasets of greenhouse gas and energy surface-atmosphere fluxes measured with the eddy-covariance technique (e.g., FLUXNET2015, AmeriFlux BASE) are widely used to benchmark models and remote-sensing products. This study addresses one of the major challenges facing model-data integration: To what spatial extent do flux measurements taken at individual eddy-covariance sites reflect model- or satellite-based grid cells? We evaluate flux footprints—the temporally dynamic source areas that contribute to measured fluxes—and the representativeness of these footprints for target areas (e.g., within 250–3000 m radii around flux towers) that are often used in flux-data synthesis and modeling studies. We examine the land-cover composition and vegetation characteristics, represented here by the Enhanced Vegetation Index (EVI), in the flux footprints and target areas across 214 AmeriFlux sites, and evaluate potential biases as a consequence of the footprint-to-target-area mismatch. Monthly 80% footprint climatologies vary across sites and through time ranging four orders of magnitude from 103 to 107 m2 due to the measurement heights, underlying vegetation- and ground-surface characteristics, wind directions, and turbulent state of the atmosphere. Few eddy-covariance sites are located in a truly homogeneous landscape. Thus, the common model-data integration approaches that use a fixed-extent target area across sites introduce biases on the order of 4%–20% for EVI and 6%–20% for the dominant land cover percentage. These biases are site-specific functions of measurement heights, target area extents, and land-surface characteristics. We advocate that flux datasets need to be used with footprint awareness, especially in research and applications that benchmark against models and data products with explicit spatial information. We propose a simple representativeness index based on our evaluations that can be used as a guide to identify site-periods suitable for specific applications and to provide general guidance for data use.

Integrating spatial heterogeneity into assessments of salt marsh biogeochemistry is becoming increasingly important because disturbances that reduce plant productivity and soil drainage may contribute to an expansion of shallow ponds. These permanently inundated and sometimes prominent landscape features can exist for decades, yet little is known about pond biogeochemistry or their role in marsh ecosystem functioning. We characterized three ponds in a temperate salt marsh (MA, USA) over alternating periods of tidal isolation and flushing, during summer and fall, by evaluating the composition of plant communities and organic matter pools and measuring surface water oxygen, temperature, and conductivity. The ponds were located in the high marsh and had similar depths, temperatures, and salinities. Despite this, they had different levels of suspended particulate, dissolved, and sediment organic matter and abundances of phytoplankton, macroalgae, and Ruppia maritima. Differences in plant communities were reflected in pond metabolism rates, which ranged from autotrophic to heterotrophic. Integrating ponds into landcover-based estimates of marsh metabolism resulted in slower rates of net production (−8.1 ± 0.3 to −15.7 ± 0.9%) and respiration (−2.9 ± 0.5 to −10.0 ± 0.4%), compared to rates based on emergent grasses alone. Seasonality had a greater effect on pond water chemistry, organic matter pools, and algal abundances than tidal connectivity. Alternating stretches of tidal isolation and flushing did not affect pond salinities or algal communities, suggesting that exchange between ponds and nearby creeks was limited. Overall, we found that ponds are heterogeneous habitats and future expansion could reduce landscape connectivity and the ability of marshes to capture and store carbon.

We studied marsh-atmosphere exchange of carbon dioxide in a high marsh dominated salt marsh during the months of May to October in 2012–2014. Tidal inundation at the site occurred only during biweekly spring tides, during which we observed a reduction in fluxes during day and night. We estimated net ecosystem exchange (NEE), gross primary production (GPP), and ecosystem respiration (Reco) using a modified PLIRTLE model, which requires photosynthetically active radiation, temperature, and normalized difference vegetation index (NDVI) as control variables. NDVI decreased during inundation, when the marsh canopy was submerged. Two-time series of NDVI, including and excluding effects of tidal inundation, allowed us to quantify the flux reduction during inundation. The effect of the flux reduction was small (2–4%) at our site, but is likely higher for marshes at a lower elevation. From May to October, GPP averaged −863 g C m−2, Reco averaged 591 g C m−2, and NEE averaged −291 g C m−2. In 2012, which was an exceptionally warm year, we observed an early start of net carbon uptake but higher respiration than in 2013 and 2014 due to higher-air temperature in August. This resulted in the lowest NEE during the study period (−255.9±6.9 g C m−2). The highest seasonal net uptake (−336.5±6.3 g C m−2) was observed in 2013, which was linked to higher rainfall and temperature in July. Mean sea level was very similar during all 3 years which allowed us to isolate the importance of climatic factors.

