Impact of dynamic feedbacks between sedimentation,sea-level rise,and biomass production on near-surface marsh stratigraphy and carbon accumulation

Salt marshes accrete both organic and inorganic sediments. Here we present analytical and numerical models of salt marsh sedimentation that, in addition to capturing inorganic processes, explicitly account for above- and belowground organic processes including root growth and decay of organic carbon. The analytical model is used to examine the bias introduced by organic processes into proxy records of sedimentation, namely ¹³⁷Cs and ²¹⁰Pb. We find that accretion rates estimated using ²¹⁰Pb will be less than accretion rates estimated using the ¹³⁷Cs peak in steadily accreting marshes if (1) carbon decay is significant and (2) data for ²¹⁰Pb extend below the ¹³⁷Cs peak. The numerical model expands upon the analytical model by including belowground processes such as compaction and root growth, and by explicitly tracking the evolution of aboveground biomass and its effect on sedimentation rates. Using the numerical model we explore how marsh stratigraphy responds to sediment supply and the rate of sea-level rise. It is calibrated and tested using an extensive data set of both marsh stratigraphy and measurements of vegetation dynamics in a Spartina alterniflora marsh in South Carolina, USA. We find that carbon accumulation in marshes is nonlinearly related to both the supply of inorganic sediment and the rate of sea-level rise; carbon accumulation increases with sea-level rise until sea-level rise reaches a critical rate that drowns the marsh vegetation and halts carbon accumulation. The model predicts that changes in carbon storage resulting from changing sediment supply or sea-level rise are strongly dependent on the background sediment supply: if inorganic sediment supply is reduced in an already sediment poor marsh the storage of organic carbon will increase to a far greater extent than in a sediment-rich marsh, provided that the rate of sea-level rise does not exceed a threshold. These results imply that altering sediment supply to estuaries (e.g., by damming upstream rivers or altering littoral sediment transport) could lead to significant changes in the carbon budgets of coastal salt marshes.     

Seasonal and annual variations in the volumetric sediment balance of a macro-tidal salt marsh

This paper examines the spatial and temporal variability in the volumetric sediment balance of Allen Creek marsh, a macro-tidal salt marsh in the Bay of Fundy. The volumetric balance was determined as the balance of inputs of sediments and organic matter via accretion on the marsh surface and outputs of sedimentary material primarily due to erosion of the marsh margin. Changes in marsh surface elevation were measured at 20 buried plates and 3 modified sediment elevation tables from 1996–2002, and detailed margin surveys were conducted in 1997, 1999 and 2001 using a differential global positioning system. Changes in surface area were calculated using GIS overlay analysis and used in conjunction with accretion and erosion data to derive volumetric estimates of gains and losses of sedimentary material in the marsh system.Currently the volumetric sediment balance at Allen Creek marsh is positive. However the processes of erosion and accretion demonstrate seasonal, annual and spatial variability. Inputs to the system include deposition on the marsh surface from sediment laden waters and from ice rafting of sediments. Sediment is deposited onto the marsh surface year round, even during the winter when vegetation cover is sparse, and the amount of deposition in general is not significantly correlated with the frequency of tidal inundations. Based on the data from 1996 to 2002, the mid and high marsh zones experience mean accretion rates of approximately 1.4 cm year^− 1 whereas accretion rates in the low marsh region are statistically significantly lower (0.8 cm year^− 1). The absolute amount of accretion varies between seasons and from year to year. The main loss to the marsh is through erosion of the marsh margin cliffs which can remove a comparatively large volume of sedimentary material in one mass wasting event and which also decreases the vegetated surface area available for deposition from sediment laden waters. The volume of material removed from the marsh margin almost tripled between 1997 (169 m³) and 2001 (502 m³) following breaching of the side of a tidal creek channel, altering the patterns of margin erosion and deposition in the marsh system. During this time, however, other sheltered areas of the marsh system, such as along the tidal creek banks, showed evidence of new vegetation growth, increasing the amount of vegetated surface area available for deposition.The processes of erosion and deposition on the marsh surface exhibit considerable spatial variability, with different regions of the marsh being more or less sensitive to seasonal variability in the dominant controls influencing sediment deposition and erosion in this system, namely wave activity, vegetation, ice and water depths. A key factor in predicting how a marsh will evolve and respond to a number of different controls, e.g. sea-level rise or reduced sediment supply, is to quantify both accretion of the marsh surface and erosion of the marsh margin, evaluating the marsh system as a volumetric whole. This study demonstrates that a marsh system should be assessed in three dimensions rather than simply as a surface of accumulation. This is particularly important for open coastal marshes exposed to the erosive action of waves.     

