Salt Marsh Tidal Channel Morphometry: Applications for Wetland Creation and Restoration

The morphometry of tidal channels in a back-barrier salt marsh in New Jersey was investigated. Characteristics of the tidal channel drainage network plan form (order, bifurcation ratio, length, sinuosity) and cross section channel form (width:depth ratio, hydraulic geometry) are compared with data from other studies. Drainage patterns follow Horton’s Law of Stream Numbers and Law of Stream Lengths. Mitigation sites should be designed in accordance with these laws. The degree to which site-specific substrate, vegetation, and flow conditions constrain or facilitate the development of sinuous, meandering channel reaches should be estimated to provide information for the design of tidal channel geometries and dimensions that will accommodate predicted discharges. Drainage networks in created and restored wetlands should reflect the spatial distribution of width and depth properties (width:depth ratios, cross-sectional areas, longitudinal slopes, hydraulic geometry) found in similar natural systems. Reproducing these characteristics will lessen the practice of oversizing channels. Hydraulic geometry relationships can facilitate the sizing of channels at Atlantic coast salt marsh mitigation sites. Recommendations are given to promote the development of drainage networks that function like the coastal back-barrier Avalon/Stone Harbor marsh in New Jersey.     

Physical Evolution of Restored Breached Levee Salt Marshes in the San Francisco Bay Estuary

Since 1972 over 940 ha (2,300 ac) of leveed former salt marsh sites around San Francisco Bay have been restored to tidal action, purposely or through natural processes. The evolution of these sites can inform predictions of rates of marshplain evolution and establishment of tidal channel systems. A review of the history of 15 re‐flooded sites ranging in size from 18 to 220 ha (45 to 550 ac) and in age from 2 to 29 years indicates that marshplain vegetation with more than 50% cover was established at nine of the sites within 4 to 20 years. The remaining six sites aged 2 to approximately 20 years continue to be less than 50% vegetated. The evolution of these sites is consistent with the following simple conceptual model of the physical evolution of restored tidal marshes in subsided breached sites. Initially, deposition of estuarine sediment builds up mudflats that allow vegetation establishment once elevations are high enough for vegetation to survive. Sites that are initially lower in the tidal frame take longer to vegetate than those that are initially higher. Three factors appear to retard the time frame for vegetation establishment: limited estuarine suspended sediment supply, erosion of deposited estuarine muds by internally generated wind waves, and restricted tidal exchange. These factors affect evolution more significantly in larger sites. The comparatively short time frame for vegetation colonization and marshplain evolution experienced in earlier, smaller, and/or less subsided breached levee restorations may not necessarily be replicable by simple levee breaching on larger subsided restoration sites now being planned. Our review of the 15 sites also indicates that the formation of tidal channels within the marshes is greatly dependent on whether and how high the site was filled before breaching. Filled sites at high intertidal elevations (above approximately 0.3 m below mean higher high water) can vegetate quickly but after several decades may show little development of tidal channels.     

Restoration of the Sieperda Tidal Marsh in the Scheldt Estuary, The Netherlands

Because of land reclamation, reinforcement of dikes, and the deepening of shipping channels, large areas of tidal marshes have been removed or eroded from the Scheldt estuary during the last two centuries. Tidal wetland restoration contributes toward compensating this loss of habitat. Not all restoration projects are meticulously planned, however; some are forced by nature. During a severe storm in 1990, a dike was breached in the brackish part of the Scheldt estuary and returned tidal influence to the Sieperda polder. In the 10 years since the dike breach, the former polder has changed into a brackish tidal marsh. Here we report on the geomorphologic and ecological developments that have taken place in the marsh. Tidal intrusion into the former polder turned crop fields into mudflats and changed pastures into salty marsh vegetation. The digging of a new creek improved marsh hydrology and enhanced tidal intrusion further into the marsh. Macrofauna typical of estuarine mudflats established rapidly in the developing marsh. Vegetation succession took place rapidly. Within 5 years, large areas of mudflats became covered with marsh vegetation. Birds characteristic of salt marshes were observed breeding or seen foraging in the marsh. The number of wading birds declined as areas of mudflat became overgrown. It is demonstrated that tidal flow is the engine to tidal marsh restoration. Tidal influence caused geomorphologic changes, which directed ecological developments in the former polder.     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

