Emission of gaseous carbon dioxide from salt-marsh sediments and its relation to other carbon losses

Rates of CO₂ emission from bare salt-marsh sediments in areas of short and tall formSpartina alterniflora were measured monthly for 1 yr. Maximum emission rates, as high as 325 ml CO₂m^?2h^?1, were observed during summer months, while minimum rates, 10.2 ml CO₂ m^?2h^?1, were observed during the winter. An exponential function of inverse soil temperature explained most of the seasonal variability, but other factors are involved in regulating CO₂ emissions as demonstrated by rates that were higher in spring than in late summer at equivalent temperatures. Annual CO₂ emissions from bare sediments were 27.3 and 18.6 mol C m^?2 yr^?1 in communities of short and tallS. alterniflora, respectively. It was estimated that losses of dissolved inorganic carbon from the turnover of pore water, up to 14.6 mol C m^?2 yr^?1 at the creek bank (tall,S. alterniflora) site, and diffusion of CO₂ from the root system ofS. alterniflora through the culms, 12.3 to 16.2 mol C m^?2 yr^?1, could also be important pathways of carbon loss from marsh sediments. If the internal flux of CO₂ from the root system through the culm is refixed within the leaves, then the observed rate of 9.8 μI CO₂ min^?1 cm^?2 of culm cross sectional area appears to make a small but significant contribution to total photosynthesis.     

A review of environmental and genetic factors that affect height inSpartina alterniflora loisel. (Salt marsh cord grass)

The salt marsh cord grass,Spartina alterniflora Loisel., occurs in markedly distinct short and tall growth forms. Both forms have the same number of chromosomes, although polyploidy is well established in the genus. Previous studies have shown that height is primarily affected by nitrate availability and environmental stresses such as increased soluble salt concentrations. These studies have shown that, within a marsh, height differences cannot be attributed to genetic, chromosomal differences or electrophoretic banding. However, more subtle genetic differences may be involved. Other studies suggest that between marshes,S. alterniflora, in response to a latitudinal gradient on the Atlantic coast, has evolved ecotypes differing in height, color and flowering period. This review paper points out that plant height inS. alterniflora may be determined by a combination of environmental and genetic factors.     

若尔盖高原沼泽湿地CH4排放研究

若尔盖高原沼泽湿地海拔 3400 m，面积 4038km²，是我国面积最大的高原沼泽湿地分布区。2001年 5～9月的非冰冻期，其主要沼泽类型木里苔草沼泽的CH₄排放通量范围是 0.51～ 8.20 m g/（m ²· h），平均值为 2.87 m g/（m ²· h）；乌拉苔草沼泽CH₄排放通量范围是 0.36～10.04 m g/（m ²· h），平均值为 4.51 m g/（m ²· h）。在空间分布上，不同沼泽类型之间CH₄排放通量具有一定的差异。在季节变化上，没有明显的排放高峰。根据代表性观测点的CH₄平均排放量、日数和沼泽总面积推算，非冰冻期若尔盖高原沼泽湿地CH₄的排放量为 0.052Tg/a。     

Aboveground Biomass Patterns of Dominant Spartina Species and Their Relationship with Selected Abiotic Variables in Argentinean SW Atlantic Marshes

