Marsh Sediments Research Articles

Late Holocene human-environment interactions on the central California coast, USA, inferred from Morro Bay salt marsh sediments

Coastal salt marshes and estuaries provide valuable ecosystem services, yet are susceptible to alteration from human activities. Records of past environmental change in these ecosystems can elucidate relationships between human activities, such as land-use practices, and physical and ecological processes, such as sediment accretion and vegetation changes. To reconstruct the environmental history of one such site, we present inferences based on analysis of sediment cores (including magnetic susceptibility, loss-on-ignition, and pollen) from the Morro Bay salt marsh, located in California’s central coast in the USA. Chronologic control for the sediments was established using radiocarbon dates, a spike in lead (Pb) sourced from gasoline combustion exhaust, and the first identified occurrences of the non-native taxa Erodium cf. cicutarium (filaree) and Eucalyptus. We demonstrate that the Morro Bay watershed was significantly altered following Spanish settlement in the region. Environmental changes associated with livestock grazing and agriculture become evident in the data starting after 1772 CE, when the Mission San Luis Obispo de Tolosa was established nearby. The most prominent changes observed are an increase in the accumulation rate of terrigenous sediment, organic matter, and carbonates, as well as a reduction in arboreal taxa concomitant with increased abundances of shrubs, herbs, and grasses. The expansion of Salicornia (pickleweed) in the 19th century suggests the salt marsh expanded at this time due to increased sediment accumulation and a resulting increase in local elevation. The timing and character of changes recorded in the Morro Bay salt marsh sediments are similar to those documented across California in estuaries, marshes, lakes, and meadows, demonstrating the magnitude of the impacts of European settlement and associated land-use practices in this region.

Spatial variation and ecological risk assessment for heavy metals in marsh sediments in Fuzhou reach of the Min River, Southeast China

To explore the pollution levels, sources and risks of heavy metals in sediments in Fuzhou reach of the Min River, the sediments involving in seven marsh types were sampled. Results showed that the concentrations of Pb, Zn and Cd in sediments declined from freshwater segment to estuarine segment. Higher levels of Cu, Cr and Ni in sediments generally occurred in estuarine segment. The highest levels of Pb and Cd were observed in bush swamp, while those of Cr, Ni, Zn and Cu occurred in mudflat. Cr, Cu, Zn and Ni probably shared common source, while Pb and Cd originated from another source. Pb and Cd were identified as heavy metals of primary concerns and the former showed high potential toxicity and high contributions to ΣTUs. Next step, the metal pollutions in sediments might be more serious if effective measures were not taken to control the loading of pollutants.

Grass Shrimp (Palaemonetes pugio) as a Trophic Link for Methylmercury Accumulation in Urban Salt Marshes.

Grass shrimp (Palaemonetes pugio) represent a potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish. Across six salt marshes subject to varying degrees of Hg contamination, MeHg concentration in grass shrimp was significantly correlated with MeHg in sediment (p < 0.05, R2 = 0.81). Bioenergetic models show that grass shrimp alone account for 12-90% of MeHg observed in YOY striped bass and 6-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.

Poor recovery of fungal denitrification limits nitrogen removal capacity in a constructed Gulf Coast marsh

