Spartina Alterniflora Marsh Research Articles

Bioaccumulation of heavy metals by Cyperus malaccensis and Spartina alterniflora in a typical subtropical estuary (Min River) of Southeast China

To understand the bioaccumulation of heavy metals by Cyperus malaccensis (CM) and Spartina alterniflora (SA) in a typical subtropical estuary (Min River) of Southeast China, the concentrations of five metals (Pb, Cr, Cu, Zn, and Ni) in plants and sediments of CM and SA marshes were determined. Two experimental plots (no flooding plot and flooding plot) were established in the intertidal zone of the Min River estuary in July 2015, and, in each plot, in situ sampling was conducted in CM and SA marshes, respectively. The concentrations of heavy metals in all samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Vertical variations of the five metals differed between marshes or plots, and, in most cases, there was no evidence of grain-size composition (particularly for clay) and sediment organic matter (SOM) contributing to the sorption of significant amounts of metals in sediments of different marshes. Flooding regime was an important factor inducing the difference of metals in sediments of the two experimental plots. Heavy metal levels in different tissues of CM and SA differed within species or plots. For the five metals, the root was generally the main stock, and, as water condition changed from non-flooding to flooding, allocations of Pb, Cu, and Zn in CM and SA decreased while those of Cr increased. The accumulation factors [AFs] of heavy metals in different tissues of CM and SA also differed between species or plots. In most cases, the [AF]plant of different metals were less than 1, implying that the metal accumulations in CM and SA of the two plots were uncommon. This study found that Cu and Zn pollution in non-flooding conditions and Pb and Cu pollution in flooding conditions might be more serious, indicating that intertidal sediments might be severely contaminated by the three metals if effective measures are not taken to control the pollutant loadings of the Min River estuary in the future. Moreover, CM generally accumulated more Pb, Cu, and Zn while SA absorbed more Cr and Ni (particularly in flooding conditions), implying that, when determining environmental pressures, the two plants could be used as specified biological indicators.

Impact of salinity and nutrients on salt marsh stability

AbstractBelowground growth in coastal plants is critical for marsh stability and the ability of coastal wetlands to keep pace with sea‐level rise. Quantifying the effects of nutrient loading on belowground plant growth is an ongoing controversy in wetland research, with previous experiments demonstrating both positive and negative impacts. Moreover, salinity may also decrease belowground growth through sulfide toxicity, or plants may increase root growth to oxidize sediments and respond to sulfide stress. Because salinity influences plant nitrogen assimilation and sediment nitrogen retention, salinity and nitrogen may interact to influence belowground plant growth. We sampled an urban‐to‐rural land‐use gradient of 11 Spartina alterniflora marshes on Long Island, New York, to look for correlates of belowground biomass. We found that belowground biomass was related positively to salinity and negatively to extractable nitrogen content in sediments. Total belowground plant biomass was reduced by 60–70% in high‐nitrogen marshes and enhanced by as much as 70% in high‐salinity marshes. Further, we found no evidence of interaction between salinity and nitrogen, indicating that these factors were independently related to belowground plant growth. Our results indicate that chronic eutrophication and increasing salinity resulting from sea‐level rise are likely to have opposing effects on future marsh stability.

Spatial variation and toxicity assessment for heavy metals in sediments of intertidal zone in a typical subtropical estuary (Min River) of China.

Sediment samples were collected in five marshes (C1, Phragmites australis marsh; C2, P. australis and Cyperus malaccensis marsh; C3, C. malaccensis marsh; C4, Spartina alterniflora marsh; and C5, Cyperus compressus marsh) respectively along two typical transects (T1 and T2) extending from the vegetated marsh to the mudflat in a typical subtropical estuary (Min River) of China in July 2015 to investigate the spatial variation and toxicity of heavy metals (Pb, Cr, Cu, Zn, and Ni) in intertidal zone. Results showed that the concentrations of heavy metal in surface sediments of the two transects were in the order of Zn>Cr>Pb>Cu>Ni and slightly decreased from the land to the sea except for Pb and Zn. The levels of the five metals at C5 marsh were generally the lowest, while those of Pb, Cr, and Zn at C2 marsh were the highest. The vertical variations of Pb, Cr, and Zn concentrations in profiles differed among marshes or transects, and in most cases, there was no evidence of sediment organic matter (SOM) contributing to the sorption of significant amount of metals in sediments along the two transects, while grain composition was a primary factor controlling the spatial variations of metals. Both the vertical distributions of Cu and Ni levels among marshes or transects showed little fluctuation, which could be better explained by the effects of sulfur on the solubility and mobility of the two metals in profiles. Heavy metal levels in sediments of the Min River estuary were much higher compared with most estuaries in Asia, Europe, Africa, and South America. The sediments in intertidal zone of the Min River estuary were moderately polluted by the five metals, and particularly, Ni was identified as heavy metal of primary concern. Both Ni and Pb in sediments of the intertidal zone showed high potential toxicity and high contributions to the sum of the toxic units (ΣTUs). In future, the metal pollutions in intertidal zone of the Min River estuary might be more serious, and in the next step, there will be long-term potential consequences for endangered animals or migrant birds if measures are not taken to rehabilitate the sediments resulting from metal pollutions.

