Estuary Salt Research Articles

Study on the structure and energy flow of the salt marsh wetland ecosystem in Liaohe Estuary based on the Ecopath model

Salt marshes are one of the three blue carbon ecosystems recognized by the Intergovernmental Panel on Climate Change (IPCC). However, coastal salt marshes in China are facing the risk of degradation. To reveal the status of the salt marsh wetland ecosystem in Liaohe Estuary, an Ecopath model composed of 14 functional groups was constructed based on the 2019 ecological survey data. A comprehensive analysis of the system’s food web structure, energy flow processes, and overall ecosystem characteristics was conducted. The results show that the energy flow in the Liaohe Estuary salt marsh wetland ecosystem is mainly distributed in three integrated trophic levels. The utilization rates of trophic levels II and III are low, easily causing blockages in the lower trophic levels of the ecosystem’s energy flow. The total system throughput of the Liaohe Estuary salt marsh wetland ecosystem is 49,099.039 t·km²·a−1;. The system connectivity index and the system omnivory index are 0.207 and 0.109, respectively. Compared with other wetland systems, the ecosystem has a larger scale, but the overall ecosystem characteristic index reveals lower stability and complexity of the Liaohe Estuary salt marsh wetland system.

Persistent Organic Pollutants in Tagus Estuary Salt Marshes: Patterns of Contamination and Plant Uptake.

The presence of anthropogenic compounds, including organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs), was studied in three salt marshes within the Tagus estuary, Portugal, along an anthropogenic pressure gradient. Results revealed differences in OCPs and PCBs among the marshes, with differing concentration levels. Specifically, one marsh, with surrounding agricultural activity, showed the highest OCP concentrations, while another, with a historical industrial past, exhibited elevated PCB levels. In contrast, a third marsh, part of a natural reserve, displayed comparatively lower concentrations of both substances. Sediment concentrations, likely influenced by agricultural practices, were found to be comparable to or higher than those observed in other Portuguese estuaries. The halophyte Spartina maritima was found to absorb OCPs, particularly in its aboveground tissues, suggesting bioaccumulation within the plant. Additionally, PCB levels appeared to be influenced by industrial history, with one marsh displaying notably higher concentrations. In conclusion, the persistence of organochlorine compounds in the salt marsh ecosystems notwithstanding the regulatory prohibitions implemented in the 1990s highlights the need for continuous monitoring and study of such sites and the necessity of remediation practices, which are imperative to mitigate ecological and health risks in these polluted salt marshes.

Salt Transport Regimes Caused by Tidal and Subtidal Processes in Narrow Estuaries

AbstractTidally averaged transport of salt in estuaries is controlled by various subtidal and tidal processes. In this study, we show the relative importance of various subtidal and tidal transport processes in a width‐averaged sense. This is done for a large range of forcing and geometric parameters, which describe well‐mixed to salt wedge estuaries. To this end, we develop a width‐averaged process‐based model aimed at conducting and analyzing a large number of experiments (∼40,000). We find that the salt transport is dominated by one of seven salt transport balances, or regimes. Four of these regimes are dominated by subtidal processes, while the other three are dominated by tidal processes. Which regime occurs in a part of an estuary depends on four dimensionless parameters, representing local geometry, and forcing conditions. One of the regimes features salt import by correlations between the depth‐averaged tidal velocity and salinity. While this mechanism was previously only associated with along‐channel geometric variations, we find it can also be a dominant mechanism in a significant part of the parameter space due to river‐induced tidal asymmetry, independent of river geometry. We apply our classification to a case study of part of the Dutch Rhine delta and compare to decomposition results of a fully realistic three‐dimensional model. We find the estuary features two regimes, with import dominated by subtidal shear transport in the seaward part of the estuary and by depth‐averaged tidal correlations in the landward part of the estuary.

