Search

The determination of rates and stocks of carbon storage in salt marshes, as well as their protection, require that we know where they and their boundaries are. Marsh boundaries are conventionally mapped through recognition of plant communities using aerial photography or satellite imagery. We examined the possibility of substituting the use of 1 m resolution LiDAR-derived digital elevation models (DEMs) and tidal elevations to establish salt marsh upper boundaries on the New Brunswick coasts of the Gulf of St. Lawrence and the Bay of Fundy, testing this method at tidal ranges from ≤2 to ≥4 m. LiDAR-mapped marsh boundaries were verified with high spatial resolution satellite imagery and a subset through field mapping of the upland marsh edge based upon vegetation and soil characteristics, recording the edge location and elevation with a Differential Geographic Positioning System. The results show that the use of high-resolution LiDAR and tidal elevation data can successfully map the upper boundary of salt marshes without the need to first map plant species. The marsh map area resulting from our mapping was ~30% lower than that in the province’s aerial-photograph-based maps. However, the difference was not primarily due to the location of the upper marsh boundaries but more so because of the exclusion of mudflats and large creeks (features that are not valued as carbon sinks) using the LiDAR method that are often mapped as marsh areas in the provincial maps. Despite some minor limitations, the development of DEMs derived from LiDAR can be applied to update and correct existing salt marsh maps along extensive sections of coastlines in less time than required to manually trace from imagery. This is vital information for governments and NGOs seeking to conserve these environments, as accurate mapping of the location and area of these ecosystems is a necessary basis for conservation prioritization indices.

We measured total carbon stocks of three marshes: Two formed in association with a developing spit along the Gulf of St. Lawrence coast of New Brunswick, Canada, and another with a lagoon on the coast of Maine, USA. Overall, 46 cores and 157 depth recordings were collected to determine depth of the marsh deposits. Total marsh soil volume was estimated by interpolation. In all marshes soil depth varied in a predictable pattern based upon marsh developmental history. In spit marshes deposit age and thickness increased towards the oldest portion of the spit. In the lagoonal marsh, soil depth was greatest in the center and declined towards both the upland and seaward margins. This same pattern held on axes perpendicular to the primary, age axis of the spit marshes. In each marsh C density did not significantly vary with depth so that marsh depth was an acceptable estimator of C stock, and therefore driven by the geomorphic context of the marshes we studied. There were major differences in C stock estimates produced using GIS interpolation, average C contained in all marsh cores, or cores along a single transect. Our study demonstrates that assuming a soil depth of just 0.5 or 1 m can substantially under- or overestimate marsh carbon stocks and the value of that stock on a carbon market.

The data presented here includes a table of soils measurements taken at high resolution depth intervals (5 cm) for three salt marshes, two along the New Brunswick coast of Canada and one on the southern coast of Maine, USA. The data includes a table which includes the bulk density, percent organic matter, percent organic carbon, carbon stock, and rhizome dominance (if identifiable) at 5 cm depth intervals for each soil core. Shapefiles are also included which indicates the GPS position of acquired cores and sites where marsh depth was measured but no material was recovered. These shapefiles also include marsh peat depth and estimates of carbon stock for each point. For further information and interpretation of the included data please see the companion research article titled “The Importance of Geomorphic Context for Estimating the Carbon Stock of Salt Marshes” [1] .

Salt marshes are highly effective carbon (C) sinks and bury more C per square meter annually than any other ecosystem. Reclamation and anthropogenic impacts, however, have resulted in extensive losses of salt marshes. Carbon credits can be generated and sold by restoring marshes, but only if C sequestration and net reductions in greenhouse gases (GHG) are reliably quantified. Restored marshes, however, may exhibit different patterns of GHG emissions than natural marshes and it is possible that they could temporarily become sources of N2O even in the usually N-limited estuarine environment. Research on short-term GHG flux following salt marsh restoration is limited to studies of two restored marshes which examined GHG flux more than six months after the return of tidal flooding. Here we report on a laboratory experiment in which soil cores collected from a drained agricultural marsh on the St. Lawrence Estuary were flooded with estuary water. Gas flux measurements immediately after flooding revealed small increases in N2O and CH4, but a large decline in CO2 yielding, from a climatic perspective, a net cooling effect over the observation period. In addition to restoring the land’s capacity to sequester C once a marsh develops, returning tidal flooding thus appears to have the added benefit of stemming large ongoing C losses. With more than 400 km2 of undeveloped dykeland, Eastern Canada is well positioned to restore large sections of marsh and contribute to reducing atmospheric CO2 concentrations.