Impact of Armored Shorelines on Shore-Zone Fish Density in a Mid-Atlantic, USA, Estuary: Modulation by Hypoxia and Temperature

Anthropogenic modifications of estuarine environments, including shoreline hardening and corresponding alteration of water quality, are accelerating worldwide as human population increases in coastal regions. Estuarine fish species inhabiting temperate ecosystems are adapted to extreme variations in environmental conditions including water temperature, salinity, and dissolved oxygen across seasonal, daily, and hourly time scales. The present research utilized quantitative sampling to examine the spatiotemporal distribution of shore-zone estuarine fish species in association with four unique shoreline types across a range of water temperature and dissolved oxygen conditions. Fish were collected from the intertidal and shallow subtidal region of four shoreline types, Spartina alterniflora marsh, Phragmites australis marsh, riprap, and bulkhead, in the summer and fall of 2009 and 2010. Analyses were performed to (1) compare mean fish density among shoreline types across all water conditions and (2) explore relationships of the complete fish assemblage, three functional species groupings, and two fish species (Fundulus heteroclitus and Menidia menidia) to unique shoreline/water conditions. Significantly greater mean fish densities were found along S. alterniflora shorelines than armored shorelines. Several metrics including fish density, species richness, and occurrence rates suggest S. alterniflora shorelines may serve as a form of refuge habitat during periods of low dissolved oxygen and high temperatures for various species, particularly littoral-demersal species including F. heteroclitus. Potential mechanisms that could contribute to a habitat providing refuge during adverse water quality conditions include tempering of the adverse condition (decreased temperatures, increased dissolved oxygen), predation protection, and increased foraging opportunities.

Changes of soil carbon, nitrogen and phosphorus and stoichiometry characteristics in marsh invaded by Spartina alterniflora

To clarify the effect of invasive alien plants on ecosystem functions, soil total carbon (TC), total carbon (TN), and total phosphorus (TP) contents were studied in the exotic invasive plant Spartina alterniflora marsh with different durations and the native species Cyperus malaccensis marsh in the Minjiang River estuary. The results showed that there were different variations of TC, TN and TP at 0-50 cm soil layer in the C. malaccensis marsh invaded by S. alterniflora, but the changes of TC and TN were similar and the change of the TP lagged behind them. Soil C/N increased continually with the increase of TC. Soil TP was a key factor affecting soil C/P and N/P during the S. alterniflora invasion, and the change of C/P and N/P followed a similar trend. The changes of TC, TN and TP were influenced by abiotic factors such as soil salinity, bulk density, soil water content and clay composition. C, N and P stoichiometric ratios were influenced mainly by soil salinity and grain composition. C/N and C/P were the good indicators for soil carbon enrichment ability in S. alterniflora marsh. S. alterniflora invasion induced the change of biomass and marsh habitats. Soil carbon, nitrogen, phosphorus contents, and their ecological stoichiometric ratios changed significantly in the C. malaccensis marsh invaded by S. alterniflora and showed diffe-rent change characteristics with different invasion duration.