Elevation trends and shrink–swell response of wetland soils to flooding and drying

Given the potential for a projected acceleration in sea-level rise to impact wetland sustainability over the next century, a better understanding is needed of climate-related drivers that influence the processes controlling wetland elevation. Changes in local hydrology and groundwater conditions can cause short-term perturbations to marsh elevation trends through shrink–swell of marsh soils. To better understand the magnitude of these perturbations and their impacts on marsh elevation trends, we measured vertical accretion and elevation dynamics in microtidal marshes in Texas and Louisiana during and after the extreme drought conditions that existed there from 1998 to 2000. In a Louisiana marsh, elevation was controlled by subsurface hydrologic fluxes occurring below the root zone but above the 4 m depth (i.e., the base of the surface elevation table benchmark) that were related to regional drought and local meteorological conditions, with marsh elevation tracking water level variations closely. In Texas, a rapid decline in marsh elevation was related to severe drought conditions, which lowered local groundwater levels. Unfragmented marshes experienced smaller water level drawdowns and more rapid marsh elevation recovery than fragmented marshes. It appears that extended drawdowns lead to increased substrate consolidation making it less resilient to respond to future favorable conditions. Overall, changes in water storage lead to rapid and large short-term impacts on marsh elevation that are as much as five times greater than the long-term elevation trend, indicating the importance of long-term, high-resolution elevation data sets to understand the prolonged effects of water deficits on marsh elevation change.     

Contributions of mineral and organic components to tidal freshwater marsh accretion

Vertical accretion in tidal marshes is necessary to prevent submergence due to rising sea levels. Mineral materials may be more important in driving vertical accretion in tidal freshwater marshes, which are found near the heads of estuaries, than has been reported for salt marshes. Accretion rates for tidal freshwater marshes in North America and Europe (n = 76 data points) were compiled from the literature. Simple and multiple linear regression analyses revealed that both organic and mineral accumulations played a role in driving tidal freshwater marsh vertical accretion rates, although a unit mass of organic material contributed ∼4 times more to marsh volume than the same mass input of mineral material. Despite the higher mineral content of tidal freshwater marsh soils, this ability of organic matter to effectively hold water and air in interstitial spaces suggests that organic matter is responsible for 62% of marsh accretion, with the remaining 38% from mineral contributions. The organic material that helps to build marsh elevation is likely a combination of in situ production and organic materials that are deposited in association with mineral sediment particles. Regional differences between tidal freshwater marshes in the importance of organic vs. mineral contributions may reflect differences in sediment availability, climate, tidal range, rates of sea level rise, and local-scale factors such as site elevation and distance to tidal creeks. Differences in the importance of organic and mineral accumulations between tidal freshwater and salt marshes are likely due to a combination of factors, including sediment availability (e.g., proximity to upland sources and estuarine turbidity maxima) and the lability of freshwater vs. salt marsh plant production.     

Spatial and temporal variations in sediment grain size in tidal wetlands, Yangtze Delta: On the role of physical and biotic controls

