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• Hydrological export of the five main carbon components from a restored saltmarsh wetland were measured. • Groundwater-driven fluxes of alkalinity, DIC, DOC, CO 2 , and CH 4 were estimated using radon. • The contribution of groundwater to surface water carbon export was determined. • SW and GW carbon exports from the restored wetland were lower than those reported for natural wetlands. Coastal wetlands play a crucial role in the global carbon cycle, yet they have been extensively degraded over the past decade. Though restoration efforts are underway worldwide, there is limited understanding of the role groundwater plays in transporting dissolved carbon within restored wetlands. Here, we address this knowledge gap by investigating water and carbon fluxes in the restored Tomago Wetlands in coastal NSW, Australia. We aim to 1) quantify surface water exports of the main carbon components, 2) estimate greenhouse gas emissions from the aquatic parts of the wetland, and 3) determine the contribution of groundwater discharge to the surface water carbon export and greenhouse gas emissions from the wetland. Sampling took place following a wet period and a freshening event. A radon mass balance model estimated groundwater discharge contributing 10 % to the surface water flow in the restored wetland. The wetland was a source of four of the five main carbon components including DOC, DIC, alkalinity, and CO 2 , with most of the exported carbon being in the form of alkalinity. Surface water DOC, DIC, alkalinity, and CO 2 exports were, 1.3 ± 0.5, 35.5 ± 13.1, 39.4 ± 14.6 and 6.5 ± 2.1 mmol/m2/d, respectively. The average water to atmosphere flux of CO 2 and CH 4 were 104 ± 209 (mmol/m2/d) and 21 ± 42 (µmol/m2/d). Groundwater-driven carbon fluxes contributed 100 % of the surface water DOC, 30 % of the DIC, 15 % of the alkalinity exports, and 5 % of the overall CO 2 losses (lateral aqueous export + atmospheric emissions) from the wetland, with minor contributions to CH 4. Carbon exports from both the wetland and groundwater observed in this restored wetland were found to be lower than those reported in the literature for natural or mature wetlands. This would have important implications when developing carbon accounting methods. Overall, our findings highlight the importance of groundwater in carbon transport within restored wetlands and emphasise the need for inclusion of shallow groundwater dynamics in carbon budgets and inventories of coastal wetlands that have or will undergo restoration. [ABSTRACT FROM AUTHOR]

Climate change driven Sea Level Rise (SLR) is creating a major global environmental crisis in coastal ecosystems, however, limited practical solutions are provided to prevent or mitigate the impacts. Here, we propose a novel eco-engineering solution to protect highly valued vegetated intertidal ecosystems. The new 'Tidal Replicate Method' involves the creation of a synthetic tidal regime that mimics the desired hydroperiod for intertidal wetlands. This synthetic tidal regime can then be applied via automated tidal control systems, "SmartGates", at suitable locations. As a proof of concept study, this method was applied at an intertidal wetland with the aim of restabilising saltmarsh vegetation at a location representative of SLR. Results from aerial drone surveys and on-ground vegetation sampling indicated that the Tidal Replicate Method effectively established saltmarsh onsite over a 3-year period of post-restoration, showing the method is able to protect endangered intertidal ecosystems from submersion. If applied globally, this method can protect high value coastal wetlands with similar environmental settings, including over 1,184,000 ha of Ramsar coastal wetlands. This equates to a saving of US$230 billion in ecosystem services per year. This solution can play an important role in the global effort to conserve coastal wetlands under accelerating SLR. [ABSTRACT FROM AUTHOR]

