Natural purification capacity of tidal flats for organic matters and nutrients: A mesocosm study

Anthropogenic nitrogen pollution impacts saltmarsh resilience with inhibition of seedling establishment and population dispersal

Saltmarsh, a prominent buffer ecosystem, has been identified as an important sink for nitrogen (N) pollutants from marine- and land-based anthropogenic activities. However, how the enriched anthropogenic N impacts saltmarsh sustainability has been neglected due to limited understanding of marsh resilience based on seedling establishment and population dispersal under anthropogenic N inputs. This study combined mesocosm experiments and model simulations to quantify the effects of increased anthropogenic N on the seedling-based vegetation expansion of Spartina alterniflora. The results indicated that seedling survivals, growth rates, and morphological indicators were inhibited by 20.08 %, 37.14 %, and > 35.56 %, respectively, under 1.5 gN/kg anthropogenic N. The sensitivity rate of vegetation expansion was increased by 70 % with 1 gN/kg increased N concentration under the scenario of low seedling density (< 15 m/yr). These findings revealed an important unidentified weakness of the marsh development process to anthropogenic N inputs. Finally, we highlighted the importance of appropriate protection measures to control nutrient pollution in salt marshes. Our study provides new insights for enhancing the resilience and sustainability of saltmarsh ecosystems.

Integrated assessment of the natural purification capacity of tidal flat for persistent toxic substances and heavy metals in contaminated sediments

Natural purification of pollutants is highly recognized as regulating ecosystem services; however, the purification capacity of tidal flats remains largely unknown and/or unquantified. A 60-day mesocosm transplant experiment was conducted in situ to assess the purification capacity of natural tidal flats. We adopted the advanced sediment quality triad approach, monitoring 10 endpoints, including chemical reduction, toxicity changes, and community recoveries. The results indicated that contaminated sediments rapidly recovered over time, particularly > 50% within a day, then slowly recovered up to ∼ 70% in a given period (60 days). A significant early reduction of parent pollutants was evidenced across all treatments, primarily due to active bacterial decomposition. Notably, the presence of benthic fauna and vegetated halophytes in the treatments significantly enhanced the purification of pollutants in both efficacy and efficiency. A forecast linear modeling further suggested additive effects of biota on the natural purification of tidal flats, reducing a full recovery time from 500 to 300 days. Overall, the triad approach with machine learning practices successfully demonstrated quantitative insight into the integrated assessment of natural purification.

Tidal flat microbial communities between the Huaihe estuary and Yangtze River estuary

Tidal flats have important ecological functions and offer great economic value. Using field sampling, numerical simulation, and high-throughput sequencing, the ecological state of typical tidal flats along the eastern coast of China was investigated. The findings demonstrated that the area may be separated into subregions with notable differences in the features of microbial communities due to the variations in water quality and total pollutant discharge of seagoing rivers. With a ratio of 62%, the development of the microbial community revealed that homogenous selection predominated. In general, the formation of microbial communities follows deterministic processes, especially those of environmental selection. The wetland microbial communities are impacted by pollutants discharged into the sea from the Huaihe River and the Yangtze River. The Yangtze River's nitrogen pollutants affected the wetland zone, and denitrification dominated. The study established ecological patterns between the river and the sea and we offer suggestions for managing watersheds and safeguarding the ecology of coastal tidal flats.

Distribution, compositional characteristics, and historical pollution records of microplastics in tidal flats of South Korea

Studies on distribution of microplastics (MPs) in sediments of tidal flats are relatively scarce compared to other coastal areas. In this study, spatial and vertical distributions and compositions of MPs in tidal flat sediments along the west coast of Korea were investigated. The abundance of MPs in surface and core sediments ranged from 20 to 325 and 14 to 483 particles per 50 g dry weight, respectively. Polypropylene (51%) and polyethylene (36%) were the most dominant MPs; the size was <0.3 mm, and the shape was mostly fragments followed by fibers. The abundance of MPs in sediments has increased rapidly since the 1970s, and recently showed a slight decrease. Surface morphology of MPs analyzed using a scanning electron microscope revealed that the MPs in tidal flats were highly weathered mechanically and/or oxidatively. The results of this study provide valid baseline data on distributions of MPs in tidal flats.