Spartina alterniflora has the highest methane emissions in a St. Lawrence estuary salt marsh

Salt marshes have the ability to store large amounts of ‘blue carbon’, potentially mitigating some of the effects of climate change. Salt marsh carbon storage may be partially offset by emissions of CH4, a highly potent greenhouse gas. Sea level rise and invasive vegetation may cause shifts between different elevation and vegetation zones in salt marsh ecosystems. Elevation zones have distinct soil properties, plant traits and rhizosphere characteristics, which affect CH4 fluxes. We investigated differences in CH4 emissions between four elevation zones (mudflat, Spartina alterniflora, Spartina patens and invasive Phragmites australis) typical of salt marshes in the northern Northwest Atlantic. CH4 emissions were significantly higher from the S. alterniflora zone (17.7 ± 9.7 mg C m−2h−1) compared to the other three zones, where emissions were negligible (<0.3 mg C m−2h−1). These emissions were high for salt marshes and were similar to those typically found in oligohaline marshes with lower salinities. CH4 fluxes were significantly correlated with soil properties (salinity, water table depth, bulk density and temperature), plant traits (rhizome volume and biomass, root volume and dead biomass volume all at 0–15 cm) and CO2 fluxes. The relationships between CH4 emissions, and rhizome and root volume suggest that the aerenchyma tissues in these plants may be a major transport mechanism of CH4 from anoxic soils to the atmosphere. This may have major implications for the mitigation potential carbon sink from salt marshes globally, especially as S. alterniflora is widespread. This study shows CH4 fluxes can vary over orders of magnitude from different vegetation in the same system, therefore, specific emissions factors may need to be used in future climate models and for more accurate carbon budgeting depending on vegetation type.

Contribution of belowground plant components to salt marsh soil volume

Accumulation of belowground plant material (live roots, rhizomes, and dead material) is critical to vertical accretion of tidal salt marsh soils and their ability to maintain elevation with respect to sea-level rise. Many studies utilize a single, global relationship between soil organic carbon and belowground volume, which does not include the pore space created by belowground growth, thus does not appropriately reflect the contribution of vegetation to soil accretion. We develop factors that relate live belowground mass to the volume of live roots and rhizomes for four species in a St. Lawrence estuary salt marsh at la Pocatière, Quebec. Spartina alterniflora, Spartina patens, and invasive Phragmites australis were identified in the aboveground growth, while only tubers and roots of Cyperus esculentus were found. Cores of the top 30 cm of soil were collected and cut into 0–15 and 15–30 cm sections from the low marsh, high marsh, and S. patens marsh invaded by P. australis. The soil was washed over a 1 mm sieve, and the organic matter was retained. The live material was sorted into fractions of roots and rhizomes by species. The volume of all components was measured by displacement in water before drying to obtain mass. An overall conversion factor of 8.2 g cm−3 reflects the volumetric contribution of live belowground plant components of all species studied. The relationship varies by species, with S. alterniflora contributing the greatest volume per unit mass (8.9 g cm−3), followed by S. patens (7.9 g cm−3), then P. australis and C. esculentus, both at 5.6 g cm−3. Our results not only demonstrate that the total mass and volume of each species for each component (rhizomes, roots, and dead material) were strongly correlated but that measurements of carbon density underestimate the true volumetric contributions of salt marsh plants to the soil.

Distribution and assessment of heavy metals in suspended particles in the Sundarban mangrove river, Bangladesh

The suspended particulate matter (SPM) is an important carrier of heavy metals transportation from land to sea, so it is significant to study the heavy metal pollution in SPM. The distribution and assessment of five heavy metals (Mn, Cr, Ni, Cu, and Pb) in SPM collected from Passur River and its estuary in Sundarban were studied in combination with water temperature, salinity, and turbidity. The results show that the heavy metal content and distribution in SPM are mainly controlled by runoff input, hydrodynamic process and the interaction process of salt and fresh water in estuaries. The quality evaluation results of heavy metals in SPM show that pollution degree is light. Studies on the heavy metals in SPM are of great significance to comprehensively evaluate regional pollution status and carry out early warning.