We investigated whether submarine groundwater discharge (SGD) traced by radon ( 222 Rn, a natural groundwater tracer) may drive carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) in surface waters in Chowder Bay, a marine embayment in Sydney Harbour, Australia. A radon mass balance revealed significant groundwater discharge rates into the bay (8.7 ± 5.8 cm d −1 ). The average CO 2 , CH 4 , and N 2 O concentrations in the subterranean estuary were 3.5, 7.2, and 2.8 times higher than the average surface water concentrations, indicating the possibility of coastal groundwater as a source of greenhouse gases to the bay. SGD-derived fluxes of greenhouse gases were 5.02 ± 2.28 mmol m −2 d −1 , 5.63 ± 2.55 μmol m −2 d −1 , and 1.72 ± 0.78 μmol m −2 d −1 for CO 2 , CH 4 and N 2 O, respectively. The average CO 2 evasion rate from surface water was 2.29 ± 0.46 mmol m −2 d −1 while CH 4 and N 2 O evasion rates were 12.89 ± 3.05 and 1.23 ± 0.25 μmol m −2 d −1 respectively. Therefore, groundwater-derived greenhouse gas fluxes accounted for >100% CO 2 and N 2 O and ∼43% of CH 4 surface water evasion, indicating SGD is likely an important source of greenhouse gases to surface waters. However, this may be due to observations being performed near the SGD source, which may overestimate its contribution to the wider Sydney Harbour. Over a 20-year time frame, the combined emissions of CH 4 and N 2 O from surface waters to the atmosphere accounted for 25% of the total CO 2 -equivalent emissions. Although this study gives preliminary insight into SGD and greenhouse gas dynamics in Sydney Harbour, more spatial and temporal resolution sampling is required to fully constrain these processes. [ABSTRACT FROM AUTHOR]

We hypothesise that mangroves play an important role in groundwater exchange processes in sub-tropical and tropical estuarine waters. To investigate this, multiple high resolution time series measurements of radium across a tidal estuary (Coffs Creek, NSW, Australia) were performed as well as a spatial survey in both bottom and surface layers. Results from the spatial survey revealed increasing radium concentrations in parts of the estuary surrounded by mangroves. The average radium concentration in estuary areas lined with mangroves was 2.5 times higher than the average concentration at the mouth of the estuary and 6.5-fold higher than upstream freshwater areas. Additionally, the area enriched in radium coincided with low dissolved oxygen concentrations, implying that porewater exchange may drive anoxia. A radium mass balance model based on 223 Ra and 224 Ra isotopes at different sections of the estuary confirmed higher porewater exchange rates from areas fringed with mangrove vegetation. Estimated porewater exchange rates were 27.8 ± 5.3 and 13.6 ± 2.1 cm d −1 (0.8 ± 0.1 and 0.4 ± 0.1 m 3 s −1 ) based on 223 Ra and 224 Ra isotopes, respectively. The average saline porewater exchange was ∼ 10-fold larger than the upstream surface freshwater inputs to the estuary. We suggest that mangrove environments within subtropical estuaries are hotspots for porewater exchange due to the complex belowground structure of crab burrows and the effect of tidal pumping. Because porewater exchange releases carbon and nitrogen from coastal sediments, development and modification of mangrove areas in subtropical estuaries have a significant effect on coastal biogeochemical cycles. [ABSTRACT FROM AUTHOR]

The role of groundwater in transporting nutrients to coastal aquatic systems has recently received considerable attention. However, the relative importance of fresh versus saline groundwater-derived nutrient inputs to estuaries and how these groundwater pathways may alter surface water N:P ratios remains poorly constrained. We performed detailed time series measurements of nutrients in a tidal estuary (Hat Head, NSW, Australia) and used radium to quantify the contribution of fresh and saline groundwater to total surface water estuarine exports under contrasting hydrological conditions (wet and dry season). Tidally integrated nutrient fluxes showed that the estuary was a source of nutrients to the coastal waters. Dissolved inorganic nitrogen (DIN) export was 7-fold higher than the average global areal flux rate for rivers likely due to the small catchment size, surrounding wetlands and high groundwater inputs. Fresh groundwater discharge was dominant in the wet season accounting for up to 45% of total dissolved nitrogen (TDN) and 48% of total dissolved phosphorus (TDP) estuarine exports. In the dry season, fresh and saline groundwater accounted for 21 and 33% of TDN export, respectively. The combined fresh and saline groundwater fluxes of NO 3 , PO 4 , NH 4 , DON, DOP, TDN and TDP were estimated to account for 66, 58, 55, 31, 21, 53 and 47% of surface water exports, respectively. Groundwater-derived nitrogen inputs to the estuary were responsible for a change in the surface water N:P ratio from typical N-limiting conditions to P-limiting as predicted by previous studies. This shows the importance of both fresh and saline groundwater as a source of nutrients for coastal productivity and nutrient budgets of coastal waters. [ABSTRACT FROM AUTHOR]

