Marsh Elevation Research Articles

Restoring marsh elevation in a rapidly subsiding salt marsh by thin-layer deposition of dredged material

Thin-layer deposition of dredged material on coastal marsh by means of high-pressure spray dredging (Jet-Spray® 2 Jet-Spray® is a registered trademark of Aztec Development Company, P.O. Box 3348, Orlando, FL 32802, USA. 2 ) technology has been proposed as a mechanism to minimize wetland impacts associated with traditional bucket dredging technologies and to restore soil elevations in deteriorated marshes of the Mississippi River delta. The impact of spray dredging on vegetated marsh and adjacent shallow-water habitat (formerly vegetated marsh that deteriorated to open water) was evaluated in a 0.5-ha Spartina alterniflora-dominated salt marsh in coastal Louisiana. The thickness of dredged sediment deposits was determined from artificial soil marker horizons and soil elevation change was determined from sedimentation–erosion tables (SET) established prior to spraying in both sprayed and reference marshes. The vertical accretion and elevation change measurements were made simultaneously to allow for calculation of shallow (∼5 m depth) subsidence (accretion minus elevation change). Measurements made immediately following spraying in July 1996 revealed that stems of S. alterniflora were knocked down by the force of the spray and covered with 23 mm of dredged material. Stems of S. alterniflora soon recovered, and by July 1997 the percent cover of S. alterniflora had increased three-fold over pre-project conditions. Thus, the layer of dredged material was thin enough to allow for survival of the S. alterniflora plants, with no subsequent colonization by plant species typical of higher marsh zones. By February 1998, 62 mm of vertical accretion accumulated at this site, and little indication of disturbance was noted. Although not statistically significant, soil elevation change was greater than accretion on average at both the spray and reference marshes, suggesting that subsurface expansion caused by increased root biomass production and/or pore water storage influence elevation in this marsh region. In the adjacent shallow water pond, 129 mm of sediment was deposited in July 1996 as a result of spraying, and despite initial shallow subsidence and continual erosion through February 1998, water bottom elevation was raised sufficiently to allow S. alterniflora to invade via rhizome growth from the adjacent marsh. Hence, thin-layer deposition of dredged material at this site was effective at restoring and maintaining marsh elevation after 1.5 years. However, if the open water sediment deposits are not soon completely stabilized via further vegetative colonization, erosion may eventually lower elevations to the level where emergent vegetation cannot persist.

Effects of Impoundment on Vertical Accretion of Coastal Marsh

Vertical accretion of impounded marsh and adjacent natural marsh at four sites in southwestern Louisiana was estimated in 1994 by determining the depth of a stratum containing137Cs deposited in 1963. With relative marsh elevation, soil bulk density, organic matter content, and organic and mineral matter accumulation rates were used to describe soil formation. Three sites were impounded in 1956 and one site in 1951. Impounded marshes had lower marsh surface elevation than natural marshes because of hydrologic isolation from tidal sediment subsidies and substrate oxidation during forced drying. The elevation of natural marshes ranged from 12 cm to 42 cm higher than the elevation of the impounded marshes in 1963 and from 20 cm to 32 cm higher in 1994. Vertical accretion between 1963 and 1994 ranged from 9 cm to 28 cm in impounded marsh and from 15 cm to 21.5 cm in natural marsh. Only in impounded marsh that remained permanently flooded was accretion greater than in natural marsh.

Interactive effects of elevation and burial with wrack on plant community structure in some Rhode Island salt marshes

1 The effects of disturbances and elevation on marsh plant communities were examined using experimental disturbances along an elevation gradient in marshes with different disturbance histories. In addition, differences in species composition among five marshes were determined at elevations at which the greatest concentration of burial by wrack occurred. 2 Experimental wrack burial generally caused significant mortality of the high‐marsh competitive dominants, Juncus gerardi and Spartina patens, and strongly increased the abundance of the fugitive perennial, Distichlis spicata. 3 The effects of experimental wrack burial interacted strongly with abiotic factors associated with elevation to influence the distributions of both competitive dominants and annual fugitive plants. 4 Frequent wrack burial in a marsh appears to lead to a persistent assemblage of plants dominated by competitively subordinate fugitives. This assemblage of fugitives tends to occur at intermediate elevations within the marsh, where wrack gets stranded for long periods of time and where the resistance of Juncus gerardi to wrack burial is lowest. 5 We suggest that wrack‐burial disturbances interact strongly with marsh elevation to influence the zonation of plants in New England salt marshes, and discuss some implications of our results.

