Silica is an essential element for marine life and plays a key role in the biogeochemistry of the ocean. Glacial activity stimulates rock weathering, generating dissolved silica that is exported to ...coastal areas along with meltwater. The magnitude of the dissolved silica export from large glacial areas such as the Greenland Ice Sheet is presently poorly quantified and not accounted for in global budgets. Here we present data from two fjord systems adjacent to the Greenland Ice Sheet which reveal a large export of dissolved silica by glacial meltwater relative to other macronutrients. Upscaled to the entire Greenland Ice Sheet, the export of dissolved silica equals 22 ± 10 Gmol Si yr−1. When the silicate‐rich meltwater mixes with upwelled deep water, either inside or outside Greenland's fjords, primary production takes place at increased silicate to nitrate ratios. This likely stimulates the growth of diatoms relative to other phytoplankton groups.
Key Points
The Greenland Ice Sheet functions as an important source of silicate for the fjords and surrounding coastal waters
Glacial meltwater does not directly stimulate phytoplankton productivity due to low nitrate concentrations
Exported silicate from the Greenland Ice Sheet is taken up by diatoms, suggesting a regulating role in phytoplankton composition
In order to evaluate the effect of improved oxygen concentrations in overlying surface water on the redox status, sediment geochemistry and metal bioavailability in metal-polluted sediments a 54 days ...lab experiment with two different experimental treatments was conducted (90% and 40% O2). Changes in redox potential (Eh) in the sediment were monitored over time. At 6 different time points (after 0, 2, 5, 12, 32 and 54 days) and at 4 sediment depths (0–1, 1–4, 4–8 and 8–15 cm), acid volatile sulfides (AVS), simultaneously extracted metals (SEM) and total organic carbon (TOC) were measured and metal release to overlying surface water was determined. Labile metal species in both water and sediment were measured using Diffusive Gradients in Thin films (DGT). Our results showed that elevated oxygen levels in overlying surface water led to an Eh increase in the sediment of the 90% O2 treatment from 0 to ±200 mV while AVS concentrations in the upper sediment layer decreased by 70%. Following AVS oxidation metal availability in the pore water was highly elevated after 54 days. However, Cu remained strongly bound to the sediment during the whole experiment. Only a limited metal release to the overlying surface water was noticed, which was due to the fact that SEMtot concentrations in the sediment did not yet exceeded AVS levels (SEMtot − AVS/fOC = 0) after 54 days. Additionally, adsorption on Fe and Mn hydroxides and particulate organic carbon also slowed down any potential metal release. Our results indicated that increasing oxygen concentrations due to general water quality improvements can enhance the mobility of trace metals which may result in the leaching of sediment-bound metals to overlying surface water, even in undisturbed watercourses.
Display omitted
► Metal contaminated sediment was exposed to elevated O2 levels in surface water. ► Changes in sediment redox potential, geochemistry and metal mobility were monitored. ► Elevated O2 concentrations resulted in AVS breakdown and increased metal mobility. ► Metal leaching was observed from sediment to surface water. ► Increasing O2 levels can enhance metal mobility, even in undisturbed watercourses.
The discrete element method is a method for simulation of a particle system. For the “soft-sphere” mechanism of particle interactions, there are several models for normal contact forces, namely ...linear spring–dashpot, and non-linear damped Hertzian spring–dashpot, among others. The focus of this paper is to determine the normal spring stiffness coefficient of the linear model through the numerical solution for the overlap between particles in non-linear models. The linear spring stiffness is determined using equivalence between the linear and the nonlinear models. Using the MFIX computational code, the proposed approach is applied in the numerical simulations of two problems: single freely falling particle and bubbling fluidized bed. A method based on mean dimensionless overlap is suggested as an initial estimate to determine the normal spring stiffness coefficient. Other possible methods for computing the stiffness coefficient are also discussed in this work, e.g., maximum dimensionless overlap and dimensionless contact duration.
Display omitted
•Method for determination of the spring stiffness coefficient of the linear model•Analytical derivation of the linear spring–dashpot model•Description of two non-linear damped Hertzian spring–dashpot models•Non-dimensional expressions, tables and graphics for the models•MFIX simulations of two problems
Integrated ecosystem management is challenging due to many, often conflicting, targets and limited resources to allocate. A valuable and straightforward approach is to integrate an ecosystem services ...assessment in a cost-effectiveness analysis as method to evaluate and compare the cost-effectiveness of several management scenarios to reach one or more objectives and take into account the potential effects on other ecosystem functions and services. Nevertheless, this method is not commonly used in ecosystem management evaluation but can provide an alternative for the frequently used but often contested cost-benefit analysis (which requires the step of assigning a monetary value to each benefit). The aim of this study is to apply the cost-effectiveness analysis in combination with an ecosystem services assessment on a real case-study (comparing alternative management strategies for estuaries) to derive lessons learned to go from theory to practice. The application of this method for the case-study reveals many remaining challenges such as data availability and knowledge to assess ecosystem effects of management measures. Nevertheless, the analysis demonstrates that this method can be used for making a more integrated evaluation and supporting better-informed management decisions.
