Groundwater discharge drives water quality and greenhouse gas emissions in a tidal wetland

Wetlands play an important role in the global carbon cycle as they can be sources or sinks for greenhouse gases. Groundwater discharge into wetlands can affect the water chemistry and act as a source of dissolved greenhouse gases, including CO2 and CH4. In this study, surface water quality parameters and CO2 and CH4 concentrations were evaluated in a tidal wetland (Hunter Wetlands National Park, Australia) using time series measurements. Radon (222Rn), a natural groundwater tracer, was used to investigate the role of groundwater as a pathway for transporting dissolved CO2 and CH4 into the wetland. In addition, water-to-air CO2 and CH4 fluxes from the wetland were also estimated. The results showed a high concentration of radon in wetland surface water, indicating the occurrence of groundwater discharge. Radon concentration had a strong negative relationship with water depth with a determination coefficient (R2) of 0.7, indicating that tidal pumping was the main driver of groundwater discharge to the wetland. Radon concentration also showed a positive relationship with CO2 and CH4 concentrations (R2 = 0.4 and 0.5, respectively), while the time series data revealed that radon, CO2, and CH4 concentrations peaked concurrently during low tides. This implied that groundwater discharge was a source of CO2 and CH4 to the wetland. The wetland had an average water-to-air CO2 flux of 99.1 mmol/(m2·d), twice higher than the global average CO2 flux from wetlands. The average CH4 flux from the wetland was estimated to be 0.3 mmol/(m2·d), which is at the higher end of the global CH4 flux range for wetlands. The results showed that groundwater discharge could be an important, yet unaccounted source of CO2 and CH4 to tidal wetlands. This work has implications for tidal wetland carbon budgets and emphasizes the role of groundwater as a subsurface pathway for carbon transport

Blue carbon ecosystem monitoring using remote sensing reveals wetland restoration pathways

In an era of climate and biodiversity crises, ecosystem rehabilitation is critical to the ongoing wellbeing of humans and the environment. Coastal ecosystem rehabilitation is particularly important, as these ecosystems sequester large quantities of carbon (known in marine ecosystems as “blue carbon”) thereby mitigating climate change effects while also providing ecosystem services and biodiversity benefits. The recent formal accreditation of blue carbon services is producing a proliferation of rehabilitation projects, which must be monitored and quantified over time and space to assess on-ground outcomes. Consequently, remote sensing techniques such as drone surveys, and machine learning techniques such as image classification, are increasingly being employed to monitor wetlands. However, few projects, if any, have tracked blue carbon restoration across temporal and spatial scales at an accuracy that could be used to adequately map species establishment with low-cost methods. This study presents an open-source, user-friendly workflow, using object-based image classification and a random forest classifier in Google Earth Engine, to accurately classify 4 years of multispectral and photogrammetrically derived digital elevation model drone data at a saltmarsh rehabilitation site on the east coast of Australia (Hunter River estuary, NSW). High classification accuracies were achieved, with >90% accuracy at 0.1 m resolution. At the study site, saltmarsh colonised most suitable areas, increasing by 142% and resulting in 56 tonnes of carbon sequestered, within a 4-year period, providing insight into blue carbon regeneration trajectories. Saltmarsh growth patterns were species-specific, influenced by species’ reproductive and dispersal strategies. Our findings suggested that biotic factors and interactions were important in influencing species’ distributions and succession trajectories. This work can help improve the efficiency and effectiveness of restoration planning and monitoring at coastal wetlands and similar ecosystems worldwide, with the potential to apply this approach to other types of remote sensing imagery and to calculate other rehabilitation co-benefits. Importantly, the method can be used to calculate blue carbon habitat creation following tidal restoration of coastal wetlands

Optimal reservoir operation using Nash bargaining solution and evolutionary algorithms

