Silty Site Research Articles

Interannual radial growth response of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) to severe droughts: an analysis along a gradient of soil properties and rooting characteristics

Key messageWe analyzed stem growth responses of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) to severe drought in 2003/04 and 2018. The results showed high drought tolerance in sandy, loamy, and most silty soils, with limitations on clayey soils. This study indicates the susceptibility of Douglas-firs with shallow root systems to extreme drought and the importance of deep rooting for high drought resilience.ContextAlthough Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is considered a more drought-tolerant substitute for Norway spruce (Picea abies (L.) Karst.) in Europe, there is considerable uncertainty about the drought tolerance of Douglas-fir under climate change, specifically concerning soil properties.AimsThis study aimed to assess the influence of soil texture, plant-available water capacity, and rooting characteristics on the interannual stem growth response of Douglas-fir when exposed to severe drought.MethodsAlong a soil texture gradient from sand to clay, we selected seven closely spaced sites at elevations of approximately 500 m a.s.l. in southern Germany. Mixed-effects models were used to analyze the effects of soil physical and rooting characteristics on growth response indices (resistance, recovery, resilience) related to the severe to extreme droughts in 2003/04 and 2018.ResultsDouglas-fir showed high drought tolerance in sandy, loamy, and most silty soils. However, the results suggest a higher drought stress risk on clayey soils, as well as at specific silty sites with shallow root systems. A higher effective rooting depth increased the resilience of Douglas-fir during the extreme drought in 2018.ConclusionDouglas-fir demonstrated its drought tolerance in most soil textures. In addition, this study supports the need for combined above- and below-ground investigations on factors influencing drought tolerance and the importance of rooting for drought resilience.

Root-derived carbon stocks in formerly deep-ploughed soils – A biomarker-based approach

Roots can add significant amounts of carbon (C) to the subsoil, which improves soil physical properties and can mitigate climate change. About 5% of croplands in Germany have been deep-ploughed (30–120 cm) at least once. This can provide better root access to the subsoil and may increase yields, but little is known on the fate of root-derived C in the subsoil (at depth greater than 30 cm) after deep-ploughing. We hypothesized that five decades after deep-ploughing, root-derived C stocks were higher than conventionally ploughed treatments due to better root development. We analysed suberin and cutin monomers as tracers for root- and shoot-derived C at three former deep-ploughed sites in Northern Germany with different soil textures and deep-ploughing depths. Concentrations of suberin monomers in the soil positively correlated with root biomass; this was most pronounced at one sandy site, but had higher variability at the other two sites due to crops with different root systems in the crop rotation, lower root development, and more favourable conditions for C decomposition. Suberin contributed more to the bulk soil organic carbon (SOC) stocks than cutin throughout the soil profile at all sites. The contribution of suberin monomers to the bulk SOC stock at silty site Banteln and the sandy site Essemühle was 38% higher in the deep-ploughed plots than at the reference plot, respectively, these differences were most visible in the subsoil of Essemühle. We conclude that as C stocks and root development increase, suberin SOC stocks also increase, especially in sandy subsoils with low pH.

Soil texture, fertilization, cover crop species and management affect nitrous oxide emissions from no-till cropland

Cover crops reduce nitrate leached, but effects on nitrous oxide (N2O) emissions are mixed. Cover crops can reduce N2O emissions by reducing levels of mineral nitrogen (N) and surface soil moisture during spring. Cover crops can also increase N2O emissions by adding organic substrates, releasing N during decomposition, or increasing summer soil water content. Winter-killed cover crops can increase soluble organic C and N during periods of typically low microbial activity. We hypothesized that planting a cover crop mix of radish (Raphanus sativus)-crimson clover (Trifolium incarnatum)-rye (Secale cereale) would increase direct N2O emissions relative to no cover crop, and result in lower direct and indirect N2O emissions than planting radish alone. We also hypothesized that extending the cover crop growing season, by planting earlier and killing later, would increase direct N2O emissions during winter, decrease direct N2O emissions during summer, and decrease indirect N2O emissions. To address these hypotheses, we conducted two field experiments (on sandy and silty soils) over four site-years. We measured cover crop biomass and N content, soil mineral N concentrations, soil moisture, green canopy cover, soil porewater nitrate, direct N2O emissions, and estimated indirect N2O emissions. Nitrous oxide emissions were ~ 7.8 times greater at the silty than the sandy sites due to greater soil moisture retention. Site-years with high radish biomass exhibited greater direct N2O emissions than sites with low radish biomass following winter-kill. Indirect N2O emissions were decreased ~7 % by planting cover crops and by ~70 % by planting cover crops early. Fertilizer induced emission peaks were 8.2 times greater than all previous N2O emissions combined at a silty site. Our results suggested that soil texture and fertilization played an important role in direct N2O emissions, while cover crop species, biomass, and timing played a more important role in NO3 leached, and thus, indirect N2O emissions.

