Spartina Alterniflora Salt Marsh Research Articles

Wave attenuation and transformation across a highly turbid muddy tidal flat-salt marsh system

Understanding wave processes within the coastal system is crucial for the appropriate design of coastal prevention engineering and offshore structures. To address the limitations of field observations and traditional methods, an array of pressure sensors and optical instruments were deployed across a shoreward intertidal flat consisting of the fluid muddy seabed, bare flat, and salt marsh in a highly turbid coastal area. We found that the Spartina alterniflora salt marsh has the strongest wave dissipation capacity. Although bare flats have the weakest wave dissipation capacity, they are the most extensive within the intertidal zone, resulting in substantial total wave energy dissipation. A noteworthy positive feedback between fluid mud and wave dissipation is observed. Additionally, the incident waves undergo complex transitions during propagation, which are strongly modulated by tidal influences. In particular, when the wave propagation direction is opposite to the tidal current, there is an amplification of wave energy within the tidal flat. This amplification is attributed to the "jacking effect" of the tidal current, hindering the transformation of the wind waves into the low-energy capillary waves. It is important to note that the high-energy infragravity waves are gradually released and steadily increase as the incident waves propagate shoreward. This implies their potentially significant role in nearshore processes, especially during extreme dynamic events or along extensive coastlines.

Canopy Heterogeneity and Environmental Variability Drive Annual Budgets of Net Ecosystem Carbon Exchange in a Tidal Marsh

AbstractTidal salt marshes are important ecosystems in the global carbon cycle. Understanding their net carbon exchange with the atmosphere is required to accurately estimate their net ecosystem carbon budget (NECB). In this study, we present the interannual net ecosystem exchange (NEE) of CO2 derived from eddy covariance (EC) for a Spartina alterniflora salt marsh. We found interannual NEE could vary up to 3‐fold and range from −58.5 ± 11.3 to −222.9 ± 12.4 g C m−2 year−1 in 2016 and 2020, respectively. Further, we found that atmospheric CO2 fluxes were spatially dependent and varied across short distances. High biomass regions along tidal creek and estuary edges had up to 2‐fold higher annual NEE than lower biomass marsh interiors. In addition to the spatial variation of NEE, regions of the marsh represented by distinct canopy zonation responded to environmental drivers differently. Low elevation edges (with taller canopies) had a higher correlation with river discharge (R2 = 0.61), the main freshwater input into the system, while marsh interiors (with short canopies) were better correlated with in situ precipitation (R2 = 0.53). Lastly, we extrapolated interannual NEE to the wider marsh system, demonstrating the potential underestimation of annual NEE when not considering spatially explicit rates of NEE. Our work provides a basis for further research to understand the temporal and spatial dynamics of productivity in coastal wetlands, ecosystems which are at the forefront of experiencing climate change induced variability in precipitation, temperature, and sea level rise that have the potential to alter ecosystem productivity.

Bioaccumulation of metals in Spartina alterniflora salt marshes in the estuary of the World's Largest Choked Lagoon.

Salt marshes are capable of mitigating metal pollution in coastal environments, yet the efficacy of this remediation is contingent upon various environmental factors and the plant species involved. This study investigates the influence of different anthropogenic activities, including industrial, urban, recreational (in an insular area), and dredging operations, on the bioaccumulation of eight metals (As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn) within Spartina alterniflora Loisel. in the Patos Lagoon estuary, Brazil. The research aims to assess the pattern of metal bioaccumulation and distribution within the plant's leaves, stems, and roots while also examining metal presence in the sediment. Our main findings reveal that S. alterniflora exhibited elevated metal levels in its plant structure directly related with the metal concentrations in the surrounding sediment, which, in turn, is related to the different anthropogenic activities. The industrial area presented the highest metal levels in sediment and plant sections, followed by dredging, insular, and urban areas. This same pattern was mirrored for the bioconcetration factors (BCF), with the BCFs consistently indicating active metal bioaccumulation across all areas and for most of the metals. This provides evidence of the metal bioaccumulation pattern in S. alterniflora, with elevated BCFs in areas affected by activities with a higher degree of impact. Translocation factors (TF) showed varying metal mobility patterns within the plant's below-ground and above-ground sections across the different areas, with only Hg exhibiting consistent translocation across all study areas. Zn was the primary metal contributor in all plant sections, followed by Pb and Cu. It is worth noting that Pb is a non-essential metal for this plant, highlighting the relationship between elevated Pb contributions in the plant sections and the bioaccumulation of this metal within the plant's structure. Overall, this study emphasizes the bioaccumulation capacity of S. alterniflora and elucidate the intrinsic connection between different anthropogenic activities and their impact on the resultant availability and bioaccumulation of metals by this salt marsh plant.