Sea level–climate correlation during the past 1400 yr

We present a new mean-high-water curve for Hammock River marsh, Clinton, Connecticut, obtained by improving the age model for an existing record of relative marsh elevation based on foraminiferal analysis of a 1.8-m-long peat core. Unlike the earlier curve, the new curve confirms trend changes in mean-high-water rise during the past 1400 yr as noted for salt marshes 15 km farther west, suggesting a regional cause. These trend changes and century-scale mean-high-water variations in the Clinton record correlate positively with large-scale regional variations in sea-surface and summer-air temperature, indicating a link between sea level and the climate-ocean system. On the basis of the Clinton mean-high-water curve, we conclude that real sea level oscillated centimeters to decimeters on a century time scale over the past 1400 yr, was 25 ± 25 cm higher ca. a.d. 1050 (Medieval Warm Period) than ca. a.d. 1650 (Little Ice Age), and rose at a mean rate of ∼1 mmṁyr−1 over the past 350 yr; there was little or no rise during the cool 1800s.

Micropatterns in Festuca rubra-dominated salt-marsh vegetation induced by sheep grazing

Micropatterns induced by sheep grazing, were studied in three consecutive years in a Festuca rubra-dominated salt marsh in a grazing trial with five different stocking rates (0, 1.5, 3, 4.5 and 10 sheep ha(-1)). The micropatterns were formed by a mosaic of short and tall F. rubra stands on a scale of square decimeters. Permanent transects of 2 m x 10 m were used to study the stability of these patterns, and to analyze interactions between the vegetation, the marsh elevation and the sheep. Micropatterns occurred only in the lightly to moderately grazed paddocks (1.5-4.5 sheep ha(-1)) with the highest spatial diversity in the 3 sheep ha(-1) transect. When grazing was excluded, micropatterns did not develop; nor did they develop in the traditionally and most intensively grazed paddock (10 sheep ha(-1)). Detailed observations in one year showed that crude-protein content did not differ between green leaves from the short and tall stands, whereas in vitro digestibility was slightly higher in the short stands. In the same year, tiller density and length of full-grown leaves increased substantially in both stands from May to September. At the same time, sheep preference shifted from tall to short stands, which suggests an interplay between intake rate and digestibility in the sheep selectivity. Seven years after establishment of the grazing trial, the 10 sheep ha(-1) transect still showed a smooth relief typical of the starting point of the other transects. These transects developed a more hummocky topography, with the highest spatial diversity occurring on the 1.5 sheep ha(-1) transect. Marsh elevations were on average up to 3 cm lower in the short than in the tall stands, which indicates that the somewhat lower-elevated patches were grazed more intensively than the higher-elevated patches. In most cases, micropatterns changed from one year to the other, probably due to weather fluctuations. The incidence of tall stands was influenced by the rainfall balance. If the incidences of both the short and the tall stands were around 50%, however, the micropatterns showed a clear correlation with the marsh elevation. The rainfall balance seemed therefore a decisive factor for a possible correspondence between micropatterns in two consecutive years. Elevation differences were so subtle that greater than average sedimentation during a winter season could change the elevation pattern. Hence both rainfall balance and winter sedimentation counteracted the stability of the micropatterns. During our three-year study period, micropatterns were only stable in one out of six possible paired comparisons. This low micropattern stability contrasts with other studies in inland environments, which shows that in more dynamic environments, abiotic processes are likely to overrule summer grazing in determining vegetation patterns.

Controls of sediment transport and deposition in an incised mainland marsh basin, southeastern North Carolina

Sedimentation on the surface of tidal marshes is a process that is controlled by the interactions of a complex set of variables. To adequately describe the patterns of sediment transport and deposition in any particular system requires extensive sampling of biological, physical, sedimentological, and geomorphological parameters. In this study, measurements of sediment deposition, marsh elevation, water level, total suspended solid (TSS) loads, overmarsh current speeds, and vegetative cover were used to determine which of these factors control sediment distribution patterns in a small marsh basin in southeastern North Carolina. The results of this study suggest that marsh elevation exerted significant control over deposition but that its effects were largely muted by other processes. Creek hydrology, sediment instability associated with areas of new creek development, plant/flow interactions, and tidal creek TSS concentrations also influenced sediment deposition in the study area. Flow patterns, resulting from the interaction between tidal stage and marsh topography, controlled the dispersal of particulate matter across the marsh surface and contributed to significant depositional differences among sampling sites as a function of tidal stage. The results of this study indicate that tidal creek geometry, creek channel position, and tidal stage interact synergistically to control sediment/particulate delivery on the surfaces of tidal wetlands.