Byers, S.E. and Chmura, G.L., 2014. Observations on shallow subsurface hydrology at Bay of Fundy macrotidal salt marshes. Hydrology is a major driver of salt marsh functions and the ecological services they support. Sediment saturation, and thus subsurface hydrology, affects salt marsh vegetation productivity, zonation, and survival. Subsurface hydrology influences rates of subsidence; concentrations of nutrients, organic matter, and oxygen; fluxes of the greenhouse gases methane and nitrous oxide; and sediment toxicity. Therefore, baseline knowledge of subsurface hydrology is crucial in predicting and managing change in salt marshes. Previous research on subsurface hydrology has mostly been conducted in organogenic, microtidal marshes that do not provide suitable models for minerogenic, macrotidal marshes such as those in the Bay of Fundy. In this study, we examine drivers of subsurface hydrology and compare two pairs of reference and recovering marshes with tidal amplitudes of 6 to 12 m. Measur...

Sea level rise and accompanying coastal squeeze will drive changes in marsh spatial configuration and connectivity affecting how fishes utilize the marsh. We investigate how sea level rise and coastal squeeze affect the spatial distribution, configuration, and connectivity within tidal marshes in a landscape context. Using spatial analyses, we compare two outputs from a process model: one with constant vertical accretion and the other with a more optimistic prediction that rates of vertical accretion will equal rates of sea level rise. We use landscape metrics on the model outputs to evaluate the landscape scale changes that occur in a selected marsh at the National Estuarine Research Reserve (NERR) site in Wells, Maine, USA. We found that moderate sea level rise would benefit marshes by expanding their area and maintaining habitat spatial complexity. However, rapid sea level coupled with stressors like coastal squeeze is likely to result in smaller and more dispersed marshes with simpler patch edges and less connected patches. Such features indicate lesser potential for food and refugia.

Tidal salt marshes provide a range of ecosystem services. The most recently recognized is their provision of highly effective sinks for atmospheric carbon dioxide, a characteristic they share with mangroves swamps which largely replace salt marshes in the intertidal zones of tropical regions. Efforts are emerging to use salt marsh preservation or restoration in carbon offset programs, similar to the REDD initiative for tropical forests, but a number of issues first must be addressed to determine if a site meets the requirements of these programs. As intertidal systems, both salt marshes and mangrove swamps are threatened by increasing rates of sea level rise, and it will be essential to determine their sustainability, thus meeting the requirement of permanence of the carbon sink. In many areas the vegetation responsible for marsh soil accretion may not survive increased flooding periods, resulting in submergence of the marsh in its present location or inability to restore a marsh at its previous elevation. However, the marsh and its carbon sink, may survive if allowed to migrate inland. Thus assessment of permanence requires a determination if inland migration will be hindered by barriers such as high slopes or development, i.e., if the ecosystem is in a coastal squeeze. Presently, the only technology that can provide elevation models at the required vertical accuracy is Lidar, which also is extremely valuable for assessing vulnerability of coastal communities to flooding. Yet, Lidar data is not available for all coastlines, even those in the developed world. To effectively plan for the future of our coastlines requires Lidar coverage on all sensitive coastlines and its free availability for impact assessments.