Fifty years after deep‐ploughing: Effects on yield, roots, nutrient stocks and soil structure

AbstractDeep‐ploughing far beyond the common depth of 30 cm was used more than 50 years ago in Northern Germany with the aim to break root‐restricting layers and thereby improve access to subsoil water and nutrient resources. We hypothesized that effects of this earlier intervention on soil properties and yields prevailed after 50 years. Hence, we sampled two sandy soils and one silty soil (Cambisols and a Luvisol) of which half of the field had been deep‐ploughed 50 years ago (soils then re‐classified as Treposols). The adjacent other half was not deep‐ploughed and thus served as the control. At all the three sites, both deep‐ploughed and control parts were then conventionally managed over the last 50 years. We assessed yields during the dry year 2019 and additionally in 2020, and rooting intensity at the year of sampling (2019), as well as changes in soil structure, carbon and nutrient stocks in that year. We found that deep‐ploughing improved yields in the dry spell of 2019 at the sandy sites, which was supported by a more general pattern of higher NDVI indices in deep‐ploughed parts for the period from 2016 to 2021 across varying weather conditions. Subsoil stocks of soil organic carbon and total plant‐available phosphorus were enhanced by 21%–199% in the different sites. Root biomass in the subsoil was reduced due to deep‐ploughing at the silty site and was increased or unaffected at the sandy sites. Overall, the effects of deep‐ploughing were site‐specific, with reduced bulk density in the buried topsoil stripes in the subsoil of the sandy sites, but with elevated subsoil density in the silty site. Hence, even 50 years after deep‐ploughing, changes in soil properties are still detectable, although effect size differed among sites.

Study on Strengthening and Waterproofing Mechanism of Calcium Lignosulfonate in Silty Soil Sites

Silty sites are affected by natural and human factors, have a low soil strength and strong water sensitivity, and are prone to cracks, soil peeling, and other failures that urgently need to be repaired. Calcium lignosulfonate (CLS) from paper plant waste fluids is a natural bio-based polymer. In this paper, against the background of the reinforcement and protection project of the Qingtai Site in Xingyang, Henan Province, silty soil was modified by adding CLS, and the material ratio with the best restoration effect was determined by carrying out a series of mechanical and waterproof tests. The mechanism of action of the modified materials was analyzed through microscopic tests such as SEM and XRD. The test results showed that a 1.0% mass fraction of CLS in the silty soil was the optimal ratio of the modified material. The mechanical properties of the modified soil first increase and then decrease with the increase of CLS content, and the waterproof properties increase with the increase of CLS content. The lignosulfonate polymer generated by the displacement reaction between CLS and the soil particles was cemented with the soil particles, meaning that the mechanical and waterproof properties of the modified soil were improved. And the content of the main elements and the mineral composition in the modified soil did not change. The research results provide a reference for the restoration and protection of silty sites.

Environmental stability and long-term variability of harpacticoid copepod assemblages

Harpacticoid copepods of the Chernaya Bay (White Sea) intertidal zone were collected in 45 surveys carried out from spring to autumn over a 25-year period (1996–2020) at three sites that differed in sediment properties. There were no significant long-term trends or seasonal cycles in total abundance. Regarding the species composition, the differences between sites were the most important source of variability over the whole period while the fine-scale (within-habitat) variability was low. Epibenthic species prevailed in fine silty sand, both burrowing and epibenthic species prevailed in medium sand, and interstitial and burrowing species prevailed in coarse sand. A comparison of the data on harpacticoid assemblages from a number of geographically remote loci corroborated the generality of this pattern. In a temporal dimension, the structure of each community was stable until the early 2000s, when the proportion of epibenthic, burrowing and interstitial species changed following changes in sediment properties (increasing siltation at sandy sites and decreasing siltation at the silty site). At each site, there was an increasing long-term trend in diversity (both in total richness and in expected species number). This increase was particularly apparent at sandy sites because of the appearance of epibenthic species. We suggest that sediment composition is the key factor determining the composition of harpacticoid assemblages in space and time. The “ecomorphological profile”, i.e., the proportion of species with different lifestyle and morphological traits, is a useful and informative indicator for describing and typifying these assemblages.

