Alterniflora Marsh Research Articles

Effects of the comprehensive elimination of Spartina alterniflora along China's coast on blue carbon and scenario prediction after ecological restoration

Salt marshes cover the largest area among the three types of traditional blue carbon ecosystems in China's coastal zone, with the introduced smooth cordgrass (Spartina alterniflora Loisel.) being dominant in these marshes. The effects of eradicating S. alterniflora nationwide and the subsequent ecological restoration on blue carbon are unclear. This paper evaluates the variation in blue carbon during the national S. alterniflora eradication campaign, which involves mechanical tillage from 2022 to 2025, and proposes three scenarios for blue carbon changes after native vegetation is reestablished by 2050. The results show that, in 2025, plant carbon stock and soil carbon stock will decrease by 1.38 Tg C and 1.21 Tg C, respectively, in the areas where S. alterniflora has been removed and managed. Although blue carbon is reduced in coastal wetlands in 2025, carbon stock is expected to increase in restored native vegetated wetlands by 2050. S. alterniflora is resilient and competitive, posing a high risk in secondary invasion. Scenario Ⅰ suggests that S. alterniflora marshes could almost recover to their original state from 2022, with 7.70 Tg C stored in plant and soil carbon stocks. Scenario Ⅱ involves native vegetated wetlands coexisting with invasive S. alterniflora marshlands, with a total carbon stock estimated at 7.15 Tg C, reflecting a decrease of 0.39 Tg C in soil carbon stock and by 0.16 Tg C in plant carbon stock. In Scenario Ⅲ, mudflats dominant and native vegetated habitats are reestablished only in suitable sites, with the total carbon stock estimated at 5.63 Tg C, a 26.9% decrease compared to 2022 levels. While eradicating invasive S. alterniflora and restoring native vegetation in China's coast enhance the ecosystem services, it reduces blue carbon stocks. Therefore, developing additional strategies to increase carbon storage in coastal wetlands is necessary.

Soil Organic Nitrogen Mineralization and N2O Production Driven by Changes in Coastal Wetlands

AbstractPlant invasion and land reclamation have drastically transformed the landscape of coastal wetlands globally, but their resulting effects on soil organic nitrogen (SON) mineralization and nitrous oxide (N2O) production remain unclear. In this study, we examined 21 coastal wetlands across southern China that have undergone habitat transformation from native mudflats (MFs) to Spartina alterniflora marshes (SAs), and subsequently to earthen aquaculture ponds (APs). We determined the SON net mineralization rate and the presence of pertinent enzyme‐encoding genes, namely chiA, pepA, and pepN. The SON net mineralization rate increased by 46.7% following the conversion of MFs to SAs but decreased by 33.1% in response to the transformation of SAs to APs. Nevertheless, there was no significant difference in the estimated mineralization efficiency of soil microbes among the habitat types. The results of structural equation modeling showed that N‐mineralization gene abundance played a major role in regulating SON mineralization. Although less than 20% of the SON was estimated to be labile/semi‐labile, SON mineralization was important in sustaining soil N2O production, with 5.8% of the mineralized N being fed into N2O production. Overall, our findings showed that the presence of S. alterniflora increased both SON content and mineralization rate, which would in turn promote further proliferation of this exotic plant along the coast. The conversion of S. alterniflora marshes to APs partially mitigated the positive effects of exotic plant invasion on SON turnover.

Deltaic marsh accretion under episodic sediment supply controlled by river regulations and storms: Implications for coastal wetlands restoration in the Yellow River Delta

