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1.
The influence of canals on vertical marsh accretion, including mineral sediment and organic matter accumulation, was evaluated at three locations along the Louisiana coast representing different geographic regions. The isotopes210Pb and157Cs were used to determine vertical accretion along transects representing a canal and a control site. Rapid rates of vertical accretion were measured at all sites and ranged from 0.47 cm yr?1 to 0.90 cm yr?1. Results indicated that there was no measurable effect of canals on marsh accretionary processes. In general, greater variation in vertical accretion, including mineral sediment deposition and organic matter accumulation, was observed between geographical regions than between canal and control sites within a region. Statistical analysis of data suggest that any difference between canal and control site would be less than 0.20 cm yr?1. Such a change in marsh surface-water level relationships as a result of any canal influence on marsh accretionary processes would be less than reported eustatic sea-level rise for the Gulf of Mexico. Results suggest that any change in the marsh surface-water level relationship could be the influence of canals on local hydrology, resulting in increased water level rather than any appreciable reduction in accretionary processes. Such changes in hydrology under certain conditions could stress vegetation, resulting in marsh deterioration.  相似文献   

2.
In order to test the assumption that accretion rates of intertidal salt marshes are approximately equal to rates of sea-level rise along the Rhode Island coast,210Pb analyses were carried out and accretion rates calculated using constant flux and constant activity models applied to sediment cores collected from lowSpartina alterniflora marshes at four sites from the head to the mouth of Narragansett Bay. A core was also collected from a highSpartina patens marsh at one site. Additional low marsh cores from a tidal river entering the bay and a coastal lagoon on Block Island Sound were also analyzed. Accretion rates for all cores were also calculated from copper concentration data assuming that anthropogenic copper increases began at all sites between 1865 and 1885. Bulk density and weight-loss-on-ignition of the sediments were measured in order to assess the relative importance of inorganic and organic accumulation. During the past 60 yr, accretion rates at the eight low marsh sites averaged 0.43±0.13 cm yr?1 (0.25 to 0.60 cm yr?1) based on the constant flux model, 0.40±0.15 cm yr?1 (0.15 to 0.58 cm yr?1) based on the constant activity model, and 0.44±0.11 cm yr?1 (0.30 to 0.59 cm yr?1) based on copper concentration data, with no apparent trend down-bay. High marsh rates were 0.24±0.02 (constant flux), 0.25±0.01 (constant activity), and 0.47±0.04 (copper concentration data). The cores showing closest agreement between the three methods are those for which the excess210Pb inventories are consistent with atmospheric inputs. These rates compare to a tide gauge record from the mouth of the bay that shows an average sea-level rise of 0.26±0.02 cm yr?1 from 1931 to 1986. Low marshes in this area appear to accrete at rates 1.5–1.7 times greater than local relative sea-level rise, while the high marsh accretion rate is equal to the rise in sea level. The variability among the low marsh sites suggests that marshes may not be poised at mean water level to within better than ±several cm on time scales of decades. Inorganic and organic dry solids each contributed about 9% by volume to low marsh accretion, while organic dry solids contributed 11% and inorganic 4% to high marsh accretion. Water/pore space accounted for the majority of accretion in both low and high marshes. If water associated with the organic component is considered, organic matter accounts for an average of 91% of low marsh and 96% of high marsh accretion. A dramatic increase in the organic content at a depth of 60 to 90 cm in the cores from Narragansett Bay appears to mark the start of marsh development on prograding sand flats.  相似文献   

3.
Analyses of organic content, pollen, and the carbon-isotopic composition of a 3.5-m sediment core collected from a subsided tidal marsh located in South San Francisco Bay, California, have provided a 500-yr record of sediment accretion and vegetation change before, during, and after a rapid 1 m increase in sea level. Core chronology was established using14C dating of fossil plant material, the first appearance of pollen types produced by plants not native to California, and changes in lead concentrations coincident with anthropogenic contamination. Prior to the mid 19th century, rates of sediment accretion were between 1 and 4 mm yr−1; sediment accretion accelerated to an average of 22 mm yr−1 following the initiation of subsidence. Changes in tidal marsh vegetation also accompanied this depositional change. Vegetation shifted from a high to low marsh assemblage, as indicated by a larger percentage of grass pollen, rhizomes ofSpartina foliosa, and a strong C4 signal. Between 1980 and 2001, Triangle marsh again developed high marsh vegetation, as indicated by higher percentages of the Amaranthaceane pollen type, seed deposition, includingSalicornia spp., and more negative carbon isotopic ratios.  相似文献   