Imaging indigenous sacred sites: A tale of clay, silt, and sand

When studying indigenous sites, especially sacred sites such as burials, the needs and wishes of the indigenous people are paramount; the site integrity must be respected, and the site must be left intact and undisturbed. Noninvasive, nondestructive geophysical imaging is well suited to such investigations, but suspicion within indigenous communities because of past transgressions is a barrier to widespread use; skepticism is not uncommon, but we have always managed to convince skeptics. We have developed an overview of results from 10 Māori (the indigenous peoples of New Zealand) sites in the South Island of New Zealand: one historic burial site, six modern burial sites that also had historical use, one historic battle site, one prehistoric site that may have been a burial site, and one prehistoric site used for food storage that was tapu (sacred). Our approach was to let them ask us to delineate areas of importance, for example, old burial sites. There are, naturally, processes and protocols, cultural and technical, that we followed. The sites comprised a range of lithologies and soil types: one site where clay soil overlays limestone, five sites where loess (airborne silt) overlays basalt, one set of inland silty soil sites, one site on peat soils overlying sandy gravels, and two sites in coastal sands. The geophysical responses of the sites cluster into three groups: Horizontal loop electromagnetic (HLEM) and magnetic field methods worked well for the clay soil site, and once the effects of the conductive clay response were removed by filtering, ground-penetrating radar (GPR) worked well. HLEM and magnetic results were good to equivocal on the silty and peaty sites, whereas GPR excelled at delineating anomalous features, particularly burials, which yielded clear characteristic diffraction responses. Finally, results for coastal sand sites were disappointing. Such sites appear to be too dynamic to yield useful results.

Forest soil penetration resistance following heavy traffic: A 10‐year field study

AbstractNatural recovery of acidic soils following heavy traffic is assumed to take several decades. The aim of this study was to monitor the recovery of soil penetration resistance (PR) after soil compaction by heavy traffic at two forest sites. The sites, located in northeast France, have soils that differ slightly in pH and silt content. The impact on soil PR following heavy traffic was dependent on the soil water content (WC) at the time of measurement, and, therefore, the entire WC range and corresponding PR values were measured. For the most acidic and silty soil, no difference in PR among treatments was observed when the soils were near saturation; however, the difference in PR values was high when soils were close to wilting point. For the least acidic and silty soil, the impact of heavy traffic on PR could be detected whatever the soil moisture, with the magnitude of the effect dependent on the soil water content and depth considered. Standardized PR values with regard to WC allowed an analysis of the evolution of the effect of heavy traffic with time. Ten years after being subjected to heavy traffic, the PR values in the 0‐ to 40‐cm layer recovered to undisturbed values at the least acidic and silty site, but not WC dynamic. The most acidic and silty soil site did not show any recovery of PR or WC dynamic ten years following traffic, although liming at this site was able to increase the regeneration of PR to undisturbed levels.

Benthic invertebrate assemblage changes in an urban bay of Lake Ontario: 1990 to 2012

Invasions by dreissenid mussels and the round goby have altered the makeup of benthos in many areas of the Great Lakes complicating the use of benthic invertebrate composition as an indicator of environmental conditions. The Ontario Ministry of the Environment, Conservation and Parks (MECP) has been monitoring the composition of benthos at nearshore stations since the 1990s. The interactive influences of environmental change and invasive species have been evident in areas of the Toronto Waterfront such as Humber Bay (HB), an area of intensifying urbanization. Dreissenid mussels were first detected in HB in 1994, became abundant in 2000, and were followed by the round goby in 2005. Both invasions overlapped with increasing population density in the watershed and efforts to reduce pollutant and nutrient inputs. Benthic invertebrate diversity increased post-dreissenids, with densities of gastropods and amphipods increasing during peak mussel abundance (2000–2003). Dreissenid abundance declined after 2005, and snails (Valvatidae) disappeared, corresponding with increasing goby abundance. Abundance of sphaeriid clams declined over the monitoring period. Water clarity increased and the chlorophyll a level declined in the mid-1990s, before dreissenids were abundant at the site. Trophic conditions inferred from invertebrate composition shifted from oligotrophic in 1990 to mesotrophic and back to oligotrophic in 2012. There was little change in sediment quality other than a subtle increase in clay and fine silt at the primarily silty site. The trajectory of changes in the benthos was more strongly correlated with the dreissenid and goby invasions than changes in environmental quality.