Summary Intermittently Closed and Open Lakes or Lagoons (ICOLLs) are dynamic coastal systems that may be vulnerable to changes in catchment hydrology. However, little is known regarding the role of groundwater on the hydrological cycles of ICOLLs. Groundwater discharge in two ICOLLs (Welsby and Mermaid) and a nearby wetland (South Welsby Lagoon) located on Bribie Island (Australia) was quantified using radon ( 222 Rn, a natural geochemical groundwater tracer) during four seasonal surveys. The distribution of radon revealed temporal and spatial changes over the study period with higher surface water radon concentrations found in winter for Welsby ICOLL and in autumn for Mermaid ICOLL. The average estimated groundwater discharge rates from a radon mass balance were 3.4 ± 2.1, 7.3 ± 8.9 and 2.6 ± 1.1 cm d −1 in Welsby, South Welsby and Mermaid Lagoons, respectively. These values are at least 8-fold greater than the average annual precipitation that falls directly over the ICOLLs (1420 mm per year, or 0.4 cm d −1 ), which, coupled with minimal surface water runoff due to the permeable sandy soils, demonstrates that these systems are groundwater-dominated. Overall, groundwater discharge rates in these ICOLLs were much larger than has been reported in other lake systems which is most likely due to the high permeability of regional sandy soils and their large shoreline to volume ratio. [ABSTRACT FROM AUTHOR]

Summary Quantifying groundwater discharge remains a challenge due to its large temporal and spatial variability. Here, we quantify groundwater discharge into a small estuary using radon ( 222 Rn) and radium isotopes ( 223 Ra and 224 Ra). High temporal resolution (30 min time steps) radon observations at 4 time series stations were used to determine where groundwater discharge is prevalent in the estuary, and to reduce mass balance model uncertainties. A three-endmember mixing model was developed based on short-lived radium isotopes (sampled at a single location) to separate the shallow saline and deep fresh sources of the discharging groundwater. The results show that using multiple 222 Rn time series stations decreased the overall uncertainty of groundwater discharge estimates from about 41% to 23%. The radon derived groundwater flux was 56 ± 13 and 35 ± 12 cm d −1 in wet and dry conditions, respectively. The spatially distributed stations detected a well-defined small area located four kilometers upstream from the mouth of the estuary as a groundwater discharging hotspot. Estimates based on a 223 Ra and 224 Ra mass balance resulted in groundwater discharge estimates of 65 ± 18 and 84 ± 48 cm d − 1 in the wet and 18 ± 5 and 20 ± 6 cm d − 1 in the dry. The mixing model revealed contrasting results for deep vs. fresh groundwater contribution in wet and dry conditions. In wet conditions, deep fresh groundwater discharging into the estuary contributed 65% compared to the shallow saline groundwater (35%), while during dry conditions a larger contribution (80%) was related to shallow groundwater. A comprehensive spatial and temporal sampling strategy can produce groundwater discharge estimates with lower uncertainty and provides additional insight on where groundwater enters surface waters. [ABSTRACT FROM AUTHOR]