To examine the spatial and temporal variability of sediment grain size in exposed tidal wetlands with ample sediment supply, we sampled sediments and measured hydrodynamics, accretion/erosion rates, and vegetation characteristics in the Yangtze Delta. Sediment grain size exhibited a landward/upward decreasing trend. This trend is mainly attributed to attenuation of hydrodynamics. A 630-day series of daily surface sediment sampling at a fixed site on an unvegetated intertidal flat revealed significant seasonal and storm-cyclic changes in grain size. This temporal variability was related to alternating accretion/erosion events, with erosion associated with coarser grain size. Such temporal dynamics were not present in vegetation, where sediment remained fine grained throughout the year. In the marsh, vegetation cover enables the trapping of fine-grained sediments in the following ways: (a) adherence of suspended sediments onto plants; (b) deposition of suspended sediments stimulated by attenuation of hydrodynamics through plant obstruction; and (c) prevention of resuspension of fine-grained deposits due to the protection of the plant canopy. The influence of vegetation on sediment grain size was clearly seen when comparing sediment trapped by different vegetation types and seasonal patterns of trapped sediment on different vegetation canopy densities. The relatively high plant biomass of the recently introduced Spartina alterniflora enhanced the trapping effect, whereas plant degradation due to buffalo grazing reduced the trapping effect. We conclude that for exposed tidal wetlands with ample sediment supply such as the Yangtze Delta, the spatial and temporal variability of sediment grain size is governed predominantly by physical controls on the unvegetated flat and predominantly by biophysical interaction of hydrodynamics and vegetation in the salt marsh, rather than by sediment supply.     

Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK

Progressive burial of artificial markers over a 5-year period is used to determine the rate and pattern of vertical accretion within a large backbarrier salt marsh on the UK east coast. Over this period, annual accretion varies spatially from 1 to 8 mm yr⁻¹. The arithmetic mean rate for the whole marsh is 3.9 mm yr⁻¹. Spatial variability in accretion is a joint function of (1) elevation-dependent inundation frequency and (2) progressive sediment removal from water masses advected across channel margins. Accretion is, therefore, inadequately represented by simple averaging of point measurements. Numerical integration of the ‘accretion surface’ results in a spatial average rate of around 3 mm yr⁻¹, well below the arithmetic mean rate.

Short-term sediment trap deployments show that local and long-range meteorological effects, and remobilisation of sediment deposited within tidal creeks, often mask the expected link between tidal height and sedimentation rate. Retention of sediment on plant surfaces is minimal, with direct settling accounting for approximately 95% of total deposition.

Time-extrapolation of weekly sediment trap data, and comparison with the 5-year marker horizon burial, shows that processes associated with ordinary tides can account for long-term accretion over most of the marsh. However, the highest surfaces receive appreciable sediment input only during aperiodic storm events.     

Development of a foraminifera-based transfer function in the Basque marshes, N. Spain: Implications for sea-level studies in the Bay of Biscay

In order to reconstruct former sea level we have developed a foraminifera-based transfer function using three models based on a modern dataset of 59 samples and 23 species obtained from four Basque marshes in Northern Spain. The relationship between observed and foraminifera-predicted elevation illustrated the strong performance of the transfer function (r²_jack ranges from 0.74 to 0.81). These results indicated that precise reconstructions of former sea levels are possible (error ranges from 0.11 to 0.19 m). The transfer function was used to calibrate the foraminiferal assemblages collected from a 50 cm salt marsh core. We placed the foraminifera-based reconstructions into a temporal framework using ¹³⁷Cs, Pb concentrations, and ²¹⁰Pb-derived sediment accumulation rates. The resulting relative sea-level curve is in good agreement with regional tide-gauge data. Both instrumental data and microfossil records suggest a rate of relative sea-level rise of approximately 2 mm yr^− 1 for the 20th century.     

黄河三角洲互花米草整治对潮滩湿地防灾功效和沉积动力的影响

互花米草作为黄河三角洲外来入侵物种, 2010年开始爆发式蔓延, 2020年面积最大时可超6 000 hm²,对潮间带生物多样性造成了严重影响。自2020年起,黄河三角洲自然保护区开始大规模的互花米草整治与盐地碱蓬生态修复工作。互花米草具有较强的消浪、缓流、促淤、固滩能力,短期内、大范围的植被去除,可能导致波流和沉积动力环境的突变。基于现场观测数据和考虑植被作用的浪、流、沙耦合数值模型,研究分析了互花米草整治和盐地碱蓬修复对黄河三角洲湿地动力过程的影响,重点关注风暴作用下黄河三角洲潮滩湿地的防灾减灾能力和沉积效应变化。研究结果表明,互花米草可有效抵御极端风暴潮,最大风暴潮位衰减率可达15 cm/km。互花米草整治将导致湿地防灾减灾能力显著下降,堤前风暴潮峰值可抬升10～30 cm,同时潮间带的泥沙捕集能力大幅下降。受限于植被密度、高度,盐地碱蓬完全修复后湿地衰减风暴潮、捕集泥沙的能力显著低于原互花米草湿地。研究建议,互花米草整治工程应考虑短期内植被去除导致的近岸风暴潮抬升、侵蚀加剧等潜在风险。     