Hydrodynamic and Biological Mechanisms for Variations in Near-Bed Suspended Sediment Concentrations in a Spartina alterniflora Marsh—a Case Study of Luoyuan Bay, China

Near-bed suspended sediment concentration (SSC) was measured at the margin and in the interior of a Spartina alterniflora marsh in Luoyuan Bay, a semi-enclosed embayment sheltered from the East China Sea, in January and July 2008. Variations in SSC at the two stations correlated to the tidal cycle: high SSC occurred in the early stage of the flood tide and the later stage of the ebb tide, while SSC was significantly lower at all other times. The SSC increased from the neap tide to the spring tide in both winter and summer. The high SSC in winter was primarily a result of stronger hydrodynamic processes, whereas the high SSC in summer was primarily attributable to biological activity. The particulate organic carbon (POC) in the S. alterniflora marsh of Luoyuan Bay primarily originates from freshwater and marine algae and phytoplankton, while the contribution of marsh detritus is limited. The contribution of POC to near-bed SSC is less than 5% in both winter and summer. During winter, the near-bed SSC was closely related to the bottom shear stress; therefore, variations in SSC were controlled by hydrodynamic processes. However, the correlation between bottom shear stress and near-bed SSC was poor in summer because of the interference of burrowing organisms during the late stage of the ebb tide. The maximum estimated contribution of turbid water flowing out of caves due to the biological activity of burrowing organisms was approximately 30.3 mg L−1 or 20.2 to 50.5% with a mean value of 34.9% of total SSC at the late stage of the ebb tide. Our findings also suggest that the relative importance of hydrodynamic processes and biological activity can vary considerably between seasons.

Marsh canopy structure changes and the Deepwater Horizon oil spill

Abstract Marsh canopy structure was mapped yearly from 2009 to 2012 in the Barataria Bay, Louisiana coastal region that was impacted by the 2010 Deepwater Horizon (DWH) oil spill. Based on the previously demonstrated capability of NASA's UAVSAR polarimetric synthetic aperture radar (PolSAR) image data to map Spartina alterniflora marsh canopy structure, structure maps combining the leaf area index (LAI) and leaf angle distribution (LAD, orientation) were constructed for yearly intervals that were directly relatable to the 2010 LAI-LAD classification. The yearly LAI-LAD and LAI difference maps were used to investigate causes for the previously revealed dramatic change in marsh structure from prespill (2009) to postspill (2010, spill cessation), and the occurrence of structure features that exhibited abnormal spatial and temporal patterns. Water level and salinity records showed that freshwater releases used to keep the oil offshore did not cause the rapid growth from 2009 to 2010 in marsh surrounding the inner Bay. Photointerpretation of optical image data determined that interior marsh patches exhibiting rapid change were caused by burns and burn recovery, and that the pattern of 2010 to 2011 LAI decreases in backshore marsh and extending along some tidal channels into the interior marsh were not associated with burns. Instead, the majority of 2010 to 2011 shoreline features aligned with vectors displaying the severity of 2010 shoreline oiling from the DWH spill. Although the association is not conclusive of a causal oil impact, the coexistent pattern is a significant discovery. PolSAR marsh structure mapping provided a unique perspective of marsh biophysical status that enhanced detection of change and monitoring of trends important to management effectiveness.

Combined influence of sedimentation and vegetation on the soil carbon stocks of a coastal wetland in the Changjiang estuary

Coastal wetlands play an important role in the global carbon cycle. Large quantities of sediment deposited in the Changjiang (Yangtze) estuary by the Changjiang River promote the propagation of coastal wetlands, the expansion of saltmarsh vegetation, and carbon sequestration. In this study, using the Chongming Dongtan Wetland in the Changjiang estuary as the study area, the spatial and temporal distribution of soil organic carbon (SOC) stocks and the influences of sedimentation and vegetation on the SOC stocks of the coastal wetland were examined in 2013. There was sediment accretion in the northern and middle areas of the wetland and in the Phragmites australis marsh in the southern area, and sediment erosion in the Scirpus mariqueter marsh and the bare mudflat in the southern area. More SOC accumulated in sediments of the vegetated marsh than in the bare mudflat. The total organic carbon (TOC) stocks increased in the above-ground biomass from spring to autumn and decreased in winter; in the below-ground biomass, they gradually increased from spring to winter. The TOC stocks were higher in the below-ground biomass than in the above-ground biomass in the P. australis and Spartina alterniflora marshes, but were lower in the below-ground biomass in S. mariqueter marsh. Stocks of SOC showed temporal variation and increased gradually in all transects from spring to winter. The SOC stocks tended to decrease from the high marsh down to the bare mudflat along the three transects in the order: P. australis marsh > S. alterniflora marsh > S. mariqueter marsh > bare mudflat. The SOC stocks of the same vegetation type were higher in the northern and middle transects than in the southern transect. These results suggest that interactions between sedimentation and vegetation regulate the SOC stocks in the coastal wetland in the Changjiang estuary.