Torio, D.D. and Chmura, G.L., 2013. Assessing coastal squeeze of tidal wetlands. Journal of Coastal Research, 29(5), 1049–1061. Coconut Creek (Florida), ISSN 0749-0208. As sea level rise accelerates and land development intensifies along coastlines, tidal wetlands will become increasingly threatened by coastal squeeze. Barriers that protect inland areas from rising sea level prevent or reduce tidal flows, and impermeable surfaces prevent wetland migration to the adjacent uplands. As vegetation succumbs to submergence by rising sea levels on the seaward edge of a wetland, those wetlands prevented from inland migration will decrease in area, if not disappear completely. Tools to identify locations where coastal squeeze is likely to occur are needed for coastal management. We have developed a ‘‘Coastal Squeeze Index’’ that can be used to assess the potential of coastal squeeze along the borders of a single wetland and to rank the threats faced by multiple wetlands. The index is based on surrounding topography and impervious surfaces derived from light detection and ranging and advanced spaceborne thermal emission and reflection radiometry imagery, respectively, and uses a fuzzy logic approach. We assume that coastal squeeze varies continuously over the coastal landscape and tested several fuzzy logic functions before assigning a continuous weight, from 0 to 1, corresponding to the influence of slope and impervious surfaces on coastal squeeze. We then combined the ranked variables to produce a map of coastal squeeze as a continuous index. Using this index, we compare the present and future threat of coastal squeeze to marshes in Wells and Portland, Maine, in the United States and Kouchibouguac National Park in New Brunswick, Canada.

Differentiation of boreal wetlands in the palaeo record is required for studies of isostacy and sea level change, landscape change, and climate dynamics. To develop palynomorph signatures and calibrate soil C–N ratios for boreal wetlands we sampled low and high elevation tidal marshes, fens and bogs along eastern James Bay, Canada. A discriminant analysis model showed that a small subset of palynomorph taxa can effectively discriminate between the four wetland types. Cyperaceae, Poaceae, and Potamogeton–Triglochin-type are important to the classification of high marshes. Bryidae spores and pollen of Myrica gale, Cyperaceae, and Poaceae are indicative of fen. Moss spores alone, primarily from Sphagnum, but also Bryidae type spores, derived from brown mosses are the indicators of bog. The use of palynomorph assemblages to discriminate among wetland types is not dependent upon the localized presence of a plant source — as it would be using macrofossil analysis. C/N of bog soils can be clearly distinguished from fen and tidal marshes and can be used as a pollen-independent proxy to indicate fen to bog wetland succession.

We monitored salinity and temperature in pools at different elevations in two marshes on the Bay of Fundy to assess variability in pool environments and how climate change might affect these ecosystems. Water temperatures reached a maximum of 36.1°C, and ice covered pools in the winter. Ice lifting out of ponds in the spring scoured bottoms. Salinity ranged from 4 to 41. Environmental variability was mainly driven by weather. Variability in pool temperature declined with decreasing elevation. Regular tidal flooding at lower elevations moderated both temperatures and salinity. Variability in pool salinity was greatest in the middle marsh region, since proximity to groundwater also moderated salinity at high elevations. Projections of milder winters should reduce formation of ice in pools, resulting in decreased scour, and possibly shallowing of pools. This, accompanied by projected increases in extreme weather, will increase salinity variability, thus increasing stress to organisms in an already s...

The supply of nitrogen to ecosystems has surpassed the Earth's Planetary Boundary and its input to the marine environment has caused estuarine waters to become eutrophic. Excessive supply of nitrogen to salt marshes has been associated with shifts in species' distribution and production, as well as marsh degradation and loss. Our study of salt marshes in agriculturally intensive watersheds shows that coastal eutrophication can have an additional impact. We measured gas fluxes from marsh soils and verified emissions of nitrous oxide (N2O) in nitrogen-loaded marshes while the reference marsh was a sink for this gas. Salt marsh soils are extremely efficient carbon sinks, but emissions of N2O, a greenhouse gas 298 times more potent than CO2, reduces the value of the carbon sink, and in some marshes, may counterbalance any value of stored carbon towards mitigation of climate change. Although more research is merited on the nitrogen transformations and carbon storage in eutrophic marshes, the possibility of significant N2O emissions should be considered when evaluating the market value of carbon in salt marshes subject to high levels of nitrogen loading.