Expected changes in the benthic fauna of Wadden Sea tidal flats as a result of sea-level rise or bottom subsidence

In different parts of the Dutch Wadden Sea, relationships between intertidal level and abundance of marine macrozoobenthos were similar. Numerical densities, biomass and species richness increased from values close to 0 at the high-water level to maximum values around mean-tide level (numbers) or halfway between this level and low-tide level (biomass). Species richness hardly declined below mean-tide level, whereas mean weight per individual continued to increase from high- to low-water level.Biomass was about 45 g ash-free dry weight per m² at its maximum and declined in an approximately linear way to values close to 0 at the high-water level and to about 7 g per m² at the low-water level. These two linear relationships were used to predict biomass changes on intertidal flats of the Wadden Sea at various scenarios of sea-level rise and bottom subsidence. Net sea-level rise is expected to result in increased amounts of intertidal zoobenthos in areas with predominantly high tidal flats, but in declines in lower areas. However, such changes will occur only if sea-level rise proceeds too fast to be compensated by extra sedimentation. Bottom subsidence as a consequence of gas extraction is expected to be too small to cause any measurable change in the benthic fauna.     

Impact of the cord-grass Spartina alterniflora on sedimentary and morphological evolution of tidal salt marshes on the Jiangsu coast, China

1Introduction Asanimportantcomponentofthecoastalsys tem,tidalflatsareformedbytidalactionunderthe conditionofabundantfine grainedsedimentsupply;theybecomeafocusofthestudyonland oceaninter actioninthecoastalzonebecausetheseareasare subjectedtointensehumanac…     

台风作用下的港湾型潮滩沉积过程 以2008年“凤凰”台风对福建省罗源湾的影响为例

通过对"凤凰"台风的现场观测和沉积物样品的分析,结果表明,在台风影响下潮水淹没时间增长,增水达1.1 m;台风期间互花米草盐沼内流速变化较复杂,而且盐沼内部流速大于盐沼边缘的;台风期间盐沼边缘潮周期平均悬沙含量是台风前2 d的7倍;台风期间十分之一波高最大为1.54 m。滩面重复测量结果显示,台风登陆期间整个光滩滩面都发生了侵蚀,盐沼内部有部分地区发生侵蚀,侵蚀深度为4.5~5.5 cm,盐沼边缘的侵蚀深度仅为0.7 cm,侵蚀原因主要是植被在风浪作用下从根部折断,从而带走了滩面的沉积物;随着台风强度的减弱,整个滩面均接受悬沙沉降;台风带来的强降雨是影响滩面沉积物活动的重要因素。     

Recent volumetric changes in salt marsh soils

Salt marsh sediment volume decreases from organic decomposition, compaction of solids, and de-watering, and each of these processes may change with age. Variability in the vertical accretion rate within the upper 2 m was determined by assembling results from concurrent application of the ¹³⁷Cs and ²¹⁰Pb dating techniques used to estimate sediment age since 1963/1964, and 0 to ca 100+ years before present (yBP), respectively. The relationship between ²¹⁰Pb and the ¹³⁷Cs dated accretion rates (Sed₂₁₀ and Sed₁₃₇, respectively) was linear for 45 salt marsh and mangrove environments. Sed₂₁₀ averaged 75% of Sed₁₃₇ suggesting that vertical accretion over the last 100+ years is driven by soil organic matter accumulation, as shown for the pre ¹³⁷Cs dated horizon. The ratio of Sed₂₁₀/Sed₁₃₇ declines with increasing mineral content. A linear multiple regression equation that includes bulk density and Sed₁₃₇ to predict Sed₂₁₀ described 97% of the variance in Sed₂₁₀. Sediments from Connecticut, Delaware and Louisiana coastal environments dated with ¹⁴C indicate a relatively constant sediment accretion rate of 0.13 cm year⁻¹ for 1000–7000 yBP, which occurs within 2 m of today's marsh surface and equals modern sea level rise rates. Soil subsidence is not shown to be distinctly different in these vastly different coastal settings. The major reason why the Sed₁₃₇ measurements indicate higher accretion rates than do the Sed₂₁₀ measurements is because the former apply to younger sediments where the effects of root growth and decomposition are greater than in the latter. The most intense rates of change in soil volume in organic-rich salt marshes sediments is, therefore, neither in deep or old sediments (>4 m; >1000 years), but within the first several hundreds of years after accumulation. The average changes in organic and inorganic constituents downcore are nearly equal for 58 dated sediment cores from the northern Gulf of Mexico. These parallel changes downcore are best described as resulting from compaction, rather than from organic matter decomposition. Thus most of the volumetric changes in these salt marsh sediments occurs in the upper 2 m, and declines quickly with depth. Extrapolation forwards or backwards, using results from the ²¹⁰Pb and the ¹³⁷Cs dating technique appear to be warranted for the types of samples from the environments described here.     