Sediment supply has long been recognized as a critical factor affecting salt marsh evolutions. Subject to fluctuating river discharges and coastal hydrodynamics, sediment supply to deltaic marshes are episodic rather than continuous. To evaluate the impacts of episodic sediment delivery under river regulations and storm events on deltaic marsh accretion, a 5-month (April 18th to September 4th, 2021) field campaign was conducted at a Spartina alterniflora marsh of the Yellow River Delta, China. Nine storm events and the Water-Sediment Regulation Scheme (WSRS) coinciding with a freshwater diversion were captured. It was found that the two phases of the WSRS differed greatly in their contribution to marsh accretion. During the Water-Regulation Phase (WRP), marsh accretion rates showed little difference with pre-WRP periods. However, the subsequent Sediment-Regulation Phase (SRP) contributed to ∼15 % of annual accretion within only 4 days, suggesting the SRP was more effective in nourishing deltaic marshes with sediment. Further analyses indicated that the higher accretion during SRP was attributed to both higher sediment concentrations and more favorable sediment pathways, influenced by delta-scale river-tide interactions. Observations also showed that storm events contributed to half of the marsh accretion during the non-WSRS period when the fluvial sediment supply was very low. During storms, wave-induced intertidal resuspension acted as the major sediment sources. Under favorable conditions (onshore wind, high water levels, high waves), tidal-averaged sediment delivery to marshes underwent as much as fifty-fold increase compared with the calm period. The observed marsh dynamics suggested that marsh sedimentations depended on not only the seaward sediment concentrations, but also the sediment transport pathways. For deltaic marsh restoration practices, we proposed that marsh-orientated schemes which align river regulations with favorable meteorological and hydrodynamic conditions, could potentially make better use of fluvial sediment for deltaic marsh nourishment.

How does exotic Spartina alterniflora affect the contribution of iron-bound organic carbon to soil organic carbon in salt marshes?

Iron-bound organic carbon (OC-FeR) is important for the stability of soil organic carbon (SOC) in salt marshes, and the Spartina alterniflora invasion reshaped local salt marshes and changed the SOC pool. To evaluate the effects of S. alterniflora invasion on the contribution of OC-FeR to SOC, we determined the OC-FeR content and soil characteristics in the 0–50 cm soil profile along the vegetation sequence, including mudflats (MF), S. alterniflora marshes established in 2003 (SA03) and 1989 (SA89), the ecotone of S. alterniflora and Phragmites australis (SE), S. salsa marsh (SS), and P. australis marsh (PA). The SOC content was 6.55–17.5 mg g−1 in the S. alterniflora marshes. Reactive iron oxides (Fed, Feo, Fep) accumulated significantly in the S. alterniflora and P. australis salt marshes. PA and S. alterniflora marshes had higher DOC contents of 0.28–0.77 mg g−1. The OC-FeR content in the 0–50 cm soil profile in these ecosystems ranged from 0.3 to 3.29 mg g−1, with a contribution to the SOC content (fOC-FeR) of approximately 11 %, which was highest in SA03 (16.3 % ~ 18.8 %), followed by SA89, SE, and PA. In addition, the molar ratios of OC-FeR to Fed were <1, indicating that the iron oxides were associated with SOC through sorption more than coprecipitation. According to the structural equation model, SOC, DOC and iron oxides were the direct driving factors of OC-FeR formation, while the vegetation zone indirectly functioned by regulating organic C inputs, iron oxide formation, and pH. This study suggested that S. alterniflora invasion promotes iron-bound organic carbon accumulation by increasing organic C inputs and regulating iron oxide formation in salt marshes, but such promotion will degenerate with development duration.

Latitudinal responses of wetland soil nitrogen pools to plant invasion and subsequent aquaculture reclamation along the southeastern coast of China

The impact of invasive species and land use change on soil nitrogen pools in coastal wetlands has been reported at local scale, but uncertainty persists for regional pattern due to geographical variability and limited field data. This study measured the top soil (upper 20 cm) organic nitrogen (SON), inorganic nitrogen (SIN) and total nitrogen (STN) concentrations and stocks across 21 coastal wetland sites in China (20°42′N-31°51′ N) that had undergone the same sequence of transformation from mudflats (MFs) to invasive Spartina alterniflora marshes (SAs) then to earthen aquaculture ponds (APs). Results showed that the conversion of MF to SA significantly increased SON and SIN concentrations and stocks by 37.7–86.1%, but subsequent conversion to APs significantly decreased them by 13.5–34.6%. SON/SIN ratio decreased upon invasion by S. alterniflora and it had a negative effect on STN accumulation, whereas conversion of SAs to APs showed the opposite trends. The change rates of SON, SIN and STN stocks showed clear decreasing trends with increasing latitude in the MF-to-SA conversion scenario, reflecting the strong influence of environmental temperatures, but weaker or insignificant trends were observed in the SA-to-AP conversion scenario, likely because of mitigating anthropogenic activities in aquaculture ponds. Our findings can be used to inform strategies to control invasive species and reduce the greenhouse gas nitrous oxide (N2O) emissions, and support global N model for climate change in response to habitat modifications in coastal wetlands.