A Multi-year Seasonal Study of Amoeboid Protists in Surface Water at the Margin of a Hudson River Estuary Salt Marsh

Marshes bordering rivers and estuaries are productive ecosystems that interact dynamically with the adjacent water mass. This is a multi-year study (2019–2022) of seasonal changes in the density of naked amoebae in monthly samples from the surface water of the Hudson estuary near Piermont, N. Y. with relationships to key environmental variables (surface water temperature, salinity, Secchi depth representing turbidity, and enterococcus bacterial counts). During the colder months (November to March), when decayed leaves and litter from the deciduous marsh grass produced organic matter in the sediment surface, the mean abundance of active amoebae ± standard error of the mean (SEM) was higher (3.07 ± 0.99 × 10⁴/ L, N = 7). In warmer months (May to September) the abundance of amoebae was lower (1.35 ± 0.29 × 10⁴ / L, N = 10). A multivariate linear regression analysis was performed relating amoeba abundance to four major water mass variables, resulting in the following statistically significant equation (p = 0.03): AD = 0.121 × T + 0.301 × L – 0.047 × S + 0.359 × C, where: AD = active amoebae density (× 10 /L), T = temperature ( C), L = tide level (m), S = Secchi disc depth (cm) and C = bacterial enterococcus concentration (number/ml). In general, given the increasing evidence of the potential importance of amoeboid protists in aquatic ecosystems, further research is warranted on their role in food webs and the carbon biogeochemical cycle within heterotrophic estuarine and coastal waters.

237Np and 241Am as Fingerprints in the Major River Basins of Southern China and North South China Sea: A Land-Sea Perspective.

During the global nuclear weapon tests, large amounts of radioactive elements are released into the environment. Long-period actinide elements such as Np, Pu, and Am exhibit different behaviors in various environments, and their isotope characteristic fingerprints are of great significance for studying the distribution, migration, and prediction of radioactive pollutants. To investigate the distribution of 241Am and 237Np in southern China, activities of 241Am (0.008 ± 0.012–0.932 ± 0.066 Bq/kg) and 237Np (0.037 ± 0.003–1.458 ± 0.063 mBq/kg), 237Np/239Pu atom ratios (0.055 ± 0.003–0.864 ± 0.054), and 241Am/239+240Pu activity ratios (0.033 ± 0.075–15.870 ± 0.477) in 95 surface sediment samples collected from the northern South China Sea and major river basins were analyzed for the first time. Due to the different scavenging mechanisms of Am and Pu, 241Am is preferentially concentrated by sinking the particulate, while plutonium is scavenged in the coastal area, resulting in a higher 241Am/239+240Pu activity ratio in estuary and coastal areas. The distribution of 237Np shows obvious spatial inhomogeneity as the high migration rate. The relevant fingerprint characteristic has changed greatly and needs to be updated urgently. As a result of the convergence of land and sea, 241Am, 239,240Pu, and 237Np are dominated by terrestrial sediments and deposited in the coastal area of southern China, which should be paid more attention to. This work can establish China’s current neptunium radioactivity database, and the difference in Np, Pu, and Am scavenging processes may be a powerful tool for evaluating the impact of the Pearl River Estuary salt tide.

Variable Effects on Benthic Community From Diking to Eradicate Invasive Plants in the Yangtze Estuary Salt Marsh

The removal of invasive plants is a global concern, and ecological restoration methods have been a major research topic in recent years. In the estuarine salt marsh of the Yangtze River, dikes are typically used in ecological restoration projects to eradicate the invasive plantSpartina alterniflora. We explored ways of optimizing dike construction and of providing an effective basis for the wetland ecological control and protection of biodiversity and analyzed the effects on the macrobenthos of fully and partially dikes. The measurement of the quantitative change in macrobenthos diversity and species composition was carried out in the project area and in a control before (2013) and after (2016) dike construction. Results showed that the number of species and average density decreased significantly in the fully diked enclosed area but increased in the partially diked semi-enclosed area. Outside the project area, all site samples showed increased species richness and average density after dike construction. This study indicated that macrobenthos was negatively affected by the dike project in the inside diked area. However, when the tidewater canals were preserved to maintain the connection between the inside and outside areas, there was a positive effect on macrobenthos in the project area. We suggest that canals are preserved while diking in the salt marsh and that gates are opened regularly to maintain the water and nutrient connectedness inside and outside the dike. The diking project mostly affected mollusks and polychaetes, which are the indispensable food sources for birds and fish. The study provides valid evidence for the management of estuarine salt marsh and the protection of macrobenthos.