The reduction of saltmarsh habitat at a global scale has seen a concomitant loss of associated ecosystem services. As such, there is a need and a push for habitat rehabilitation. This study examined an innovative saltmarsh restoration project in Australia which sought to address the threats of mangrove encroachment and sea level rise. The project was implemented in 2017, using automated hydraulic control gates, termed "SmartGates," to lower the tidal regime over one site, effectively reversing sea level rise at a local level. Measured indicators of saltmarsh cover, number of species, seedling counts, and saltmarsh assemblages all showed significant positive development over time, with trends varying based on saltmarsh zone. The saltmarsh, predominantly Sarcocornia quinqueflora, developed from remnant supralittoral (previously high) marsh which remained at 45% cover to achieve over 15% coverage across the cleared habitat after 3 years. Slower development in the low marsh (<5%) compared to other zones contrasts with other saltmarsh restoration studies which may be due to the unique nature of the restoration method or the nature of Australian saltmarsh species which favor higher elevations and drier conditions. The development of saltmarsh at the treatment site was found to track toward that at comparison sites over time, becoming similar to some comparison sites by the studies end. This study highlights the usefulness of the novel restoration method used and of the measured indicators for assessing saltmarsh development. This innovative tidal control method could play an important role in the future of saltmarsh restoration worldwide. [ABSTRACT FROM AUTHOR]

• Heat tracer characterised vertical flow; Darcy results represented lateral flow. • Radon quantified both vertical and lateral flow including recirculated seawater. • Heat and head methods may underestimate flows in tidal settings. • Solute flux estimates in tidal wetlands should be based on a multi-tracer approach. Subsurface flow plays an important role in the functioning of wetlands and in the maintenance of their ecosystem services. Specifically, the transport and exchange of dissolved matter between sediments and surface waters is regulated by subsurface flow, which can strongly affect ecological zonation and productivity. Having a quantitative understanding of this subsurface flow is therefore important. Field techniques based on Darcy's equation or natural tracers are often used separately to assess flows. Here, radon and heat (both natural groundwater tracers) and Darcy's equation are used simultaneously to quantify the subsurface flow in a tidal wetland (Kooragang Island, Newcastle, Australia) and the results of the independent methods are compared. A steady-state radon mass balance model indicated an overall net subsurface exfiltration of 10.2 ± 4.2 cm/d while a 1D, vertical fluid heat transport model indicated a net exfiltration of 4.3 ± 2.9 cm/d. Flow estimated from analysis of hydraulic heads indicated an exfiltration rate of 3.2 ± 1.8 cm/d. The difference in flow rates is likely due to the localised measurement of the heat and head methods relative to radon, and therefore, these methods are less likely to capture zones of preferential subsurface flow. The main advantage of radon is that it provides the total subsurface flow regardless of the driving force. While head gradient or heat tracer method have the advantage of temporally quantify infiltration and exfiltration, we highlight that these methods may underestimate subsurface flows in highly dynamic coastal systems, such as tidal wetlands where a large portion of the subsurface flow is recirculated seawater. This could potentially lead to errors in solute flux estimates. This study highlights the importance of employing a multi-tracer approach and has implications towards quantifying the hydrological export of dissolved constituents (e.g., carbon and nitrogen) in coastal wetlands. [ABSTRACT FROM AUTHOR]

Porewater exchange is usually the least quantified process in delivering dissolved material from wetlands to coastal waters, although it has been recognised as an important pathway for the transport of trace metal, carbon and nutrient to the ocean. Here, surface water fluxes of dissolved manganese (Mn), iron (Fe), dissolved organic/inorganic carbon (DOC/DIC), total dissolved nitrogen (TDN) and phosphorous (TDP) were estimated from a temperate mangrove wetland (Kooragang Island, Newcastle, Australia). Radon (222Rn, a natural groundwater tracer) was used to develop a mass balance model to quantify porewater exchange rates and evaluate the contribution of porewater-derived dissolved material to the overall wetland surface water export. A 25-h time series dataset depicted a clear peak of Mn, Fe, TDN, DOC and radon during ebb tides which related to porewater discharge. Porewater exchange rates were estimated to be 14.0 ± 6.3 cm/d (0.18 ± 0.08 m3/s), mainly driven by tidal pumping, and facilitated by a large number of crab burrows at the site. Results showed that the wetland was a source of Mn, Fe, TDN and DOC to the adjacent river system and a sink for TDP and DIC. Surface water Mn, Fe, TDN and DOC exports were 4.0 ± 0.6, 6.6 ± 1.6, 23.9 ± 3.6 and 197.7 ± 29.7 mmol/m2 wetland/d, respectively. Porewater-derived Mn, Fe, TDN and DOC accounted for ~95, 100, 89 and 54% of the wetland surface water exports demonstrating its significant contribution. Our study indicates that temperate mangrove wetlands can be a major source of dissolved metal, carbon and nutrient delivery to coastal waters and that mangrove porewater exchange significantly contributes to this process. Image 1 • Radon (a natural tracer) was used to quantify porewater exchange in a tidal wetland. • Surface & porewater solute fluxes were measured at a temperate mangrove wetland. • Temperate mangrove wetlands can be a source of metal, C and N to coastal waters. • Porewater TDN and DOC fluxes accounted for 89 and 54% of surface water fluxes. • Porewater driven solute inputs should be accounted for in coastal wetland management actions. [ABSTRACT FROM AUTHOR]