Display omitted
•An ecosystem services assessment is integrated in a cost-effectiveness analysis.•The method is applied on 4 management alternatives in estuaries.•Despite data limitations, the cost-effectiveness is calculated for four ecosystem services.•The main challenge of assessing management effects is finding a common indicator.•Flood control areas are more cost-effective than traditional dikes.
The morphogenesis of tidal channel networks that dissect intertidal flats and marshes has been studied especially by morphodynamic modeling, while relatively few empirical data exist on ...high-resolution field observations. Here we measured the spontaneous formation and evolution of a tidal channel network in a newly constructed tidal marsh (Scheldt estuary, Belgium) over a period of 4years, by high-accuracy topographic surveying with a temporal resolution of 1year at high spatial resolution considering all channels deeper than 0.1m. As a reference, topographic measurements with a similar high resolution were performed in a nearby, mature natural tidal marsh network. Based on the field surveying and additional GIS processing, we derived several geometric and hydraulic parameters (channel width, depth, eroded volumes, cross-sectional area, length profiles, drainage density, mainstream length, tidal discharge and watershed area), and compared the evolution of geometric relationships in the constructed marsh with the natural tidal marsh. In this way we have evaluated how fast an equilibrium state was attained. We found that after 2 to 3years of tidal working the cross-sectional areas of former ditches in the constructed marsh were in equilibrium with the corresponding tidal discharge. Furthermore we observed that the mainstream lengths and the drainage densities for the smaller watershed areas were comparable with the natural tidal marsh, demonstrating the rapid headward growth of newly forming channels and tributary channel formation near the channel heads. Newly formed channels preferentially developed in the low elevation zones of the constructed marsh and channel extension was not significantly influenced by the presence or absence of vegetation. However, the overall channel drainage density and channel cross-sectional areas of the newly formed channels were still lower compared to the natural tidal marsh after 4years. This indicates that further channel network extension and continued channel deepening can be expected in the coming years.
► After a few years spontaneous channel formation occurred in a constructed tidal marsh, ► featured by rapid headward growth and tributary formation near the channel heads. ► Newly formed channels preferentially developed in the low elevated zones. ► Drainage density and cross-sectional area of new channels not yet in equilibrium
Wetland communities are shaped by high levels of stress, disturbance and competition. Using South African palmiet wetlands as a case study (
Prionium serratum
dominated valley-bottom wetlands), we ...explore whether autogenic or allogenic succession is the dominant process driving community dynamics in valley-bottom wetlands in drylands. Several wetland rehabilitation programmes in South Africa use the dominant wetland species palmiet (
P. serratum
) as a pioneer to facilitate recolonization. However research is needed on palmiet wetland dynamics and formation to guide these restoration efforts. We explore vegetation patterns by analyzing which environmental parameters drive dominance of palmiet, resulting in the characteristic patchiness of palmiet wetlands, and which plant functional traits account for this. In 20 plots from three palmiet wetlands distributed across the Cape Floristic Region of South Africa, key soil, groundwater and vegetation parameters, as well as community composition were measured. Twenty-two dominant species were selected and 13 functional traits measured. Soil pH and relative groundwater depth were the main environmental parameters driving community assembly in palmiet wetlands. Palmiet-dominated communities were characterized by greater stem diameter, leaf length–width ratio, leaf area and cellulose and lignin concentration compared to fynbos communities. These traits suggest adaptations to disturbances such as fires (thicker stems) and floods (long, thin leaves, flexible shoots and thicker stems). We propose three hypotheses of palmiet wetland development which shed light on palmiet wetland restoration and highlight gaps for future research.
Besides well‐known effects of climate and parent material on silicate weathering the role of land use change as a driver in the global silicon cycle is not well known. Changes in vegetation cover ...have altered reservoirs of silicon and carbon in plants and soils. This has potential consequences for plant‐Si availability, agricultural yields, and coastal eutrophication, as Si is a beneficial element for many crop plants and an essential nutrient for diatom growth. We here examined the role of sustained and intensive land use and human disturbance on silicon (Si) pool distribution in soils with similar climatological and bulk mineralogical characteristics. We show that land use impacts both biogenic and nonbiogenic Si pools. While biogenic Si strongly decreases along the land use change gradient (from forest to croplands), pedogenic silica fractions (e.g. pedogenic clays) increase in topsoils with a long duration of cultivation and soil disturbance. Our results suggest that nonbiogenic Si pools might compensate for the loss of reactive biogenic silicon in temperate zones.