Widespread degradation and destruction of coastal wetlands over the last century have spurred on the practice of creating salt marshes to mitigate losses of wetland area and ecosystem function. Constructed marshes can quickly recover plant biomass, but biogeochemical functions, such as nitrogen removal capacity through denitrification, can take decades to centuries to recover. One potential mechanism for this uncoupling of structural and functional recovery is that an impaired microbial community subsequently impedes the nitrogen removal capacity of constructed marshes. While both bacteria and fungi can contribute to nitrogen removal via denitrification, little is known about fungal denitrification in wetlands, even though it has the potential to contribute to climate change via the production of the greenhouse gas nitrous oxide. Here, we measured fungal and bacterial denitrification potential rates and fungal biomass in sediments collected from a 33-year-old constructed marsh and a reference natural marsh to assess both a) the relative contribution of each group to total nitrogen removal and b) whether fungal biomass accrual is a driver of functional recovery in salt marshes. To assess the relative contributions of fungi and bacteria to denitrification, we added selective inhibitors (antifungal, antibacterial, or both) to sediments from each marsh and measured potential fungal and bacterial denitrification rates. We also measured sediment ergosterol concentrations seasonally as a proxy for fungal biomass. In the absence of inhibitors, denitrification potential rates in the constructed marsh were three times lower than in natural marsh sediment. Further, denitrification rates in the constructed marsh remained similarly low regardless of increasing inhibitor concentration, whereas denitrification rates in the natural marsh declined with increasing inhibitor concentration. When either inhibitor was added alone, denitrification was stimulated in the natural marsh, but was suppressed in the constructed marsh, suggesting an apparent competitive release, where the reduction of either bacterial or fungal competition for substrate thereby allowed the remaining competitor greater access to the substrate, in the natural marsh. We observed that compared to the constructed marsh, fungi in the natural marsh contributed ∼30% more (50% vs. 21%) to denitrification, and that fungal biomass was generally higher. Collectively, these data suggest that 3 decades post construction, denitrification rates and the contribution of fungal denitrification to total denitrification are lower in the constructed marsh than the natural marsh. These data also suggest that fungal biomass may limit the recovery of nitrogen removal in constructed marshes and that fungal denitrification can be an important pathway of N removal in salt marsh ecosystems.

Seasonal and Zonal Succession of Bacterial Communities in North Sea Salt Marsh Sediments.

Benthic microbial communities of intertidal zones perform important biogeochemical processes and provide accessible nutrients for higher organisms. To unravel the ecosystem services of salt marsh microbial communities, we analyzed bacterial diversity and metabolic potential along the land–sea transition zone on seasonal scales on the German North Sea Island of Spiekeroog. Analysis of bacterial community was based on amplicon sequencing of 16S rRNA genes and –transcripts. Insights into potential community function were obtained by applying the gene prediction tool tax4fun2. We found that spatial variation of community composition was greater than seasonal variations. Alphaproteobacteria (15%), Gammaproteobacteria (17%) and Planctomycetes (11%) were the most abundant phyla across all samples. Differences between the DNA-based resident and RNA-based active communities were most pronounced within the Planctomycetes (17% and 5%) and Cyanobacteriia (3% and 12%). Seasonal differences were seen in higher abundance of Gammaproteobacteria in March 2015 (25%) and a cyanobacterial summer bloom, accounting for up to 70% of the active community. Taxonomy-based prediction of function showed increasing potentials for nitrification, assimilatory nitrate and sulfate reduction from sea to land, while the denitrification and dissimilatory sulfate reduction increased towards the sea. In conclusion, seasonal differences mainly occurred by blooming of individual taxa, while the overall community composition strongly corresponded to locations. Shifts in their metabolism could drive the salt marsh’s function, e.g., as a potential nitrogen sink.

“Slow” and “fast” in blue carbon: Differential turnover of allochthonous and autochthonous organic matter in minerogenic salt marsh sediments

AbstractSalt marshes are recognized as blue carbon ecosystems for their ability to rapidly sequester organic carbon (OC) via sedimentation. For this reason, there is growing interest in managing these systems to help mitigate climate change. However, in order for sediment accretion to offset ongoing CO2 emissions, organic matter undergoing burial must be derived from contemporary primary production. To investigate the provenance of OC sequestered by salt marshes, we analyzed the concentration and isotope composition (δ13C and Δ14C) of sedimentary OC in two minerogenic marshes in San Francisco Bay, California, USA. Data from the low marsh dominated by Spartinaspp. show that autochthonous OC is entering the sediment matrix and may in part be stabilized through incorporation into the mineral‐bound pool. However, the sedimentary OC stock as a whole is dominated by material with low values of δ13C (~ −20 to −25‰) and Δ14C (~ 100 to −300‰) indicating a preponderance of allochthonous OC that was retained in other OC reservoirs for 102 to 103 years prior to deposition in the marsh. Diagenetic modeling of OC and its δ13C values further shows that autochthonous OC is lost from the sediment profile at a much faster rate than allochthonous OC. The rapid turnover of contemporary autochthonous OC (“fast carbon”) relative to the apparent stability of old allochthonous OC (“slow carbon”) suggest that carbon burial at these sites is largely decoupled from ongoing CO2 emissions.