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.

Conversion of natural coastal wetlands to mariculture ponds dramatically decreased methane production by reducing substrate availability

Coastal wetlands are increasingly being converted into aquaculture ponds to meet growing global demand for fish protein. Coastal wetlands conversion has been predicted to result in significant carbon (C) emissions; however, a mechanistic understanding of the effects of coastal wetland conversion on soil’s capacity to produce greenhouse gases remains lacking. Here, integrated biogeochemical investigations on methanogenic substrates, community structures, and activities were conducted in a coastal Spartina alterniflora saltmarsh and three saltmarsh-converted mariculture ponds aged 6, 13, and 20 years. Saltmarsh conversion into mariculture ponds decreased CH4 production potential from 353 to 16.3–78.3 µg CH4 kg−1 d−1. The concentrations of dissolved organic C and non-competitive methanogenic substrate, trimethylamine, were reduced by 84.8% and 79.7%, respectively, whereas methanogens abundance decreased by 43.7%, probably due to the decreased C input following conversion. Among the ponds, CH4 production potential, methanogenic substrates (acetate and trimethylamine), and methanogen abundance decreased with conversion chronosequence. However, the dominant methanogen group did not shift after land conversion, and the potential methylamine-utilizing Methanosarcinaceae accounted for 86.5% and 85.3–89.7% of the total populations in the S. alterniflora saltmarsh and mariculture ponds, respectively, implying that methylotrophic methanogenesis was the predominant pathway of CH4 production in both ecosystems. The results indicated that the conversion of natural coastal saltmarsh to mariculture ponds decreased CH4 production by reducing substrate availability and methanogen abundance, and CH4 production potential in mariculture ponds was more efficiently suppressed with conversion chronosequence.

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.

Changes of Inundation Frequency in the Yellow River Delta and Its Response to Wetland Vegetation

The spatiotemporal changes of inundation frequency in the Yellow River Delta (YRD) have profound influences on sustainable ecological protection and are also closely relevant to economic development scarcity on the coast of China. However, long-term changes of inundation frequency have remained poorly characterized. Using the Google Earth Engine (GEE) cloud platform, this study processed Landsat images to explore inundation frequency changes from 1990 to 2020 in the YRD. The results indicated that (1) The existing water index combining the classification results based on the MLM (Maximum Likelihood Method) is suitable for the mapping of the long-term water bodies, especially in the coastal regions; (2) The inundation frequency showed a clearly uneven temporal–spatial distribution. The low inundation area (LIA) is mainly intertidal natural wetlands with a declining trend, while the high inundation area (HIA) is dominated by constructed wetlands with a rising trend; (3) The use frequency of artificial ponds determines the inundation frequency of the constructed wetland. The industry development has gradually matured, causing the inundation frequency from dispersion to concentration in the constructed wetland; and (4) In the natural wetland, the LIA have increased since 2010 and have accounted for 30% in 2020. The large-scale appearance of LIA occurs with the emergence of high vegetation abundance. Spartina alterniflora salt marshes with strong reproductive ability and high abundance lead to the difficulty of wetland flooding and reduce the inundation frequency.