Salt marsh zonation patterns generate different abiotic and biotic conditions that can accentuate species inherent differences in primary production and biomass. In South West Atlantic marshes, there are two Spartina species: Spartina alterniflora in the low intertidal and Spartina densiflora in the high intertidal. These two species are generally found in all marshes but with different dominance: In some marshes, the S. densiflora zone occupies higher extents, and in others, the S. alterniflora zone is the one that prevails. We found through field sampling that, in six studied marshes, there is greater S. densiflora live and total (i.e., dead+live) aboveground biomass (g m^?2) in the marshes dominated by S. densiflora than in the ones dominated by S. alterniflora. Spartina alterniflora had similar aboveground biomass in the six marshes, regardless of the dominance of each species. When comparing the two Spartina species within each marsh, S. densiflora had greater live and total biomass in the marshes it dominates. In the marshes dominated by S. alterniflora, both species had similar live and total biomass. In all marshes, there was greater dead S. densiflora biomass. A multivariate analysis using selected abiotic factors (i.e., salinity, latitude, and tidal amplitude) showed that S. alterniflora aboveground biomass patterns are mainly correlated with salinity, while S. densiflora live biomass is mainly correlated with salinity and latitude, dead biomass with salinity and tidal amplitude, and total biomass with salinity alone. We conclude that in S. densiflora dominated marshes, the main processes of that species zone (i.e., nutrient accumulation) will be accentuated because of its higher biomass. We also conclude that climatic conditions, in combination with specific Spartina biotic and ambient abiotic parameters, can affect marsh ecological functions.     

Predation by the black-clawed mud crab,<Emphasis Type="Italic">Panopeus herbstii</Emphasis>, in Mid-Atlantic salt marshes: Further evidence for top-down control of marsh grass production

Although top-down control of plant growth has been shown in a variety of marine systems, it is widely thought to be unimportant in salt marshes. Recent caging experiments in Virginia and Georgia have challenged this notion and shown that the dominant marsh grazer (the periwinkle,Littoraria irrorata) not only suppresses plant growth, but can denude marsh substrate at high densities. In these same marshes, our field observations suggest that the black-clawed mud crab,Panopeus herbstii, is an abundant and potentially important top-down determinant of periwinkle density. No studies have quantitatively examinedPanopeus distribution or trophic interactions in marsh systems, and its potential impacts on community structure remained unexplored. We investigated distribution and feeding habits ofPanopeus in eight salt marshes along the Mid-Atlantic seashore (Delaware-North Carolina). We found that mud crabs were abundant in tall (4–82 ind m^?2), intermediate (0–15 ind m^?2), and short-form (0–5 ind m^?2)Spartina alterniflora zones in all marshes and that crab densities were negatively correlated with tidal height and positively correlated with bivalve density. Excavation of crab lairs r?utinely produced shells of plant-grazing snails (up to 36 lair^?1) and bivalves. Lab experiments confirmed that mud crabs readily consume these abundant marsh molluscs. To experimentally examine potential community effects of observed predation patterns, we manipulated crab and periwinkle densities in a 1-mo field experiment. Results showed thatPanopeus can suppress gastropod abundance and that predation rates increase with increasing snail density. In turn, crab suppression of snail density reduces grazing intensity on salt marsh cordgrass, suggesting presence of a trophic cascade. These results indicate that this previously under-appreciated consumer is an important and indirect determinant of community structure and contribute to a growing body of evidence challenging the long-standing notion that consumers play a minor role in regulating marsh plant growth.     

Wetland Loss Patterns and Inundation-Productivity Relationships Prognosticate Widespread Salt Marsh Loss for Southern New England

Tidal salt marsh is a key defense against, yet is especially vulnerable to, the effects of accelerated sea level rise. To determine whether salt marshes in southern New England will be stable given increasing inundation over the coming decades, we examined current loss patterns, inundation-productivity feedbacks, and sustaining processes. A multi-decadal analysis of salt marsh aerial extent using historic imagery and maps revealed that salt marsh vegetation loss is both widespread and accelerating, with vegetation loss rates over the past four decades summing to 17.3 %. Landward retreat of the marsh edge, widening and headward expansion of tidal channel networks, loss of marsh islands, and the development and enlargement of interior depressions found on the marsh platform contributed to vegetation loss. Inundation due to sea level rise is strongly suggested as a primary driver: vegetation loss rates were significantly negatively correlated with marsh elevation (r ²?=?0.96; p?=?0.0038), with marshes situated below mean high water (MHW) experiencing greater declines than marshes sitting well above MHW. Growth experiments with Spartina alterniflora, the Atlantic salt marsh ecosystem dominant, across a range of elevations and inundation regimes further established that greater inundation decreases belowground biomass production of S. alterniflora and, thus, negatively impacts organic matter accumulation. These results suggest that southern New England salt marshes are already experiencing deterioration and fragmentation in response to sea level rise and may not be stable as tidal flooding increases in the future.     