Invasive Spartina alterniflora changes the Yangtze Estuary salt marsh from CH4 sink to source

Expansion of Spartina alterniflora salt marshes in the East China Sea coast is changing the ecosystem and thus, uncertainty has hampered methane (CH4) flux accounts in these areas. To analyse seasonal and diurnal patterns of the CH4 fluxes and their relationship with environmental factors, four plots were established in a salt marsh of the Nanhui coast in the southern fringe of the Yangtze River estuary, differing in sediment salinity and vegetation history and including one bare mudflat. Monthly studies from March 2017 to January 2018 using a chamber technique showed that CH4 fluxes from the plots ranged from −1.7 to 72.2 mg m−2 h−1. The mature Spartina sites showed higher CH4 emission, peaking in the summer. In the mudflat CH4 consumption was observed in January and the summer. The seasonal CH4 fluxes showed positive correlation (p < 0.05) with temperature, and plant development (height of vegetation), and negative correlation (p < 0.05) with water salinity. Various diurnal cycles in the CH4 fluxes were observed at different seasons. Average CH4 emissions were higher during the daytime than at night, however, without significant difference. Thus, the CH4 fluxes started to rise at noon, and the maximum CH4 flux was observed after the ebb tide (at 18:00) during nighttime. The diurnal variation in CH4 fluxes showed a significant correlation with season but not with temperature.

Methane Emissions during the Tide Cycle of a Yangtze Estuary Salt Marsh

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.

Circulation and transport processes in a tidally forced salt-wedge estuary: The São Francisco river estuary, Northeast Brazil

The São Francisco river flow is regulated by a series of dams that work in cascade for hydro-power generation, maintaining the flow <1000 m3 s−1 all year long. This caused the dampening of the flood peaks and perpetuated the salt intrusion, changing the function of the system as a material source to the coastal ocean. Here we present an assessment of the estuary salt intrusion, hydrodynamics, mixing processes, and the transport of scalars based on field observations. The mains goal is to evaluate its present role as a sink or source of materials to the adjacent sea. Three tidal surveys were carried out to gather data on the water level, cross-section discharge, current velocity and direction, salinity, temperature, concentration of suspended particulate matter (SPM), dissolved oxygen, and chlorophyll. Data were recorded from moored instrumentations in different locations, from boat-based sampling in a cross-section, and also in along-estuary surveys. The estuary shows a highly stratified salinity distribution, with the salt intrusion excursion of 7 km during the spring tide, reaching ∼11 km at high tide. The salt intrusion is laterally asymmetric, influenced by the system morphology. There is a strong tidal attenuation along the estuary, even though the flow regime is sub-critical (Richardson Number > 0.25). The estuary presented lateral variability of currents modulated by the tide, being horizontally sheared during spring tide and vertically sheared during neap. The transport of scalars was modulated by the tide, and the estuary exported SPM, dissolved oxygen, and chlorophyll both during neap (44.4, 12.9 and 3.6x10−3 kg s−1, respectively) and spring tide (31.8, 7.4 and 2x10−3 kg s−1), despite the severe flow regulation, meaning that the system still functions as a source of materials to the coastal sea.

Surface properties and changes in morphology of microplastics exposed &amp;lt;italic&amp;gt;in-situ&amp;lt;/italic&amp;gt; to Chinese coastal wetlands