Distribution characteristics and influencing factors of soil organic carbon in tidal flat wetland of central Jiangsu, China.

We measured winter and summer soil organic carbon (SOC) contents in two typical coastal wetlands, the Spartina alterniflora salt marsh and the non-vegetation mudflat, on the south side of the Chuandong River Estuary in Yancheng, Jiangsu Province. We investigated the spatiotemporal variations of soil organic carbon contents and its driving factors. The results showed that SOC content ranged from 0.75 to 2.38 g·kg-1 in the mudflat area and from 2.07 to 18.59 g·kg-1 in the S. alterniflora salt marsh area, showing a decreasing trend towards the sea. The SOC content in the S. alterniflora salt marsh area was approximately 2.5 to 3.5 times of that in the mudflat area. Within a depth range of 1 m, there was no vertical variation in SOC content in the mudflat area, but an increasing and then decreasing pattern in the S. alterniflora marsh area with the peak occurring in the depth range of 20 to 30 cm. Soil organic carbon content exhibited significant seasonal difference, with higher value in summer than in winter. The summer SOC content was 5% to 10% higher than that in winter in the S. alterniflora marsh area, while it was 43% higher in summer than in winter in the mudflat area. In the S. alterniflora marsh area, soil organic carbon content was positively correlated with soil moisture and salinity, but negatively correlated with sediment particle size. In contrast, there was no significant correlation between soil organic carbon content and soil physicochemi-cal factors in the mudflat area. Those results indicated that the correlation between various soil physicochemical factors and SOC is established on the basis of vegetation cover in coastal wetlands. Our findings could provide valuable insights for the conservation of blue carbon ecosystems in coastal wetlands in China.

Projecting future wave attenuation by vegetation from native and invasive saltmarsh species in the United States

Saltmarshes are naturally found along gently sloped coastlines of temperate zones including low-energy intertidal estuaries, shallow bays, and on the landward side of barrier islands. Often referred to as one of the most productive, dynamic, and valuable ecosystems on Earth, saltmarshes have been increasingly used as natural and nature-based features (NNBFs) for coastal defense. In preserved conditions, low-lying, regularly flooded marshes in the United States are naturally dominated by a single native species, Spartina alterniflora. However, over the last century the invasive Common Reed (i.e., Phragmites australis), which is directly associated with decreases in plant biodiversity and disruption of natural biochemical cycles, has become the dominant marsh species in North America. Our study quantified the influence of Phragmites australis invasion on future wave attenuation by vegetation when compared to native Spartina alterniflora marshes. We combined the coupled ADCIRC+SWAN with the point-based landscape model SLAMM and field-work-based explicit representation of vegetation to develop a series of modeling simulations based on a combination of low and high-intensity storms, SLR projections, and marsh migration scenarios. Wave attenuation by different marsh species was investigated using average vegetation characteristics (stem height, density, and diameter) for both Phragmites australis-dominated and Spartina alterniflora-dominated bay-wide modeling scenarios. Results are presented as spatially distributed differences in wave attenuation and average wave attenuation curves. Our results show that under current SLR conditions Phragmites australis-dominated marshes can provide significantly more wave attenuation than native Spartina alterniflora marshes during extreme hurricane events. On the contrary, under high-frequency and low-intensity storm events, Spartina alterniflora-dominated marshes are slightly more efficient than invasive Phragmites australis marshes. With the addition of SLR and the increase in the incoming wave heights Phragmites australis marshes can support more wave attenuation than Spartina alterniflora.