Loss of geomorphic diversity in shallow tidal embayments promoted by storm-surge barriers.

Coastal flooding prevention measures, such as storm-surge barriers, are being widely adopted globally because of the accelerating rise in sea levels. However, their impacts on the morphodynamics of shallow tidal embayments remain poorly understood. Here, we combine field data and modeling results from the microtidal Venice Lagoon (Italy) to identify short- and long-term consequences of flood regulation on lagoonal landforms. Artificial reduction of water levels enhances wave-induced sediment resuspension from tidal flats, promoting in-channel deposition, at the expense of salt marsh vertical accretion. In Venice, we estimate that the first 15 closures of the recently installed mobile floodgates operated between October 2020 and January 2021 contributed to a 12% reduction in marsh deposition, simultaneously promoting a generalized channel infilling. Therefore, suitable countermeasures need to be taken to offset these processes and prevent significant losses of geomorphic diversity due to repeated floodgate closures, whose frequency will increase as sea levels rise further.

REE Geochemistry of Neogene–Holocene Sediments of La Fontanilla Cove (Tinto Estuary, SW Spain)

The Tinto and Odiel rivers (SW Spain) drain from a vast sulfide mining district and join at a 20-km-long estuary that enters the Atlantic Ocean. In this work, the contents of rare earth elements (REE) and fractionation in Neogene–Holocene sediment cores from La Fontanilla cove (Tinto estuary) were studied. The sediments were collected from a depth of 18 m at different distances from the recent river flow and were analyzed for new information on the temporal development of the REE load in the sediment column. Results show that the ∑ REE is higher in the finer sediments and during periods of mining activity from prehistoric to recent times. Marine influence appears to increase the light REE (LREE) relative to the heavy REE (HREE). The REE patterns of these estuarine sediments show convex curvatures in the MREE relative to the LREE and HREE, indicating the presence of acid-mixing processes between the fluvial waters affected by acid mine drainage (AMD) and seawater, as well as the precipitation of poorly crystalline mineral phases. Significant positive Eu anomalies were found in ebb-tide channels and marsh deposits, which can reflect the mineralogical composition and/or a strong localized salinity gradient combined with organic matter degradation. Sedimentological characteristics of the deposits appear to play the main role in accumulation and fractionation of the REE.

Beyond habitat boundaries: Organic matter cycling requires a system‐wide approach for accurate blue carbon accounting

AbstractAs coastal ecosystems become widely recognized for their capacity to sequester carbon (blue carbon), standard accounting methodologies for the generation of carbon credits are being developed. To ensure the applicability of these standards across blue carbon ecosystems, we investigated organic carbon provenance and burial in salt marshes and seagrass meadows of an arid, upwelling‐dominated Eastern Pacific lagoon. We found low carbon density in benthic sediments of Bahía de San Quintín (5.9 ± 0.5 mg C cm−3), only marginally higher in Zostera marina beds (6.9 ± 0.5 mg C cm−3), likely due to remineralization and hydrodynamically driven export of seagrass material, resulting in low carbon burial rates (4.5 ± 2.5 g C m−2 yr−1). Sediment organic carbon is mainly controlled by the fraction of fine sediment and its source is largely allochthonous, although sources differ spatially. Salt marshes at San Quintín derive 40% of their organic matter from autochthonous material and exhibit higher carbon burial rates (up to 414.7 ± 28.6 g C m−2 yr−1) and sediment carbon densities (32.0 ± 0.7 mg C cm−3) compared to benthic sediments. This study emphasizes the connectivity of blue carbon habitats with marsh plant detritus supplementing benthic carbon burial and incorporation of detrital eelgrass in marsh sediments. Our findings highlight the importance of allochthonous organic matter for carbon sequestration in blue carbon habitats, suggesting standard accounting practices that deduct allochthonous organic matter would miss the full potential for carbon burial.