Coastal wetlands are key areas of accumulation of microplastics. However, until now only a few studies have focused on the surface properties and morphological changes in microplastics in the real coastal wetland environment. Here, two typical biogeographic coastal soils, the Yellow River Estuary salt marsh wetland in the temperate zone and the Beibu Bay mangrove wetland in the subtropical zone, were selected for study. Polystyrene foams and polyethylene films were used and exposed within two coastal wetlands sites through in situ soil burial (underground exposure) and surface placement (above-ground exposure). The samples were sampled after 6, 12, 18 and 24 months of exposure to reveal the characteristics of the surface properties and morphological changes in microplastics in typical wetlands from the southern and northern biogeographic coastal zones. The surface morphology, microstructures and attached materials were observed using scanning electron microscopy using an energy dispersive spectrometer. Surface properties of the microplastics, i.e. the surface roughness, specific surface area, pore size distribution, functional groups and hydrophobicity, were analyzed by using atomic force microscopy, a surface area analyzer, a mercury porosimeter, Fourier transform infrared spectrometry and a contact angle meter. The surface morphology of the polystyrene foams in the Beibu Bay mangrove wetland exhibited more pits and holes than those in the Yellow River Estuary salt marsh wetland. The polystyrene foams exposed above-ground in the Beibu Bay mangrove wetland showed embrittlement and exfoliation after 18 months, while those exposed underground did not show such features. The specific surface areas of the polystyrene foams and the polyethylene films in the Yellow River Estuary salt marsh wetland were higher than those in the Beibu Bay mangrove wetland. The pore distributions on the surfaces of the two microplastic types mainly comprised macropores and mesopores. However, the porosity of the polyethylene film in the Yellow River Estuary salt marsh wetland was slightly higher than in the Beibu Bay mangrove wetland. The porosities in both regions were higher than in the original control samples. In terms of carbonyl index, rates of change in the Yellow River Estuary salt marsh wetland were higher than those in the Beibu Bay mangrove wetland. The surface hydrophobicity of the polyethylene film in the two regions declined with increasing exposure time. The changes in surface morphology of the polystyrene foams were more rapid than those in the polyethylene films, but the degree of change in specific surface area of the polyethylene films was greater than in the case of the polystyrene foams. It can be concluded that the surface properties and changes in morphology of microplastics in the coastal soil environment are related to multiple factors including the types and conditions of the wetlands, types of microplastics, exposure mode and exposure time. However, the specific mechanisms of these surface changes require further study. In summary, this study provides a scientific basis for research on the chemical processes of the micro-interfaces on the microplastic surfaces and environmental behavior and risk assessment of microplastics in the Chinese coastal zone.

Invasive Phragmites Increases Blue Carbon Stock and Soil Volume in a St. Lawrence Estuary Marsh

AbstractOur study of a St. Lawrence Estuary marsh reveals that, compared to native Spartina patens‐dominated vegetation, invasive Phragmites australis makes a greater contribution to soil volume and carbon stock (referred to as blue carbon). Phragmites' contributions to soil volume enhance marsh sustainability in face of sea level rise, and its greater contribution to soil carbon helps to reduce the atmospheric concentration of CO2. Phragmites australis (common reed) is a cosmopolitan species growing in fresh to brackish wetlands. An invasive genetic strain, introduced from Europe or Asia, has expanded extensively along the St. Lawrence River in the last few decades but has been little studied on the estuarine portion. We collected soil cores from three sites within an invasive Phragmites stand and one site within S. patens‐dominated stand in a St. Lawrence Estuary salt marsh near la Pocatiere, Quebec. We measured the bulk density, carbon content, volume, and mass of belowground organic matter in 2‐cm‐thick soil layers of three cores at each Phragmites site. Bulk density and carbon content were measured in 5‐cm‐thick soil layers of three cores at S. patens site. Results showed that soil in the Phragmites stands held 37–77% more blue carbon than in the S. patens‐dominated marsh. Based upon their diameter size, Phragmites rhizomes could be contributing 7.4–10.2 cm to the thickness in the upper 20 cm of soil. We suggest that any management of invasive Phragmites include consideration of its role in increasing blue carbon stocks and marsh resilience along with other ecosystem services.

Characteristics of organic matter sources from Guadiana Estuary salt marsh sediments (SW Iberian Peninsula)

Estuaries are dynamic interfaces between land, rivers and the ocean that play major roles in the global carbon cycle. These coastal wetlands store huge amounts of organic carbon (OC), commonly known as “blue carbon” and excellent places to study C cycling. The Guadiana river estuary is among the most important tidal salt marshes in the South - Iberian coastal margin. Here, a detailed organic geochemical study is described that includes the identification of sedimentary OM composition at a molecular and isotopic level. Total organic carbon content (TOC) of core sediments ranged from 0.39 to 2.23% and stable carbon isotope composition (δ13C) also showed a wide range between −22.4 and −27.0‰. A13C depletion trend observed from the surface to bottom in the core sediments profiles, reflects loss of labile biogenic 13C enriched compounds i.e. polysaccharides and a selective preservation of more depleted compounds with depth i.e. lignin and lipids in the core sediments. Series of n-alkanes were found in the range from C10 to C31. Carbon preference index ratio (CPI) calculated for long-chain n-alkanes (C24–C31) that ranged between 1.17 and 1.94 reflecting diverse OM inputs to the sediments. A study of the lignin-derived phenolic composition pointed to a recalcitrant OM derived from both gymnosperm and angiosperm plants. Moreover, high abundance of vinyl phenol and vinyl guaiacol points to a dominant contribution of lignins from grasses and aquatic macrophytes to the sediments. A well-resolved series of long-chain linear alkyl benzenes (LABs from C4 to C22) were recorded in all core sediment samples indicating direct discharge of untreated domestic and/or industrial effluents to the estuary. Polycyclic aromatic hydrocarbons (PAHs) with more than 3 rings reflects the pyrogenic origin of a portion of the sedimentary OM. This study highlights the importance of different OM sources to the lower Guadiana estuarine sediments and contributes to a better knowledge about its origin, dynamics and fate.