Change in the community structure and organic carbon content of meio- and macrobenthos between tidal flat and salt marsh areas colonized by Spartina alterniflora in the Bahía Blanca estuary (SW Atlantic)

Salt marshes are regarded as among the most productive coastal ecosystems, important “blue carbon” sinks and a support for benthic communities with large abundances, whose structure may be strongly influenced by salt marsh vegetation. During the last few decades, Spartina alterniflora has been colonizing bare mudflats in the Bahía Blanca estuary, and a large increase in the area covered by salt marshes has been reported. This colonization can strongly influence the structure of benthic fauna and its role in the carbon cycle. The hypothesis of this study was that the community structure and the organic carbon contained in the meio- and macrobenthos change between tidal flats and salt marshes recently colonized by S. alterniflora. Response variables studied to compare the tidal flat and salt marsh were density, biomass and production to biomass (P/B) ratio of macro- and meiobenthos. Density and biomass of Gastropoda and P/B ratio of Nematoda were higher on the salt marsh than on the tidal flat. By contrast, density and biomass of Polychaeta were higher on the tidal flat. These results suggest that the expansion of S. alterniflora marshes on tidal flats produces changes in the structure of the macro- and meiobenthos community (taxonomic composition and biomass) that have an influence on carbon cycling.

Community assembly of comammox Nitrospira in coastal wetlands across southeastern China.

Complete ammonia oxidizers (comammox Nitrospira) are ubiquitous in coastal wetland sediments and play an important role in nitrification. Our study examined the impact of habitat modifications on comammox Nitrospira communities in coastal wetland sediments across tropical and subtropical regions of southeastern China. Samples were collected from 21 coastal wetlands in five provinces where native mudflats were invaded by Spartina alterniflora and subsequently converted to aquaculture ponds. The results showed that comammox Nitrospira abundances were mainly influenced by sediment grain size rather than by habitat modifications. Compared to S. alterniflora marshes and native mudflats, aquaculture pond sediments had lower comammox Nitrospira diversity, lower clade A.1 abundance, and higher clade A.2 abundance. Sulfate concentration was the most important factor controlling the diversity of comammox Nitrospira. The response of comammox Nitrospira community to habitat change varied significantly by location, and environmental variables accounted for only 11.2% of the variations in community structure across all sites. In all three habitat types, dispersal limitation largely controlled the comammox Nitrospira community assembly process, indicating the stochastic nature of these sediment communities in coastal wetlands. IMPORTANCE Comammox Nitrospira have recently gained attention for their potential role in nitrification and nitrous oxide (N2O) emissions in soil and sediment. However, their distribution and assembly in impacted coastal wetland are poorly understood, particularly on a large spatial scale. Our study provides novel evidence that the effects of habitat modification on comammox Nitrospira communities are dependent on the location of the wetland. We also found that the assembly of comammox Nitrospira communities in coastal wetlands was mainly governed by stochastic processes. Nevertheless, sediment grain size and sulfate concentration were identified as key variables affecting comammox Nitrospira abundance and diversity in coastal sediments. These findings are significant as they advance our understanding of the environmental adaptation of comammox Nitrospira and how future landscape modifications may impact their abundance and diversity in coastal wetlands.

Denitrification Potential of Surface Soils of Constructed Wetlands in Newtown Creek, an Urban Superfund Site.

Denitrification, the anaerobic microbial conversion of nitrate (NO3 - ), a common water pollutant, to nitrogen (N) gases, is often high in the soil of natural wetlands. In areas where natural wetlands have been degraded or destroyed, constructed and restored wetlands have been used to restore ecosystem services like denitrification. Thus, denitrification in restored and constructed wetlands could play an important role in treating anthropogenic N sources such as combined sewer overflow (CSO) discharges which can be high in NO3 - . In this study, we measured denitrification potential using an anaerobic slurry assay and made a suite of ancillary measurements (soil moisture content, microbial biomass carbon (C) and N content, potential net N mineralization and nitrification, soil inorganic N pools and soil respiration) in four constructed salt marsh wetlands, and a series of wetland habitat basins in Newtown Creek, NY, an urban superfund site. Samples were also taken from natural salt marshes located at Paerdegat Basin, Jamaica Bay, NY. Our results show that constructed Spartina alterniflora marshes in ultra-urban Newtown Creek support denitrification potential equivalent to rates of natural marshes in Jamaica Bay and reference marshes in other urban estuaries. There were significant positive correlations between microbial biomass C and N content and organic matter content and denitrification potential. Results suggest that constructed wetlandscan support wetland vegetation, soils, and microbial life and contribute to N removal under ultra-urban conditions. This article is protected by copyright. All rights reserved.