An estuarine fine-particle budget determined from radionuclide tracers

The sedimentary distributions of radiocesium and plutonium have been used to determine patterns of fine-particle accumulation, estimate net sediment fluxes from different sources, and develop a fine-particle budget for the Hudson-Raritan estuary. It is proposed that the rates and patterns of fineparticle accumulation reflect a sediment surface in dynamic equilibrium with the wave and current regimes. Rates of accumulation in most estuarine areas appear to equal the rate to sea-level rise or land subsidence. Localized areas, which have not yet attained or are presently out of equilibrium, serve as fine particle traps.     

Marsh vertical accretion via vegetative growth

Coastal marshes accrete vertically in response to sea-level rise and subsidence. Inadequate accretion and subsequent conversion of coastal marshes to open water generally is attributed to inadequate mineral sedimentation because mineral sedimentation is widely assumed to control accretion. Using ¹³⁷Cs dating to determine vertical accretion, mineral sedimentation, and organic matter accumulation, we found that accretion varied with organic accumulation rather than mineral sedimentation across a wide range of conditions in coastal Louisiana, including stable marshes where soil was 80% mineral matter. These results agreed with previous research, but no mechanism had been proposed to explain accretion via vegetative growth. In an exploratory greenhouse experiment, we found that flooding stimulated root growth above the marsh surface. These results indicated the need for additional work to determine if flooding controls accretion in some marshes by stimulating root growth on the marsh surface, rather than by mineral accumulation on the marsh surface. Restoration or management that focus on mineral sedimentation may be ineffective where a relationship between accretion and mineral sedimentation is assumed rather than tested.     

Winter processes on northern salt marshes: Evaluating the impact of in-situ peat compaction due to ice loading, Wells, ME

New England salt marshes are dominated by the supratidal high marsh grass, Spartina patens. This grass forms a nearly planar surface, which makes it highly vulnerable to the predicted regime of accelerated sea-level rise (SLR). If the high marsh cannot keep pace with rising sea level it will be transformed to intertidal environments, leading to unusually rapid coastal evolution. Winter processes such as ice-loading of surface peat may degrade the marsh surface. Large volumes of snow and ice compress peat, resulting in shallow compaction and a net loss of elevation in some areas.On the Webhannet marsh in Maine, a simulated ice compaction experiment indicates that thick ice can compress marsh peat (46 cm simulated ‘ice’ produced 6.9 ± 0.3 mm of compaction; 24 cm ‘ice’ produced 3.0 ± 0.8 mm). Experimental data suggest that ice thicknesses greater than 10 cm depress the marsh surface by 2 mm for each cm of total ice thickness. However, surface elevations rebounded to near-control levels within 2 weeks of the removal of simulated ‘ice’ from the surface of the marsh. Normal winter ice accumulations on New England marshes, therefore, do not appear to be sufficient to permanently compact marsh surface peat and lead to loss in marsh surface elevation.     