Optimizing reservoir operation is critical to ongoing sustainable water resources management. However, different stakeholders in reservoir management often have different interests and resource competition may provoke conflicts. Resource competition warrants the use of bargaining solution approaches to develop an optimal operational scheme. In this study, the Nash bargaining solution method was used to formulate an objective function for water allocation in a reservoir. Additionally, the genetic and ant colony optimization algorithms were used to achieve optimal solutions of the objective function. The Mahabad Dam in West Azerbaijan, Iran, was used as a case study site due to its complex water allocation requirements for multiple stakeholders, including agricultural, domestic, industrial, and environmental sectors. The relative weights of different sectors in the objective function were determined using a discrete kernel based on the priorities stipulated by the government (the Lake Urmia National Restoration Program). According to the policies for the agricultural sector, water allocation optimization for different sectors was carried out using three scenarios: (1) the current situation, (2) optimization of the cultivation pattern, and (3) changes to the irrigation system. The results showed that the objective function and the Nash bargaining solution method led to a water utility for all stakeholders of 98%. Furthermore, the two optimization algorithms were used to achieve the global optimal solution of the objective function, and reduced the failure of the domestic sector by 10% while meeting the required objective in water-limited periods. As the conflicts among stakeholders may become more common with a changing climate and an increase in water demand, these results have implications for reservoir operation and associated policies

Innovative Tidal Control Successfully Promotes Saltmarsh Restoration

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 quinque flora, 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 (\u3c5%) 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

Sea level rise will change estuarine tidal energy: A review

Climate change induced sea level rise (SLR) is likely to impact estuarine hydrodynamics and associated processes, including tidal energy. In this study, a hierarchy of factors influencing the future of estuarine tidal energy resources is proposed based on their relevance to SLR. These include primary factors (e.g., tidal prism, tidal range, tidal current, tidal asymmetry), secondary factors (e.g., sediment transport), and tertiary factors (e.g., shifts in estuarine shape/landform). The existing uncertainty regarding SLR impacts on tidal energy resource is high, given the spatial variability of estuaries. SLR may cause tidal ranges or currents to strengthen or weaken, depending on estuarine shape and boundary conditions (e.g., presence or absence of levees and adjacent low-lying areas). To date, local site studies have not resulted in an overarching assessment of SLR effects on tidal energy resources and comparative studies encompassing different regions and estuary types are recommended in order to address the existing knowledge gaps and provide insights for policymakers and stakeholders. SLR implications to estuarine tidal energy resources may be particularly important as SLR-induced changes can alter the available resource within a renewable energy development's operational lifetime (-20-30 years for tidal stream devices and-120 years for tidal barrages). In this regard, broader environmental impacts, as well as technoeconomic assessments, are difficult to predict and long-term management decisions associated with harnessing the potential of tidal energy schemes within estuaries should be made with caution

Coastal wetlands can be saved from sea level rise by recreating past tidal regimes

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

The evolving landscape of sea-level rise science from 1990 to 2021

As sea-level rise (SLR) accelerates due to climate change, its multidisciplinary field of science has similarly expanded, from 41 articles published in 1990 to 1475 articles published in 2021, and nearly 15,000 articles published in the Web of Science over this 32-year period. Here, big-data bibliometric techniques are adopted to systematically analyse this large literature set. Four main research clusters (themes) emerge: (I) geological dimensions and sea-level indicators, (II) impacts, risks, and adaptation, (III) physical components of sea-level change, and (IV) coastal ecosystems and habitats, with 16 associated sub-themes. This analysis provides insights into the evolution of research agendas, the challenges and opportunities for future assessments (e.g. next IPCC reports), and growing focus on adaptation. For example, the relative importance of sub-themes evolves consistently with a relative decline in pure science analysis towards solution-focused topics associated with SLR risks such as high-end rises, declining ecosystem services, flood hazards, and coastal erosion/squeeze

A large-scale review of wave and tidal energy research over the last 20 years

Over the last two decades, a large body of academic scholarship has been generated on wave and tidal energy related topics. It is therefore important to assess and analyse the research direction and development through horizon scanning processes. To synthesise such large-scale literature, this review adopts a bibliometric method and scrutinises over 8000 wave/tidal energy related documents published during 2003–2021. Overall, 98 countries contributed to the literature, with the top ten mainly developed countries plus China produced nearly two-thirds of the research. A thorough analysis on documents marked the emergence of four broad research themes (dominated by wave energy subjects): (A) resource assessment, site selection, and environmental impacts/benefits; (B) wave energy converters, hybrid systems, and hydrodynamic performance; (C) vibration energy harvesting and piezoelectric nanogenerators; and (D) flow dynamics, tidal turbines, and turbine design. Further, nineteen research sub-clusters, corresponding to broader themes, were identified, highlighting the trending research topics. An interesting observation was a recent shift in research focus from solely evaluating energy resources and ideal sites to integrating wave/tidal energy schemes into wider coastal/estuarine management plans by developing multicriteria decision-making frameworks and promoting novel designs and cost-sharing practices. The method and results presented may provide insights into the evolution of wave/tidal energy science and its multiple research topics, thus helping to inform future management decisions