Effects of landscape modification on coastal sediment nitrogen availability, microbial functional gene abundances and N2O production potential across the tropical-subtropical gradient

Wetland sediment is an important nitrogen pool and a source of the greenhouse gas nitrous oxide (N2O). Modification of coastal wetland landscape due to plant invasion and aquaculture activities may drastically change this N pool and the related dynamics of N2O. This study measured the sediment properties, N2O production and relevant functional gene abundances in 21 coastal wetlands across five provinces along the tropical-subtropical gradient in China, which all had experienced the same sequence of habitat transformation from native mudflats (MFs) to invasive Spartina alterniflora marshes (SAs) and subsequently to aquaculture ponds (APs). Our results showed that change from MFs to SAs increased the availability of NH4+-N and NO3−-N and the abundance of functional genes related to N2O production (amoA, nirK, nosZ Ⅰ, and nosZ Ⅱ), whereas conversion of SAs to APs resulted in the opposite changes. Invasion of MFs by S. alterniflora increased N2O production potential by 127.9%, whereas converting SAs to APs decreased it by 30.4%. Based on structural equation modelling, nitrogen substrate availability and abundance of ammonia oxidizers were the key factors driving the change in sediment N2O production potential in these wetlands. This study revealed the main effect patterns of habitat modification on sediment biogeochemistry and N2O production across a broad geographical and climate gradient. These findings will help large-scale mapping and assessing landscape change effects on sediment properties and greenhouse gas emissions along the coast.

Invasive Spartina alterniflora marshes in China: a blue carbon sink at the expense of other ecosystem services

Coastal (marine) wetlands are recognized as one of the world's most efficient sinks for organic carbon (“blue carbon”), and credits for their restoration and conservation can be obtained from carbon markets. We reviewed 50 studies to compare the climate mitigation potential of the invasive cordgrass Spartina alterniflora to that of native vegetation in marshes along the coast of China. The importance of S alterniflora marshes as a carbon sink varied geographically; however, at all sites soils associated with S alterniflora emitted substantially more methane than soils populated with native plants. Because the species was deliberately introduced, the carbon stored in S alterniflora marshes could qualify for inclusion within China's national inventory of greenhouse‐gas emissions. However, in locations where it significantly increases carbon stocks, its dominance results in the loss of other ecosystem services. Therefore, if included in a national inventory, S alterniflora marshes could conflict with China's action plan for meeting UN Sustainable Development Goals.

A new perspective on the impacts of Spartina alterniflora invasion on Chinese wetlands in the context of climate change: A case study of the Jiuduansha Shoals, Yangtze Estuary

Spartina alterniflora, an invasive plant, was introduced to the Chinese coastal zone in the early 90s. As an eco-engineering species, S. alterniflora not only alters saltmarsh species distributions, previously described as habitat degradation, but it also plays a vital role in coastal protection, especially for the development of recently emerged intertidal shoals. To provide a reference for coastal management under global change, we quantified the impact of the invasion process on provided ecological and coastal protection functions, exemplified at the emerging Jiuduansha Shoals (JDS) in the Yangtze Estuary. Results obtained by high-precision satellite monitoring and numerical modelling showed that the establishment and growth of S. alterniflora can exert considerable changes on local environment. The invasion of S. alterniflora to JDS wetland can be divided into three distinct phases, (1) establishment 1998–2003, (2) expansion 2003–2009, and (3) dominant 2009–2018 stages according to the changes in saltmarsh composition. Spatially, S. alterniflora continuously replaced Scirpus mariqueter, forcing S. mariqueter and Phragmites australis slowly to the lower and higher intertidal habitats, respectively. Notably, S. alterniflora expansion was the main driver that contributed to over 70 % of recent JDS wetland expansion even under sediment deficit conditions. Established S. alterniflora marsh (directly) dampens more waves because of aboveground stems, but it also causes more accretion and indirectly leads to higher “morphological” wave dampening. Thus, it increases coastal defense provided by the saltmarsh in the context of sea-level rise and strengthening storms. In conclusion, the role of S. alterniflora invasion to the local environment under global changes is controversial. For sustainable coastal management, we need context-dependent S. alterniflora management to maximize the benefit of coastal protection and minimize the impact on local ecology, especially in sediment-starving estuaries with expected coastline retreat.