潮间带盐沼植物黏附悬浮颗粒物的差异性研究

为了研究潮间带盐沼植物黏附悬浮颗粒物的差异性,在长江口选择了三种盐沼植物群落对它们黏附的颗粒物质量进行测定,结果表明:(1)植物群落距潮沟或光滩越近,生长位置的滩面高程越低,则黏附颗粒物越多,而在盐沼前缘单位滩地面积上植物黏附颗粒物的质量以1%~3%/m(单位水平距离)的速率从水体悬浮颗粒物含量相对较高的盐沼外缘光滩或潮沟向盐沼内部减小;(2)植物黏附的颗粒物量在垂向上从上到下急剧增大,通常在靠近滩面5~10 cm的部分植物的黏附量占植物黏附总量的三分之一以上;(3)相邻群落单位滩地面积的互花米草[(220.6±172.7)g/m2]的总黏附量明显多于芦苇[(64.9±38.1)g/m2]和海三棱藨草[(45.2±31.7)g/m2],而按单位生物量来说单位滩地面积上三种盐沼植物黏附的颗粒物以海三棱藨草最多[(150.5±134.8)g/kg],芦苇最少[(28.8±22.8)g/kg],互花米草介于两者之间[(57.5±32.9)g/kg];(4)海三棱藨草的黏附量在季节上差异性明显,秋初(9月)是春末(5月)的6倍,在冬季该植物消失,其黏附颗粒物的功能也消失。造成盐沼植物黏附悬浮颗粒物差异的根本原因是生物量、悬浮颗粒物含量和淹没条件(淹没的深度、时间、频率)的不同。     

Temporal variation of accumulation rates on a natural salt marsh in the 20th century — The impact of sea level rise and increased inundation frequency

Salt marshes are potentially threatened by sea level rise if sediment supply is unable to balance the rising sea. A rapid sea level rise is one of the pronounced effects of global warming and global sea level is at present rising at an elevated rate of about 3.4 mm y^? 1 on average. This increasing rate of sea level rise should make it possible to study the effect of rapidly rising sea level on salt marsh accumulation. However, such an understanding is generally hampered by lack of available data with sufficient precision. Here we present a high-precision dataset based on detailed radiometric measurements of ¹³⁷Cs in 10 sediment cores retrieved at a natural and unmanaged micro tidal salt marsh. Two distinct ¹³⁷Cs-peaks were found in all cores, one peak corresponding to the 1963-maximum caused by testing of nuclear weapons in the atmosphere and the other to the Chernobyl accident in 1986. Salt marsh accretion has generally kept pace with sea level rise since 1963 but comparison of the accumulation rates of minerogenic material in the period 1963–1986 and 1986–2003 revealed a slight decrease in accumulation with time in spite of an observed increase in inundation frequency. The observed decrease in sediment deposition is significant and gives reason for concern as it may be the first sign of a sedimentation deficiency which could be threatening this and other salt marshes in the case of a rapidly rising sea level. Our work demonstrates that the assumption of a constant relationship between salt marsh inundation and sediment deposition is not necessarily valid, even for a salt marsh that receives most of its allocthonous sediment from the adjacent sea. The apparent decrease in sediment deposition indicates that the basic assumption of sufficient sediment supply used in contemporary models dealing with salt marsh accretion is most probably not valid in the present case study and it may well be that this is also the case for many other salt marshes, especially if sea level continues to rise rapidly as indicated by some climate change scenarios.     

An equilibrium profile model for retreating marsh shorelines in southeast Louisiana