Few studies exist on the contribution of subterranean estuaries (STEs) to the oceanic uranium (U) budget. Here, we estimate the dissolved U fluxes out of a quartz sand STE located on the east coast of Australia. Our results indicate that the advective flow of seawater in permeable sands enhances cycling of U in the STE. Dissolved U concentrations ranged from 25 nM in the STE to an effective zero salinity end-member of 3.8 nM in the surface estuary. The dissolved U (salinity corrected) concentrations were positively correlated to Fe (r 2 = 0.49 p < 0.001) during a shallow beach time series experiment. These results indicate that reductive dissolution of Fe oxides may be an important process maintaining high concentrations of U in shallow permeable sand STEs. The U export rates from the STE to the surface estuary in this study were estimated to be 1.8 μmol U m −2 day −1 based on shallow saline groundwater exchange pathways and 0.4 μmol U m −2 day −1 based on deep fresh submarine groundwater discharge (SGD). Uranium's behavior in STEs is diverse and site specific. Out of the seven investigations available here and in the literature, three suggested a SGD-derived U source to the coastal ocean, while four suggested a U sink within STEs removing seawater U. Therefore, it remains unclear whether SGD is a source or sink of U to the ocean and additional investigations in contrasting settings are required to resolve the global contribution of SGD to the marine U cycle. [ABSTRACT FROM AUTHOR]

Land reclamation projects and the installation of drainage infrastructure has impacted coastal wetlands worldwide. By altering water levels and inundation extent, these activities have changed the viable ecosystems onsite and resulted in the proliferation of freshwater species. As more than 50% of tidal wetlands have been degraded globally over the last 100 years, the importance of this issue is increasingly being recognised and tidal wetland restoration projects are underway worldwide. However, there are currently limited sites where large-scale reintroduction of tidal flushing has been implemented with the explicit aim to foster the growth of a threatened ecosystem. In this study, the tidal restoration of an internationally recognised Ramsar listed wetland in eastern Australia is described to highlight how coastal saltmarsh can be targeted by mimicking inundation depths and hydroperiod across the 410-ha site. Coastal saltmarsh is particularly important to this site as it is part of the east Australasian flyway for migratory birds and the minimum saltmarsh extent, as listed within the Ramsar's limits of acceptable change, have been breached. To recreate coastal saltmarsh habitat onsite, water level and hydroperiod criteria were established based on similar vegetation patterns within the adjacent estuary. A calibrated 2D hydrodynamic model of the site was then used to test how the preferred inundation criteria could be applied to the largest possible restored wetland area. Once optimised, a synthetic tidal signal was implemented onsite via automated hydraulic controls. The onsite vegetation response over an 8-year period was assessed to highlight the ecosystem response to controlled tidal inundation and denoted substantial saltmarsh expansion during the period. The techniques applied onsite have successfully met the restoration targets and can be applied at similar sites worldwide, offsetting sea level rise impacts to natural inundation hydroperiod. [ABSTRACT FROM AUTHOR]