Salt marshes are highly effective carbon (C) sinks and have higher rates of soil C burial (per square meter) than terrestrial ecosystems. Marsh reclamation and anthropogenic impacts, however, have resulted in extensive losses of salt marshes. Restoration of marshes drained and “reclaimed” for agriculture (referred to in Canada as dykelands) and degraded marshes can generate C credits, but only if C burial is reliably quantified. To date, studies reporting on C burial rates have been limited primarily to restored marshes which are more than 10 years old. Here we report on a study which assessed C burial six years after the return of tidal flooding to a section of dykeland in Aulac, New Brunswick on Canada’s Bay of Fundy. The C burial rate in the restored marsh averaged 1 329 g C m-2 yr-1, more than five times the rate reported for a nearby mature marsh. Carbon density in the recovering marsh was relatively consistent with depth and although salt marsh cordgrass (Spartina alterniflora) became established in 2012, the bulk of the C in the new marsh deposit is assumed to be allochthonous. Financial constraints are a barrier to marsh restoration projects and C markets could provide a considerable source of funding for restoration work in the future. For marsh restoration projects to be recognized in C crediting systems, however, it must also be demonstrated that the allochthonous C would not otherwise have been sequestered; the potential for this is discussed.

Reports of shifts in rates of aggradation in a salt marsh deposit at Amherst Point, Nova Scotia have been linked with fluctuations in eustatic sea level, crustal subsidence and, possibly, changes in tidal amplitude. However, evidence for the most recent fluctuation is an artifact of early Acadian land reclamation that occurred in the seventeenth century at Amherst Point. Acadian settlers had constructed dikes to prevent regular tidal flooding, but probably allowed occasional flooding of the surface by tidal waters, thus maintaining the salt marsh signature of the deposit. Statistical analysis of the pollen assemblage from the uppermost deposit interpreted as a regressive sequence aligns critical depths with pollen assemblages retrieved from modern soils of diked and drained marshes. Inspection of historical aerial photographs confirms that the dikes persisted on the site until at least 1939. Interpretations of earlier fluctuations recorded in deposits below this regressive sequence remain valid. It is possible that the Bay of Fundy experienced a regressive period just prior to Acadian diking, but future research on unexamined salt marsh deposits here or elsewhere must include an exhaustive search of historical documents to avoid misinterpretation of anthropogenic modifications.

We assess the status of channel networks and pools of two tidal salt marshes recovering from more than a century of agricultural reclamation on the Bay of Fundy, Canada. A process of largely unmanaged restoration occurred at these sites since abandonment of agricultural activities during the first half of the twentieth century. Each recovering marsh was compared to a reference marsh that was never drained or ditched. We field mapped channel networks at all marshes and used aerial photographs to map the pre-abandonment channel network at one of the sites. The recovering marshes have hybrid channel networks that feature highly variable channel morphologies, loss of original channels, and incorporation of drainage ditches. Although channel networks in recovering marshes integrate agricultural ditches, the recovering marsh networks may not be substantially increased in length or density. Our aerial photograph analysis shows that channel density at one of the recovering marshes is comparable to the pre-abandonment density, but with reduced sinuosity. Field mapping of permanent tidal pools on the lower Bay marshes revealed that pools cover 13% of the recovering marsh, compared to ∼5% of the reference marsh. This study demonstrates that these essential marsh features can be regained through restoration or simple abandonment of drainage infrastructure.

We measured the amount of arsenic, chromium, copper, lead, nickel, vanadium, and zinc accumulated over a five-year period from 1997 to 2002 in surface sediments of seven salt marshes along the New Brunswick coast of the Bay of Fundy, Canada. Study sites extended from outer to inner Bay, spanning a gradient in tidal range (6–12 m) and mean sediment deposition rate (0.27–1.76 cm yr−1). In each study site, metal concentrations were measured in low and high marsh areas. Concentrations of chromium, nickel, and zinc appear to be within their natural range, while arsenic, lead, and vanadium are enriched in some sites. Calculated sediment metal loadings rates showed variability among marsh sites that closely followed sediment deposition patterns, suggesting sediment deposition rate is the driving factor of short-term metal accumulation in Fundy marshes. The value of salt marshes as a sink for metals may be enhanced by high sedimentation rates.