Factors controlling aboveground Spartina alterniflora (Smooth cordgrass) tissue element composition and production in different-age barrier island marshes

Aboveground production and tissue element composition of Spartina alterniflora were compared in bareier island marshes of different age off the Eastern Shore of Virginia. The marshes were also characterized by physical and chemical parameters of the substrate. The results suggest that sediment nutrient stock do not directly control the spatial pattern of element content or production of S. alterniflora between these marshes. Elevated salinity likely limits the nitrogen uptake capability of S. alterniflora in the high marsh, which, in turn, controls leaf tissue nitrogen content of plants within individual sites. Low substrate redox potential may control the spatial pattern of nitrogen uptake between the different-age marsh sites, loading to more favorable growing conditions at the low stations of the young marsh sites where values of tissue nitrogen and production are highest. Tissue phosphorus did not differ between, or within the marsh sites. The result of a fertilization experiment suggest that nitrogen, and not phosphorus, is the primary limiting nutrient in this sytem. This indicates that nutrient limitation and other stresses work in conjunction to control tissue element content and macrophyte production at these marsh sites. Spatial variability of factors that control leaf tissue nitrogen and production is likely related to topography and grain size of an individual marsh, which is a function of marsh age. Most studies in different-age marshes have compared transplanted marshes to older, natural marshes. This work is one of few studies comparing developing and mature natural, marshes on barrier islands.     

Productivity of cordgrass,Spartina alterniflora,estimated from live standing crops,mortality, and leaf shedding in a Virginia salt marsh

Net annual productivity of tall and medium form cordgrass,Spartina alterniflora, was estimated by a new clip sampling method in a sloping foreshore salt marsh at Wallops Island, Virginia. This method measured live standing crops only, to avoid problems of measuring dead biomass inherent in other methods. Losses from live standing crops by shoot mortality and by leaf shedding were estimated from these measurements and added separately to production of live tillers and of live culms. This allowed quantification of various components of production.Spartina tillering in different zones of the marsh produced 62 to 211 g dry weight per m² per yr. Tiller mortality removed 37 to 106 g per m² per yr from live standing crops. Culms produced 348 to 1,132 g per m² before flowering and die-back. Culm mortality removed 28 to 246 g per m² before flowering. Leaf shedding removed an additional 83 g per m² in tall formSpartina. Altogether, net annual productivity These estimates are much higher than previous estimates of productivity and standing crops inSpartina marshes nearby.     

Growth dynamics of cordgrass,Spartina alterniflora loisel., on control and sewage sludge fertilized plots in a Georgia salt marsh

Seasonal plant growth dynamics were followed for a year in undisturbed plots of tall and short formSpartina alterniflora Loisel. and in plots of short formS. alterniflora which were enriched with sewage sludge at a rate of 100 g dry sludge m^?2wk^?1, corresponding to a nitrogen enrichment of 2 g N m^?2wk^?1. Monthly determinations of aboveground live and dead biomass, density of live stems, the ratio of number of young shoots to total number of shoots, and belowground mass of macro-organic matter to a depth of 30 cm were made for each area. Sludge fertilization increased the live biomass of the short formS. alterniflora by up to 150% of the control live biomass, but had little effect on the dead biomass, stem density, or proportion of young shoots. There was a trend of increased amount of belowground macro-organic matter in fertilized compared to control plots during the last 6 months of the study. In all areas, there was a marked decrease in the proportion of young shoots from winter to early summer, followed by a rapid increase in the percent of young shoots from late summer to fall. Sampling of plots 7 and 20 months after termination of sludge enrichment showed higher plant biomass and % N content in surface soils, but no difference in N content of live plant tissue, in fertilized compared to unfertilized marsh. After 20 months, about half of the sludge nitrogen remaining in the soils of the fertilized plots had disappeared.     