Seasonal soil-plant nitrogen dynamics of a cordgrass salt marsh in response to coastal embankments in Eastern China

The effects of coastal embankments on nitrogen (N) cycling in the Spartina alterniflora salt marsh have been extensively reported. However, it remains unclear effects of the embankment on the sizes of diverse N subpools in the plant-soil subsystems year-round. This study examined seasonal changes in various N subpools of plant subsystems; soil subsystems [e.g., soil organic N (SON), recalcitrant organic N (RON), labile organic N (LON), dissolved organic N (DON), ammonium N(NH+4 N), and nitrate N(NO3-N)]; N mineralization [e.g., soil net ammonification (RA) and nitrification (RN) rate]; and immobilization [e.g., microbial biomass N (MBN)] in embanked and adjacent S. alterniflora natural salt marshes on the coast of Eastern China. The embankment significantly reduced the litter N storage by 62.7–71.8% over the four seasons and decreased the root N storage by 53.0% during winter. The SON, LON, RON, and N H +4 N concentrations declined significantly by 43.0–60.2%, 35.8–64.8%, 44.9–59.0%, and 20.8–42.2%, respectively, over the four seasons following the embankment construction. Furthermore, the embankment dramatically reduced the DON concentrations by 21.9% in spring, 14.6% in summer, and 10.4% in winter, while notably diminishing the NO3 N concentrations by 33.4% in autumn and 44.9% in winter, and the RA and RN in spring and summer. However, the embankment clearly increased the MBN concentrations during summer and autumn, the NO3 N concentrations in spring, and the RA and RN in winter at different levels. Due to the decreased soil N inputs from plants, the embankment decreased the organic and inorganic N subpools every season to varying degrees, except for the NO3 N concentration in spring. We suggest that the decreased soil salinity following embankment establishment might increase the uptake of ions by microbes, while stimulating the production of MBN. Ultimately, the NO3 N and DON were two vital N sources for S. alterniflora, and plants absorbed N from the soil to promote their biomass, as well as N concentration and storage. This study is conducive toward understanding the mechanisms behind the effects of coastal embankments on the N transfer among various N subpools in the plant and soil systems.

Exogenous nitrogen from riverine exports promotes soil methane production in saltmarshes in China

Methane emissions from saltmarshes can potentially promote climate warming. Soil methane production is positively correlated with methane emissions from saltmarshes. Understanding the factors influencing soil methane production will improve the prediction of methane emissions, but an investigation of these factors has not been conducted in saltmarshes in China. We collected soils from native Phragmites australis and invasive Spartina alterniflora saltmarshes along the coast of China; the soil potential methane production (PMP) was determined by incubation experiments. The large-scale investigation results showed that the ratios of methanogens relative to sulfate-reducing bacteria (RMRS) and total organic carbon (TOC) were positively correlated with soil PMP for both species. Dissolved inorganic nitrogen (DIN) was positively correlated with the soil PMP of P. australis saltmarshes, and plant biomass was positively correlated with the soil PMP of S. alterniflora saltmarshes. Our results showed that exogenous nitrogen from riverine exports was positively correlated with DIN and plant biomass in both P. australis and S. alterniflora saltmarshes. In addition, exogenous nitrogen was also positively correlated with TOC in S. alterniflora saltmarshes. Consequently, exogenous nitrogen indirectly promoted soil methane production in P. australis saltmarshes by increasing the DIN and promoted soil methane production in S. alterniflora saltmarshes by enhancing the TOC and plant biomass. Moreover, we found that the promoting effect of DIN on the soil PMP of P. australis saltmarshes increased when the incubation temperature increased from 15 °C to 25 °C. Thus, the promoting effect of exogenous nitrogen on the soil methane production in P. australis saltmarshes might be strengthened in the peak of growing season. Our findings are the first to confirm that exogenous nitrogen inputs from rivers indirectly promote soil methane production in P. australis and S. alterniflora saltmarshes and provide new insights into the factors responsible for soil methane production in saltmarshes.

BERM: a Belowground Ecosystem Resiliency Model for estimating Spartina alterniflora belowground biomass.

Spatiotemporal patterns of Spartina alterniflora belowground biomass (BGB) are important for evaluating salt marsh resiliency. To solve this, we created the BERM (Belowground Ecosystem Resiliency Model), which estimates monthly BGB (30-m spatial resolution) from freely available data such as Landsat-8 and Daymet climate summaries. Our modeling framework relied on extreme gradient boosting, and used field observations from four Georgia salt marshes as ground-truth data. Model predictors included estimated tidal inundation, elevation, leaf area index, foliar nitrogen, chlorophyll, surface temperature, phenology, and climate data. The final model included 33 variables, and the most important variables were elevation, vapor pressure from the previous four months, Normalized Difference Vegetation Index (NDVI) from the previous five months, and inundation. Root mean squared error for BGB from testing data was 313gm-2 (11% of the field data range), explained variance (R2 ) was 0.62-0.77. Testing data results were unbiased across BGB values and were positively correlated with ground-truth data across all sites and years (r= 0.56-0.82 and 0.45-0.95, respectively). BERM can estimate BGB within Spartina alterniflora salt marshes where environmental parameters are within the training data range, and can be readily extended through a reproducible workflow. This provides a powerful approach for evaluating spatiotemporal BGB and associated ecosystem function.