A hybrid shoreline stabilization technique: Impact of modified intertidal reefs on marsh expansion and nekton habitat in the northern Gulf of Mexico

To mitigate shoreline erosion numerous armoring techniques have been employed extensively along the degrading shores of the Gulf of Mexico (GoM). Shoreline armoring strategies incorporating built vertical structures have resulted in numerous undesired ecological consequences. Bioengineering hybrid techniques consisting of “living shorelines” are emerging as an alternative option to mitigating shoreline loss and overcoming ecological shortcomings of hardened structures. Hitherto, only a few studies have assessed efficacy of hybrid techniques on shoreline stabilization and adjacent habitat enhancement. In this study, we integrated permeable intertidal reef-breakwaters (also known as wave attenuation units or WAUs) and predominantly restored native Spartina alterniflora marsh vegetation to mitigate erosion along severely degrading shores of a narrow peninsula in the northern GoM. Particularly, we evaluated impacts of a large-scale WAU reef deployment on a range of physical and biological parameters including erosion mitigation (shoreline stabilization), facilitation of created marsh expansion and habitat provision to marsh-utilizing nekton. We compared WAU reefs to adjacent gap areas without WAUs to evaluate the effects of tidal openings on the metrics measured. Our results of over 3 years suggest that, intertidal WAU reefs facilitate in created marsh expansion and the tidal openings between the reef complexes allow free movement of marsh-utilizing nekton fauna. Based on our results, we conclude that hybrid restoration technique is highly efficacious on erosion mitigation, adjacent marsh expansion and habitat creation. However, more works in other coastal systems are required to confirm the impacts of hybrid techniques on erosion mitigation and consequently on marshes and marsh-utilizing nekton.

Living Shorelines: Coastal Resilience with a Blue Carbon Benefit.

Living shorelines are a type of estuarine shoreline erosion control that incorporates native vegetation and preserves native habitats. Because they provide the ecosystem services associated with natural coastal wetlands while also increasing shoreline resilience, living shorelines are part of the natural and hybrid infrastructure approach to coastal resiliency. Marshes created as living shorelines are typically narrow (< 30 m) fringing marshes with sandy substrates that are well flushed by tides. These characteristics distinguish living shorelines from the larger meadow marshes in which most of the current knowledge about created marshes was developed. The value of living shorelines for providing both erosion control and habitat for estuarine organisms has been documented but their capacity for carbon sequestration has not. We measured carbon sequestration rates in living shorelines and sandy transplanted Spartina alterniflora marshes in the Newport River Estuary, North Carolina. The marshes sampled here range in age from 12 to 38 years and represent a continuum of soil development. Carbon sequestration rates ranged from 58 to 283 g C m-2 yr-1 and decreased with marsh age. The pattern of lower sequestration rates in older marshes is hypothesized to be the result of a relative enrichment of labile organic matter in younger sites and illustrates the importance of choosing mature marshes for determination of long-term carbon sequestration potential. The data presented here are within the range of published carbon sequestration rates for S. alterniflora marshes and suggest that wide-scale use of the living shoreline approach to shoreline management may come with a substantial carbon benefit.

Program Profile: The Georgia Coastal Ecosystems Long Term Ecological Research Project (GCE‐LTER)

Program Profile: The <scp>G</scp>eorgia Coastal Ecosystems Long Term Ecological Research Project (GCE‐LTER)

Microbial Community Composition and Extracellular Enzyme Activities Associated with Juncus roemerianus and Spartina alterniflora Vegetated Sediments in Louisiana Saltmarshes.

Saltmarshes are typically dominated by perennial grasses with large underground rhizome systems that can change local sediment conditions and be important in shaping the sediment microbial community. Factors such as salinity that control plant zonation in saltmarshes are also likely to influence the microbial community, but little is known as to whether microbial communities share distribution patterns with plants in these systems. To determine the extent to which microbial assemblages are influenced by saltmarsh plant communities, as well as to examine patterns in microbial community structure at local and regional scales, we sampled sediments at three saltmarshes in Louisiana, USA. All three systems exhibit a patchy distribution of Juncus roemerianus stands within a Spartina alterniflora marsh. Sediment samples were collected from the interior of several J. roemerianus stands as well as from the S. alterniflora matrix. Samples were assayed for extracellular enzyme activity and DNA extracted to determine microbial community composition. Denaturing gradient gel electrophoresis of rRNA gene fragments was used to determine regional patterns in bacterial, archaeal, and fungal assemblages, while Illumina sequencing was used to examine local, vegetation-driven, patterns in community structure at one site. Both enzyme activity and microbial community structure were primarily influenced by regional site. Within individual saltmarshes, bacterial and archaeal communities differed between J. roemerianus and S. alterniflora vegetated sediments, while fungal communities did not. These results highlight the importance of the plant community in shaping the sediment microbial community in saltmarshes but also demonstrate that regional scale factors are at least as important.