4.
Throughflow marsh flumes were used to measure total sediment exchanges (TSS) between the marshes and water column of two Louisiana estuaries. One, the Barataria Basin estuary, is isolated from significant riverine sediment input. There were significant (p<0.05) imports of 33.9 to 443 mg TSS m?2 h?1 at the Barataria Basin brackish marsh (BM) site. The Barataria Basin saltmarsh (SM) site exported TSS in two summer samplings, but significant uptake was measured in April (166 mg m?2 h?1) and November (45 mg m?2 h?1) during a winter frontal passage event. The other estuary, Fourleague Bay, receives large sediment inputs from the Atchafalaya River, and TSS imports of 22.5 to 118.5 mg m?2 h?1 were measured at the BM site here. We calculated sediment accumulation from fluxes quantified in marsh flumes using site-specific sedimentological data and flooding regimes at each site. Water level records from May 1987 to April 1989 showed an extended period of unusually low flooding frequencies. As a result, calculated accretion rates were low, with monthly rates of 0.02 to 0.11 mm and ?0.06 to 0.06 mm at the Barataria BM and SM sites, respectively, and ?0.18 to 0.08 mm at the Fourleague Bay marsh flume site. Actual net sediment deposition, determined by feldspar marker horizon analysis, was 0.7–1.6 mm mo?1 at the Barataria SM and 0.2–1.3 mm mo?1 at the Fourleague Bay BM. Even the highest calculated accretion rates, based on flume measurements, were half to one order of magnitude lower than actual measured sediment deposition. This discrepancy was probably because: 1) most sedimentation occurs during episodic events, such as Hurricane Gilbert in September 1988, which deposited 3.5–15.5 mm of sediment on the Barataria Basin saltmarsh, or 2) most vertical accretion in Louisiana marshes occurs via deposition of in situ organic matter rather than by influx of allochthonous sediment. Our results affirm the variability of short-term sediment transport and depositional processes, the close coupling of meteorologic forcing and flooding regime to sediment dynamics, and the importance of understanding these interrelated mechanisms in the context of longer term measurements.  相似文献   

5.
The suitability of marsh sites for sea‐level studies was examined based on field experiments along a transect from low to high marsh. Bead distributions were determined both seasonally and after 7 years. Seasonal sediment mixing was greatest in the low marsh and in the late spring and early summer, when biological activity is greatest. However, after an initial interval of relatively intense reworking, the bead concentrations reached an approximate equilibrium profile characteristic of each marsh environment as reflected by the profiles obtained after 7 years. Mixed‐layer thickness is greatest (>10 cm) in the intermediate and low marsh, and burial rates are rapid (3.7–11.1 mm yr?1). Moreover, burial rates are comparable to or even surpass longer‐term (30 to >150 yr) radiotracer‐derived sediment accumulation rates and rates of local and regional sea‐level rise (～4 mm yr?1). Therefore, sediment accumulation rates appear to reflect primarily sediment resuspension/redeposition within the system due to bioturbation. Thus, bioturbation may be critical to the ability of marshes to keep pace with sea level, while seemingly precluding the use of low marsh for high‐resolution sea‐level studies. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

6.
In an attempt to characterize localized rates of sediment accretion, 10 sediment cores were collected from the lower reach of the Passaic River, a major tributary of Newark Bay, New Jersey. Sediments were assayed for 210Pb activity at predetermined depths and the rate of sediment accretion (cm yr?1) was estimated from the least squares regression of the log of unsupported activity versus depth. Sediment accretion rates, derived from 210Pb measurements (RPb) were used to predict the depth interval within the core containing sediments deposited around 1954; subsequent 137Cs analyses were focused on this depth interval. Sediment accretion rates derived from 137Cs measurements (RCs) were extrapolated from the depth of the 1954 horizon. Lead-210 derived sediment accretion rates in cores collected from a sediment bench extending along the inside bend on the southern shore of a meander in the river, ranged from 4.1 cm yr?1 to 10.2 cm yr?1 and averaged 6.8 cm yr?1. The RCs estimates for cores from this area ranged from 3.8 cm yr?1 to 8.9 cm yr?1 and averaged 6.6 cm yr?1. The RCs for cores collected in a more hydrologically dynamic reach of the river upstream of the sediment bench, were only 0.41 cm yr?1 and 0.66 cm yr?1. The results of this investigation indicate that this reach of the lower Passaic River is an area of high sediment accumulation, retaining much of the sediment load deposited from upstream and downstream sources. The rates of sediment accretion in the lower Passaic River are among the highest reported anywhere in the Newark Bay estuary.  相似文献   

