The effect of a small vegetation dieback event on salt marsh sediment transport

Vegetation is a critical component of the ecogeomorphic feedbacks that allow a salt marsh to build soil and accrete vertically. Vegetation dieback can therefore have detrimental effects on marsh stability, especially under conditions of rising sea levels. Here, we report a variety of sediment transport measurements associated with an unexpected, natural dieback in a rapidly prograding marsh in the Altamaha River Estuary, Georgia. We find that vegetation mortality led to a significant loss in elevation at the dieback site as evidenced by measurements of vertical accretion, erosion, and surface topography compared to vegetated refer- ence areas. Below-ground vegetation mortality led to reduced soil shear strength. The dieback site displayed an erosional, concave-up topographic profile, in contrast to the reference sites. At the location directly impacted by the dieback, there was a reduction in flood dominance of suspended sediment concentration. Our work illustrates how a vegetation disturbance can at least temporarily reverse the local trajectory of a prograding marsh and produce complex patterns of sediment transport

Sea Level-Driven Marsh Migration Results in Rapid Net Loss of Carbon

Sea level rise alters coastal carbon cycling by driving the rapid migration of coastal ecosystems, salinization of freshwater systems, and replacement of terrestrial forests with tidal wetlands. Wetland soils accumulate carbon (C) at faster rates than terrestrial soils, implying that sea level rise may lead to enhanced carbon accumulation. Here, we show that carbon stored in tree biomass greatly exceeds carbon stored in adjacent marsh soils so that marsh migration reduces total carbon stocks by 50% in less than 100years. Continued marsh soil carbon accumulation may eventually offset forest carbon loss, but we estimate that the time for replacement is similar to estimates of marsh survival (i.e., centuries), which suggests that forest C may never be replaced. These findings reveal a critical C source not included in coastal C budgets driven by migrating ecosystems and rapidly shifting allocations between carbon stored in soils and biomasS

Reconciling models and measurements of marsh vulnerability to sealevel rise

Tidal marsh survival in the face of sea level rise (SLR) anddeclining sediment supply often depends on the ability ofmarshes to build soil vertically. However, numerical models typically predict survival under rates of SLR that farexceedfield-based measurements of vertical accretion. Here, we combine novel measurements from sevenU.S. Atlantic Coast marshes and data from 70 additional marshes from around the world to illustrate that—over con-tinental scales—70% of variability in marsh accretion rates can be explained by suspended sediment concentratin(SSC) and spring tidal range (TR). Apparent discrepancies between models and measurements can be explained bydiffering responses in high marshes and low marshes,the latter of which accretes faster for a given SSC andTR. Together these results help bridge the gap between models and measurements, and reinforce the paradigm thatsediment supply is the key determinant of wetland vulnerability at continental scales

Inferring tidal wetland stability from channel sediment fluxes : observations and a conceptual model

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Earth Surface 118 (2013): 2045–2058, doi:10.1002/jgrf.20143.Anthropogenic and climatic forces have modified the geomorphology of tidal wetlands over a range of timescales. Changes in land use, sediment supply, river flow, storminess, and sea level alter the layout of tidal channels, intertidal flats, and marsh plains; these elements define wetland complexes. Diagnostically, measurements of net sediment fluxes through tidal channels are high-temporal resolution, spatially integrated quantities that indicate (1) whether a complex is stable over seasonal timescales and (2) what mechanisms are leading to that state. We estimated sediment fluxes through tidal channels draining wetland complexes on the Blackwater and Transquaking Rivers, Maryland, USA. While the Blackwater complex has experienced decades of degradation and been largely converted to open water, the Transquaking complex has persisted as an expansive, vegetated marsh. The measured net export at the Blackwater complex (1.0 kg/s or 0.56 kg/m2/yr over the landward marsh area) was caused by northwesterly winds, which exported water and sediment on the subtidal timescale; tidally forced net fluxes were weak and precluded landward transport of suspended sediment from potential seaward sources. Though wind forcing also exported sediment at the Transquaking complex, strong tidal forcing and proximity to a turbidity maximum led to an import of sediment (0.031 kg/s or 0.70 kg/m2/yr). This resulted in a spatially averaged accretion of 3.9 mm/yr, equaling the regional relative sea level rise. Our results suggest that in areas where seaward sediment supply is dominant, seaward wetlands may be more capable of withstanding sea level rise over the short term than landward wetlands. We propose a conceptual model to determine a complex's tendency toward stability or instability based on sediment source, wetland channel location, and transport mechanisms. Wetlands with a reliable portfolio of sources and transport mechanisms appear better suited to offset natural and anthropogenic loss.Funding was provided by the USGS Coastal and Marine Geology Program and the Climate and Land Use Change Research and Development Program.2014-04-0

Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Coupling between barrier islands and their associated backbarrier environments (salt marsh, tidal flats) leads to complex ecogeomorphic feedbacks that are proposed to control the response of barrier island systems to relative sea-level rise. This study tests the applicability of these still-theoretical concepts through investigation of the Virginia barrier islands (eastern United States), which are located in an area of accelerated sea-level rise. Using historical maps and photographs from A.D. 1851 to 2010, we determine that rapid landward island migration (1–6 m yr–1) is leading to backbarrier area reduction and large-scale salt marsh loss (63 km2 or 19%) at a rate of 0.45 km2 yr–1. Landward barrier island migration far outpaces upland marsh migration and is responsible for 51% of marsh loss; the remainder is due to backbarrier processes (e.g., edge erosion). In direct contrast to proposed ecogeomorphic feedbacks linking barrier island and backbarrier environments, shoreline retreat rates were not related to changes in backbarrier marsh, open-water areas, or tidal prism. Rather, these results indicate that, for barrier island systems already undergoing migration, the primary barrier-backbarrier coupling is the loss of marsh and tidal-flat area because of barrier island migration

Asynchronous nitrogen supply and demand produce nonlinear plant allocation responses to warming and elevated CO2

Terrestrial ecosystem responses to climate change are mediated by complex plant–soil feedbacks that are poorly understood, but often driven by the balance of nutrient supply and demand. We actively increased aboveground plant-surface temperature, belowground soil temperature, and atmospheric CO2 in a brackish marsh and found nonlinear and nonadditive feedbacks in plant responses. Changes in root-to-shoot allocation by sedges were nonlinear, with peak belowground allocation occurring at +1.7 °C in both years. Above 1.7 °C, allocation to root versus shoot production decreased with increasing warming such that there were no differences in root biomass between ambient and +5.1 °C plots in either year. Elevated CO2 altered this response when crossed with +5.1 °C, increasing root-to-shoot allocation due to increased plant nitrogen demand and, consequently, root production. We suggest these nonlinear responses to warming are caused by asynchrony between the thresholds that trigger increased plant nitrogen (N) demand versus increased N mineralization rates. The resulting shifts in biomass allocation between roots and shoots have important consequences for forecasting terrestrial ecosystem responses to climate change and understanding global trends

Temperature sensitivity of organic-matter decay in tidal marshes

Approximately half of marine carbon sequestration takes place in coastal wetlands, including tidal marshes, where organic matter contributes to soil elevation and ecosystem persistence in the face of sea-level rise. The long-term viability of marshes and their carbon pools depends, in part, on how the balance between productivity and decay responds to climate change. Here, we report the sensitivity of labile soil organic-matter decay in tidal marshes to seasonal and latitudinal variations in temperature measured over a 3-year period. We find a moderate increase in decay rate at warmer temperatures (3-6% per degrees C, Q(10) = 1.3-1.5). Despite the profound differences between microbial metabolism in wetlands and uplands, our results indicate a strong conservation of temperature sensitivity. Moreover, simple comparisons with organic-matter production suggest that elevated atmospheric CO2 and warmer temperatures will accelerate carbon accumulation in marsh soils, and potentially enhance their ability to survive sea-level rise

Safety intelligence : An exploration of senior managers' characteristics

Peer reviewedPostprin

Preservice teachers’ pictorial strategies for a multistep multiplicative fraction problem

Previous research has documented that preservice teachers (PSTs) struggle with under- standing fraction concepts and operations, and misconceptions often stem from their understanding of the referent whole. This study expands research on PSTs’ understanding of wholes by investigating pictorial strategies that 85 PSTs constructed for a multistep fraction task in a multiplicative context. The results show that many PSTs were able to construct valid pictorial strategies, and the strategies were widely diverse with respect to how they made sense of an unknown referent whole of a fraction in multiple steps, how they represented the wholes in their drawings, in which order they did multiple steps, and which type of model they used (area or set). Based on their wide range of pictorial strategies, we discuss potential benefits of PSTs’ construction of their own representations for a word problem in developing problem solving skills

Carbon nanotube: a low-loss spin-current waveguide

We demonstrate with a quantum-mechanical approach that carbon nanotubes are excellent spin-current waveguides and are able to carry information stored in a precessing magnetic moment for long distances with very little dispersion and with tunable degrees of attenuation. Pulsed magnetic excitations are predicted to travel with the nanotube Fermi velocity and are able to induce similar excitations in remote locations. Such an efficient way of transporting magnetic information suggests that nanotubes are promising candidates for memory devices with fast magnetization switchings