The expansion of Spartina alterniflora marsh in response to tidal flat reclamation, central Jiangsu coast, eastern China

The central Jiangsu coast is typical of China's coastal muddy silt with an average tidal range of 4 m. The shoreline has been gradually advancing seaward as the coast was supplied by a large amount sediment from ancient Yangtze River and Yellow River. As a result, a lot of land resources are fostered in the coast. Therefore tidal flat reclamation activities gradually intensified to the demand of economic development. A large- scale reclamation plan in Jiangsu coast has been implemented since the 21 st century and Spartina alterniflora marsh has become the main target of tidal flat reclamation. Thus how Spartina alterniflora marsh will expand during the reclamation period is worthy of study. Thirteen phases of satellite imageries from 2001 to 2013 and field observation data are analyzed to quantify the expansion rate of Spartina alterniflora marsh in association with the reclamation activities. The result shows that Spartina alterniflora expansion during 2001- 2013 can be divided into three periods. During 2001- 2007,the Spartina alterniflora area outside the seawall was relatively large and the influence on the tidal flat sedimentary environment by reclamation did not hamper its expansion. As a result,both the annually increased Spartina alterniflora area and the expansion rate keep pace with reclamation. During 2008- 2010, the Spartina alterniflora area outside the seawall decreased significantly; nevertheless, the mean annual expansion rate of the Spartina alterniflora marsh was still relatively high. During 2011- 2013, the area of Spartina alterniflora marsh became small because of the reclamation activities during 2001-2007 and 2008-2010 periods. Spartina alterniflora expansion became stabilized. As such, the pattern of the marsh expansion differs in response to the reclamation rate and stages. If a sufficiently wide marsh can be maintained during reclamation, then the marsh will continue to grow. The underlying mechanism is that the Spartina alterniflora marsh is influenced by the generation of seedling and the sediment types and bed elevation, while reclamation will affect all these factors. The relationship between marsh expansion and reclamation rate for the period of 2001- 2007 indicates that, for sustainable reclamation on the Jiangsu coast, a width of 0.9-1 km for the Spartina alterniflora marsh should be maintained.

Weak Correlation Between Methane Production and Abundance of Methanogens Across Three Brackish Marsh Zones in the Min River Estuary, China

Methane production is influenced by the abundance of methanogens and the availability of substrates. Sulfate-reducing bacteria (SRB) also play an important role in the anaerobic decomposition of organic matter. However, the relationships between methane production and abundance of methanogen and pore water substrates in estuarine brackish marshes are poorly characterized, and even to our knowledge, no published research has explored the relationship between methane production rate and pore water dimethyl sulfide (DMS) concentration. We investigated methane production rate, abundances of methanogens and SRB, sediment organic carbon contents, and concentrations of pore water substrates (acetate and DMS) and terminal electron acceptors (SO4 2−, NO3 −, and Fe3+) at a brackish marsh landscape dominated by Phragmites australis marsh, Cyperus malaccensis marsh, and Spartina alterniflora marsh in the Min River estuary, southeast China. The average rates of methane production over the entire 30 cm of the sediment profile (5 cm sampling interval) in the three marshes were 0.142, 0.058, and 0.067 μg g−1 day−1, respectively. The abundance of both methanogens and SRB in the sediment of the P. australis marsh with the highest sediment organic carbon content was higher than in the C. malaccensis and S. alterniflora marshes. Mean pore water DMS concentrations over the entire 30 cm of the sediment profile under the S. alterniflora marsh were higher than those in the C. malaccensis marsh and P. australis marsh. Methane production rate correlated weakly with the abundance of methanogens across the three marsh zones together, but did not correlate with the concentrations of pore water acetate and DMS. Our results suggest that the abundance of methanogens is controlled by sediment organic carbon supply, and further, methane production is affected by the abundance of methanogens in the subtropical estuarine brackish marshes.