Decomposition of Saltmeadow Cordgrass (Spartina patens) in Louisiana Coastal Marshes

In Louisiana, plant production rates and associated decomposition rates may be important in offsetting high rates of land loss and subsidence in organic marsh soils. Decomposition ofSpartina patens shoot and leaf material was studied by using litter bags in mesohaline marshes in the Barataria and Terrebonne basins of coastal Louisiana.Spartina patens decomposed very slowly with an average decay constant of 0.0007, and approximately 50% of the material remained after 2 years in the field. Material at the Barataria site decomposed faster than did Terrebonne material with trend differences apparent during the first 150 days. This difference might be explained by the higher content of phosphorus in the Barataria material or a flooding period experienced by the Barataria bags during their first 10 days of deployment. Nitrogen and carbon content of the plant material studied did not differ between the two basins. We detected no consistent significant differences in decomposition above, at, or below sediment/water level. BecauseS. patens is the dominant plant in these marshes, and because it is so slow to decompose, we believe thatS. patens shoots are an important addition to vertical accretion and, therefore, marsh elevation.

Habitat Persistence Underlies Intraspecific Variation in the Dispersal Strategies of Planthoppers

Dispersal is considered a vital life history characteristic for insects exploiting temporary habitats, and life history theorists have often hypothesized an inverse relationship between dispersal capability and habitat persistence. Most often, this hypothesis has been tested using interspecific comparisons of dispersal capability and qualitative estimates of habitat persistence. Consequently, most assessments have failed to control for possible phylogenetic nonindependence and they also lack quantitative rigor. We capitalized on existing intraspecific variation in the dispersal capability of Prokelisia planthoppers to examine the relationship between habitat persistence and dispersal, thereby minimizing possible phylogenetic effects. Two congeneric species (Prokelisia marginata and P. dolus) occur in the intertidal marshes of North America, where they feed exclusively on cordgrasses (Spartina). Because these planthoppers exhibit wing dimorphism, flight—capable adults (macropters with fully developed wings) are easily differentiated from flightless adults (brachypters with reduced wings). Thus, dispersal capability can be readily estimated by the percentage of macropters in a population. At a regional spatial scale, we found a highly significant negative relationship between dispersal capability (percent macroptery) and habitat persistence. In this system, habitat persistence is influenced by a combination of marsh elevation, winter severity, and tidal range, which interact to determine the ability of planthoppers to endure through winter in their primary habitat for development. P. marginata develops primarily in low—marsh habitats during summer, habitats that can be subjected to pronounced winter disturbance due to ice scouring and/or extensive tidal inundation. Levels of winter disturbance of the low marsh are extreme along the Atlantic coast, intermediate along the Pacific, and low along the Gulf. Both the failure of P. marginata populations to remain through winter in this habitat, and the dispersal ability of these populations (92%, 29%, and 17% macroptery, respectively), are correlated with levels of disturbance. Thus, in regions where winter disturbance is high, levels of dispersal are correspondingly high to allow for recolonization of extirpated habitats from overwintering sites on the high marsh. Unlike P. marginata, P. dolus develops primarily in high—marsh habitats, which are much less disturbed on all coasts during winter. Consequently, this species remains year—round in its primary habitat for development, and most populations exhibit relatively low levels of macroptery (<10%). When raised under common garden conditions, many more macropters of both species were produced from Atlantic compared to Gulf populations. Thus the proportion of macropters produced from the populations used in this experiment paralleled the incidence of macroptery measured in the field, providing evidence that the geographic variation in dispersal capability in both species has in part a genetic basis. The results of this study provide strong intraspecific evidence for an inverse relationship between the dispersal capability of insects and the persistence of their habitats.