The influence of tidal marshes on upland groundwater discharge to estuaries

We investigated subsurface hydrology in two fringing tidal marshes and in underlying aquifers in the coastal plain of Virginia. Vertical distributions of hydraulic conductivity, hydraulic head and salinity were measured in each marsh and a nearby subtidal sediment. Discharge of hillslope groundwater into the base of the marshes and subtidal sediment was calculated using Darcy's law. In the marshes, fluxes of pore water across the sediment surface were measured or estimated by water balance methods. The vertical distribution of salt in shoreline sediments was modeled to assess transport and mixing conditions at depth. Hydraulic gradients were upward beneath shoreline sediments; indicating that groundwater was passing through marsh and subtidal deposits before reaching the estuary. Calculated discharge (6 to 10 liters per meter of shoreline per day) was small relative to fluxes of pore water across the marsh surface at those sites; even where discharge was maximal (at the upland border) it was 10 to 50 times less than infiltration into marsh soils. Pore water turnover in our marshes was therefore dominated by exchange with estuarine surface water. In contrast, new interstitial water entering subtidal sediments appeared to be primarily groundwater, discharged from below. The presence of fringing tidal marshes delayed transport and increased mixing of groundwater and solute as it traveled towards the estuaries. Soil-contact times of discharged groundwater were up to 100% longer in marshes than in subtidal shoreline sediments. Measured and modeled salinity profiles indicated that, prior to export to estuaries, the solutes of groundwater, marsh pore water and estuarine surface water were more thoroughly mixed in marsh soils compared to subtidal shoreline sediments. These findings suggest that transport of reactive solutes in groundwater may be strongly influenced by shoreline type. Longer soil-contact times in marshes provide greater opportunity for immobilization of excess nutrients by plants, microbes and by adsorption on sediment. Also, the greater dispersive mixing of groundwater and pore water in marshes should lead to increased availability of labile, dissolved organic carbon at depth which could in turn enhance microbial activity and increase the rate of denitrification in situations where groundwater nitrate is high.     

Dynamics of intertidal marshes near shallow estuaries in Louisiana

A three-year (1991–1993) field investigation was conducted to quantify the hydrodynamics of intertidal marshes adjacent to tidal channels and shallow bays within two Louisiana coastal regions: (1) the sediment-rich Atchafalaya Basin, and, (2) the sediment-poor Terrebonne Basin with relatively minor riverine inflow. The Terrebonne Basin marsh is regularly inundated and flooding is characterized by sporadic draining interspersed by prolonged flooding events. The maximum water depth on the marsh surface exceeds 50 cm, the flow velocity across marsh surface reaches 10 cm sec^–1, and the sediment deposition rate varies from 10 to 90 g m^–2 per tidal cycle. This rather high sediment deposition rate occurs during winter storms with strong southerly winds. In contrast, the marsh site within the sediment-rich Atchafalaya Basin is irregularly inundated and characterized by sporadic flooding interspersed by prolonged draining. There the marsh flooding depth rarely exceeds 25 cm, the over-marsh flow velocity barely reaches 2.5 cm sec^–1, and the sediment deposition rate ranges from 5 to 50 g m^–2 per tidal cycle. The surprisingly low rate of sediment deposition in a marsh within a sediment-rich region is largely due to the man-made canals that alter the hydrologic regime in the upper reaches of the tidal channel.     