Sonar observations of biological activity in marine sediments

This talk will begin with a review of past experiments using bottom-mounted, rotating sonars to observe temporal change in the seafloor. These measurements were conducted at 40 kHz and 300 kHz, and provide images of change as represented by ping-to-ping correlation. Time scales of change vary greatly with frequency and sediment type, with changes occurring over weeks at 40 kHz at a silty site and over minutes at 300 kHz at a sandy site. Finally, the level of scattering due to relic tubes, shells, etc., will be compared to that of live animals. The conclusion is that it is easiest to observe the cumulative effects of biological activity, but techniques are available to isolate current activity.

Geoacoustic inversion in shallow tropical waters for a silty and sandy seabed

Seabed parameters are inverted from ambient noise measurements at two shallow tropical environments with dissimilar seabed characteristics, a silty and a sandy seabed, using an approach that matches the measured and modeled complex vertical coherence. Coherence is modeled using the Green's function output from the model oases, along with theoretical formulation, for a range independent environment. Genetic algorithm is used to search the model parameter space consisting of sound speed, density, and attenuation in the sediment layers and half-space. Reasonable estimates have been obtained for the silty site, whereas the sandy site gave relatively poor parameter estimates due to reflective seabed and shipping interference.

Keeping agricultural soil out of rivers: Evidence of sediment and nutrient accumulation within field wetlands in the UK

Intensification of agriculture has resulted in increased soil degradation and erosion, with associated pollution of surface waters. Small field wetlands, constructed along runoff pathways, offer one option for slowing down and storing runoff in order to allow more time for sedimentation and for nutrients to be taken up by plants or micro-organisms. This paper describes research to provide quantitative evidence for the effectiveness of small field wetlands in the UK landscape. Ten wetlands were built on four farms in Cumbria and Leicestershire, UK. Annual surveys of sediment and nutrient accumulation in 2010, 2011 and 2012 indicated that most sediment was trapped at a sandy site (70 tonnes over 3 years), compared to a silty site (40 tonnes over 3 years) and a clay site (2 tonnes over 3 years). The timing of rainfall was more important than total annual rainfall for sediment accumulation, with most sediment transported in a few intense rainfall events, especially when these coincided with bare soil or poor crop cover. Nutrient concentration within sediments was inversely related to median particle size, but the total mass of nutrients trapped was dependent on the total mass of sediment trapped. Ratios of nutrient elements in the wetland sediments were consistent between sites, despite different catchment characteristics across the individual wetlands. The nutrient value of sediment collected from the wetlands was similar to that of soil in the surrounding fields; dredged sediment was considered to have value as soil replacement but not as fertiliser. Overall, small field wetlands can make a valuable contribution to keeping soil out of rivers.

Evaluation of field wetlands for mitigation of diffuse pollution from agriculture: Sediment retention, cost and effectiveness

Diffuse pollution, and the contribution from agriculture in particular, has become increasingly important as pollution from point sources has been addressed by wastewater treatment. Land management approaches, such as construction of field wetlands, provide one group of mitigation options available to farmers. Although field wetlands are widely used for diffuse pollution control in temperate environments worldwide, there is a shortage of evidence for the effectiveness and viability of these mitigation options in the UK. The Mitigation Options for Phosphorus and Sediment Project aims to make recommendations regarding the design and effectiveness of field wetlands for diffuse pollution control in UK landscapes. Ten wetlands have been built on four farms in Cumbria and Leicestershire. This paper focuses on sediment retention within the wetlands, estimated from annual sediment surveys in the first two years, and discusses establishment costs. It is clear that the wetlands are effective in trapping a substantial amount of sediment. Estimates of annual sediment retention suggest higher trapping rates at sandy sites (0.5–6 t ha−1 yr−1), compared to silty sites (0.02–0.4 t ha−1 yr−1) and clay sites (0.01–0.07 t ha−1 yr−1). Establishment costs for the wetlands ranged from £280 to £3100 and depended more on site specific factors, such as fencing and gateways on livestock farms, rather than on wetland size or design. Wetlands with lower trapping rates would also have lower maintenance costs, as dredging would be required less frequently. The results indicate that field wetlands show promise for inclusion in agri-environment schemes, particularly if capital payments can be provided for establishment, to encourage uptake of these multi-functional features.