A comparison of passerine foraging habits in two tidal marshes of different salinity

Foraging habits of dominant passerine species of the Savannah River National Wildlife Refuge, SC, were compared in freshwater and brackish marshes. Temporal or spatial partitioning was evaluated using discriminant function analysis of the distribution among feeding time, foraging height, marsh elevation zone, and plant-type habitat variables by red-winged blackbirds (Agelaius phoeniceus), boat-tailed grackles (Quiscalus major), long-billed marsh wrens (Cistothorus palustris), and common yellowthroats (Geothlypis trichas). Marsh elevation zone and plant type often occurred together as significant variables due to the zonal distribution of marsh plants, especially in the freshwater marsh. Likewise, foraging height often occurred as a significant variable along with plant type due to the dependence of foraging height on plant type and location of preferred food on a plant species. Partitioning of habitat variables was observed more frequently in the brackish marsh when compared with the freshwater marsh. Lower diversity of plant species and food items, combined with a greater zonation of plant communities in the brackish marsh, is proposed to account for the differences observed between the two marshes. Differences in community structure and scale of partitioning between the freshwater and brackish marsh also suggest that the brackish marsh may be more advantageous to generalist species contrasted with specialists. We postulate that conversion of freshwater marshes to brackish marshes, as might be caused by hydrologic alterations, ground-water withdrawal, or global sea-level rise, may create conditions that would favor generalist species.

Monitoring flooding in coastal wetlands by using radar imagery and ground-based measurements

Abstract Satellite radar was used in a Florida Juncus roemerianusmarsh to map tidal flooding, a critical control of coastal vegetation distribution. Radar images taken during a time of near-continuous recordings of ground-based hydrology measurements directly linked marsh flooding to lowered radar returns and indicated a negative covariation between flood frequency and radar return. Flood-extent contours extracted from the radar images and calibrated with point depth measurements showed marsh elevation could be estimated to about 8 cm compared to the 150 cm topographic contours currently available.

Effects of tidally mediated litter moisture content on decomposition of Spartina alterniflora and S. patens

Relationships between flooding frequency, flooding duration, litter moisture levels, and litter decay rates were investigated across the natural hydrologic gradient common to intertidal salt marshes. The effects on litter decay of natural and experimental alterations of litter moisture content were assessed in both field litterbag experiments (conducted in a southern New Jersey salt marsh from 1989 to 1990) and laboratory incubations (1990). Overall, tidally mediated litter moisture content was the dominant factor controlling litter decay throughout the vegetated marsh. Rates of carbon loss were most closely related to litter moisture levels (r=0.84), which were directly related to flooding frequency (r=0.66) and duration (r=0.63). Litter moisture levels were related to elevation within the tidal range due to increasing surface levation from creekbank to high marsh (ca. 54 cm) and height of litter above the sediment surface. Carbon losses from litter of short and tall form Spartina alterniflora Loisel. and S. patens (Aiton) Muhl. along the marsh elevation gradient indicate that while some of the variations in decay rates may be due to litter type, litter moisture accounted for most of the observed variation between marsh zones and within each litter type. Mousture levels are also affected by the water retention capacity of each litter type, which may also secondarily influence decay rates. Short-term incubations of litter indicated that CO2 evolution was positively related to moisture content with negligible respiration at moisture levels below 15% (fresh mass), increasing to a maximum between 65 and 75% depending upon litter type. Since most Spartina spp. litter remains above the marsh surface where it maintains a lower moisture content than surface litter, the use of surface litterbags may overestimate rates of carbon loss in some systems. In addition, since changes in elevation of only a few centimeters had significant effects on both litter moisture levels and decay rates, slight changes in tidal regime may have important consequences for organic matter cycling in salt marshes by affecting litter decomposition processes.

Sexual Productivity and Spring Intramarsh Distribution of a Key Salt-Marsh Microbial Secondary Producer