Tens of thousands of hectares of salt marsh have formed along eastern Canada's macro-tidal Bay of Fundy. Most of these marshes have been diked and converted to agricultural land since the arrival of Acadian settlers in the 17th century. Although there are currently no controlled salt marsh restoration efforts underway in the Bay of Fundy, there are isolated examples of salt marshes that were restored as a result of unplanned dike breaches. In an effort to develop pollen modern analogues of local dikeland and salt marsh land uses, surface soil samples were collected from John Lusby Marsh, a dikeland to which tidal flooding was restored in the late 1940s, and two nearby diked marshes, one left fallow and one under active agricultural management. Using discriminant analysis (DA) and principal components analysis (PCA) we successfully separated pollen assemblages representing three grassland land use types: salt marsh, farmed dikeland and fallow dikeland. The farmed dikeland land use was further divided into grazed or manured land use and hayed or unmanured land use based on the presence and relative proportions of coprophilous fungal spores such as Sordaria-type, Tripterospora-type, Cercophora-type, Chaetomium-type, Podospora-type and Sporormiella-type. This work marks the first attempt at incorporating dung fungal spores into a palynological analysis using controlled conditions and demonstrates how the inclusion of dung fungal spore abundances can relay information on past land uses. This brings a new interpretive dimension to palynological analyses without requiring any additional sample collection or preparation.

Modifications of the hydrology of the Florida Everglades began in the 1880s. Impacts due to changes in the hydrological regime, as well as nutrient enrichment are clear in the northern Everglades. Although these changes were more subtle in the southern Everglades, altered hydroperiods and plant communities are targeted for restoration. Identification of restoration goals requires knowledge of past conditions, but detailed vegetation surveys prior to the pre-drainage era are lacking. Palynological studies can help fill this void. Interpretations based upon pollen and plant spores alone are limited by low pollen production of the dominant marsh species and low plant diversity in the ecosystem. In some regions the occurrence of other microfossils present in palynological preparations has been used to help interpret hydrological regimes and nutrient status and here we explore that potential for paleoecological records from the Everglades. In this study we report the distribution of various microfossils found within pollen zones identified in sediments cored from three tree islands in Shark River Slough, Everglades National Park. Using published modern pollen analogues we classified paleovegetation zones as slough, sawgrass marsh, tree island or tree island edge and documented the abundances of microfossils of algae, fungi, cyanobacteria and testate amoebae. Tree island zones, indicative of relatively short hydroperiods, were characterized by highest abundances of fungal material, testate amoebae and the base of the deposits, by the cyanobacterium, Gloeotrichia. Much of the fungal material appears to be associated with plant roots, and the mycorrhizal Glomus is locally abundant. The taxonomic affinity of the testate amoebae has not been determined, but some were arcellaceous forms that included charcoal particles in their tests. Gloeotrichia also is found in marsh and slough deposits. Its occurrence in the latter probably signals minimal water depth. Botryococcus and Zynemataceae zygospores are found in all deposits, but are less abundant in the tree island deposits. Both groups are probably part of the periphyton flora, but Botryococcus has not been recorded in phycological studies of the Everglades. Observation of these algae in the seasonally dry Everglades is consistent with other studies where they have been reported to represent shallow, slow moving waters, but inconsistent with the assumptions of meso- or eutrophic conditions commonly made in other studies. Our observation of the alga Botryoccocus and the meroplanktonic cyanobacterium Gloeotrichia in the historically oligotrophic Everglades challenges their status as indicators of nutrient enrichment.

We examined the distribution of vegetation and pollen in surface soils of three salt marshes on the outer Bay of Fundy to assess the value of pollen in paleoenvironmental reconstructions. We examined the relationship of pollen to vegetation using linear regression at two scales: vegetation zones dominated by graminoids and forb patches within these zones. Of the dominant graminoids, only pollen of Cyperaceae was a dependable indicator of source vegetation. In contrast, at least seven common forbs were significantly represented by their pollen at one or both scales. Because forbs are more abundant in northern marshes, increases in forb pollen abundance in buried sediments of southern marshes could signal colder climate regimes. Other studies have shown Plantago maritima L. and Ligusticum scoti cum L. to be associated with distinct ranges in elevation, and Triglochin maritima L. with soil anoxia. Thus, shifts in forb pollen abundance may indicate changes in tidal flooding frequency and sea levels.Key words: Potentilla anserina, Glaux maritima, Spergularia canadensis, wetland, palynology.