Profiling resilience and adaptation in mega deltas: a comparative assessment of the Mekong, Yellow, Yangtze, and Rhine deltas

River deltas and estuaries are disproportionally-significant coastal landforms that are inhabited by nearly 600 M people globally. In recent history, rapid socio-economic development has dramatically changed many of the World's mega deltas, which have typically undergone agricultural intensification and expansion, land-use change, urbanization, water resources engineering and exploitation of natural resources. As a result, mega deltas have evolved into complex and potentially vulnerable socio-ecological systems with unique threats and coping capabilities. The goal of this research was to establish a holistic understanding of threats, resilience, and adaptation for four mega deltas of variable geography and levels of socio-economic development, namely the Mekong, Yellow River, Yangtze, and Rhine deltas. Compiling this kind of information is critical for managing and developing these complex coastal areas sustainably but is typically hindered by a lack of consistent quantitative data across the ecological, social and economic sectors. To overcome this limitation, we adopted a qualitative approach, where delta characteristics across all sectors were assessed through systematic expert surveys. This approach enabled us to generate a comparative assessment of threats, resilience, and resilience-strengthening adaptation across the four deltas. Our assessment provides novel insights into the various components that dominate the overall risk situation in each delta and, for the first time, illustrates how each of these components differ across the four mega deltas. As such, our findings can guide a more detailed, sector specific, risk assessment or assist in better targeting the implementation of risk mitigation and adaptation strategies

Evaluation and analysis of acid sulphate soil remediation via tidal restoration

The effectiveness of a floodgate modification strategy that involved restoring tidal flushing via modified floodgates to improve surface water quality and combat acid sulphate soil leachate was investigated in a low-lying flood mitigation drain near Berry in southeastern NSW, Australia. Prior to floodgate modifications, extensive soil, groundwater and surface water analysis indicated ubiquitous acidic conditions that were exacerbated by low drain water levels maintained by the one-way floodgates. The floodgates also increased a range of environmental problems including metal flocculation, low dissolved oxygen levels, pyrite oxidation, and acid transport. Temporal data showed that total drain water acidity was highest after rainfall but decreased when buffering agents inherent in brackish creek water penetrated upstream of the floodgate. Before floodgate modifications were undertaken, tidal restoration techniques and criteria were developed to address surface water quality, hydraulics, and design concerns. First, an ion-association mixing program was developed and calibrated to simulate water quality and reaction kinetics within the drain. Second, hydraulic calculations and GIS mapping techniques indicated that full tidal restoration could be permitted solely within the primary drain. Floodgate design criteria were then developed and two modified floodgates were constructed and installed. One of these designs, the automated \u27Environmentally Controlled Smart Gate System\u27, was developed to permit tidal flushing based on real-time environmental parameters while utilising state-of-the-art technologies. After floodgate modifications, tidal buffering and dilution improved drain water quality during all flow regimes and was most effective during prolonged dry periods. Tidal flushing also decreased the acid reservoir effect, permitted fish passage, reduced soluble iron concentrations and elevated the groundwater table. The impact of tidal forcing on the groundwater table was investigated biogeochemically through multi-port piezometers, while saline intrusion was simulated using 3-Dimensional finite element techniques. These investigations showed that tidal forcing at the study site was limited due to moderate lateral soil hydraulic conductivity values. Furthermore, while calibrated for the study site conditions, the finite element model and tidal buffering techniques can be easily adopted to other low-lying estuarine sites throughout Australia. The floodgate modification strategy was shown to improve drain water quality, decrease groundwater drawdown, and improve drain hydraulics. However, before future floodgate modifications are undertaken several concerns should be addressed. These concerns and the findings from the entire thesis were summarised within a comprehensive best management practice developed to assist in effectively, efficiently, and safely restoring tidal flushing to additional sites