7.
We studied organic matter cycling in two Gulf Coast tidal, nonsaline marsh sites where subsidence causes marine intrusion and rapid submergence, which mimics increased sea-level rise. The sites experienced equally rapid submergence but different degrees of marine intrusion. Vegetation was hummocked and much of the marsh lacked rooted vegetation. Aboveground standing crop and production, as measured by sequential harvesting, were low relative to other Gulf CoastSpartina patens marshes. Soil bulk density was lower than reported for healthyS. alterniflora growth but that may be unimportant at the current, moderate sulfate levels. Belowground production, as measured by sequential harvesting, was extremely fast within hummocks, but much of the marsh received little or no belowground inputs. Aboveground production was slower at the more saline site (681 g m?2 yr?1) than at the less saline site (1,252 g m?2 yr?1). Belowground production over the entire marsh surface averaged 1,401 g m?2 yr?1 at the less saline site and 585 g m?2 yr?1 at the more saline site. Respiration, as measured by CO2 emissions in the field and corrected for CH4 emissions, was slower at the less saline site (956 g m?2 yr?1) than at the more saline site (1,438 g m?2 yr?1), reflecting greater contributions byS. alterniflora at the more saline site which is known to decompose more rapidly thanS. patens. Burial of organic matter was faster at the less saline site (796 g m?2 yr?1) than at the more saline site (434 g m?2, yr?1), likely in response to faster production and slower decomposition at the less saline site. Thus vertical accretion was faster at the less saline site (1.3 cm yr?1) than at the more saline site (0.85 cm yr?1); slower vertical accretion increased flooding at the more saline site. More organic matter was available for export at the less saline site (1,377 g m?2 yr?1) than at the more saline site (98 g m?2 yr?1). These data indicated that organic matter production decreased and burial increased in response to greenhouse-like conditions brought on by subsidence. *** DIRECT SUPPORT *** A01BY069 00016  相似文献   

8.
N2 fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N2 fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C2H4 cm?2 h?1 to 14 nmol C2H4 cm?2 h?1, while in the transplanted marsh mean AR rates ranged from 1 nmol C2H4 cm?2 h?1 to 33 nmol C2H4 cm?2 h?1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N2 fixation is estimated to be 2.6 g N m?2 yr?1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m?2 yr?1. These estimates should be added to previous estimates of N2 fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.  相似文献   

9.
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.  相似文献   

10.
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.  相似文献   

11.
Belowground production of roots and rhizomes in the top 20 cm of soil was 2.2 kg m?2 yr?1 based on a maximum minus minimum estimation procedure in a giant cordgrass (Spartina cynosuroides (L.) Roth) marsh in Mississippi. Approximately 1.9 kg m?2 (86%) of this production occurred in late spring-summer and 0.3 kg m?2 in late fall. This estimate ignores any production below 20 cm depth and is thus an underestimate. Production values increased to 4.0 kg m?2 yr?1 using Smalley’s technique and accounting for decomposition. Aboveground tissues (leaves and stems) were depleted in nitrogen in July which corresponded to peaks in both above- and belowground biomass. The low root/shoot ratio (2.6) on this marsh does not suggest that growth is nutrient limited. Indeed, total productivity (above- and belowground) for this marsh was high (between 4.4 and 6.2 kg m?2 yr?1).  相似文献   

12.
Salt marsh resilience to sea-level rise depends on marsh plain elevation, tidal range, subsurface processes, as well as surface accretion, of which suspended-sediment concentration (SSC) is a critical component. However, spatial and temporal patterns of inorganic sedimentation are poorly quantified within and across Salicornia pacifica (pickleweed)-dominated marshes. We compared vertical accretion rates and re-examined previously published suspended-sediment patterns during dry-weather periods at Seal Beach Wetlands, which is characterized by a mix of Spartina foliosa (cordgrass) and pickleweed, and for Mugu Lagoon, where cordgrass is rare. Mugu Lagoon occurs higher in the tidal frame and receives terrigenous sediment from an adjacent creek. Feldspar marker horizons were established in winter 2013–2014 to measure accretion. Accretion rates at Seal Beach Wetlands and Mugu Lagoon were 6 ± 0.5 mm/year (mean ± SE) and 2 ± 0.3 mm/year. Also, the estimated sediment flux (g/year) across the random feldspar plots was 3.5 times higher at Seal Beach Wetlands. At Mugu Lagoon, accretion was higher near creeks, although not statistically significant. Dry-weather SSC showed similar concentrations at transect locations across sites. During wet weather, however, SSC at Mugu Lagoon increased at all locations, with concentrations decaying farther than 8 m from tidal creek edge. Based on these results from Mugu Lagoon, we conclude accretion patterns are set by infrequent large flooding events in systems where there is a watershed sediment source. Higher accretion rates at Seal Beach Wetlands may be linked to lower-marsh elevations, and thus more frequent inundation, compared with Mugu Lagoon.  相似文献   