We used137Cs-dating to determine vertical accretion rates of 15 salt marshes on the Bay of Fundy, the Gulf of St. Lawrence, and the Atlantic coast of Nova Scotia. Accretion rates are compared to a number of factors assumed to influence vertical marsh accretion: rates of relative sea-level rise, climatic parameters (average daily temperatures and degree days) and latitude (related to insolation and day length), sediment characteristics (organic matter inventory, bulk, mineral, and organic matter density), distance of the core site from the nearest source of tidal waters, and the tidal range. Uniques to our study is a consideration of climatic parameters and latitude, which should influence organic matter production, and thus vertical accretion rates. Significant predictors of accretion rates (in order of importance) were found to be organic matter inventory, distance from a creek, and range of mean tides. Contrary to conclusions from previous studies, we found that accretion rates decreased with increasing tidal range, probably because we considered a wider span of tidal ranges, from micro- to macrotidal. Although four marshes with low organic matter inventories also show a deficit in accretion with respect to relative sea-level rise, organic matter is not limiting in two-thirds of the marshes studied, despite shorter growing seasons.

Transformation of agricultural land to natural terrestrial vegetation has been suggested as a means to increase soil carbon storage. However, the capacity for carbon storage in terrestrial soils is limited as compared to soils of tidal salt marshes, the original vegetation of many coastal agricultural lands. In a number of countries, tidal salt marshes have been ''reclaimed,'' that is drained and diked to prevent tidal flooding and create suitable conditions for dry land agriculture. In this study we examine spatial and temporal patterns of carbon accumulation in tidal salt marshes of the Bay of Fundy and estimate the carbon storage potential of the bay's extensive area of reclaimed marsh. Rates of carbon accumulation vary from the upper to the outer Bay, over which there is a gradient of decreasing tidal range and suspended sediment supply. In the outer bay, high-marsh densities are highest (0.042 ± 0.009 g C cm ˇ3 ), but carbon accumulation rates over the past 30 years are lowest (76 g C m ˇ2 yr ˇ1 ). The reverse pattern occurs in the upper bay where carbon densities in the high-marsh environment are lowest (0.036 ± 0.002 g C cm ˇ3 ), but carbon accumulation rates over the past 30 years may be as high (184 g C m ˇ2 yr ˇ1 ). Compared to other ecosystems, the rates of carbon accumulation presented in this study were similar over timescales of years to centuries. Increases in relative sea level (over time) and suspended sediment supply (across the bay) positively affect the marsh soil accumulation rate and the rate of carbon sequestration. Parameters such as %C are not useful predictors of a marsh's potential for carbon sequestration. Soil carbon densities of functioning marshes and reclaimed marsh soils are similar, but marsh soils have a storage capacity that increases with rising sea level, while agricultural soils, such as those in reclaimed marshes, have a fixed (or possibly decreasing in reclaimed marshes) volume over time.

We examine rates of salt marsh accumulation in three marshes of the outer Bay of Fundy. At each marsh we selected a site in the high marsh with similar vegetation, and thus similar elevation. Accretion rates were estimated by 137Cs, 210Pb, and pollen stratigraphy to estimate rates of change over periods of 30, 100, and ~170 years, respectively. These rates are compared with records from the two closest tide gauges (Saint John, New Brunswick, and Eastport, Maine) to assess the balance of recent marsh accretion and sea-level change. Averaged marsh accretion rates have ranged from 1.3 ± 0.4 to 4.4 ± 1.6 mm·year–1 over the last two centuries. Recent rates are similar to the rate of sea-level change recorded at Eastport, Maine, suggesting that they are in step with recent sea-level change but very sensitive to short-term variation in relative sea level. Based on the pollen stratigraphy in the marsh sediments, the marsh accretion rate was higher during the late 18th to early 19th century. Higher rates probably were due to local increases in relative sea level as a result of neotectonic activity and may have been enhanced by increased sediment deposition through ice rafting.

In this study we examine the nature of microforaminifera test linings found in palynological preparations and demonstrate the potential of these palynomorphs for paleoenvironmental interpretation. Sediment samples from four marsh salinity zones of the Barataria Basin, Mississippi Delta, were processed by conventional palynological techniques which included HC1 and acetolysis treatments. Taxonomic identification was not attempted, but it is assumed that Rotalidae forams, Miliammina fusca and Ammonia beccari, are important constituents of the test lining assemblages. Multi‐chambered forms were classed by morphotype, size, number of chambers, spacing of chambers, and condition. The absence of a relationship between size and chamber number within each morphotype implies the presence of multiple species populations. There is also no relationship between foram concentration and salinity. Using stepwise discriminant analysis, six classes were selected which enable successful discrimination among the fou...