Saltmarsh sediments with wastewater input emit more carbon greenhouse gases but less N2O than mangrove sediments

Wastewater discharge and salt marsh invasion are two main disturbances in global mangrove wetland ecosystems, but their combined effects on greenhouse gas (GHG) emissions remain unclear. Here, we performed a laboratory incubation experiment by adding wastewater (aquaculture and domestic wastewater) to mangrove and salt marsh sediments. Dissolved GHGs (CO2, CH4 and N2O) and environmental parameters were measured over a time series of 50 days. The nitrification and denitrification rates and corresponding functional gene abundances (AOA, AOB, nirS, and narG) were also quantified. The results show that wastewater addition significantly increased GHG emissions and global warming potential (GWP) in wetland sediments (p < 0.05 for both). Compared with mangroves, salt marsh sediments produced more carbon GHGs (CO2 and CH4) but less N2O after wastewater input. These different stimulatory effects could be explained by the nitrogen and carbon substrate availability, organic matter quality, and functional gene abundances in sediments. Assessing GHG emissions as CO2 equivalents, future scenarios of increasing salt marsh invasion with wastewater addition would increase the GWP caused by CO2 and CH4, decrease blue carbon sequestration potential in wetlands, and decrease the GWP caused by N2O and total GWP, indicating a great impact on global climate change and important implications for managing coastal wetlands.

Estimation of the natural background of phosphate in a lowland river using tidal marsh sediment cores

Abstract. Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. In order to underpin the definition of current limits, this study was set up to identify the pre-industrial background PO4 concentration in surface water of the Scheldt River, a tidal river in Flanders. We used the sedimentary records preserved in tidal marsh sediment cores as an archive for reconstructing historical changes in surface water PO4. For sediment samples at sequential depths below the sediment surface, we dated the time of sediment deposition and analysed the extractable sediment P. The resulting time series of sediment P was linked to the time series of measured surface water-PO4 concentrations (data 1967–present). By combining those datasets, the sorption characteristics of the sediment could be described using a Langmuir-type sorption model. The calibrated sorption model allowed us to estimate a pre-industrial background surface water PO4 levels, based on deeper sediment P that stabilized at concentrations smaller than the modern. In three out of the four cores, the sediment P peaked around 1980, coinciding with the surface water PO4. The estimated pre-industrial (∼1800) background PO4 concentration in the Scheldt River water was 62 [57; 66 (95 % CI)] µg PO4-P L−1. That concentration exceeds the previously estimated natural background values in Flanders (15–35 µg TP L−1) and is about half of the prevailing limit in the Scheldt River (120 µg PO4-P L−1). In the 1930s, river water concentrations were estimated at 140 [128; 148] µg PO4-P L−1, already exceeding the current limit. The method developed here proved useful for reconstructing historical background PO4 concentrations of a lowland tidal river. A similar approach can apply to other lowland tidal rivers to provide a scientific basis for local catchment-specific PO4 backgrounds.