Composition and abundance of resident marsh-surface nekton: comparison between tidal freshwater and salt marshes in Virginia,USA

Previous research on intertidal nekton communities has identifiedimportant determinants of community structure and distribution; however, fewstudies have compared nekton utilization of disparate marsh habitats. Inthis study, abundance and distribution patterns of resident nekton werecompared between tidal freshwater marsh and salt marsh surfaces varying inflooding depth and duration. Nekton were collected in pit traps installedalong elevational transects at four marshes in coastal Virginia (twofreshwater, two saline) from April through November 1992–1993. Thedominant fish collected at all sites was the mummichog Fundulusheteroclitus. The daggerblade grass shrimp Palaemonetes pugio was thedominant nekton species collected at salt marsh sites, and was seasonallyabundant on tidal freshwater marshes. A positive correlation betweenflooding depth and nekton abundance was observed on salt marshes; anopposite pattern was observed on tidal freshwater marshes. Tidal floodingregime influences the abundance of resident nekton, however, the effect maybe confounded by other environmental variables, including variation insurface topography and seasonal presence or absence of submerged aquaticvegetation (SAV) in adjacent subtidal areas. In mid-Atlantic tidalfreshwater wetlands, SAV provides a predation refuge and forage site forearly life stages of marsh-dependent nekton, and several species utilizethis environment extensively. Salt marshes in this region generally lackdense SAV in adjacent subtidal creeks. Consequently, between-sitedifferences in species and size-specific marsh surface utilization byresident nekton were observed. Larvae and juveniles represented 79%and 59% of total fish collected at tidal freshwater and salt marshsites, respectively. The resident nekton communities of tidal freshwater andsalt marsh surfaces are characterized by a few ubiquitous species with broadenvironmental tolerances. This revised version was published online in July 2006 with corrections to the Cover Date.     

黄河三角洲潮间带不同类型湿地景观格局变化与趋势预测

以1979—2013年7期卫星遥感影像(Landsat TM)为数据源,结合野外实地调查,通过建立黄河三角洲潮间带湿地数据库,探讨不同类型湿地的景观格局以及自然与人为因素对景观格局变化的影响,并基于Markov模型对未来20年三角洲潮间带不同类型湿地的景观格局进行了趋势预测。结果表明,三角洲的潮间带湿地面积在1979—2013年间整体呈先降低后增加变化。其中,1979—2010年的湿地面积持续减少,由1050.28 km~2减少为575.39 km~2,减少率为45.22%;2010—2013年的湿地面积略有增加,由575.39 km~2增加为596.17 km~2,增长率为0.36%。1979—2013年,潮间带主要湿地景观类型随距海远近均呈明显带状分布,但芦苇湿地面积呈明显降低趋势(减少273.53 km~2,减少率为79.68%),盐田养殖池面积呈显著增加趋势(增长12.04km~2,增长率为1584.21%),而光滩、碱蓬湿地、碱蓬-柽柳湿地和柽柳-芦苇湿地等其它类型湿地面积整体均呈波动减少趋势。未来20年,潮间带湿地面积整体将呈降低趋势,其值将由2010年的575.39 km~2减少为2030年的546.98 km~2,减少率为6.60%。芦苇湿地面积将继续减少(减少30.16 km~2,减少率为24.12%),盐田养殖池面积将持续增长(增加3.71 km~2,增长率为38.61%),而光滩、碱蓬湿地、碱蓬-柽柳湿地和柽柳-芦苇湿地等其它类型湿地面积均将呈小幅波动变化。研究发现,尽管自然与人为驱动力的双重作用决定了1979—2013年间潮间带的湿地景观格局及其动态变化,但黄河年输沙量(x_1)、区域GDP(x_2)和水产品产量(x_3)对潮间带湿地面积变化(y)的影响更为重要(y=733.192+35.317 x_1-0.005 x_2-4.085 x_3,P=0.00010.05),其对过去30多年间潮间带湿地面积变化的解释贡献高达76.7%。随着黄河三角洲高效生态经济区国家战略的实施,为实现潮间带区域的可持续发展,潮间带湿地的保护与生态保育应给予特别重视。     