The first national scale evaluation of total nitrogen stocks and burial rates of intertidal sediments along the entire coast of South Korea

The regulating ecosystem services, such as water purification, that tidal flats provide by nitrogen (N) burial are being increasingly recognized; yet, quantitative estimates remain limited. Here, we first present nationwide evaluation of total N stocks and burial rates in the Korean tidal flats, based on a 3 year long monitoring assessment combined with remote sensing approach. A total of 20 intertidal flats representing 7 provinces of South Korea were extensively surveyed in 2018-20. The sediment textural type (sand, mixed, and mud) classified from remotely sensed imagery was significantly correlated to that identified from field data (p < 0.01), warranting a nationwide estimate of total N stocks. The estimated total N stocks and burial rates in the tidal flats of Korea were 1.5 Tg N and 8,192 Mg N yr^-1, respectively. Total N stocks significantly varied by region, province, morphology, salinity, and land use type adjacent to the corresponding tidal flats. In general, the N stocks of tidal flats were influenced by the degree of terrestrial N inputs to the ocean. The N stocks were significantly correlated with several physical parameters, such as precipitation (p < 0.05) and tide (p < 0.01). Among the sediment properties, the mud content was found to be the key factor determining the total N stocks across the 20 intertidal flats (p < 0.01). Finally, by applying the environmental value for N removal (USD km^-2 yr^-1) to the tidal flat area (km²), the economic value of the total N removal was estimated as ~233 Million USD yr^-1 in Korea and ~15 Billion USD yr^-1 globally. Overall, the present work confirms the valuable ecosystem service of tidal flat's cost-efficient N removal capacity, highlighting marine ecosystem service.

A solution for restoration of critical wetlands and waterbird habitats in coastal deltaic systems

Loss of coastal wetland habitats has been directly linked to a decline in waterbird populations including migratory species, leading to calls to reverse this trend in part by restoring these habitats. However, distinct "sediment scarcity" has hindered coastal habitat restoration. Here, taking the Yangtze River Delta, China as an example, we put forward a feasible solution to solve the sediment shortage in habitat restoration so necessary to restore migratory waterbird numbers. Four biological indices including total wetland area, wetland vegetation area and waterbird species richness and abundance, were used to compare and assess the restorative efforts. Three solutions were adopted for the rehabilitation sites, including promoting sediment deposition and settlement through engineering intervention in Chongming Dongtan (CD) and Eastern Nanhui (EN), and using dredged sediments to nourish and create new habitats in Hengsha Eastern Shoal (HES). The mean wetland area increased 19.66 km²/yr in EN, 8.78 km²/yr in HES and 3.83 km²/yr in CD after rehabilitation. Along with the increase of wetlands and habitats, the abundance of waterbirds increased 1.3 times, 121 times and 1.5 times in EN, HES and CD, respectively. In contrast, in the site of Fengxian and Jinshan (FJ) where no any rehabilitation measure was taken after reclamation, the habitats were lost almost completely and the waterbird abundance dropped drastically. The comparison and assessment results demonstrate that proper coastal silting structures and ecological utilization of nearby dredged sediments are the feasible and effective solutions to retain sediments, restore coastal habitats and increase waterbird diversity and abundance.

Sea level rise impacts on estuarine dynamics: A review

Sea level rise (SLR) poses a hazard to ecosystems and economies in low-lying coastal and estuarine areas. To better understand the potential impacts of SLR in estuaries, a comprehensive review of existing theory, literature, and assessment tools is undertaken. In addition, several conceptual models are introduced to assist in understanding interlinked estuarine processes and their complex responses to SLR. This review indicates that SLR impacts in estuaries should not be assessed via static (bathtub) approaches as they fail to consider important hydrodynamic effects such as tidal wave amplification, dampening, and reflection. Where hydrodynamic models are used, the existing literature provides a relatively detailed understanding of how SLR will affect estuarine hydrodynamics (e.g., tides and inundation regimes). With regards to the current understanding of, and ability to model, the connections between altered hydrodynamics (under SLR) and dependent geomorphic, ecological, and bio-geochemical processes, significant knowledge gaps remain. This is of particular concern as there is currently a paradigm shift towards more integrated and holistic management of estuaries. Estuarine management under accelerating SLR is likely to become increasingly complex, as decision-making will be undertaken with uncertainty. As such, this review highlights that there is a fundamental requirement for more sophisticated and interdisciplinary studies that integrate physical, ecological, bio-geochemical, and geomorphic responses of estuaries to SLR.

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.