The influence of groundwater and pore‐water exchange on dissolved organic matter (DOM) dynamics in coastal surface waters remains poorly understood. Here, we combine bottom up (i.e., groundwater‐derived flux estimates) and top down (i.e., water column response) evidence to assess whether groundwater exchange drives DOM dynamics in shallow coastal waters. We rely on automated chromophoric DOM (CDOM, a proxy for DOM) and radon (222Rn, groundwater proxy) measurements over tidal time scales in 12 shallow systems, including tidal freshwater wetlands, estuaries, mangroves, coral reefs, coastal lakes, a saltmarsh, and a residential canal estate. Groundwater‐derived dissolved organic carbon (DOC) fluxes ranged from 2 ± 2 mmol m−2 d−1 in a coral reef to 1941 ± 1325 mmol m−2 d−1 in a mangrove tidal creek. These groundwater fluxes replaced surface water DOC inventories on time scales ranging from ~ 0.5 d to several weeks. Systems with short replacement times displayed positive correlations between radon and CDOM in surface waters. Groundwater exchange diluted surface water DOC in four systems. Using multiple lines of evidence, we interpreted groundwater to be an important source of DOM to surface waters in 4 out of the 12 systems, including an offshore coral reef lagoon with low surface water DOC concentrations. Groundwater discharge was a negligible source of DOM in systems with high surface water DOC and CDOM concentrations such as tidal freshwater wetlands and coastal lakes. This investigation highlights the high variability in groundwater‐derived DOC fluxes and responses in the water column, and demonstrates that submarine groundwater discharge and advective pore‐water exchange should be considered in coastal carbon budgets. [ABSTRACT FROM AUTHOR]

Many coal mining subsidence lakes have formed in eastern China due to underground coal mining. Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco-environment of lakes. However, LGD in coal mining area is rarely reported. In the study, we quantified groundwater discharge into a Huainan coal mining subsidence lake (China) using water stable (D and 18O) and radioactive (222Rn) isotopes. 222Rn and 18O mass balance models were used to independently estimate LGD. The lake water was sampled based on the horizontal and vertical profile sampling. The isotope depth profiles were weighted according to the lake bathymetry to obtain a representative isotope inventory by geographic information system (GIS) analysis. The LGD rates estimated from two models were comparable, and the values were 18 mm d−1 for the 18O and 14 ± 8 mm d−1 for the 222Rn. Compared with the LGD of Ammelshainer See lake (Germany), a subsidence lake formed by coal mining, the LGD in the study was larger, which may be related to the different hydrological conditions in different areas. By integrating the LGD rates from these two models, this study quantitatively analyzed the role of groundwater in maintaining the water balance in subsidence lakes, which is of great significance to water resources assessment and sustainable utilization of coal mining subsidence lakes in eastern China. • Using radioactive and stable isotopes to estimate LGD rate of the coal mining subsidence lake. • Combining horizontal and vertical sampling improves the accuracy of radon decay loss assessment. • Isotope depth profile is weighted to obtain representative isotope inventory. • Groundwater discharge contributes a large amount of water to coal mining subsidence lakes. [ABSTRACT FROM AUTHOR]

Estuaries are an important source of greenhouse gases to the atmosphere, but uncertainties remain in the flux rates and production pathways of greenhouse gases in these dynamic systems. This study performs simultaneous high resolution measurements of the three major greenhouse gases (carbon dioxide, methane, and nitrous oxide) as well as carbon stable isotope ratios of carbon dioxide and methane, above and below the pycnocline along a salt wedge estuary (Yarra River estuary, Australia). We identified distinct zones of elevated greenhouse gas concentrations. At the tip of salt wedge, average CO2 and N2O concentrations were approximately five and three times higher than in the saline mouth of the estuary. In anaerobic bottom waters, the natural tracer radon (222Rn) revealed that porewater exchange was the likely source of the highest methane concentrations (up to 1302 nM). Isotopic analysis of CH4 showed a dominance of acetoclastic production in fresh surface waters and hydrogenotrophic production occurring in the saline bottom waters. The atmospheric flux of methane (in CO2 equivalent units) was a major (35-53%) contributor of atmospheric radiative forcing from the estuary, while N2O contributed <2%. We hypothesize that the release of bottom water gases when stratification episodically breaks down will release large pulses of greenhouse gases to the atmosphere. [ABSTRACT FROM AUTHOR]