Modeling Habitat Change in Salt Marshes After Tidal Restoration

Salt marshes continue to degrade in the United States due to indirect human impacts arising from tidal restrictions. Roads or berms with inadequate provision for tidal flow hinder ecosystem functions and interfere with self‐maintenance of habitat, because interactions among vegetation, soil, and hydrology within tidally restricted marshes prevent them from responding to sea level rise. Prediction of the tidal range that is expected after restoration relative to the current geomorphology is crucial for successful restoration of salt marsh habitat. Both insufficient (due to restriction) and excessive (due to subsidence and sea level rise) tidal flooding can lead to loss of salt marshes. We developed and applied the Marsh Response to Hydrological Modifications model as a predictive tool to forecast the success of management scenarios for restoring full tides to previously restricted areas. We present an overview of a computer simulation tool that evaluates potential culvert installations with output of expected tidal ranges, water discharges, and flood potentials. For three New England tidal marshes we show species distributions of plants for tidally restricted and nonrestricted areas. Elevation ranges of species are used for short‐term (<5 years) predictions of changes to salt marsh habitat after tidal restoration. In addition, elevation changes of the marsh substrate measured at these sites are extrapolated to predict long‐term (>5 years) changes in marsh geomorphology under restored tidal regimes. The resultant tidal regime should be designed to provide habitat requirements for salt marsh plants. At sites with substantial elevation losses a balance must be struck that stimulates elevation increases by improving sediment fluxes into marshes while establishing flooding regimes appropriate to sustain the desired plants.     

Consolidation and volumetric soil-water content of salt marsh soils following habitat modification for mosquito control

The runnelling form of habitat modification for mosquito control in saltmarsh increases tidal frequency, and may affect soil properties such as volumetric soil-water content and consolidation. The effects of habitat modification on soil properties are in turn likely to affect ecological processes. Runnels constructed mechanically to a depth of no more than 0.3 m with smooth, spoon shaped edges linked isolated mosquito-breeding pools in the high marsh to the tidal source at the saltmarsh/mangrove interface. The physical design of runnels may result in a significant increase in the frequency of flooding tidal events that flush isolated mosquito-breeding pools. Impacts of the runnelling technique were determined at three marshes using two sampling protocols: (a) comparisons between modified and unmodified shores and (b) comparisons with lateral distance from a runnel. At one marsh, volumetric water content was significantly higher at runnelled than at unrunnelled sites after tides that only partly inundated the marsh, but this pattern was not found at the other marshes. Soil consolidation was greater further from the shore, but was not different between runnelled and unrunnelled shores. Measurements at different lateral distances from runnels demonstrated higher water content levels and lower consolidation up to 5 m from runnels and no effect further away. The varied responses to runnelling at different marshes may reflect specific site characteristics such as slope and hydraulic tidal forces. Remedial strategies for similar mosquito control techniques, based on habitat modification, should include dynamic classifications of saltmarshes.     

A biogeographic pattern in sparrow bill morphology: parallel adaptation to tidal marshes

The study of ecological convergence, the evolution of similar traits on multiple occasions in response to similar conditions, is a powerful method for developing and testing adaptive hypotheses. However, despite the great attention paid to geographic variation and the foraging ecology of birds, surprisingly few cases of convergent or parallel feeding adaptations have been adequately documented. In this study, we document a biogeographic pattern of parallel bill morphology across 10 sparrow taxa endemic to tidal marshes. All North American tidal marsh sparrows display parallel differentiation from close relatives in other habitats, suggesting that selection on bill morphology is strong. Relative to their body mass, tidal marsh sparrows have longer, thinner bills than their non-tidal marsh counterparts, which is likely an adaptation for consuming more invertebrates and fewer seeds, as well as for probing in sediment crevices to capture prey. Published data on tidal marsh food resources and diet of the relevant taxa support this hypothesis. This morphological differentiation is most pronounced between sister taxa with the greatest estimated divergence times, but is found even in taxa that show little or no structure in molecular genetic markers. We, therefore, speculate that tidal marsh ecosystems are likely settings for ecological speciation.     