[1] Year-round flooding provides a common land management practice to reestablish the natural carbon dioxide (CO2) sink function of drained peatlands. Here we present eddy covariance measurements of net CO2 exchange from a temperate fen during three consecutive growing seasons (May–October) that span a period of conversion from moderately rewetting to flooding. When we started our measurements in 2009, the hydrological conditions were representative for the preceding 20 years with a mean growing season water level (MWGL) of 0 cm but considerably lower water levels in summer. Flooding began in 2010 with an MWGL of 36 cm above the surface. The fen was a net CO2 sink throughout all growing seasons (2009: −333.3 ± 12.3, 2010: −294.1 ± 8.4, 2011: −352.4 ± 5.1 g C m−2), but magnitudes of canopy photosynthesis (CP) and ecosystem respiration (Reco) differed distinctively. Rates of CP and Reco were high before flooding, dropped by 46% and 61%, respectively, in 2010, but increased again during the beginning of growing season 2011 until the water level started to rise further due to strong rainfalls during June and July. We assume that flooding decreases not only the CO2 release due to inhibited Reco under anaerobic conditions but also CO2 sequestration rates are constricted due to decreased CP. We conclude that rewetting might act as a disturbance for a plant community that has adapted to drier conditions after decades of drainage. However, if the recent species are still abundant, a rise in CP and autotrophic Reco can be expected after plants have developed plastic response strategies to wetter conditions.

The methane exchange in an oligotrophic mire complex was measured on the ecosystem and microform scale with the eddy covariance (EC) and the closed chamber technique, respectively. Information about the distribution of three distinct microform types in the area of interest and in each 30min EC flux source area was derived from a high-resolution (1m) landcover map in combination with an analytical source weight model (Kormann and Meixner, 2001). The mean weighted coverage of flark, lawn and hummock microforms in the EC source area (0.3%:57%:43%) closely mirrors the overall distribution in the area of interest (0.5%:50.1%:49.4%), despite great differences in microform coverage between individual 30min EC source areas. The measured ecosystem flux was fitted to the sum of three microform flux models based on environmental variables and weighted by their fractional coverage in the EC source area. This method resulted in a better representation of the ecosystem flux compared to an approach based on only one flux model for the whole ecosystem (R=0.87, RMSE=0.44 vs. R=0.74, RMSE=0.61, n=5181) and thus constitutes a successful down-scaling of measured ecosystem scale flux to the microform scale. A comparison of down-scaled and measured microform fluxes reveals a good agreement for lawn microforms and systematic differences for flark and hummock microforms. Reasons for the differences are thought to be the limited resolution of the landcover classification and the systematic underestimation of hummock fluxes by the closed chamber technique. As a result, hummock fluxes derived by down-scaling of EC fluxes are considered to be more dependable than closed chamber fluxes. The seasonal ecosystem methane budget from gap-filled EC measurements was 9.4±0.2gCHm; the budget derived from up-scaled microform measurements was 8.0±0.8gCHm. The lower value of the latter budget is attributed to the underestimation of flark and hummock fluxes by closed chamber measurements and to the microform gap-filling procedure. Generally, estimates from up-scaled microform measurements are found to be less certain than estimates from EC measurements.

The closed-chamber method is the most common approach to determine CH4 fluxes in peatlands. The concentration change in the chamber is monitored over time, and the flux is usually calculated by the slope of a linear regression function. Theoretically, the gas exchange cannot be constant over time but has to decrease, when the concentration gradient between chamber headspace and soil air decreases. In this study, we test whether we can detect this non-linearity in the concentration change during the chamber closure with six air samples. We expect generally a low concentration gradient on dry sites (hummocks) and thus the occurrence of exponential concentration changes in the chamber due to a quick equilibrium of gas concentrations between peat and chamber headspace. On wet (flarks) and sedge-covered sites (lawns), we expect a high gradient and near-linear concentration changes in the chamber. To evaluate these model assumptions, we calculate both linear and exponential regressions for a test data set (n = 597) from a Finnish mire. We use the Akaike Information Criterion with small sample second order bias correction to select the best-fitted model. 13.6%, 19.2% and 9.8% of measurements on hummocks, lawns and flarks, respectively, were best fitted with an exponential regression model. A flux estimation derived from the slope of the exponential function at the beginning of the chamber closure can be significantly higher than using the slope of the linear regression function. Non-linear concentration-over-time curves occurred mostly during periods of changing water table. This could be due to either natural processes or chamber artefacts, e.g. initial pressure fluctuations during chamber deployment. To be able to exclude either natural processes or artefacts as cause of non-linearity, further information, e.g. CH4 concentration profile measurements in the peat, would be needed. If this is not available, the range of uncertainty can be substantial. We suggest to use the range between the slopes of the exponential regression at the beginning and at the end of the closure time as an estimate of the overall uncertainty.