A creek draining a coastal wetland was studied to determine dissolved iron fluxes to the ocean, and whether these fluxes may be driven by submarine groundwater discharge (SGD). Dissolved metals (i.e., Fe, Mn, Ba, and U), natural groundwater tracers ( 224 Ra and 226 Ra), and dissolved organic carbon (DOC) were measured every 1 h for 30 h during both summer (wet season) and winter (dry season). The average dissolved Fe concentrations at the mouth of the creek were 8020 and 5630 nM during the summer and winter, respectively. Dissolved Fe concentrations in groundwater were 4060 nM and SGD-derived Fe fluxes were 1.2 mol km − 2 year − 1 (catchment area) or 205 mol km − 2 year − 1 (creek area). Groundwater seepage played a minor role on the total coastal wetland creek export of Fe to the ocean. The surface water dissolved Fe fluxes to the ocean were approximately 9 × 10 5 mol km − 2 (estuary area) year − 1 and ~ 50,000 mol km 2 year − 1 on a catchment area basis. If our results are comparable to other small estuarine systems and considering the global areas of coastal wetlands (~ 660,000 km 2 ), short creeks that drain coastal wetlands may discharge a large amount of dissolved Fe to the ocean (32.2 Gmol year − 1 or ~ 1.8 Tg year − 1 ) which is comparable to the global river estimates (~ 1.5 Tg year − 1 ). The high dissolved Fe concentrations associated with high DOC concentrations and no significant Fe removal within the estuarine gradient seem to be responsible for these large potential Fe fluxes. [ABSTRACT FROM AUTHOR]

Methane (CH4) emissions from estuarine wetlands were proved to be influenced by tide movement and inundation conditions notably in many previous studies. Although there have been several researches focusing on the seasonal or annual CH4 emissions, the short-term CH4 emissions during the tide cycles were rarely studied up to now in this area. In order to investigate the CH4 emission pattern during a tide cycle in Yangtze Estuary salt marshes, frequent fixed-point observations of methane flux were carried out using the in-situ static closed chamber technique. The results indicated that the daily average CH4 fluxes varied from 0.68 mgCH4·m−2·h−1 to 4.22 mgCH4·m−2·h−1 with the average flux reaching 1.78 mgCH4·m−2·h−1 from small tide to spring tide in summer. CH4 fluxes did not show consistent variation with both tide levels and inundation time but increased steadily during almost the whole research period. By Pearson correlation analysis, CH4 fluxes were not correlated with both tide levels (R = −0.014, p = 0.979) and solar radiation (R = 0.024, p = 0.865), but significantly correlated with ambient temperature. It is temperature rather than the tide level mainly controlling CH4 emissions during the tide cycles. Besides, CH4 fluxes also showed no significant correlation with the underground pore-water CH4 concentrations, indicating that plant-mediated transport played a more important role in CH4 fluxes compared with its production and consumption. [ABSTRACT FROM AUTHOR]

Shipping water, the flow washing over and impacting the upper decks of ships and offshore structures, occurs frequently during their service life and often causes structural problems. For engineers to design safe floating structures subjected to shipping water it is essential to gain an in-depth understanding of its depth and flow field, and the resulting impact forces. In this work, Computational Fluid Dynamics (CFD) is applied to understand the physics of shipping water washing over a stepped platform. We find that the most accurate solutions are obtained with the k − ε turbulence closure. The hydrodynamic load generated by the shipping water is found to strongly depends on the kinematic energy of the water hitting the step. It is shown that with smaller values of the freeboard a more dynamic flow ensues, with a stronger vortex and larger velocity gradient resulting in deeper shipping water and a larger impact force. • A CFD model is used to numerically simulate shipping water propagating over a step. • The turbulence assumption leads to a better fit with the experimental data. • Shipping water becomes more dynamic as freeboard decreases, creating a larger impact. • As water flows on the step, a large velocity gradient and vortices are generated. [ABSTRACT FROM AUTHOR]