The influence of tidal forcing on groundwater flow and nutrient exchange in a salt marsh-dominated estuary

Data from salt marshes in the U.S. Southeast show that long-term variations in mean water level (MWL) correlate strongly with salt marsh productivity and porewater salinity. Here we used numerical models of tidally-driven groundwater flow to assess the effect of variations in tidal amplitude and MWL on porewater exchange between salt marshes and tidal creeks. We modeled homogeneous and layered stratigraphy and compared flat and sloped topography for the marsh surface. Results are consistent with field observations and showed that increases in tidal amplitude increased groundwater flushing, particularly when increasing the tidal amplitude caused the marsh platform to be inundated at high tide. Increases in MWL caused groundwater flushing to increase if that rise caused greater areas of the marsh to be inundated at high tide. Once the marsh was fully inundated at high tide, further increases in MWL caused groundwater flushing to decrease. Results suggest that small increases in MWL associated with sea level rise could increase nutrient export significantly in marshes with elevations that are equilibrated near mean high water, but rising sea level could decrease the export of nutrients to, and thus fertility in, estuaries adjacent to marshes that are equilibrated lower in the tidal frame. Likewise, macrotidal estuaries are predicted to be subject to much larger groundwater and nutrient exchange than similar microtidal estuaries. We speculate that the early stages of rising relative sea level may significantly impact water quality in estuaries that are not river-dominated by raising the discharge of nutrients from coastal wetlands.     

Salt Marsh Restoration in Connecticut: 20 Years of Science and Management

In 1980 the State of Connecticut began a tidal marsh restoration program targeting systems degraded by tidal restrictions and impoundments. Such marshes become dominated by common reed grass (Phragmites australis) and cattail (Typha angustifolia and T. latifolia), with little ecological connection to Long Island Sound. The management and scientific hypothesis was that returning tidal action, reconnecting marshes to Long Island Sound, would set these systems on a recovery trajectory. Specific restoration targets (i.e., pre‐disturbance conditions or particular reference marshes) were considered unrealistic. However, it was expected that with time restored tides would return ecological functions and attributes characteristic of fully functioning tidal salt marshes. Here we report results of this program at nine separate sites within six marsh systems along 110 km of Long Island Sound shoreline, with restoration times of 5 to 21 years. Biotic parameters assessed include vegetation, macroinvertebrates, and use by fish and birds. Abiotic factors studied were soil salinity, elevation and tidal flooding, and soil water table depth. Sites fell into two categories of vegetation recovery: slow, ca. 0.5%, or fast, more than 5% of total area per year. Although total cover and frequency of salt marsh angiosperms was positively related to soil salinity, and reed grass stand parameters negatively so, fast versus slow recovery rates could not be attributed to salinity. Instead, rates appear to reflect differences in tidal flooding. Rapid recovery was characterized by lower elevations, greater hydroperiods, and higher soil water tables. Recovery of other biotic attributes and functions does not necessarily parallel those for vegetation. At the longest studied system (rapid vegetation recovery) the high marsh snail Melampus bidentatus took two decades to reach densities comparable with a nearby reference marsh, whereas the amphipod Orchestia grillus was well established on a slow‐recovery marsh, reed grass dominated after 9 years. Typical fish species assemblages were found in restoration site creeks and ditches within 5 years. Gut contents of fish in ditches and on the high marsh suggest that use of restored marsh as foraging areas may require up to 15 years to reach equivalence with reference sites. Bird species that specialize in salt marshes require appropriate vegetation; on the oldest restoration site, breeding populations comparable with reference marshland had become established after 15 years. Use of restoration sites by birds considered marsh generalists was initially high and was still nearly twice that of reference areas even after 20 years. Herons, egrets, and migratory shorebirds used restoration areas extensively. These results support our prediction that returning tides will set degraded marshes on trajectories that can bring essentially full restoration of ecological functions. This can occur within two decades, although reduced tidal action can delay restoration of some functions. With this success, Connecticut's Department of Environmental Protection established a dedicated Wetland Restoration Unit. As of 1999 tides have been restored at 57 separate sites along the Connecticut coast.     