Response of the Akrotiri Marsh, island of Cyprus, to Bronze Age climate change

Here we present two multiproxy records covering the last 5000 years from the Akrotiri Marsh in southern Cyprus. Pollen and diatom analysis of radiocarbon dated marsh sediments with an average chronological resolution of one date every thousand years, reveal expansion and contraction of the marsh in response to mid-late Holocene climate events, with peak aridity reconstructed in the early Bronze Age, inferred between 4.3 and 4.1 cal ka BP, and repeated dry intervals in the Late Bronze Age, inferred between 3.4 and 3.1 cal ka BP. The record provides important contextual climate data to the debate surrounding reported Early and Late Bronze Age societal collapse events present in numerous archaeological archives throughout the Mediterranean and Near Eastern region. This is the first multiproxy record from the island of Cyprus potentially displaying the 4.2 ka BP event. The characteristics of the inferred 3.2 ka BP event are very similar to the manifestation of the event in the east of Cyprus and on the southern Levantine mainland. These results contribute to the regional understanding of Bronze Age climates on Cyprus, give insight into the expression of global climate forcing mechanisms such as the North Atlantic Oscillation and the Siberian High, and provide potential evidence for reduced anthropogenic land-use during the 4.2 ka BP and 3.2 ka BP Events supporting what is documented in archaeological archives. • Two 5000-year-old multiproxy records from the Akrotiri Marsh, southern Cyprus • Both the 4.2 K and 3.2 K climate events had an impact on the island's water balance. • Pollen & diatoms infer Early Bronze Age peak aridity between 4.3 & 4.1 cal ka BP • Repeat dry intervals in Late Bronze Age from 3.4 to 3.1 cal ka BP, W-Shaped event • Insight into the expression of the North Atlantic Oscillation and the Siberian High

Spartina Alterniflora Invasion Changed Enrichment and Coupling Characteristics of Multiple Elements in Coastal Marsh Sediment of the Yellow River Delta

Spartina Alterniflora Invasion Changed Enrichment and Coupling Characteristics of Multiple Elements in Coastal Marsh Sediment of the Yellow River Delta

Benthic Biofilm Potential for Organic Carbon Accumulation in Salt Marsh Sediments

Coastal salt marshes are productive environments with high potential for carbon accumulation and storage. Even though organic carbon in salt marsh sediment is typically attributed to plant biomass, it can also be produced by benthic photosynthetic biofilms. These biofilms, generally composed of diatoms and their secretions, are known for their high primary productivity and contribution to the basal food web. The growth of biofilms and the preservation of carbon produced by biofilms depends on the amount of sedimentation; low sedimentation rates will favor decomposition, while high sedimentation rates could decrease biofilm productivity. In this study, we conducted laboratory experiments to test (1) if biofilms can potentially accumulate carbon in marsh soil and (2) how different sedimentation rates affect the amount of carbon accumulation. Containers filled with a settled mud bed were inoculated with natural biofilms collected from a marsh surface and allowed to grow with favorable light exposure, nutrient supply, and absence of grazing. Mud was added weekly in different amounts, resulting in an equivalent sedimentation rate from 12 to 189 mm/yr. After 11 weeks, the sediment columns were sampled and analyzed for chl a, organic matter via loss on ignition (LOI), and total organic carbon (TOC). Chl a accumulation rates ranged from 123 to 534 mg/cm2/yr, organic matter accumulation ranged from 86 to 456 g/m2/yr, and TOC accumulation rates ranged from 31 to 211 g/m2/yr. These values are on the same order of magnitude of marsh carbon accumulation rates measured in the field. All three metrics (chl a, organic matter, and TOC) increased with increased sedimentation rate. These results show that biofilms can potentially contribute to carbon accumulation in salt marsh soils. Furthermore, areas with high sedimentation rates have the potential for higher amounts of organic matter from biofilms in the sediment.