Phaeosphaeria spartinicola is known to be an important fungal (ascomycetous) secondary producer in the smooth-cordgrass (Spartina alterniflora) decomposition system of western Atlantic salt marshes, yet its degree of predominance among the ascomycete assemblages of salt marshes and the concentration of its sexual reproductive structures (ascomata) have been largely unknown. During May–June, we measured by direct microscopy the percent occupancy of leaf abaxial area and concentrations of ascomata in leaf blades of smooth cordgrass at three elevations in three drainage systems within the marshes of Sapelo Island, Georgia, United States. We also measured in water-saturation chambers the rates at which the ascomata expelled ascopoores (sexual propagules) out of decaying leaves from marsh sites containing or not containing shredder gastropods.P. spartinicola ascomata were found at averages of 36% to 93% of grid-circles (3-mm radius) on decaying leaf blades, with lower values at points directly adjacent to the leaf sheath, on leaves at earlier stages of decay, and at elevational subsites where shredder snails were more active. Marsh elevation had no effect of its own on percent occupation. No other species of ascomycetes were found at overall avarage frequencies greater than 3%. Average concentration of ascomata along the intervascular rows where they were located was 1 ascoma per 0.5 mm row (∼1000 cm−2 abaxial leaf surface, translating to production of 1.6×107 ascomata m−2 intermediate-height marsh per standing crop of living stems). The fraction of total fungal production allocated to ascomata is speculatively and crudely estimated at about 9%, without taking into account potential loss to invertebrate shredders. At sites with abundances of snails >-50 m−2 peaks of ascospore expulsion (about 50–75 spores cm−2 leaf h−1, 3–5× the overall average rate) observed at snail-free sites were absent. Our measured rate of ascospore expulsion (averaged over snail-free and high-snail sites, and possibly an underestimate) translated to 6.5×104 spores m−2 marsh h−1 for times of freshwater saturation of leaves, and one-third that value for times of saltwater wetting.

Early Use by Fish of a Mitigation Salt Marsh, Humboldt Bay, California

Fish numbers and biomass in a mitigation salt marsh, Humboldt Bay, California, were examined from July 1981 to October 1982 and were compared with a nearby established marsh to determine whether the restored marsh provided adequate mitigation for habitat lost due to construction of a nearby marina. The use by fish of channels adjacent to the two marshes and the channel at the Woodley Island Marina construction site, for which mitigation was required, were also compared. The mitigation marsh, adjacent to Freshwater Slough channel, was 5.2 km from the marina site. Fishes were sampled by otter trawl, ichthyoplankton net, fixed channel nets, drop traps, and beach seines. Thirty-one fish species and two crab species were collected. Wide ranges in seasonal salinities and water temperatures, and differences in marsh elevation influenced fish use of the mitigation marsh area. The intertidal area of the mitigation marsh, dominated by euryhaline sticklebacks and topsmelts, did not replace intertidal and subtidal habitat lost by marina construction, which had more stable salinities and water temperatures and was used extensively by juvenile English sole.

Vegetation Change on a Northeast Tidal Marsh: Interaction of Sea‐Level Rise and Marsh Accretion

Increasing rates of relative sea—level rise (RSL) have been linked to coastal wetland losses along the Gulf of Mexico and elsewhere. While such losses have yet to be reported for New England tidal marshes, rapidly rising RSL may still be affecting these systems. Studies of the Wequetequock—Pawcatuck tidal marshes over four decades have documented dramatic changes in vegetation that appear to be related primarily to differential rates of marsh accretion and sea—level rise. Other environmental factors such as sediment supply and anthropogenic modifications of the system may be involved as well. When initially studied in 1947—1948 the high marsh supported a Juncus gerardi—Spartina patens belting pattern typical of many New England salt marshes. On the most of the marsh complex the former Juncus belt has been replaced by forbs, primarily Triglochin maritima, while the former S. patens high marsh is now a complex of vegetation types–stunted Spartina alterniflora, Distichlis Spicata, forbs, and relic stands of S. patens. These changes are documented by vegetation sampling that closely followed the 1947—1948 methods and by peat core analysis. Marsh elevations were determined by leveling, and the mean surface elevation of areas where the vegetation has changed is significantly lower than that of areas still supporting the earlier pattern (4.6 vs. 13.9 cm above mean tide level). The differences in surface elevation reflect differences in accretion of marsh peat. Calculations based on sandy overwash layers deposited during historically recorded storms as well as on experimentally placed marker horizons of known age indicate that stable areas have been accreting at the rate of local sea—level rise, 2.0—2.5 mm/yr at least since 1938; changed areas have accreted at about one half that rate. Lower surface elevations result in greater frequency and duration of tidal flooding, and thus in increased peat saturation, salinity, and sulfide concentrations, and in decreased redox potential, as directly measured over the growing season at both changed and stable sites. It is proposed that these edaphic changes have combined to favor establishment of a wetter, more open vegetation type dominated by to distinctive communities–Stunted S. alterniflora and forbs. Changes documented on the Wequetequock—Pawcatuck system have been observed on the other Long Island Sound marshes and may serve as a model for the potential effects of seal—level rise on New England tidal salt marshes.

Flood tolerance and the distribution of Iva frutescens across New England salt marshes.