Potential impacts of siltation by Spartina alterniflora on nutrient dynamics in its decomposing litters in coastal marsh of the Min River estuary, southeast China

The Spartina alterniflora started to invade the Min River estuary (Southeast China) in 2002 and, thereafter, its invasion area showed an increasing trend. Since the siltation depths caused by S. alterniflora in the Min River estuary were much higher (4.8–7.2 cm yr−1) than the values reported in other coastal regions of China (3.5–6.5 cm yr−1), the impacts of siltation on nutrient cycle processes in this region might be more evident. In order to explore the potential effects of siltation by S. alterniflora on nutrient ((carbon (C), nitrogen (N), phosphorous (P) and sulfur (S)) variations in its decaying litters, three one-off siltation treatments (no siltation scenario (0 cm yr−1, NSS), current siltation scenario (5 cm yr−1, CSS) and strong siltation scenario (10 cm yr−1, SSS)) were designed in coastal marsh of the Min River estuary and the in-situ decomposition experiment was conducted from February 2016 to February 2017 by litterbag technique. Results showed that the siltation caused by S. alterniflora showed significant impact on its decomposition rate, following the sequence of NSS (0.005638 d−1) > SSS (0.003005 d−1) > CSS (0.002478 d−1) (p < 0.05). The total carbon (TC) contents in decomposing litters in the three siltation treatments showed dissimilar fluctuations and significantly higher values were observed in the CSS and SSS treatments compared to the NSS treatment. The contents of total nitrogen (TN) and total sulfur (TS) in decomposing detritus in the three siltation treatments generally showed increasing trend during the whole decomposition, while those of total phosphorus (TP) showed increasing trend after decomposing for 30 days. The differences in nutrient variations among the three siltation treatments, to a great extent, rested with the alterations of substrate quality in detritus during the experiment. Although the stocks of C, N, P and S in detritus in the three siltation treatments evidenced the release from litters to the surroundings during decomposition, the release amounts of these nutrients in some periods were at a lower level. With increasing siltation depths, the release of C, N and P from detritus was generally restrained during the whole decomposition, while that of S from decaying litters was inhibited only at the late stage of decomposition. This paper found that the siltation caused by S. alterniflora reduced the nutrient return (particularly for C, N and P) from its detritus, which, in turn, might greatly alter the nutrient cycle in S. alterniflora marsh.

Landscape Change Affects Soil Organic Carbon Mineralization and Greenhouse Gas Production in Coastal Wetlands

AbstractPlant invasion and aquaculture activities have drastically modified the landscape of coastal wetlands in many countries, but their impacts on soil organic carbon (SOC) mineralization and greenhouse gas production remain poorly understood. We measured SOC mineralization rate and soil CO2 and CH4 production rates in three habitat types from 21 coastal sites across the tropical and subtropical zones in China: native mudflats (MFs), Spartina alterniflora marshes (SAs), and aquaculture ponds (APs). Landscape change from MFs to SAs or APs increased total and labile fraction of SOC, as well as carbon mineralization rate and greenhouse gas production, but there were no discernible differences in SOC source‐sink dynamics between SAs and APs. SOC mineralization rate was highest in SAs (20.4 μg g−1 d−1), followed by APs (16.9 μg g−1 d−1) and MFs (11.9 μg g−1 d−1), with CO2 as the dominant by‐product. Bioavailable SOC was less than 2% and was turned over within 60 days in all three habitat types. Proliferation of S. alterniflora marshes and expansion of AP construction had resulted in a net increase in soil CO2‐eq production of 0.4–4.3 Tg yr−1 in the last three decades. Future studies will benefit from better census and monitoring of coastal habitats in China, complementary in situ measurements of greenhouse gas emissions, and more sampling in the southern provinces to improve spatial resolution.