13.
Accretion rates were measured in fringe and basin mangrove forests in river and tidally dominated sites in Terminos Lagoon, Mexico, and a basin mangrove forest in Rookery Bay, Florida, USA. Accretion rates were determined using the radionuclides210Pb and137Cs. Consolidation-corrected accretion rates for the Rookery Bay cores, ranged from 1.4 to 1.7 mm yr?1, with an average rate of 1.6 mm yr?1. Rates at the Mexico sites ranged from 1.0 to 4.4 mm yr?1, with an average of 2.4 mm yr?1. Determination of rates in these mangrove forests was greatly affected by the consolidation corrections which decreased the apparent accretion rate by over 50% in one case. Accretion rates at basin sites compare favorably with a reported 1.4 to 1.6 mm yr?1 rate of sea-level rise, indicating little or no subsidence at inland locations. Accretion rates in fringe sites are generally greater than basin sites, indicating greater subsidence rates in these sediments over longer time intervals.  相似文献   

14.
The accumulation of selected plant nutrients and heavy metals in a rapidly accreting Louisiana salt marsh was examined. Sedimentation processes were shown to be supplying large amounts of plant nutrients to the marsh. Accumulation of heavy metals was low and appeared to be associated with the natural heavy metal content of incoming sediment rather than from a pollution source. A large portion of organic carbon from primary production remained in the marsh, contributing to the aggradation process of vertical marsh accretion. Nitrogen accumulated in the marsh at rates as great as 21 g per m2 per yr.  相似文献   

15.
A coastwide study of the relationship between marsh aggradation and water level changes along the rapidly deteriorating Louisiana gulf coast was conducted. Rate of vertical marsh accretion determined from137Cs dating was compared to water level changes or submergence. Results identified marsh locations that are not keeping pace with submergence. Coastwide vertical accretion rates on the order of 0.7–0.8 cm/yr are not sufficient to keep pace with water level increases occurring at rates in most locations of over 1.0 cm/yr. Submergence rates were four to five times greater than eustatic sea level change for the Gulf of Mexico. Louisiana gulf coast marshes are likely to continue deteriorating unless means are implemented for distributing Mississippi River sediment to the marsh. It is estimated that sediment equivalent to less than 10 percent of the present annual suspended load of the Mississippi would provide enough sediment for marsh accretionary processes to compensate for submergence or water level increase.  相似文献   

16.
A 115-year-old railroad levee bisecting a tidal freshwater marsh perpendicular to the Patuxent River (Maryland) channel has created a northern, upstream marsh and a southern, downstream marsh. The main purpose of this study was to determine how this levee may affect the ability of the marsh system to gain elevation and to determine the levee’s impact on the marsh’s long-term sustainability to local relative sea level rise (RSLR). Previously unpublished data from 1989 to 1992 showed that suspended solids and short-term sediment deposition were greater in the south marsh compared to the north marsh; wetland surface elevation change data (1999 to 2009) showed significantly higher elevation gain in the south marsh compared to the north (6?±?2 vs. 0?±?2 mm year?1, respectively). However, marsh surface accretion (2007 to 2009) showed no significant differences between north and south marshes (23?±?8 and 26?±?7 mm year?1, respectively), and showed that shallow subsidence was an important process in both marshes. A strong seasonal effect was evident for both accretion and elevation change, with significant gains during the growing season and elevation loss during the non-growing season. Sediment transport, deposition and accretion decreased along the intertidal gradient, although no clear patterns in elevation change were recorded. Given the range in local RSLR rates in the Chesapeake Bay (2.9 to 5.8 mm year?1), only the south marsh is keeping pace with sea level at the present time. Although one would expect the north marsh to benefit from high accretion of abundant riverine sediments, these results suggest that long-term elevation gain is a more nuanced process involving more than riverine sediments. Overall, other factors such as infrequent episodic coastal events may be important in allowing the south marsh to keep pace with sea level rise. Finally, caution should be exercised when using data sets spanning only a couple of years to estimate wetland sustainability as they may not be representative of long-term cumulative effects. Two years of data do not seem to be enough to establish long-term elevation change rates at Jug Bay, but instead a decadal time frame is more appropriate.  相似文献   