We assessed the impact of nutrient additions on greenhouse gas fluxes using dark static chambers in a microtidal and a macrotidal marsh along the coast of New Brunswick, Canada approximately monthly over a year. Both were experimentally fertilized for six years with varying levels of N and P. For unfertilized, N and NPK treatments, average yearly CO2 emissions (which represent only respiration) at the microtidal marsh (13, 19, and 28 mmoles CO2 m(-2) hr(-1), respectively) were higher than at the macrotidal marsh (12, 15, and 19 mmoles m(-2) hr(-1), respectively, with a flux under the additional high N/low P treatment of 21 mmoles m(-2) hr(-1)). Response of CH4 to fertilization was more variable. At the macrotidal marsh average yearly fluxes were 1.29, 1.26, and 0.77 μmol CH4 m(-2) hr(-1) with control, N, and NPK treatments, respectively and 1.21 μmol m(-2) hr(-1) under high N/low P treatment. At the microtidal marsh CH4 fluxes were 0.23, 0.16, and -0.24 μmol CH4 m(-2) hr(-1) in control, N, and NPK and treatments, respectively. Fertilization changed soils from sinks to sources of N2O. Average yearly N2O fluxes at the macrotidal marsh were -0.07, 0.08, and 1.70, μmol N2O m(-2) hr(-1) in control, N, NPK and treatments, respectively and 0.35 μmol m(-2) hr(-1) under high N/low P treatment. For the control, N, and NPK treatments at the microtidal marsh N2O fluxes were -0.05, 0.30, and 0.52 μmol N2O m(-2) hr(-1), respectively. Our results indicate that N2O fluxes are likely to vary with the source of pollutant nutrients but emissions will be lower if N is not accompanied by an adequate supply of P (e.g., atmospheric deposition vs sewage or agricultural runoff). With chronic fertilization the global warming potential of the increased N2O emissions may be enough to offset the global cooling potential of the C sequestered by salt marshes.

Cesium-137 activity and ash contents were measured in shallow cores from marshes on a salinity gradient in the Mississippi Delta plain. Average accretion rates calculated from 137 Cs activity profiles are similar to those previously reported. Our study, however, demonstrates anomalies associated with storm deposits which may help in interpretation of activity profiles in other regions. We also find trends in activity profiles related to the salinity gradient which had not previously been described in subaerial sediments. Our results suggest that there is a major difference between 137 Cs accumulation patterns in systems restricted to relatively fine-grained sediments and those with coarse-grained sediments. To understand and properly interpret patterns of 137 Cs activity for sedimentary accretion rates in such systems it is imperative that relative concentrations of clastic material be determined and storm deposits be recognized.

Chmura, G.L., Costanza, R. and Kosters, E., 1992. Modelling coastal marsh stability in response to sea level rise: a case study in coastal Louisiana, USA. Ecol. Modelling, 64: 47-64. In some regions coastal marsh stability is threatened by high rates of sea level rise. The deltaic plain of the Mississippi River is a natural laboratory for the study of marsh stability under conditions of rising sea level because it has been experiencing high rates of local submergence which cause relatively high rates of apparent sea level rise. We constructed a dynamic simulation model to study the relationship of accretion to three components of relative sea level rise: compaction, eustatic rise and submergence. The model is then used to project marsh stability under various future scenarios of sea level rise as well as enhancement of sediment supplies and marsh accretion. The model was calibrated to a 14C-dated sediment deposit which provides a long-term record of sediment accretion. Results indicate that an equilibrium between relative sea level and accretion rates can be attained, but that in this region of coastal Louisiana only the most optimistic assumptions yield coastal marshes that are stable in the long term.