Effect of neap-spring hydrodynamics on salt and suspended sediment transport in multi-branched urban estuaries

Unique neap and spring tide hydrodynamic features were studied in the estuarine areas of multi-branched urban rivers in Tokyo, Japan. Intensive measurements of salinity, turbidity, dissolved oxygen, velocity, particulate organic matter (POM), and nutrients were conducted for 13h in five stations simultaneously on July 3 and July 11, 2017. Water sampling, analysis and calculations of salt and sediment transport were performed. Results showed that the Arakawa and Sumida Rivers follow the typical natural estuary hydrodynamics while Shakujii River followed a typical artificial urban estuary. Shakujii River upstream estuary had lower flow velocity during spring tide (−0.05 to 0.04m/s) than neap tide (−0.09 to 0.16m/s) because of the channel slope that does not allow the transit of water to upstream even during high tide. Shakujii River downstream estuary had hypoxic (DO<2mg/L) bottom depths during neap and spring tide. Sumida River and Shakujii River are adjacent yet the nutrient and POM had different nature. Shakujii River is highly influenced by freshwater from combined sewer systems carrying POM, nutrients and sediments. The freshwater was 54% (14,650m3) and 100% (28,671.1m3) by volume during neap and spring tide, respectively. The POM and nutrients in Sumida River is influenced by tidal processes and tributary rivers. SS was transported from Sumida to Shakujii River during flood phase of spring tide. Freshwater from Arakawa River flows to Sumida River during ebb tide. Salt (−0.066kg/m/s) and SS (−0.16 to −2.435kg/m/s) were retained in the estuaries which may lead to river deterioration and occurrence of scum, hypoxia and odor. The results of the study, particularly on Shakujii River, are significant and could serve as basis for water quality management in similar urban estuaries.

Preliminary Insight into Winter Native Fish Assemblages in Guadiana Estuary Salt Marshes Coping with Environmental Variability and Non-Indigenous Fish Introduction

This work aims to undertake a preliminary characterization of winter fish assemblages in the salt marsh areas of Guadiana lower estuary (South-East Portugal) and discusses the potential risks of habitat dominance by a non-indigenous species (NIS). To this effect, six field campaigns were carried out in four sampling sites during winter season targeting the collection of fish species. A total of 48 samples were collected. Individuals from seven different taxa (marine and estuarine) were collected, although the assemblage was dominated by two estuarine species—the native Pomatoschistus sp. (goby) and the NIS Fundulus heteroclitus (mummichog). Goby was the most abundant taxa in the majority of salt marsh habitats, except for one specific, marsh pool, where extreme environmental conditions were registered, namely high temperature and salinity. Such conditions may have boosted the intrusion of mummichog in this area. This species is well adapted to a wide range of abiotic factors enabling them to colonize habitats where no predators inhabit. Impacts of mummichog introduction in the Guadiana salt marsh area are still unpredictable since this is the first time they have been recorded in such high density. Nevertheless, in scenarios of increased anthropogenic pressure and, consequently, habitat degradation, there is a potential risk of mummichog spreading to other habitats and therefore competing for space and food resources with native species.