Spartina alterniflora saltmarsh soil organic carbon properties and sources in coastal wetlands

Spartina alterniflora saltmarsh soil organic carbon properties and sources in coastal wetlands

Artificial modification on lateral hydrological connectivity promotes range expansion of invasive Spartina alterniflora in salt marshes of the Yellow River delta, China

‘Invasibility’, or the extent to which a habitat is prone to being invaded by plants, is a measure of the resistance of that ecosystem to biological invasion: a limited extent represents abiotic conditions unsuitable for invasion by invasive species; however, human activity can change that and make a habitat prone to rapid invasion. Field surveys and greenhouse experiments were carried out to explore, using spatial analysis, how a strong invader, namely Spartina alterniflora, is assisted by such activities as constructing levees and digging trenches, ditches, and pits in a tidal salt marsh. These activities changed the lateral hydrological connectivity of a salt marsh. The invasibility was then estimated based on the probability of seed dispersal and retention using the classical probabilistic method, and the rate of seedling emergence using threshold analysis. Changes in lateral hydrological connectivity led to more seeds of the invading species being retained, especially in high marshes, and promoted the emergence of its seedlings by making the soil more moist and less saline. The results suggest that changes in the lateral hydrological connectivity in a salt marsh can make it more prone to being invaded. The results have important implications for the control of invasive plants by limiting human activity and thereby regulating lateral hydrological connectivity in coastal ecosystems.

A Comparison between the MODIS Product (MOD17A2) and a Tide-Robust Empirical GPP Model Evaluated in a Georgia Wetland

Despite the importance of tidal ecosystems in the global carbon budget, the relationships between environmental drivers and carbon dynamics in these wetlands remain poorly understood. This limited understanding results from the challenges associated with in situ flux studies and their correlation with satellite imagery which can be affected by periodic tidal flooding. Carbon dioxide eddy covariance (EC) towers are installed in only a few wetlands worldwide, and the longest eddy-covariance record from Georgia (GA) wetlands contains only two continuous years of observations. The goals of the present study were to evaluate the performance of existing MODIS Gross Primary Production (GPP) products (MOD17A2) against EC derived GPP and develop a tide-robust Normalized Difference Moisture Index (NDMI) based model to predict GPP within a Spartina alterniflora salt marsh on Sapelo Island, GA. These EC tower-based observations represent a basis to associate CO2 fluxes with canopy reflectance and thus provide the means to use satellite-based reflectance data for broader scale investigations. We demonstrate that Light Use Efficiency (LUE)-based MOD17A2 does not accurately reflect tidal wetland GPP compared to a simple empirical vegetation index-based model where tidal influence was accounted for. The NDMI-based GPP model was capable of predicting changes in wetland CO2 fluxes and explained 46% of the variation in flux-estimated GPP within the training data, and a root mean square error of 6.96 g C m−2 in the validation data. Our investigation is the first to create a MODIS-based wetland GPP estimation procedure that demonstrates the importance of filtering tidal observations from satellite surface reflectance data.

Uptake and accumulation of metals in Spartina alterniflora salt marshes from a South American estuary