The natural regeneration of salt marsh on formerly reclaimed land

Question: Does the vegetation of restored salt marshes increasingly resemble natural reference communities over time? Location: The Essex estuaries, southeast England. Methods: Abandoned reclamations, where coastal defences had been breached in storm events, and current salt marsh recreation schemes were surveyed giving a chronosequence of salt marsh regeneration from 2 to 107 years. The presence, abundance and height of plant species were recorded and comparisons were made with adjacent reference salt marsh communities at equivalent elevations. Results: Of the 18 paired sites surveyed, 13 regenerated marshes had fewer species than their adjacent reference marsh, three had an equal number and two had more. The plant communities of only two de‐embankment sites matched that of the reference community. 0–50 year old sites and 51–100 year old sites had fewer species per quadrat than the 101+ year sites and the reference salt marshes. There was a weak relationship between differences in species richness for regenerated and reference marshes and the time since sites were first re‐exposed to tidal inundation. Cover values for the invasive and recently evolved Spartina anglica were greater within regenerated than reference marshes. Conclusions: Salt marsh plants will colonise formerly reclaimed land relatively quickly on resumption of tidal flooding. However, even after 100 years regenerated salt marshes differ in species richness, composition and structure from reference communities.     

Ecological responses to tidal restorations of two northern New England salt marshes

Efforts are underway to restore tidal flow in New England salt marshes that were negatively impacted by tidal restrictions. We evaluated a planned tidal restoration at Mill Brook Marsh (New Hampshire) and at Drakes Island Marsh (Maine) where partial tidal restoration inadvertently occurred. Salt marsh functions were evaluated in both marshes to determine the impacts from tidal restriction and the responses following restoration. Physical and biological indicators of salt marsh functions (tidal range, surface elevations, soil water levels and salinities, plant cover, and fish use) were measured and compared to those from nonimpounded reference sites. Common impacts from tidal restrictions at both sites were: loss of tidal flooding, declines in surface elevation, reduced soil salinity, replacement of salt marsh vegetation by fresh and brackish plants, and loss of fish use of the marsh.Water levels, soil salinities and fish use increased immediately following tidal restoration. Salt-intolerant vegetation was killed within months. After two years, mildly salt-tolerant vegetation had been largely replaced in Mill Brook Marsh by several species characteristic of both high and low salt marshes. Eight years after the unplanned, partial tidal restoration at Drakes Island Marsh, the vegetation was dominated bySpartina alterniflora, a characteristic species of low marsh habitat.Hydrologic restoration that allowed for unrestricted saltwater exchange at Mill Brook restored salt marsh functions relatively quickly in comparison to the partial tidal restoration at Drakes Island, where full tidal exchange was not achieved. The irregular tidal regime at Drakes Island resulted in vegetation cover and patterns dissimilar to those of the high marsh used as a reference. The proper hydrologic regime (flooding height, duration and frequency) is essential to promote the rapid recovery of salt marsh functions. We predict that functional recovery will be relatively quick at Mill Brook, but believe that the habitat at Drakes Island will not become equivalent to that of the reference marsh unless the hydrology is further modified.Corresponding Editor: R.E. Turner Manuseript     

Ecological responses to tidal restorations of two northern New England salt marshes