[1] Wetlands represent the largest component of the terrestrial biological carbon pool and thus play an important role in global carbon cycles. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon-storing ecosystems such as tidal saline wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°S in the Indian Ocean to 55.5°N in the northeastern Atlantic. The average soil carbon density of mangrove swamps (0.055 ± 0.004 g cm−3) is significantly higher than the salt marsh average (0.039 ± 0.003 g cm−3). Soil carbon density in mangrove swamps and Spartina patens marshes declines with increasing average annual temperature, probably due to increased decay rates at higher temperatures. In contrast, carbon sequestration rates were not significantly different between mangrove swamps and salt marshes. Variability in sediment accumulation rates within marshes is a major control of carbon sequestration rates masking any relationship with climatic parameters. Globally, these combined wetlands store at least 44.6 Tg C yr−1 and probably more, as detailed areal inventories are not available for salt marshes in China and South America. Much attention has been given to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In contrast to peatlands, salt marshes and mangroves release negligible amounts of greenhouse gases and store more carbon per unit area.

The northeastern Everglades contain a vast (originally ca. 800 km2), densecluster of red bay and dahoon holly tree islands on deep peats (≥1.5 m). Unlike most other parts of the Everglades, the specific locations and origins of these tree islands appear unrelated to local mounds or depressions on the buried limestone bedrock or sand substrates, and instead solely reflect peatland accretionary and successional processes. A very broad shallow peat swale has maintained deeper-water marsh vegetation with abundant waterlilys and much less sawgrass in this region when compared to adjacent Everglades terrain to the west. Protection from fire and promotion of floating peat-island formation are some apparent ecological linkages between tree islands and waterlily marshes.

Conversion of wetlands by drainage for agriculture or other anthropogenic activities could have a negative or positive feedback to global warming (GWF). We suggest that a major predictor of the GWF is salinity of the wetland soil (a proxy for available sulfate), a factor often ignored in other studies. We assess the radiative balance of two northern salt marshes with average soil salinities >20 ppt, but with high (macro-) and low (micro-) tidal amplitudes. The flux of greenhouse gases from soils at the end of the growing season averaged 485 ± 253 mg m −2 h −1 , 13 ± 30 µ gm −2 h −1 , and 19 ± 58 µ gm −2 h −1 in the microtidal marsh and 398 ± 201 mg m −2 h −1 ,2 ± 26 µ gm −2 h −1 , and 35 ± 77 µ gm −2 h −1 in the macrotidal marsh for CO2 ,N 2O, and CH4, respectively. High rates of C sequestration mean that loss of these marshes would have a radiative balance of −981CO2 eq. m −2 yr −1 in the microtidal and −567CO2 eq. m −2 yr −1 in the macrotidal marsh.

Fluxes of methane (CH4) and carbon dioxide (CO2) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH4 fluxes ranged from 0.2 mg m−2 d−1 to 11.0 mg m−2 d−1, with an overall average of 1.6 mg m−2 d−1. Flux of CH4 was inversely correlated (r2=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO4 2− in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH4 emission. Mean soil flux of CO2 ranged from 0.3 g m−2 d−1 to 3.7 g m−2 d−1, with an overall average of 2.5 g m−2 d−1; it was correlated with aboveground biomass (positive, r2=0.38, p = 0.001) and position of the water table (negative, r2 = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO2 flux. There were high variations in gas flux within the six plant communities. The sequences were CH4: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO2: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH4 and CO2.

Organic carbon-rich sediments from the surface of fresh, intermediate, brackish and salt marshes of coastal Louisiana were sampled and analyzed for their 13C content. The average ∂13C from all sites within each wetland type was-27.8‰,-22.1‰,-16.9‰, and-16.2‰, for fresh, intermediate, brackish and salt marshes, respectively. Means from the fresh, intermediate and brackish marshes were significantly different at the 0.01 level. A mixing model using measurements of standing crop and ∂13C of plant carbon was applied to estimate the contribution of each species to the sedimentary carbon at four of the marsh sites. Sedimentary ∂13C values generally reflected that of the dominant species present at each site. Brackish and salt marsh samples, however, showed a negative shift of ∂13C with respect to whole plant carbon. We interpret these depeleted ∂13C values to be the result of more extensive organic matter decomposition and selective preservation of 13C-depleted refractory components in sediments from saline sites. The results of this study suggest that ∂13C composition of sedimentary carbon may offer a valuable tool for distinguishing subtle changes in paleohydrology of wetlands resulting from relative sea level changes.