Vegetation Dynamics in Rhode Island Salt Marshes During a Period of Accelerating Sea Level Rise and Extreme Sea Level Events

Sea level rise is a major stressor on many salt marshes, and its impacts include creek widening, ponding, vegetation dieback, and drowning. Marsh vegetation changes have been associated with sea level rise across southern New England, but most of these studies pre-date the current period of rapidly accelerating sea level rise coupled with episodic events of extreme increases in water levels. Here, we combine data from two salt marsh monitoring and assessment programs in Rhode Island that were designed to assess marsh responses to sea level rise and use these data to document temporal and spatial patterns in marsh vegetation during the current period of extreme water level increases. Vegetation monitoring at two Narragansett Bay salt marshes confirms the ongoing decline of the salt meadow species Spartina patens during this period as it becomes replaced by Spartina alterniflora. Bare ground resulting from vegetation dieback was significantly related to mean high water levels and led to the rapid conversion of mixed Spartina assemblages to S. alterniflora monocultures. A broader spatial assessment of RI marshes shows that S. alterniflora dominance increases at lower elevation marshes toward the mouth of Narraganset Bay. Our data provide additional evidence that S. patens continues to decline in southern New England marshes and show that losses can accelerate during periods of extreme high water levels. Unless adaptive management actions are taken, we predict that marshes throughout RI will continue to lose salt meadow habitat and eventually resemble lower elevation marshes that are already dominated by S. alterniflora monocultures.     

Relationships of soil,plant, and microbial characteristics in silt-clay and sand,tall-formSpartina alterniflora marshes

The relationships between soil texture, plant growth, and anaerobic microbial activity in two tall-formSpartina alterniflora marshes on Sapelo Island, Georgia, were compared. The soil of one marsh was composed of typical silt-clay-sized particles; the soil of the other marsh consisted of >90% sand-sized particles. The two soils supported similar biomasses ofS. alterniflora, however, plants were taller and more robust in the silt-clay-soil than in the sand soil. Total microbial adenosine triphosphate concentrations in the silt-clay and sand soils averaged 5.71 and 1.64 μg per cm³, respectively. Seawater slurries of both soils exhibited potential for microbial sulfate reduction, methanogenesis, and glucose fermentation; rates for the processes averaged 2.03 and 0.33 nmol S-cm³ per h; 1.20 and 0.87 μmol CH₄ per cm³ per h; and 0.04 and 0.12 per min (rate constant) for the sand and silt-clay soils, respectively.     

Ecological Impacts of Macroalgal Blooms on Salt Marsh Communities

Despite excessive growth of macroalgae in estuarine systems, little research has been done to examine the impacts of increased algal biomass that drifts into nearby salt marshes and accumulates on intertidal flats. The accumulation of macroalgal mats and subsequent decomposition-related releases of limiting nutrients may potentially alter marsh communities and impact multiple trophic levels. We conducted a 2-year in situ study, as well as laboratory mesocosm experiments, to determine the fate of these nutrients and any bottom-up impacts from the blooms on the dominant salt marsh plant (Spartina alterniflora) and herbivores. Mesocosm results showed that macroalgal decomposition had a positive impact on sediment nitrogen concentrations, as well as S. alterniflora growth rates. In contrast, our in situ results suggested that S. alterniflora growth was hindered by the presence of macroalgal mats. From our results, we suggest that macroalgal accumulation and subsequent release of nitrogen during decomposition may be beneficial in nitrogen limited areas. However, as marshes are becoming increasingly eutrophic, releasing lower marsh plants from nitrogen limitation, this accumulation of macroalgal biomass may hinder S. alterniflora growth through smothering and breakage of culms. As macroalgal blooms are predicted to intensify with rising temperatures and increased eutrophication, the ecological impacts associated with these changes need to be continuously monitored in order to preserve these fragile ecosystems.     