Nekton use of oligohaline Phragmites australis and Spartina alterniflora marsh in Chesapeake Bay and North Carolina, USA

Oligohaline Phragmites australis and Spartina alterniflora marshes with similar physical conditions were examined within two different geographic regions: 1) related to marsh nekton and marsh-edge finfish species abundance, biomass and community structure, 2) to determine whether marsh functional patterns are consistent over large-scale geographic settings, and 3) to assess consistency of nekton use for each macrophyte type between geographic regions. Few significant differences regarding abundance, biomass, community structure, and catch composition between P. australis and S. alterniflora marshes for marsh nekton and marsh-edge finfish species were observed. P. australis and S. alterniflora marshes, regardless of geographic region, provided similar functions for marsh nekton and estuarine finfish communities. However, differences between Chesapeake Bay and North Carolina geographic regions for marsh nekton use were pronounced. Significantly more nekton species utilized P. australis and S. alterniflora marshes in North Carolina versus Chesapeake Bay. Also, the abundance of seven of 12 marsh nekton species common to both regions within S. alterniflora marshes, and four of 12 species within P. australis marshes, differed significantly between regions. It is important to consider at what point differences between estuarine habitats warrant the selection of one functional habitat over another, or conversion of functional habitat for restoration of another. Instead of determinations based on one or a few species of interest, clearly defined criteria should be used to determine necessity for replacement of one habitat type for another, and these should consider ecological values and attributes necessary for ecosystem function.

Structure and diversity of nirK-type denitrifying microbial community in marsh soils at different invasion stages of Spartina alterniflora in the Minjiang River estuary, China.

To explore the differences in structure and diversity of nirK-type denitrifying microbial community in marsh soils at different invasion stages of Spartina alterniflora, the mudflat (MF, before invasion) and the S. alterniflora marsh after seaward invasion for 1-2 years (SAN) and 6-7 years (SA) in Shanyutan of the Minjiang River estuary were investigated by high-through put sequencing method. Results showed that the seaward invasion of S. alterniflora reduced the richness and diversity of nirK-type denitrifying microbial community in marsh soils. The nirK-type denitrifying microbial community in soils at different invasion stages included Proteobacteria and Actinobacteria, with Proteobacteria as the dominant one. The seaward invasion of S. alterniflora greatly altered the composition of nirK-type denitrifying microbial community in marsh soils. The highest relative abundance of genus in soils from different invasion stages were Bradyrhizobium, Mesorhizobium and Alcaligenes, respectively. The seaward invasion of S. alterniflora increased the spatial heterogeneity of nirK-type denitrifying microbial community composition in marsh soils. In SAN plot, the enhancement of spatial heterogeneity was primarily due to higher environmental disturbances in plots and the increased spatial heterogeneity of environmental variables caused by the seaward invasion of S. alterniflora. The seaward invasion of S. alterniflora altered the physico-chemical properties (e.g., grain composition, pH and moisture) and N nutrient conditions (total N, NH4+-N and NO3--N) in marsh soils, which greatly altered the structure and diversity of nirK-type denitrifying microbial community. Our findings reveal the microbial mechanism of denitrification process in marsh soils during the seaward invasion of S. alterniflora.

Phenological heterogeneities of invasive Spartina alterniflora salt marshes revealed by high-spatial-resolution satellite imagery