Salt marshes are capable of reducing metal pollution in coastal waters, but this capacity is highly dependent on the metal, the physico-chemical characteristics of the sediment, the plant species, the production of biomass, the time of the year, etc. The aim of this study was to assess the uptake and accumulation of Pb, Ni, Cu and Zn in Spartina alterniflora from three salt marshes within the Bahía Blanca estuary (BBE), a human-impacted Argentinean system. Metal concentrations in sediments and plants showed the same order at all sites: Zn > Cu > Pb ≥ Ni. The site with lower organic matter and fine sediment content had lower metal concentrations in the sediments, but not a lower metal content in the plant tissues, meaning that the sediment characteristics influenced the metal concentrations in the sediment and their uptake by plants. Despite differences in sediment characteristics between sites, metals were always higher in the belowground tissues than in aboveground ones and, in general, higher in dead than in live tissues. Some metals were accumulated in plant tissues, but not others, and this is dependent on the metal and the sediment characteristics. Allocation patterns of metals in tissues of S. alterniflora were mainly dependent on metal concentrations, determining higher belowground pools, but the aboveground pools were important in some cases due to higher biomass. Partitioning of metals in above or belowground pools determines their fate within the estuarine system, since tissues can decompose in situ (belowground) or be exported (aboveground). Seasonal dynamics were important for some variables but were less noticeable than the differences between sites and tissues. Our results indicate that S. alterniflora from the BBE is efficient in accumulating some metals, despite usually low metal concentrations in sediments and plants. This accumulation capacity has implications for the whole system through the fate of the tissues.

Ecological structure and function in a restored versus natural salt marsh.

Habitat reconstruction is commonly employed to restore degraded estuarine habitats and lost ecological functions. In this study, we use a combination of stable isotope analyses and macrofauna community analysis to compare the ecological structure and function between a recently constructed Spartina alterniflora salt marsh and a natural reference habitat over a 2-year period. The restored marsh was successful in providing habitat for economically and ecologically important macrofauna taxa; supporting similar or greater density, biomass, and species richness to the natural reference during all but one sampling period. Stable isotope analyses revealed that communities from the natural and the restored marshes relied on a similar diversity of food resources and that decapods had similar trophic levels. However, some generalist consumers (Palaemonetes spp. and Penaeus aztecus) were more 13C-enriched in the natural marsh, indicating a greater use of macrophyte derived organic matter relative to restored marsh counterparts. This difference was attributed to the higher quantities of macrophyte detritus and organic carbon in natural marsh sediments. Reduced marsh flooding frequency was associated with a reduction in macrofaunal biomass and decapod trophic levels. The restored marsh edge occurred at lower elevations than natural marsh edge, apparently due to reduced fetch and wind-wave exposure provided by the protective berm structures. The lower elevation of the restored marsh edge mitigated negative impacts in sampling periods with low tidal elevations that affected the natural marsh. The results of this study highlight the importance of considering sediment characteristics and elevation in salt marsh constructions.

The Tidal Marsh Inundation Index (TMII): An inundation filter to flag flooded pixels and improve MODIS tidal marsh vegetation time-series analysis

Remote sensing in tidal marshes can provide regional assessments of wetland extent, phenology, primary production, and carbon sequestration. However, periodic tidal flooding reduces spectral reflectance, especially in the near and short-wave infrared wavelengths. Consequently, marsh vegetation time-series products that lack tidal filtering, such as those provided by MODIS (Moderate Resolution Imaging Spectroradiometer), may not reflect true vegetation trends. We created a new Tidal Marsh Inundation Index (TMII) for processing daily 500-m MODIS surface reflectance data and calibrated it with a Spartina alterniflora salt marsh pixel on Sapelo Island, GA. Ground-truth data for TMII was extracted from a PhenoCam, which collected high frequency digital photography of the TMII calibration pixel. To identify the best wavelengths to include in the TMII, we compared goodness of fit metrics from generalized linear models (GLMs). Predictors for these GLMs included suites of normalized difference indices from the literature as well as other band combinations. We also explored including a phenology parameter that could scale TMII relative to vegetation development. Ultimately, TMII was based on the normalized difference of green and shortwave infrared reflectance in combination with a phenology parameter composed of the moving average of the normalized difference of near infrared and shortwave infrared reflectance. This final index allowed a single optimized decision boundary to identify flooding across the annual growth cycle. When compared to ground-truth data from the PhenoCam, the TMII classified flooded conditions with 67–82% and dry conditions with 75–81% accuracy, respectively, across training, testing and validation datasets. We applied TMII to new S. alterniflora marsh MODIS pixels on Sapelo Island, GA as well as on Plum Island, MA. For these new pixels, TMII classified marsh flooding with 77–80% overall accuracy. We also demonstrated how users can apply TMII filtering in a MODIS workflow to create vegetation time-series composites within S. alterniflora, Spartina patens and Juncus roemerianus marshes. We showed how a new user can validate and optimize TMII in their application, either by comparing it to inundation data or by validating the filtered vegetation time series against field data. We also compared TMII-filtered composites to the existing MODIS MOD13 16-d Normalized Difference Vegetation Index (NDVI) product. TMII-filtered composites generated less noisy time-series that fit field data better than MOD13. TMII filtering was most important on Sapelo Island, where the tide range was high and vegetation was sparse. Results were less dramatic when TMII was applied to different marsh species within the Gulf Coast sites with lower tidal ranges, but TMII-filtering still improved vegetation time series. Thus, preprocessing MODIS imagery with the TMII effectively identified most inundated pixels. The TMII represents a step forward for wetland remote sensing that will be useful for improved estimation of phenology, biomass and carbon storage in coastal marshes.