The influence of morphology in determining the decomposition of two salt marsh macrophytes

Annual decomposition rates of Spartina alterniflora height forms and Juncus roemerianus were determined in situ in three North Carolina salt marshes using the litter bag method. The decomposition of Spartina was significantly influenced by size, i.e., height form, with the taller plants which had greater amounts of stem tissue, being more resistant to decay. Instantaneous decay rates for short and medium Spartina were not significantly different at any site, but they were both significantly greater than that of the tall form at two of the three study sites. Juncus decomposed more slowly than Spartina during the first 8 months, but had decomposed as completely as all three height forms of Spartina at two of the study sites by the end of the 13-month study period. Constant submergence appeared to inhibit decomposition since there was twice as much undecomposed plant material remaining in bags placed in tidal creeks as in those on the marsh surface.     

Role of Spartina alterniflora on sediment dynamics of coastal salt marshes-case study from central Jiangsu and middle Fujian coasts

Coastal salt marshes represent an important coastal wetland system. In order to protect coastlines from erosion and rapid increase in accumulation rate, Spartina alterniflora (S. alterniflora) was introduced into the Chinese coast. Two study areas (Wanggang and Quanzhou Bay) were selected that represent the plain type and embayment type of the coastal salt marshes. In situ measurements show that the tidal current velocities are stronger on the intertidal mudflat without S. alterniflora than that with S. alterniflora, and the velocity above the canopy surface is larger than that in the salt marsh canopy. The existence of S. alterniflora also influences the velocity structure above the bare flat during ebb tide. With the decrease in current flow velocity when seawater enters into the S. alterniflora marsh, suspended sediments are largely entrapped on the marsh surface, leading to increase in sedimentation rates and change in physical evolution processes of the coastal salt marshes. The highly developed root systemof S. alterniflora induces sediment mixing and exchange between subsurface sediment strata and affects the vertical sediment distribution remarkably. The sedimentation rate of S. alterniflora marsh at the Wanggang area is much higher than the relative sea level rise rate, where rapid progradation of theWanggang saltmarshes that is protecting the coast from sea erosion is observed.     

The potential role of roots and rhizomes in structuring salt-marsh benthic communities

The density of the Carolina marsh clam,Polymesoda caroliniana (Bosc), was determined in three adjacent tidal marsh communities which differed only in plant species composition. Clam density was inversely related to the density (biomass) of plant roots and rhizomes in sediments and directly related to density of plant stems (numbers). Clam abundance was not related to the basal area of plant stems. Each plant community contained clams of various ages from juvenile to adult indicating continued recruitment and survival. These data suggest thatP. caroliniana is most abundant inJuncus roemerianus marshes because there are fewer roots and rhizomes (mean of 2.5 kg m^?2) to hamper burrwing as compared toSpartina alterniflora andcynosuroides (5.1 and 6.3 kg m^?2, respectively) dominated marshes. Salinity, floding frequency, distance from flooding water, and sediment type were essentially constant among the three plant communities. Root/rhizome density should be collected along with other environmental parameters during studies of benthic organisms on marshes because it potentially limits the occurrence or abundance of some species.     

Exchange of microbial biomass between aSpartina alterniflora marsh and the adjacent tidal creek

Salt marsh systems of the southeastern United States are characterized by extensive stands ofSpartina alterniflora. These marshes may influence the concentrations of material suspended in flooding and receding tidal waters. The ability of aSpartina alterniflora-dominated marsh to influence the concentration of suspended microbial biomass was investigated through the use of a 142-m long flume. The flume extended through stands of tall-, medium-, and short-heightSpartina. Water passing through the tallSpartina lost a considerable portion of microbial biomass. Initial samples from medium-heightSpartina were collected from water that had already passed through the tall grass. These samples contained 20 to 70% less microbial biomass than did water entering the tallSpartina. Calculations of mass transport suggest that the tallSpartina zone of marsh acts as a sink for microbial biomass while the short-heightSpartina tends to export biomass (to the tallSpartina zone). The marsh as a whole acts as a sink for microbial biomass. Transport estimates from 32 individual tide cycles were modeled to obtain an annual estimate of transport. As a consequence of high variability among individual transport estimates, no annual transport estimate could be distingushed from a net-zero transport.     