The carbon (C) balance of boreal peatlands is mainly the sum of three different C fluxes: carbon dioxide (CO2), methane (CH4) and dissolved organic carbon (DOC). Intra- and inter-annual dynamics of these fluxes are differentially controlled by similar factors, such as temperature and water-table. Different climatic conditions within and between years might thus result in varying absolute and relative contributions of each flux to net ecosystem productivity (NEP). In this study CO2 fluxes were measured at a boreal peatland in eastern Finland during a dry year (2006) and a wet year (2007) and combined with DOC and CH4 fluxes from the same site. CO2 uptake in the wet year was 65% higher than in the dry year, caused by higher water table (WT) and subsequently reduced rates of soil respiration. Two to three-fold increases in DOC and CH4 fluxes in the wet year did not completely offset the higher CO2 uptake in that year, resulting in NEP of -83.7±14 g C m(-2) in the dry and -134.5±21 g C m(-2) in the wet year. Thus, in our study, WT was identified as the most important factor responsible for variations in the C balance between the observed years.

Le bilan hydrique, la chimie des eaux et les transferts en solution ont été étudiés dans le bassin versant de Kidisjoki (18 km2 ; 75 à 365 m d’altitude ; 69º47’N et 27º05’E) en Laponie finlandaise. L’aire d’étude fait partie du Bouclier baltique, d’âge précambrien et composé essentiellement de gneiss et de granulites. Nous estimons que la dénudation chimique dans le bassin versant est de l’ordre de 2,9 t.km2.a-1. La variabilité spatio-temporelle de la charge en solution dans le bassin versant semble être influencée par des variations spatiales de la persistance du gélisol hivernal et de l’épaisseur du régolithe. En dépit de la faible intensité de l’altération chimique et de la faible concentration des solutions dans les eaux superficielles, l’altération chimique semble être plus importante que l’érosion fluviale et pourrait être le principal processus de dénudation de cet environnement subarctique. Water balance, water chemistry, and solute fluxes have been investigated in the Kidisjoki catchment (18 km2 ; 75 to 365 m a.s.l. ; 69°47’N, 27°05’E) in subarctic Finnish Lapland. The study area is part of the Precambrian Baltic Shield and is composed lithologically of gneisses and granulites. It is estimated that chemical denudation in the catchment is on the order of 2.9 t.km2.yr-1. Spatio-temporal variability of solute yields within the catchment seems to be influenced by spatial variations of winter ground frost duration and regolith thickness. In spite of the low intensity of chemical weathering and the low solute concentrations in the surface water, chemical denudation seems to be more important than mechanical fluvial erosion, and so might be the most important denudational process in this subarctic environment.

In Central Europe, most bogs have a history of drainage and many of them are currently being restored. Success of restoration as well as greenhouse gas exchange of these bogs is influenced by environmental stress factors as drought and atmospheric nitrogen deposition. We determined the methane and nitrous oxide exchange of sites in the strongly decomposed center and less decomposed edge of the Pietzmoor bog in NW Germany in 2004. Also, we examined the methane and nitrous oxide exchange of mesocosms from the center and edge before, during, and following a drainage experiment as well as carbon dioxide release from disturbed unfertilized and nitrogen fertilized surface peat. In the field, methane fluxes ranged from 0 to 3.8 mg m−2 h−1 and were highest from hollows. Field nitrous oxide fluxes ranged from 0 to 574 μg m−2 h−1 and were elevated at the edge. A large Eriophorum vaginatum tussock showed decreasing nitrous oxide release as the season progressed. Drainage of mesocosms decreased methane release to 0, even during rewetting. There was a tendency for a decrease of nitrous oxide release during drainage and for an increase in nitrous oxide release during rewetting. Nitrogen fertilization did not increase decomposition of surface peat. Our examinations suggest a competition between vascular vegetation and denitrifiers for excess nitrogen. We also provide evidence that the von Post humification index can be used to explain nitrous oxide release from bogs, if the role of vascular vegetation is also considered. An assessment of the greenhouse gas release from nitrogen saturated restoring bogs needs to take into account elevated release from fresh Sphagnum peat as well as from sedges growing on decomposed peat. Given the high atmospheric nitrogen deposition, restoration will not be able to achieve an oligotrophic ecosystem in the short term.

The response of evapotranspiration (ET) to warming is of critical importance to the water and carbon cycle of the boreal biome, a mosaic of land cover types dominated by forests and peatlands. The effect of warming-induced vapour pressure deficit (VPD) increases on boreal ET remains poorly understood because peatlands are not specifically represented as plant functional types in Earth system models. Here we show that peatland ET increases more than forest ET with increasing VPD using observations from 95 eddy covariance tower sites. At high VPD of more than 2 kPa, peatland ET exceeds forest ET by up to 30%. Future (2091–2100) mid-growing season peatland ET is estimated to exceed forest ET by over 20% in about one-third of the boreal biome for RCP4.5 and about two-thirds for RCP8.5. Peatland-specific ET responses to VPD should therefore be included in Earth system models to avoid biases in water and carbon cycle projections. Climate warming increases evapotranspiration (ET) more in boreal peatlands than in forests. Observations show that peatland ET can exceed forest ET by up to 30%, indicating a stronger warming response in peatlands. Earth system models do not fully account for peatlands and hence may underestimate future boreal ET.