Tidal flooding is widely believed to be an important determinant of marsh plant distributions but has rarely been tested in the field. In New England the marsh elder Iva frutescens often dominates the terrestrial border of salt marshes and we examined its flood tolerance and distribution patterns. Marsh elders only occur at elevations where their roots are not subject to prolonged water table flooding. Consequently they are found on the terrestrial border of marshes and at lower elevations associated with drainage ditches and locally elevated surfaces. Marsh elders transplanted to elevations lower than they normally occur died within a year with or without neighbors and greenhouse tests revealed that I. frutescens is much less tolerant of flooded soil conditions than plants found at lower marsh elevations. We also manipulated the water table level of field plots and found that increasing or decreasing water table drainage led to enhanced and diminished I. frutescens performance, respectively. Our results demonstrate the importance of water table dynamics in generating spatial patterns in marsh plant communities and provide further evidence that supports the hypothesis that the seaward distributional limits of marsh plant populations are generally dictated by physical processes.

Salt Marsh Plant Zonation: The Relative Importance of Competition and Physical Factors

In Carpinteria Salt Marsh, Salicornia virginica (pickleweed) grows at lower marsh elevations than does Arthrocnemum subterminalis (Parish's glasswort). Standing biomass of both species was greatest immediately adjacent to their abrupt border, suggesting that conditions for plant growth were best here. We utilized field experiments, in which growth rates of naturally occurring and transplanted individuals of both species were measured in four marsh zones, to investigate the role of edaphic factors and competition in maintaining this zonation pattern. The frequency of flooding, and hence soil waterlogging, was greatest at lower marsh elevations, whereas salinity was highest at higher marsh elevations. Consequently, it was not clear, a priori, which part of the marsh had the most severe physical conditions. In our field experiments, both Salicornia and Arthrocnemum grew better in the two middle marsh zones (high Salicornia zone and Arthrocnemum zone) than in either the low marsh (low Salicornia zone), where flooding was frequent and soils were waterlogged, or the high marsh (transition zone), where soil salinity was extremely high during much of the year and plant water potentials very low. However, Salicornia appeared better able to tolerate flooding, and so persisted in the low Salicornia zone, whereas Arthrocnemum appeared better able to tolerate high salinities, and so persisted in the transition zone. Interspecific competition was most important in the relatively benign middle marsh zones, where each species excluded the other from a portion of this prime habitat. In this marsh, flooding, soil salinity, and competition all interacted to determine plant zonation patterns, but the relative importance of these factors varied at different elevations.

Population variation in Spartina anglica C. E. Hubbard

summaryClonal replicates of Spartina anglica C. E. Hubbard were reciprocally transplanted into cleared plots and undisturbed vegetation in three adjacent zones on the Dee Estuary salt marsh, Cheshire, England. The aim of the investigation was to examine whether clones from these populations show a differential response to the range of naturally occurring environmental variation. Clone survival was significantly lower in the pioneer zone relative to the sward and mature marsh zones and was greatest in plants from the pioneer population in all transplant sites. Tiller number per plant was consistently greater in the pioneer population, increased with increasing marsh elevation and the removal of natural vegetation in the mature zone, and, for the pioneer and mature populations, showed a significant clone x environment interaction effect. The norms of reaction showed marked differences between clones in the pattern of response to environmental variation, but joint regression of clone means on overall means showed that very few clones had a linear response. Several morphological characters showed significant interactions with environmental effects in ANOVA, but only for total plant biomass in the pioneer population did this reflect a linear increase in more favourable environments. Differences in overall stature quantified by the pattern of morphological character correlations indicated the occurrence of different morphological forms within the populations. The differences in performance and plasticity are discussed in the context of the successional development of these populations, and the possible influence of age‐related somatic variation.

Understanding Salt-Marsh Accretion, Scolt Head Island, Norfolk, England

Measurements of surface sediment accretion have been obtained for Hut Marsh, Scolt Head Island, Norfolk, England, using sand marker horizons. More than eighty 1-m2 marker sites were deployed in October 1983, and vertical accretion measured at them in April and October 1986 and April 1987. Elevations relative to Ordnance Datum were established by levelling each site at the time of deployment. Data collected by Steers between 1935 and 1957 on the same marsh show a clear relationship between marsh elevation and sedimentation. The results of the current more detailed study, presented as mean annual accretion rates, indicate a complex pattern of sediment distribution at this macrotidal location. These data, together with the results of tidal flow monitoring in the creeks on Hut Marsh, show the importance of spatial and temporal patterns of sediment delivery and variations in depositional processes in determining the distribution of sedimentation across the marsh surface.