Occurrence ofSpartina macrodetritus in Bay of Fundy waters

Suspended macrodetritus (>600 μm) from extensiveSpartina alterniflora marshes is widespread in the Cumberland Basin region of the macrotidal upper Bay of Fundy. The average concentration is about 5 mg C per m³ and most is found in the lower two-thirds of the water column. The total amount in the Basin at high tide is approximately 8 metric tonnes of organic carbon which is less than 1% of the annual net aerial primary production of the fringingS. alterniflora marshes. The high visibility ofSpartina detritus in Cumberland Basin is caused by the extreme tides (average tidal range about 11 m) which export marsh vegetation and distribute it widely about the estuary, and by the pronounced turbidity which has been shown to limit phytoplankton growth.     

The relationship of smooth cordgrass (Spartina alterniflora) to tidal datums: A review

An analysis of data relatingSpartina alterniflora Loisel. to tidal elevations along the Atlantic and Gulf coasts demonstrated that although this species is primarily confined to the intertidal zone, its elevational limits. of occurrence do not correspond to a consistent elevation relative to a tidal datum in all marsh locations. The variation in the vertical distribution of this species reported among marsh studies was attributed primarily to differences in mean tide range (MTR). A positive correlation between MTR and elevational growth range (r=0.91) demonstrated that theSpartina alterniflora zone expands with increasing tidal amplitude. Differences in MTR among marsh locations accounted for 70 and 68% of the statistical variation in the upper and lower limits, respectively, ofS. alterniflora growth. Among marshes of similar tidal amplitudes, the upper limit of occurrence ofS. alterniflora in northern marshes was significantly lower than that in marshes at lower latitudes. These results, in combination with regional differences in plant species distribution across the upper intertidal zone, suggested that some of the variation in the upper limit was due to latitudinal differences in growth conditions and/or differences in interspecific competition. Local and regional differences in other factors such as salinity, nutrients, or physical disturbance may have also contributed to the variation in the limits of growth relative to a tidal plane within and among marshes.     

Aboveground and belowground productivity ofSpartina alterniflora (Smooth Cordgrass) in natural and created Louisiana salt marshes

In Louisiana, salt marshes are being created in an effort to offset the large loss of such habitat that has occurred over the last 50 yr. Primary productivity is an important function and indicator of success for salt marsh creation and restoration projects. The aim of this study was to determine whether the aboveground and belowground productivity of the dominant salt marsh grassSpartina alterniflora in created marshes in southwest Louisiana began to approximate productivity levels in natural marshes, over time. Net annual aboveground primary productivity (NAPP) was measured by a harvest technique, while the ingrowth core method was used to estimate net annual belowground primary productivity (NBPP). NAPP levels were similar to those found in other, Louisiana salt marshes, while NBPP levels were similar to or higher than the reported range forS. alterniflora studied along the Atlantic and Gulf of Mexico coasts. NAPP tended to decrease as the created marshes aged, but the levels in the oldest, 19 year old, created marsh were still well above values measured in the, natural marshes. It was estimated that it would take 35 yr after marsh creation for NAPP in the created marshes to become equivalent to that in natural marshes. NBPP in the created marshes became equivalent to levels found in the natural marshes after 6–8 yr, but then belowground production increased with marsh age, reaching an asymptote that surpassed natural marsh levels. Equivalency in primary productivity has not been reached in these marshes. Elevation also affected productivity, as higher elevational sites with greater topographic heterogeneity had significantly lower aboveground and belowground biomass levels than those with elevations closer to mean sea level. This underscores the need to construct marshes so that their mean elevation and degree of topographic heterogeneity are similar to natural marshes.