Abundance and Diet of Predatory Fishes in Phragmites, Treated Phragmites, and Natural Spartina Marshes in Delaware Bay

Abundance and diet of larger predatory fishes in Phragmites marsh creeks were compared with those in natural Spartina alterniflora marshes and Phragmites marshes exposed to herbicides and burning (treated marshes). The most abundant fishes (n = 3,107) caught in gill nets from June to November 2001 and May to November 2002 were Morone americana (white perch; 82 %) and Ictalurus punctatus (channel catfish; 6 %). Annual abundance of species varied inconsistently across habitats. Morone americana caught in Phragmites marshes had lower percent piscivory and gut fullness than those caught in Spartina or treated marsh creeks, perhaps due to reduced fish prey availability. The diet in Phragmites marshes also showed comparatively higher occurrences and dry weights of the dominant marsh surface fish Fundulus heteroclitus (mummichog). With the few exceptions noted for M. americana, larger, piscivorous fishes appeard to function similarly in all three habitats, perhaps because their relatively large home ranges may cross habitat boundaries. More research, at broader spatial scales, is needed to link the effects of marsh restoration on piscivorous fish behavior and movement.

Distribution and deposition characteristics of carbon and nitrogen in sediments in a semi-closed bay area, southeast China

In this study, we analyzed carbon and nitrogen parameters (e.g., total organic carbon (TOC), total nitrogen (TN), stable isotopes of organic matter (δ13C, δ15N), and carbon–nitrogen ratio (C/N)), grain-size parameters and deposition rate, as well as their variations in the surface layer and on the profile of the sediment cores in various ecological zones of Luoyuan Bay. The results showed that the sedimentary type of Luoyuan Bay was clay silt. The TOC, TN, δ13C, and δ15N were in the range of 0.450–0.955%, 0.054–0.101%, −23.75 to −19.47‰, and 3.57–6.72‰, respectively; the C/N was in the range of 8.80–13.78. The grain-size parameters of the Spartina alterniflora marsh and transition zone were similar, whereas a similarity in the carbon and nitrogen parameters between the transition zone and mudflat was observed. The correlation of TOC and TN was different between the fresh organic matter and the obsolete organic matter. The particle size was not the main factor that controlled the TOC and TN contents in the sediments; the δ13C indicated the organic matter was dominated by marine sources. The average deposition rates in the Spartina alterniflora marsh, transition zone, and mudflat were 2.47, 2.79, 1.16gcm−2y−1, respectively. In the Spartina alterniflora marsh, the TOC and TN content increased by 96% and 104%, respectively, from 1955 to the present. Compared with the mudflat, the TOC and TN content in the layer between the surface and the 40-cm depth of the Spartina alterniflora marsh were 26% and 13% higher, respectively. The introduction of Spartina alterniflora and the metabolism of their roots had a significant effect on the carbon and nitrogen deposition in the layer at 0–40cm depth. The carbon sequestration rate of the salt marsh wetland in Luoyuan Bay was comparable to the carbon sequestration of global marshes. The deposition rates of TOC and TN in the Spartina alterniflora marsh and transition zone were greater than twice that in the mudflat.

Marsh Dieback, loss, and recovery mapped with satellite optical, airborne polarimetric radar, and field data

Landsat Thematic Mapper and Satellite Pour l'Observation de la Terre (SPOT) satellite based optical sensors, NASA Uninhabited Aerial Vehicle synthetic aperture radar (UAVSAR) polarimetric SAR (PolSAR), and field data captured the occurrence and the recovery of an undetected dieback that occurred between the summers of 2010, 2011, and 2012 in the Spartina alterniflora marshes of coastal Louisiana. Field measurements recorded the dramatic biomass decrease from 2010 to 2011 and a biomass recovery in 2012 dominated by a decrease of live biomass, and the loss of marsh as part of the dieback event. Based on an established relationship, the near-infrared/red vegetation index (VI) and site-specific measurements delineated a contiguous expanse of marsh dieback encompassing 6649.9ha of 18,292.3ha of S. alterniflora marshes within the study region. PolSAR data were transformed to variables used in biophysical mapping, and of this variable suite, the cross-polarization HV (horizontal send and vertical receive) backscatter was the best single indicator of marsh dieback and recovery. HV backscatter exhibited substantial and significant changes over the dieback and recovery period, tracked measured biomass changes, and significantly correlated with the live/dead biomass ratio. Within the context of regional trends, both HV and VI indicators started higher in pre-dieback marshes and exhibited substantially and statistically higher variability from year to year than that exhibited in the non-dieback marshes. That distinct difference allowed the capturing of the S. alterniflora marsh dieback and recovery; however, these changes were incorporated in a regional trend exhibiting similar but more subtle biomass composition changes.