Efforts are underway to restore tidal flow in New England salt marshes that were negatively impacted by tidal restrictions. We evaluated a planned tidal restoration at Mill Brook Marsh (New Hampshire) and at Drakes Island Marsh (Maine) where partial tidal restoration inadvertently occurred. Salt marsh functions were evaluated in both marshes to determine the impacts from tidal restriction and the responses following restoration. Physical and biological indicators of salt marsh functions (tidal range, surface elevations, soil water levels and salinities, plant cover, and fish use) were measured and compared to those from nonimpounded reference sites. Common impacts from tidal restrictions at both sites were: loss of tidal flooding, declines in surface elevation, reduced soil salinity, replacement of salt marsh vegetation by fresh and brackish plants, and loss of fish use of the marsh. Water levels, soil salinities and fish use increased immediately following tidal restoration. Salt-intolerant vegetation was killed within months. After two years, mildly salt-tolerant vegetation had been largely replaced in Mill Brook Marsh by several species characteristic of both high and low salt marshes. Eight years after the unplanned, partial tidal restoration at Drakes Island Marsh, the vegetation was dominated bySpartina alterniflora, a characteristic species of low marsh habitat. Hydrologic restoration that allowed for unrestricted saltwater exchange at Mill Brook restored salt marsh functions relatively quickly in comparison to the partial tidal restoration at Drakes Island, where full tidal exchange was not achieved. The irregular tidal regime at Drakes Island resulted in vegetation cover and patterns dissimilar to those of the high marsh used as a reference. The proper hydrologic regime (flooding height, duration and frequency) is essential to promote the rapid recovery of salt marsh functions. We predict that functional recovery will be relatively quick at Mill Brook, but believe that the habitat at Drakes Island will not become equivalent to that of the reference marsh unless the hydrology is further modified.     

A Pleistocene disturbance event describes modern diversity patterns in tidal marsh birds

There is growing evidence to support that paleo‐timescale events are important determinants in the present‐day distribution of organisms. We explored the relationship between community composition of tidal marsh birds in the northeastern United States and potential drivers of biodiversity patterns across timescales to explore the relevance of historical contingency in this ecosystem. These potential predictors represent some of the major known influences on biodiversity in tidal marshes, including 1) a recent, intense hurricane event driving a large‐scale perturbation of this ecosystem (4 yr), 2) gradual modification of marshes through installation of human infrastructure (～ 150 yr), and 3) marsh formation and development after the Last Glacial Maximum (LGM, ～ 20 000 yr). We surveyed > 1300 locations in tidal marshes from 2011–2014 using passive point count methods to measure bird community composition at these points. We found that rarefied richness, total individuals (N), and total biomass were best explained by a quadratic relationship with marsh age peaking at 40° latitude, the location of the Last Glacial Maximum of the Laurentide ice sheet. We hypothesize that formation of marsh millennia earlier in the southern part of our survey area allowed for earlier evolution of specialization to tidal marsh by bird species than those occupying much younger, northern marshes, which could have then driven differential rates of colonization in the north (by habitat generalists) and competitive exclusion in the south (by habitat specialists). We tested this theory using a novel functional diversity metric (community habitat specialization index, or CHSI) and find that community specialization decreased linearly with marsh age, supporting our hypothesis. Our findings highlight the importance for consideration of historical contingency in biodiversity research and further exploration of mechanisms operating across geological timescales.     

Long-term response of fishes and other fauna to restoration of former salt hay farms: multiple measures of restoration success

This synthesis brings together published and unpublished data in an evaluation of restoration of former salt hay farms to functioning salt marshes. We compared nine years of field measurements between three restored marshes (Dennis, Commercial, and Maurice River Townships) and a reference marsh (Moores Beach) in the mesohaline portion of Delaware Bay. In the process, we compared channel morphology, geomorphology, vegetation, sediment organic matter, fish assemblages, blue crabs, horseshoe crabs, benthic infauna, and diamondback terrapins. For fishes we compared structural (distribution, abundance) and functional (feeding, growth, survival, reproduction, production) aspects to evaluate the restored marshes in an Essential Fish Habitat context. Marsh vegetation and drainage density responded gradually and positively with restored marshes approximating the state of the reference marsh within the nine-year study period. The fauna responded more quickly and dramatically with most measures equal or greater in the restored marshes within the first one or two years after restoration. Differences in response time between the vegetation and the fauna imply that the faunal response was more dependent on access to the shallow intertidal marsh surface and intertidal and subtidal creeks than on characteristics of the vegetated marsh. The fishes in created subtidal creeks in restored marshes responded immediately and maintained fish assemblages similar to the reference marsh over the study period. The intertidal creek fish assemblages tended to become more like the reference marsh in the last years of the comparison. Overall, these results document the success of the restoration and how marshes function for both resident and transient fauna, especially fishes.     