Louisiana's coastal marshes are experiencing the highest wetland loss rates in the U.S., in part due to subsidence-driven relative sea-level rise. These marshes are also vulnerable to the erosive power of wave attack: 1) on the marsh edge adjacent to open-water bodies, and 2) after the marsh platform is submerged. Marsh shorelines in Barataria Bay, Breton Sound, and the active Balize delta of southeastern Louisiana were examined in areas where the subaerial marsh platform had disappeared since 1932. Vibracore transects of marsh and adjacent bay surface sediments (to 2 m depth) were analyzed using geotechnical, stratigraphic, and radiochemical (137-Cs and 210-Pb) methods, and the subaerial-to-subaqueous transition of the marsh was mapped for elevation using standard stadia rod transit and fathometer measurements. Results indicate that marsh edge erosion of the platform takes place subaqueously until water depths of 1.5 m are reached. This is observed even in interior pond regions, but the shoreface elevation profiles are a function of fetch: exposed open bay sites display greater incision (depth and rate) of the marsh platform than protected interior bay or pond sites. Core stratigraphy reveals that the outer part of the subaqueous platform switches from erosional to depositional as retreat proceeds, covering the incised marsh deposits unconformably with estuarine shelly muds. 137-Cs and excess 210-Pb activity indicates that these muds are deposited within a few decades of subaerial marsh loss. The consistency of the cross-shore profile results suggests that a single profile of equilibrium can approximate the morphology of eroding marsh edges in southeast Louisiana: platform stratigraphy and resistance to erosion have a limited effect on profile shape. This equilibrium profile and remote sensing images of shoreline change are used to estimate the sediment yield to adjacent estuarine areas by this process. On average, 1.5 m³ of sediment are yielded per m shoreline length annually from both Barataria Bay and Breton Sound. Due to the highly organic nature of the eroded sediment (30%), this supply of organic-rich material could significantly impact estuarine productivity and hypoxia on the Louisiana continental shelf.     

Managed realignment as compensation for the loss of intertidal mudflat: A short term solution to a long term problem?

Intertidal mudflats are critical to the functional ecology of estuaries yet large areas are being lost as a result of land claim, erosion and coastal squeeze. This study examines whether managed realignment (at Paull Holme Strays, Humber estuary) can realistically achieve compensation for the loss of intertidal mudflat in the long term. Typical estuarine species quickly colonised the site with the total number of species recorded from the site as a whole being almost equal to that in the reference area within one year. Comparable biomass between the two areas was achieved after 2 years. However, organism abundance remains an order of magnitude lower within the realignment site compared to outside. Community structure within the realignment has changed from one characterised by terrestrial/freshwater organisms and early colonising species to one composed of typically estuarine species. However, the developing benthic communities only represent those typical of the estuary in areas of low elevation and high inundation frequency. Rapid accretion has favoured saltmarsh colonisation in much of the realignment site and this is expected to increase as accretion proceeds with invertebrate colonisation being inhibited by increasing elevation. Hence, realignment to restore intertidal mudflats can only be a short term solution in sites of high tidal elevation and in a dynamic and turbid estuary with high natural accretion rates, such as the Humber.     

Accretion of a New England (U.S.A.) Salt Marsh in Response to Inlet Migration, Storms, and Sea-level Rise

Sediment accumulation rates were determined at several sites throughout Nauset Marsh (Massachusetts, U.S.A.), a back-barrier lagoonal system, using feldspar marker horizons to evaluate short-term rates (1 to 2 year scales) and radiometric techniques to estimate rates over longer time scales (¹³⁷Cs,²¹⁰Pb,¹⁴C). The barrier spit fronting theSpartina-dominated study site has a complex geomorphic history of inlet migration and overwash events. This study evaluates sediment accumulation rates in relation to inlet migration, storm events and sea-level rise. The marker horizon technique displayed strong temporal and spatial variability in response to storm events and proximity to the inlet. Sediment accumulation rates of up to 24 mm year⁻¹were recorded in the immediate vicinity of the inlet during a period that included several major coastal storms, while feldspar sites remote from the inlet had substantially lower rates (trace accumulation to 2·2 mm year⁻¹). During storm-free periods, accumulation rates did not exceed 6·7 mm year⁻¹, but remained quite variable among sites. Based on¹³⁷Cs (3·8 to 4·5 mm year⁻¹) and²¹⁰Pb (2·6 to 4·2 mm year⁻¹) radiometric techniques, integrating sediment accumulation over decadal time scales, the marsh appeared to be keeping pace with the relative rate of sea-level rise from 1921 to 1993 of 2·4 mm year⁻¹. At one site, the²¹⁰Pb-based sedimentation rate and rate of relative sea-level rise were nearly similar and peat rhizome analysis revealed thatDistichlis spicatarecently replaced this onceS. patenssite, suggesting that this portion of Nauset Marsh may be getting wetter, thus representing an initial response to wetland submergence. Horizon markers are useful in evaluating the role of short-term events, such as storms or inlet migration, influencing marsh sedimentation processes. However, sampling methods that integrate marsh sedimentation over decadal time scales are preferable when evaluating a systems response to sea-level rise.