The rapid expansion of invasive Spartina alterniflora in China’s coastal wetlands has seriously threatened regional ecosystem health. Characterizing phenology of S. alterniflora salt marshes supports the studies on plant invasion mechanism and carbon fluxes in coastal wetlands. To date, little is known about the capacity of high-spatial-resolution imagery in revealing local scale heterogeneities and temporal variations in the phenology; in addition, the driving factors of such heterogeneities are not well understood. This study took Yellow River Estuary (YRE) as the study site where S. alterniflora area has expanded since 2008. We retrieved start of season (SOS), end of season (EOS), length of growing season (LOS), and maximum Normalized Difference Vegetation Index (NDVImax) from time series Sentinel-2 imagery (10-meter resolution) during 2017 ∼ 2019 with assist of a rule-based data noise removal approach. Results showed that phenological metrics differ with invasion years. The differences in the median NDVImax reached 0.13, in the median SOS reached 26 days, and in the median LOS reached 51 days. S. alterniflora with 5-year invasion had the highest NDVImax, and younger S. alterniflora marshes had earlier SOS and longer LOS. Both SOS and EOS showed negative relationships with NDVImax, indicating the marshes with greater green vegetation cover had earlier SOS and EOS. More spatial details can be revealed from higher-resolution PlanetScope imagery (3-meter resolution), showing the influence of tidal creeks on phenology. The spatial heterogeneity in phenology was associated with environmental factors such as tidal inundation, elevation, micro-climate, nutrients, and canopy structure. S. alterniflora showed markedly higher NDVImax and earlier EOS in 2018 compared to 2017 and 2019, associated with larger river water discharge and sediment load brought by Water Sediment Regulation Scheme, greater precipitation in the wettest season and lower preseason temperature in 2018. Large phenological variations at both spatial and temporal scales suggest that S. alterniflora marshes are sensitive to the local environment and climate change. This study has the potential to provide references for the management of invasive plants threatening coastal wetlands in the world.

Phosphorus forms in marsh soils with different years of Spartina alterniflora invasion in the Minjiang River estuary, China

We examined the effects of Spartina alterniflora invasion on phosphorus forms of marsh soils, based on the method of space-for-time substitution by selecting S. alterniflora marshes with different invasion years (SA1, 5-6 years; SA2, 8-10 years; and SA3, 12-14 years) in Shanyutan of the Minjiang River estuary. The results showed that in marsh soils of different invasion years, the proportion of hardly decomposable phosphorus (HCl-Pi and Residual-P) was the highest (46.4%-46.7%), followed by moderately decomposable phosphorus (NaOH-Pi, NaOH-Po and Sonic-Pi) (40.0%-44.0%), and the easily decomposable phosphorus (Resin-Pi, NaHCO3-Pi and NaHCO3-Po) was the lowest (9.5%-13.3%). With increasing invasion years of S. alterniflora, soil phosphorus forms and their spatial distributions were greatly altered. The contents of moderately decomposable phosphorus, hardly decomposable phosphorus, and total phosphorus (TP) generally increased, while easily decomposable phosphorus content generally decreased. Compared with SA1, the contents of moderately decomposable phosphorus, hardly decomposable phosphorus and TP in SA2 increased by 11.5%, 9.7% and 10.5%, while those in SA3 increased by 24.8%, 13.2% and 13.5%, respectively. The distribution of phosphorus forms was greatly altered with increasing invasion years, which was dependent on the variations of key factors such as EC, pH value and grain composition. The implementation of regular mowing activities for S. alterniflora in the Minjiang River estuary in recent years, to some extent, reduced the return of phosphorus from residues to soils and decreased the availability of the easily decomposable phosphorus in soils.

Benthic macrofauna of Spartina alterniflora marshes and nearby unvegetated tidal flats of Paranagua Bay (SE Brazil)

The composition and distribution of benthic macro fauna in Spartirui alterniflora marshes and nearby unvegetated tidal flats of Paranagua Bay (SE, Brazil) were investigated. Samples were taken along a salinity and physical gradient, from the inner to the outer bay. The bay was divided into sectors according to its physico-chemical and faunal characteristics. Salt marshes were affected by the gradient, with the tallest formations developing in the mesohaline sector and the lowest ones in the euhaline sector. Macrobenthic associations were also affected, with the mesohaline sector presenting a low species richness of predominantly epibenthie forms. In this sector, the composition and abundance of benthic macrofauna of the salt marshes and unvegetated flats were distinct. Species richness of the polyhaline sector, particularly of the infaunal taxa, was higher than that of the mesohaline sector. Differences between the fauna of marshes and flats became increasingly evident towards the euhaline, high energy sector, characterized by the highest species richness along the bay. Benthic associations from marshes and unvegetated flats were clearly structured by the salinity and physical gradient. Although salt marsh and flats present a relative homogeneity in physico-chemical parameters, the composition and abundance of associated benthic fauna can vary widely, particularly in poly- and euhaline sectors.