Sediments in Marsh Ponds of the Gulf Coast Chenier Plain: Effects of Structural Marsh Management and Salinity

Physical characteristics of sediments in coastal marsh ponds (flooded zones of marsh associated with little vegetation) have important ecological consequences because they determine compositions of benthic invertebrate communities, which in turn influence compositions of waterbird communities. Sediments in marsh ponds of the Gulf Coast Chenier Plain potentially are affected by (1) structural marsh management (levees, water control structures and impoundments; SMM), and (2) variation in salinity. Based on available literature concerning effects of SMM on sediments in emergent plant zones (zones of marsh occasionally flooded and associated with dense vegetation) of coastal marshes, we predicted that SMM would increase sediment carbon content and sediment hardness, and decrease oxygen penetration (O₂ depth) and the silt-clay fraction in marsh pond sediments. Assuming that freshwater marshes are more productive than are saline marshes, we also predicted that sediments of impounded freshwater marsh ponds would contain more carbon than those of impounded oligohaline and mesohaline marsh ponds, whereas C:N ratio, sediment hardness, silt-clay fraction, and O₂ depth would be similar among pond types. Accordingly, we measured sediment variables within ponds of impounded and unimpounded marshes on Rockefeller State Wildlife Refuge, near Grand Chenier, Louisiana. To test the above predictions, we compared sediment variables (1) between ponds of impounded (IM) and unimpounded mesohaline marshes (UM), and (2) among ponds of impounded freshwater (IF), oligohaline (IO), and mesohaline (IM) marshes. An a priori multivariate analysis of variance (MANOVA) contrast indicated that sediments differed between IM and UM marsh ponds. As predicted, the silt-clay fraction and O₂ depth were lower and carbon content, C:N ratio, and sediment hardness were higher in IM than in UM marsh ponds. An a priori MANOVA contrast also indicated that sediments differed among IF, IO, and IM marsh ponds. As predicted, carbon content was higher in IF marsh ponds than in ponds of other impounded marsh types. In contrast to our predictions, C:N ratio and sediment hardness were lowest and silt-clay fraction and O₂ depth were highest in IO and IM marsh ponds. Our results indicated that SMM has affected physical properties of sediments in coastal marsh ponds. Moreover, sediments in IF marsh ponds were affected more so than were those in IO and IM marsh ponds. Our results, in conjunction with those of previous studies, indicated that sediments of marsh ponds and emergent plant zones differed greatly. We predict that changes in pond sediments due to SMM will promote greater epifaunal macroinvertebrate biomass, which in turn should attract larger populations of wintering waterbirds. However, waterbirds that filter or probe soft sediments may be negatively affected by SMM because of the expected decrease in infaunal invertebrate biomass.     

Dynamics of biogenic Si in freshwater tidal marshes: Si regeneration and retention in marsh sediments (Scheldt estuary)

The sequestration and recycling of biogenic silica (BSi) in freshwater tidal marshes was modelled through the combination of short-term year round sediment trap data with a long-term sedimentation model, MARSED. The modelling was implemented through the complete evolution from a young rapidly rising marsh to a marsh with an elevation close to mean high water. BSi in imported suspended matter was higher in summer (10.9 mg BSi g⁻¹ sediment) than winter (7.6 mg BSi g⁻¹ sediment). However, the deposition of BSi on the marsh surface was higher in winter compared to summer, due to the higher sedimentation rates. Deposition of BSi was correlated to the suspended matter deposition. In the old marsh, yearly about 40 g BSi m⁻² was deposited, while in the young marsh deposition could rise up to 300 g m⁻². Young marshes retained up to 85% of the imported biogenic silica. Recycling efficiency (60%) increased drastically for older marshes. The study shows that marshes act as important sinks for BSi along estuaries. The recycling of the imported BSi to DSi in summer and spring is most likely an essential factor in the buffering role of tidal marshes for estuarine DSi concentrations.