A 1900 Year Sediment Record Suggests Recent Establishment of Black Mangrove (Avicennia Germinans) Stands within a Salt Marsh in St. Augustine, Florida, USA

Salt marshes and mangroves are currently being affected by rising temperatures. Mangroves thrive below −29° N latitude in Florida, USA, and have a low tolerance for extreme cold events, whereas salt marshes dominate further north. One potential effect of climate change is a reduction in the frequency of extreme cold events, which may lead to mangrove expansion into salt marsh systems. Our research identified sediment proxy indicators of salt marsh and mangrove environments. These indicators were applied to soil cores from intertidal wetlands near the current northern limit of mangrove presence on the east coast of Florida, to determine if mangrove expansion into salt marsh environments has precedence in the deeper past. Our findings suggest that mangrove and salt marsh sediments can be distinguished using a combination of stable carbon isotope ratios of sedimentary organic matter and macroscopic plant fragments, and our results showed that a mangrove stand that we cored established only recently. This result is consistent with other work in the southeastern United States that suggests that mangroves established at the current boreal limit only recently after the end of the Little Ice Age, and that the current mangrove expansion may be fueled by anthropogenic climate change.

Vegetation interactions with geotechnical properties and erodibility of salt marsh sediments

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a global scale phenomenon, of great socio-economic concern due to the substantial benefits that they provide. However, the causes of spatial variability in marsh loss rates are inadequately understood for the purposes of predicting future ecosystem distributions and functions under global environmental change. This study investigated the relationship between the presence of different saltmarsh plants and the mechanical properties of the underlying substrate that relate to its vulnerability to erosion. Relationships between three halophytes (Puccinellia spp., Spartina spp. and Salicornia spp.) and sediment stability were assessed and compared to unvegetated substrates using in-situ and laboratory tests of substrate geotechnical properties and sediment characteristics. Sampling was conducted at two UK sites with contrasting sedimentology, one sand-dominated and one clay-rich. Sediment samples, collected simultaneously with measurements of shear strength, were analysed for moisture content, particle size and organic, carbonate and mineral compositions. These data were then used to explore the contribution of plant type, alongside the sedimentological parameters, to measured shear strength.Shear strength of the sediment varied between and, to a lesser extent, within sites, with the four cover types having a similar effect on shear strength within sites relative to each other. Sediments covered by Puccinellia spp exhibit the highest shear strength, while bare sediments exhibit the lowest. The effect of vegetation type on shear strength was greater in the coarser sediments of Warton Sands. Surface cover type made a significant contribution to exploratory statistical models developed for the prediction of sediment shear strength. The findings support existing recognition that vegetation can enhance sediment shear strengths but extend the insight to reveal differences in this effect that show generality between sedimentological settings. Further, the combination of methods provides insight into the fundamental mechanics by which various measures of sediment stability may be affected by different surface cover types. Cohesion appears to be a more appropriate descriptor of sediment erodibility than shear strength or friction angle and is most greatly enhanced by the presence of a fine, fibrous root system such as that of Puccinellia. A more detailed understanding of the multi-scale mechanisms by which plants confer strength to substrates is needed to better anticipate their impact on sediment erodibility, and therefore salt marsh vulnerability.

Changes in sediment characteristics in the first year of a realigned saltmarsh

Saltmarsh restoration is described as a cost-effective response to coastal biodiversity loss and flood management, although, to date such conservation practices yield different characteristics than natural marshes, such as in plant communities and sediment properties. The outcome of these differences is thought to be influenced by the initial conditions set within the first year of marsh re-establishment, which are often overlooked or neglected as part of the restoration process. Therefore, our study aimed to address this gap by examining the spatial and temporal variations in sediment characteristics of a newly (<15 months) realigned saltmarsh in Essex, UK and the effects of previous land use (arable and pasture). Sediment properties (bulk density, nutrients and pH) were sampled monthly from the realigned site as well as from an adjacent reference natural marsh. After 14 months from initial inundation the two realigned marsh regions (arable and pasture) behaved similarly despite different starting points in terms of pH, water content, bulk density and nutrient load. Physical aspects of the realigned sediments (bulk density and water content) matched those of the reference natural marsh within 14 months post-breach in the 0–5 cm depth range. By contrast, the lower depth sediments within realigned sites remained more dense and drier compared to natural marsh sediments. Chemical properties of the realigned sediments matched those of the natural marsh throughout the sediment profile 14 months post-breach. Success of restoration is measured in the ability of the site to provide the desired ecosystem services and short-term recoveries and set goals may not imply long term sustainability as these observed differences may have ramifications for future biological community development and the success of a restoration.