Interactive salt—Alkali stress and exogenous Ca2+ effects on growth and osmotic adjustment of Lolium multiflorum in a coastal estuary

Salt and pH levels in coastal estuaries are very high. In our study, different degrees of interactive salt − alkali stress were simulated to determine the effects of interactive salt − alkali stress on the growth and osmotic regulation of Lolium multiflorum (group A experiment). In addition, Ca2+ solutions were prepared to examine the mitigative effects of exogenous Ca2+ on interactive salt − alkali stress (group B experiment). In group A, five salinity levels (0‰, 5‰, 10‰, 15‰, 20‰) were set. Each salinity level was further categorized at three pH levels: 7±0.12, 8±0.22, and 9±0.16. In group B, five Ca2+ levels (0, 2, 4, 6, and 8mM) were added to the experiment. Damage of L. multiflorum by interactive salt − alkali stress was more pronounced than by single salt stress. The tolerable range was defined for pH 7 and salinity ≤10‰, and pH 8 and salinity ≤5‰, respectively, when the leaf relative electrical conductivity was not significantly increased, the chlorophyll content and root vigor were not significantly reduced, and the mortality rate of L. multiflorum was low. However, growth was decreased by more than 90% (at pH≥8 and salinity ≥15‰), and osmotic regulation of L. multiflorum was drastically impaired beyond the tolerance range. Under single salt stress, the content of proline, Na+/K+ ratio and Cl− content were lower than that under interactive salt − alkali stress, while the contents of Ca2+ and Mg2+ were higher. The optimum exogenous Ca2+ concentration was 6mM for L. multiflorum. At this concentration, leaf relative electrical conductivity, proline content and Na+/K+ were lowest, and chlorophyll content, root vigor and Mg2+ content were highest. Overall, L. multiflorum could tolerate a certain degree of interactive salt − alkali stress, which can be beneficial for the ecological restoration of coastal waters.

Numerical analysis of inter-specific relationships in the estuary salt marsh plant community of southern Chongming Dongtan, Shanghai.

The salt marsh plant communities were investigated with quadrats in the southern Chongming Dongtan. Based on the vegetation coverage and the 2×2 contingency table, 8 common species among the 17 higher plants recorded were analyzed. The variance ratio of overall association, Chi-square test and Spearman rank correlation coefficient were used to describe the relevance and correlations between species pairs. The results showed that W (48.61), a statistical index to test the variance ratio (VR=0.61), fell outside of the range of Chi-square test, indicating that the overall correlation of all vegetation species was significantly negative. According to the environment adaptation mode of dominant species and the main influencing factors, the species were divided into 4 ecological groups, i.e., Phragmites australis, Carex scabrifolia-Scirpus triqueter - Juncellus serotinus, Spartina alterniflora - Scirpus mariqueter, Echinochloa crusgalli - Imperata cylindrica, based on the ranking of Spearman correlation coefficient. The inter-specific relationships in the salt marsh plant community of southern Chongming Dongtan were complicated and extremely unstable with species sensitive to environmental impacts. According to the analysis of relationships between the species and their pre-sent distribution, we suggested using S. mariqueter as target species to provide strategies for protecting native species based habitats.

Effects of Zn Stress on Growth, Zn Accumulation, Translocation, and Subcellular Distribution of Spartina alterniflora Loisel

Spartina alterniflora plays an important role in heavy metal phytoremediation of estuary salt marshes. A study quantifying the effects of different Zn concentrations (100, 500, and 1000 μg · g−1) on growth, Zn accumulation, translocation, and subcellular distribution of S. alterniflora was conducted. The results showed that plant height and dry biomass increased under the 100 and 500 μg · g−1 Zn stress test. The Zn concentration in the same tissues increased with increasing Zn concentration. In addition, the Zn concentrations under the same Zn stress increased in the following order: Fine roots &gt; leaves &gt; stems &gt; rhizomes. For example, it was found that the Zn concentration in fine roots reached up to 60.64 μg · g−1, and this concentration was significantly higher than in other tissues. However, the highest accumulation was detected in leaves under the 500 μg · g−1 Zn stress test, regardless of other tissues and Zn treatments. The Zn translocation factor increased with increasing Zn treatment concentrations and reached 1.05 under 1000 μg · g−1 Zn stress. This result suggests that Zn was easily transported to the shoots of S. alterniflora. Moreover, the cell wall and cytoplasmic supernatant played important roles in Zn stabilization and detoxification. This was a result of Zn mainly distributed in the cell wall and cytoplasmic supernatant but not in cell organelles.