Heavy metal contamination and ecological risk in Spartina alterniflora marsh in intertidal sediments of Bohai Bay, China

To investigate the effects of Spartina alterniflora on heavy metals pollution of intertidal sediments, sediment cores of a S. alterniflora salt marsh and a mudflat in Bohai Bay, China were analyzed. The results showed that S. alterniflora caused higher total C and P, but lower bulk density and electrical conductivity. The levels of Cd, Cu and Pb were higher in S. alterniflora sediment. Both Cd and Zn were higher than the probable effect level at both sites, indicating their toxicological importance. The geo-accumulation and potential ecological risk indexes revealed higher metal contamination in S. alterniflora sediment. Multivariate analysis implied that anthropogenic activities altered mobility and bioavailability of heavy metals. The percentage of mobile heavy metals was higher in S. alterniflora sediment, indicating improvement of conversion from the immobilized fraction to the mobilized fraction. These findings indicate that S. alterniflora may facilitate accumulation of heavy metals and increase their bioavailability and mobility.

Differences in burrow morphology of crabs between Spartina alterniflora marsh and mangrove habitats

Density and morphology of crab burrows were compared using resin casts between exotic Spartina alterniflora marsh (SPA) and four mangrove habitats (Avicennia marina forest, AM; Kandelia obovata forest, KO; Aegiceras corniculatum forest, AC; Mudflat, MF) at a subtropical mangrove estuary, Yunxiao, China. Five burrow-morphological parameters (density, opening diameter, depth, branching and total burrow volume per m2) were measured. The results showed that there were significant differences in the morphology (depth and volume) of crab burrows between SPA and mangrove habitats. The depth (ANOVA, d.f.=27, F=24.212, P=0.000) and total burrow volume (ANOVA, d.f.=23, F=3.666, P=0.022) per m2 in SPA were significantly lower than those in the other four mangrove habitats. Correlation analysis indicated significant negative correlation between burrow depth and root density. We concluded that over-dense roots and subterranean stems of Spartina alterniflora impeded crabs excavating burrows extensively, both in the vertical and horizontal directions. Consequently, the invasion of Spartina alterniflora weakened the ecosystem engineering effects of mangrove crabs. The burrows in KO and AM had significantly higher burrow complexity than those in the other three habitats. We speculated that mangrove root was the main environmental variable resulting in the varied complexity of crab burrows. Higher root density or lower root density both impede crabs constructing complex burrows, while intermediate root density contributes to the development of higher borrow complexity.

Land cover changes in tidal salt marshes of the Bahía Blanca estuary (Argentina) during the past 40 years

The present work describes land cover changes in the inner section of Canal Principal, in the Bahía Blanca estuary. The study area is subjected to a rising relative sea level, large interanual variations in rainfall, and recent changes human in land use. We used historical aerial photographs, high resolution satellite images, and GIS to quantify changes in land cover for the years 1967, 1996, and 2005. The replacement of Sarcocornia perennis marshes and halophytic shrub-like steppes by mudflats is a recurring pattern through the area. We estimated a total loss to mudflats of 33 and 6% of the area of marshes and steppes, respectively, and it may reflect increased erosion of relict Holocene coastal terraces in response to a rising sea level. Human activities have played a significant role in reshaping coastal landscape, particularly in the harbor area. Fifty percent of the area originally covered by shrub-like steppes and 33% of the Sarcocornia perennis marshes were replaced by human land uses. Major changes correspond to dredged spoil deposition and landfilling. One of the most striking changes observed is the increase of the area covered by Spartina alterniflora marshes. This type of replacement suggests the occurrence of depositional environments that, at least locally, allow bed elevation and vegetation growth. In the harbor area, an enhanced sedimentation may result from maintenance dredging. At the mouth of Maldonado channel, sediment deposition may occur during extraordinary heavy rainfall associated to El Niño.