Sedimentary processes and products in a mesotidal salt marsh environment: insights from Groves Creek, Georgia

Southeastern salt marshes are important repositories of sediment and carbon, and their formation is heavily dependent on deposition and accumulation of inorganic sediment. This study examined Groves Creek marsh near Savannah, GA, a typical Spartina alterniflora salt marsh of the southeastern US. Analyses were focused on the character, deposition and accumulation of material within the marsh on daily, monthly, decadal and centennial timescales, to determine the dominant factors in material supply and redistribution, and on its stratigraphy to determine the 1,000-year history of Groves Creek salt marsh development. Modern processes create gradients in grain size, which shows little variation from the tidal channel flanks up to mean sea level, and which coarsens with distance into the marsh from mean sea level to mean high water. This unexpected result suggests that, although floc transport is an important mechanism of sediment supply near the channel margins, energetic events must supply coarser materials to the marsh platform, where they are not readily removed by typical energy regimes. Daily deposition can approach ~3 g/cm2 year; however, centennial accumulation rates are orders of magnitude lower (0.11±0.05 g/cm2 year) and are similar to those present over the past 300 years (0.05–0.2 g/cm2 year), indicating that much of the daily deposition is remobilized. Stable isotopic δ13C (average –18.7‰) and δ15N (average 5.7‰) values most likely indicate a large contribution from S. alterniflora as a carbon source throughout the marsh, although heavier δ15N on the channel flanks suggest that benthic algae may be locally important. Geologic, geochemical and microfossil evidence suggests that depositional conditions in the Groves Creek marsh have changed significantly over the past ~1,000 years, creating a distinct fining-upward sequence. This sequence preserves the signature (from bottom to top) of subtidal flats grading to intertidal sandflats, an erosional lag created by a migrating tidal channel, point bar deposits, channel fill deposits, and a capping salt marsh. A significant change in environmental conditions occurred between 360–500 cal years BP, which changed the character of geological, geochemical and microfossil components in the marsh, and, because this observation is robust throughout the southeastern US, hints at a major change in estuarine and coastal systems at this time.

Seawall construction alters soil carbon and nitrogen dynamics and soil microbial biomass in an invasive Spartina alterniflora salt marsh in eastern China

Seawalls have increasingly been constructed to control the invasion of an exotic perennial grass, Spartina alterniflora, in coastal wetlands of eastern China. We investigated soil organic carbon (C) and nitrogen (N), available and microbial C and N, soil microbial community composition and biomass in a seawall-reclaimed S. alterniflora salt marsh compared with an adjacent natural S. alterniflora salt marsh. Seawall reclamation in S. alterniflora salt marsh significantly decreased soil salinity, moisture, litter and root biomass, and strongly decreased soil total organic C by 57% and total organic N by 59%, and also lowered soil available C and N in S. alterniflora salt marsh. Seawall reclamation significantly decreased soil microbial biomass C and the quantities of the total phospholipid fatty acids (PLFAs), and bacterial, fungal, gram-negative bacterial, gram-positive bacterial, saturated straight-chain, and monounsaturated PLFAs in deeper soil layers (10–30cm). Our results suggested that seawall construction could greatly decrease soil C and N accumulation of S. alterniflora salt marsh by decreasing S. alterniflora residuals input into the soil and lowering soil salinity and moisture, which further decreased soil microbial biomass by lowering soil available C and N in S. alterniflora salt marsh. Changes in soil microbial community composition and biomass could in turn affect soil C and N accumulation in a seawall-reclaimed S. alterniflora salt marsh in eastern China.