Key Points
Silicon pools are studied in soils along a land use change gradient
Cultivation alters biogenic and nonbiogenic silicon pools
Pedogenic clays may buffer Si depletion in temperate zones
There is growing consensus that human interventions can fundamentally change fine sediment transport in estuaries. Critical transitions in response to human interventions have been hypothesized based ...on indirect observational evidence and theoretical understanding. So far direct evidence has been lacking. Based on a 20 year data-set of surface suspended particulate matter (SPM) concentrations, we present empirical evidence of critical transitions in a temperate meso-tidal estuary. In 2008-2009 the SPM dynamics of the Scheldt estuary (Belgium/The Netherlands) changed dramatically. Not only did the total amount of sediment in suspension increase, a new maximum turbidity zone (MTZ) at typical winter discharges appeared. At intermediate and low summer discharges the longitudinal distribution of SPM now flickers between two markedly different states. Our data suggest that a range of human interventions (fairway widening and deepening, dredging and dumping activities) set the scene leading to the observed transitions. Moreover the freshwater MTZ in the Scheldt and in its major tributary exhibit an increasing sensitivity towards freshwater discharge, coinciding with water quality improvements. This suggests large scale impacts of changes in eutrophication status on estuarine sediment dynamics. This has largely been a blind spot in morphodynamic research.
Vegetation is increasingly recognized as an important control on flow and landform patterns in many landscape types. Field studies on the landscape-scale effect of vegetation in fluvial and tidal ...floodplains are relatively scarce while insights are especially based on flume and numerical models. Large-scale flow patterns and landforms were measured on a vegetated tidal marsh and unvegetated tidal flat, in particular dynamic changes in two-dimensional water surface slopes, flow velocities and directions on an extensive network of locations. It was found that during flooding and drainage of the vegetated tidal marsh, the flow was concentrated in and routed through bare tidal channels, whereas on the unvegetated tidal flat more homogeneous sheet flow occurred. On the marsh the peak flow velocities were lower on the vegetated platform (<0.1m/s) compared to their adjacent tidal channels (0.3−1m/s), while on the bare tidal flat the channel and platform flow velocities were not significantly different from each other (0.1−0.4m/s). These differences in flow patterns have important implications for the landform differences between vegetated and bare tidal floodplains: (1) for tidal channels with channel widths smaller than 10m, width-to-depth ratios (=β) were smaller for the tidal marsh channels (β ~3) than for the tidal flat channels (β ~9) (due to flow concentration towards the channels in the marsh); and (2) the vegetated marsh platform exhibits a clear levee-basin topography (due to vegetation-induced platform flooding from the channels), while there is no levee-basin topography on the bare flat (due to sheet flow). This study not only emphasizes previous findings on the direct effect of vegetation, acting as frictional constraints on flow in tidal marshes, it also suggests that the indirect effect of vegetation, namely its induced micro-topography, influences the flooding and drainage behavior of marsh systems potentially altering feedbacks caused by direct vegetation effects.
•We investigate the landscape-scale effect of vegetation through field measurements.•On vegetated tidal marshes flow was concentrated in the channels.•On unvegetated tidal flats homogeneous sheet flow occurred.•Differences in flow patterns precipitate differences in landforms.•Levee-basin topographies observed in the marsh are absent on the unvegetated flat.•Microtopography as an indirect effect of vegetation on marsh flooding and drainage.
Accurate measurement of water content (θ), apparent electrical conductivity (EC
a
) of soils, and soil solution electrical conductivity (EC
w
) is critical for a better management of irrigation water ...and the effective monitoring and control of soil salinity. In this paper, we demonstrate the design and validation of an identical time domain reflectometry (TDRWO) probe for accurate and non-destructive measurements of ECw. Two calibration functions were used to determine the soil-water content by TDR. Theses relations were verified by laboratory experiment over a wide range of θ and bulk density (BD) for dune sand. The exact length and the impedance of all TDR-probes were calibrated using the WinTDR procedure. Estimates of ECw were based on θ and the bulk ECa using a special modeling approach which applies long-time TDR waveform analysis. The geometrical disposition of the matrix phase factors (τ and β) was calculated by immersing the TDR probe in sodium chloride (NaCl) solutions of different electrical conductivities in pressure cells wetted and drained with each of these NaCl solutions. The reliability of the TDRWO was validated/proven under laboratory conditions. The laboratory experiment consisted in inserting the TDR probe into a pressure cell packed with mixed sand and 2-mm sieved loam soil that was subsequently wetted and drained with different NaCl solutions at various pressure heads. The EC
w
estimated by TDRWO was compared to the EC
w
measured in the draining solutions after they stabilized in the soil porous system. The τ and β factors calculated for the ceramic disks sets were 1.991 and 4.312, respectively. In the laboratory experiments, high and therefore significant correlations (R
2
= 0.986; P < .001) were found between the EC
w
estimated by the TDRWO and the corresponding EC
w
values measured in the column-drainage. These results demonstrate that the TDRWO is an instrument which allows researchers to accurately estimate soil solution salinity independently of the soil water content and the porous medium in which the TDR probe is installed. Test results showed that the TDR method has high accuracy for monitoring soil water content but is less accurate in monitoring soil electrical conductivity across a water content range of about ≤0.03cm
3
/cm
3
. Regression analysis between TDR-Volumetric water content (ϴ
v
) and weighing method - ϴ
v
showed good correlation with an R
2
of 0.97 and 0.98 during two laboratory experiments.