STUDY TO DETERMINE THE RECOVERY OF A LOUISIANA MARSH FROM AN OIL SPILL

ABSTRACT A pipeline break in April 1985 near the Shell Pipe Line Corporation pumping station at Nairn, Louisiana, resulted in the release of approximately 300 barrels of crude oil in a south Louisiana marsh. A study was conducted three months after the spill to assess the effects of the oil on the marsh community. Results showed that the vegetation was affected by the oil.3 A follow-up study was conducted in fall 1986 to determine the extent of recovery of the marsh vegetation. Color infrared aerial photographs taken in October 1986 revealed an increase in healthy vegetation and a decrease in stressed vegetation, indicating that some marsh recovery had occurred. However, some vegetated areas were converted to mudflats or open water after the spill. A review of historical aerial photographs taken between 1952 and 1978 showed that deterioration of the marsh had been occurring at an estimated annual rate of one percent since 1972 and may have increased to three percent since the early 1980s. A vegetation survey conducted in September 1986 also showed recovery of some of the marsh vegetation. The extent of recovery appeared to be a function of location of the vegetation within the marsh. Sampling plots located on the western side of the study area (adjacent to Bay Vacherie) exhibited substantial recovery, while some of the plots on the eastern half (near a flood protection levee) did not recover. The greater impact of the oil on the vegetation adjacent to the levee appears to be attributable to the prevalent wind direction at the time of the oil spill; the lower marsh elevation, causing waterlogging of the vegetation and concentration of the oil; and use of marsh buggies near the levee during the cleanup. Some additional recovery may still occur in the future in areas that have suitable surface elevation for reestablishment of the vegetation.

Role of Salt-Marsh Erosion in Barrier Island Evolution and Deterioration in Coastal Louisiana: ABSTRACT

ABSTRACT Barrier shoreline erosion in Louisiana reaches over 33 ft/yr (10 m/yr) and island area decreased by 40% between 1880 and 1979. Salt marsh erosion is an important factor in evolutionary barrier shoreline development and is presently contributing, both directly and indirectly, to the deterioration of Louisiana's barrier islands. The marshes originally developed as fresh marshes associated with regression of Mississippi River delta lobes. After fluvial abandonment, salinity gradually increased and natural habitat change occurred as subsidence of deltaic sediments and transgression of the coastline by marine processes proceeded. The marsh surface is subjected to relative sea-level rise and, unless sufficient sedimentation maintains marsh elevation, erosional processes become dominant. Increased inundation of marsh vegetation stresses even halophytic vegetation and ultimately leads to plant death. Examination of variations in marsh topography over an area of approximately 2.45 acres (1 hectare) revealed marked variations in the frequency and duration of tidal inundation. Increased flooding of lower areas can be sufficient to cause plant death and the opening of marsh ponds. Fragmentation of the marsh by these subsidence-induced processes is part of the evolution of morphostratigraphic forms in the Mississippi deltaic plain, from erosional headland with flanking barriers to barrier island arc. Coastal bays in the Mississippi deltaic plain, many of which originated as interdistributary bays before transgression, are also expanding at the expense of the surrounding salt marshes. Measurements on the margins of Terrebonne/Timbalier Bay, approximately 77 mi2 (200 km2) in area, show that the rate of marginal erosion varies according to the exposure of the bay shoreline to prevailing southeasterly winds, and fetch length across the bay. In the most exposed areas erosion rates of over 13 ft/yr (4 m/yr) have been monitored and even the north-facing bay shorelines of barrier islands experience erosion rates of up to 6.6 ft/yr (2 m/yr). The expansion of coastal bays and the fragmentation of interior marshes by pond formation leads to escalation of marsh erosion and increased threats to barrier islands. Bay margins disintegrate to a series of small islands which are rapidly eroded by wave attack. As islands disappear, wave fetch across the bay lengthens and wave attack on these island shorelines, as well as back barrier marshes, increases. Erosion of back barrier marshes both deceases island area and reduces the foundation for landward migration of the barrier sand body. Plans to restore Louisiana's barrier islands must address the problem of bay expansion and back barrier marsh erosion in order to maintain a viable barrier beach system.