Gulf Ribbed Mussels (Geukensia granosissima) Increase Methane Emissions from a Coastal Spartina alterniflora Marsh

Understanding methane emissions from natural sources is becoming increasingly important with future climactic uncertainty. Wetlands are the single largest natural source of methane; however, little attention has been given to how biota and interactions between aboveground and belowground communities may affect methane emission rates in these systems. To investigate the effects of vegetative disturbance and belowground biogeochemical alterations induced by biota on methane emissions in situ, we manipulated densities of Littoraria irrorata (marsh periwinkle snails) and Geukensia granosissima (gulf ribbed mussels) inside fenced enclosures within a Spartina alterniflora salt marsh and measured methane emissions and sediment extracellular enzyme activity (phosphatase, β-glucosidase, cellobiohydrolase, N-acetyl-β-D-glucosaminidase, peroxidase, and phenol oxidase) over the course of a year. Changes in snail density did not have an effect on methane emission; however, increased densities of ribbed mussels significantly increased the emission of methane. Sediment extracellular enzyme activities for phosphatase, cellobiohydrolase, N-acetyl-β-D-glucosaminidase, and phenol oxidase were correlated to methane emission, and none of the enzymes assayed were affected by the snail and mussel density treatments. While methane emissions from salt marsh ecosystems are lower than those from freshwater systems, the high degree of variability in emission rates and the potential for interactions with naturally occurring biota that increase emissions warrant further investigations into salt marsh methane dynamics.

Accumulation of soil carbon drives denitrification potential and lab-incubated gas production along a chronosequence of salt marsh development

We measured sediment organic carbon and nitrogen accumulation and rates of denitrification enzyme activity and greenhouse gas (CO2, CH4, N2O) production from slurries of sediments of a mudflat that formed in 2002, a young (8-year-old) natural Spartina alterniflora salt marsh that developed on part of the mudflat, and four mature (>200 years old) salt marshes in southeastern Georgia to examine microbial processes related to carbon (C) and nitrogen (N) cycling during succession from mudflat to mature marsh. Soil organic C and N and C: N ratio (0–30 cm) increased across the chronosequence from mudflat (791 ± 35 g C/m2, 125 ± 17 g N/m2) to young marsh (2520 ± 131 g C/m2, 190 ± 10 g N/m2) to mature marshes (5827 ± 250 g C/m2, 372 ± 20 g N/m2). After 8 years of colonization by S. alterniflora, sediment organic carbon increased 3.2 times, and nitrogen increased 1.5 times relative to the mudflat. The high rate of organic C and N accumulation based on time series measurements (188 g C/m2/yr, 7.8 g N/m2/yr) and feldspar marker layers (359 g C/m2/yr, 26.2 g N/m2/yr) was attributed to high accretion (3 cm/yr) in this low elevation (0.18 m NAVD88) emerging marsh. Carbon dioxide production increased with increasing sediment organic C from mudflat to mature marshes. Un-amended denitrification enzyme activity, measured in slurry incubations, ranged from an average of 0.020 ± 0.005 μg g−1 hr−1 in the mature marshes to 0.094 ± 0.03 μg g−1 hr−1 in the young marsh. We also measured denitrification potential in slurry incubations amended with C (glucose), N (nitrate), and C + N to assess the potential for substrate limitations. Denitrification potential in the mudflat did not show strong nutrient limitation. In the young marsh, denitrification potential was C-limited, and in the mature marsh, it was co-limited by C and N. In July samples, CO2 production showed a statistically significant increase with age from the mudflat to the mature marshes. However, in both months, CO2 production efficiency (expressed on a per g C basis) was significantly higher in the mudflat sediment slurries than in the young marsh and mature marsh samples. Spartina colonization of mudflats and the subsequent accumulation of organic matter are key to enriching sediment organic C and N pools that control microbial heterotrophy, particularly denitrification and CO2 production, which play important roles in marsh C and N cycling.