Geological evidence of an unreported historical Chilean tsunami reveals more frequent inundation

Assessing tsunami hazards commonly relies on historical accounts of past inundations, but such chronicles may be biased by temporal gaps due to historical circumstances. As a possible example, the lack of reports of tsunami inundation from the 1737 south-central Chile earthquake has been attributed to either civil unrest or a small tsunami due to deep fault slip below land. Here we conduct sedimentological and diatom analyses of tidal marsh sediments within the 1737 rupture area and find evidence for a locally-sourced tsunami consistent in age with this event. The evidence is a laterally-extensive sand sheet coincident with abrupt, decimetric subsidence. Coupled dislocation-tsunami models place the causative fault slip mostly offshore rather than below land. Whether associated or not with the 1737 earthquake, our findings reduce the average recurrence interval of tsunami inundation derived from historical records alone, highlighting the importance of combining geological and historical records in tsunami hazard assessment.

Mercury exposure of tidal marsh songbirds in the northeastern United States and its association with nest survival.

The biogeochemistry of tidal marsh sediments facilitates the transformation of mercury (Hg) into the biologically available form methylmercury (MeHg), resulting in elevated Hg exposures to tidal marsh wildlife. Saltmarsh and Acadian Nelson's sparrows (Ammospiza caudacutua and A. nelsoni subvirgatus, respectively) exclusively inhabit tidal marshes, potentially experiencing elevated risk to Hg exposure, and have experienced range-wide population declines. To characterize spatial and temporal variation of Hg exposure in these species, we sampled total mercury (THg) in blood collected from 9 populations spanning 560 km of coastline, including individuals resampled within and among years. Using concurrent nesting studies, we tested whether THg was correlated with nest survival probabilities, an index of fecundity. Blood THg ranged from 0.074-3.373 µg/g ww across 170 samples from 127 individuals. We detected high spatial variability in Hg exposure, observing differences of more than 45-fold across all individuals and 8-fold in mean blood THg among all study plots, including 4-fold between study plots within 4 km. Intraindividual changes in blood Hg exposure did not vary systematically in time but were considerable, varying by up to 2-fold within and among years. Controlling for both species differences and maximum water level, the dominant driver of fecundity in this system, nest survival probability decreased by 10% across the full range of female blood THg concentrations observed. We conclude that Hg has the potential to impair songbird reproduction, potentially exacerbating known climate-change driven population declines from sea-level rise in saltmarsh and Acadian Nelson's sparrows.

Tidal Marsh Sediment and Carbon Accretion on a Geomorphologically Dynamic Coastline

AbstractWe examined the sediment profiles, surface accretion rates, and soil carbon (C) accumulation rates of two brackish tidal marshes on Kilbella and Wannock Rivers of the central coast of British Columbia, Canada, a region where such tidal marsh studies are rare. Sediment grain size is variable within these deposits and is a significant predictor of soil C density. The average soil C density of the Kilbella (0.027 ± 0.01 g C cm−³) and the Wannock marsh (0.029 ± 0.007 g C cm−³) are within the range of reported global averages. Compared to global estimates, the depth weighted mean rates of soil C accumulation are low, between 29 and 50 g C m−2 year−1, although high sectional rates (up to 490 ± 230 g C m−³ year−1) occur in deeper segments of the marsh profile at Kilbella. Modern accretion rates are below moderate to high sea level rise scenarios. The Kilbella marsh contains sand layers from periods of mass sedimentation events (landslides and tsunamis), which provide the marsh with elevation capital that can be critical to its sustainability. The Kilbella profile demonstrates that variation in carbon density with depth cannot be predicted in marshes on geomorphologically dynamic coasts.