17.
The Susquehanna River is the major contributor to sediment loadings in the Chesapeake Bay. Because many environmental contaminants are associated with suspended particulates, the degree of particle retention within the reservoirs of the lower Susquehanna River is an important consideration in evaluating contaminant loadings to the Chesapeake Bay. Profiles of weapons-test Cs-137, nuclear power plant-related Cs-134 and Cs-137, and naturally-derived Pb-120 were used to estimate rates of sediment accretion in the conowingo Reservioir, an impoundment of the Susquehanna River along the Maryland-Pennsylvania border. Net accretion rates ranged from about 2 cm yr?1 downstream of a nuclear power plant cooling discharge to a high of about 7 cm yr?1 at the mount of an incoming creek. Slight, but consistent, increases in the annual rate of accretion since the creation of the reservoir in 1928 are apparent. The current net average annual sediment load reatined by the reservoir is estimated to be 0.4×106 to 1.5 × 106 metric tons yr?1. The retained sediment load represents about 8–23% of the long-time average sediment input to the reservoir.  相似文献   

18.
Flax Pond is a small (0.5 km2) salt marsh on the north shore of Long Island, New York. Two 1 m2 plots within each of the following environments were covered with a marker layer of either brick dust or aluminum glitter: 1) bare mud flats; 2) areas newly colonized by Spartina alterniflora; and 3) high intertidal. S. alterniflora peat surfaces. Monthly cores revealed the amount of sediment that accumulated since placement of the marker. Accretion rates from October, 1974 to February, 1976 were as follows: bare mud flats ?20.5 to 45.5 mm/yr; recently vegetated mud flats ?9.5 to 37.0 mm/yr; and high intertidal peat surfaces ?2.0 to 4.25 mm/yr. Sedimentation rates decrease with increasing elevation because of the reduced tidal submergence time and decreased height of the overlying water column. In areas of low elevation, ice and storms cause either erosion or a reduced rate of accretion during the winter months. The average mud accretion rate over the past 173 years is 3.4 mm/yr. Differences between the short-term rate and the long-term rate indicate substantial annual variation in the accumulation of mud in salt marshes. Short-term rates of peat accretion are similar to long-term estimates, indicating that rates of peat accretion are relatively constant over long intervals.  相似文献   

19.
This paper documents the role of salt marsh algal mats in the productivity of a southern California tidal wetland. The productivity of the mats, which are composed of filamentous bluegreen and green algae and diatoms, varies both temporally and spatially in relation to tidal inundation and overstory vegetation. The estimates of net primary productivity (NPP) were highest under the canopy ofJaumea carnosa (Less.) Gray (341 g C m?2 yr?1) at low elevation. Elsewhere, NPP appeared to be limited by low light (276 g C m?2 yr?1 underSpartina foliosa Trin.) and desiccation (185 g C m?2 yr?1 underBatis martima L. and 253 g C m?2 yr?1 underMonanthochloe littoralis Engelm). Algal NPP was from 0.8 to 1.4 times that of the vascular plant overstory NPP. It is hypothesized that the arid environment of southern California and resulting hypersaline soils reduce vascular plant cover, which leads to high algal productivity.  相似文献   

20.
One year’s measurements of surficial sedimentation rates (1986–1987) for 26 Maine marsh sites were made over marker horizons of brick dust. Observed sediment accumulation rates, from 0 to 13 mm yr?1, were compared with marsh morphology, local relative sea-level rise rate, mean tidal range, and ice rafting activity. Marshes with four different morphologies (back-barrier, fluvial, bluff-toe, and transitional) showed distinctly different sediment accumulation rates. In general, back-barrier marshes had the highest accumulation rates and blufftoe marshes had the lowest rates, with intermediate values for transitional and fluvial marshes. No causal relationship between modern marsh sediment accumulation rate and relative sea-level rise rate (from tide gauge records) was observed. Marsh accretionary balance (sediment accumulation rate minus relative sea-level rise rate) did not correlate with mean tidal range for this meso- to macro-tidal area. Estimates of ice-rafted debris on marsh sites ranged from 0% to >100% of measured surficial sedimentation rates, indicating that ice transport of sediment may make a significant contribution to surficial sedimentation on Maine salt marshes.  相似文献   

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