Small-scale spatial variability of zoobenthic communities in a commercial Mediterranean port

In the context of the limited information on the ecology of port communities, the present work aims at assessing the small-scale spatial variability of zoobenthos inhabiting hard and soft substrata, in a Mediterranean port with high levels of commercial shipment. Samples were collected in summer from three stations and four depth levels, using core and quadrate samplers. A total of 34,578 individuals were collected, identified to 118 animal species. Soft substratum communities were impoverished and their structure varied spatially according to sediment composition. At a functional level deposit feeders dominated; their abundance decreased at the silty sites. Biotic indices were found inadequate for the assessment of ecological quality, due to the very low abundance of the fauna. Fouling communities varied spatially in vertical scales; diversity indices and the abundance of Bivalvia varied also in horizontal scales. Suspension and deposit feeders dominated showing a decreasing trend with depth. Two animal-dominated communities, serpulid blocks in the lower midlittoral zone and mussel beds in the sublittoral, substituted an algal-dominated community, which has been previously recorded from the same port quays. This substitution may be due to the intensive mussel farming in the nearby area contributing to the rapid expansion of mussels and of their serpulid biofoulers. Despite the existence of biogenic substrata, which enhance habitat complexity, the diversity of the associated fauna decreased and most species were tolerant to organic pollution. Recursive biomonitoring seems necessary to assess the ecological status of communities and to develop integrated management plans for temperate ports.

Dynamic compaction of saturated sands and silty sands: results

Numerical simulations of dynamic compaction of three loose saturated sands and silty sand sites are presented. The theory associated with the numerical model is presented in a companion paper. First, a case history of dynamic compaction in a clean sand site without pre-installed wick drains is considered. Second, a case history involving a sandy silt site with pre-installed wick drains is simulated. Third, a silty sand site without pre-installed wick drains is simulated. In all cases the numerical results for post-improvement cone resistance and standard penetration test (SPT) penetration resistance profiles agree fairly well with observed field data. The results show the effectiveness of pre-installed wick drains for dynamic compaction of saturated silty sands. The numerical simulation tool presented is a valuable tool for feasibility studies of dynamic compaction of saturated sands and silty sands.

Community structure of marine sedimentary protists in relation to flow and grain size

Populations of unicellular, marine sedimentary protists are constrained by a variety of physical environmental factors, but influences of flow regime have rarely been studied. We compared community structure among 3 subtidal sites differing in flow strength and grain size in a coastal bay. We used denaturing gradient gel electrophoresis (DGGE) to assess eukaryotic diversity based on 18S rDNA, and quantitative Protargol staining (QPS) to examine ciliate communities by microscopy. Sedimentary 18S rDNA in mid-summer was dominated by diatoms. Analyses of gel bands by presence/absence among sites, dendogram, and multidimensional scaling showed that eukaryotic community structure was related to grain size more strongly than to flow regime. Among bands identified as diatoms by recovery and sequence analysis, 4 taxa (40%) differed among sites in relation to flow strength, and 2 taxa (29%) differed in relation to grain size. No bands had sequences matching ciliates, but QPS showed that 6 ciliate species (20 %) were distributed in relation to flow, and 10 species (33%), in relation to grain size. Ciliate species richness and community similarity were greatest for the 2 strong-flow sites, despite differences in mean grain size. The strong-flow, silty site had the high-est concentrations of chlorophyll a, total ciliates, karyorelictids, and scuticociliates, and the lowest ciliate species diversity. DGGE was run again for this site 1 mo later and revealed a shift in the rDNA pool to dominance by metazoans. Flow regime and grain size may be important factors structuring subtidal communities of sedimentary protists.

The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments

To advance the present understanding of the frequency dependence of compressional wave velocity and attenuation in marine sediments a series of well-constrained in situ acoustic transmission experiments (16 to 100kHz) were performed on intertidal sediments. The processing techniques incorporated in situ spreading losses, sediment to transducer coupling and thorough error analyses. Significant variations in velocity and attenuation were observed over scales of tens of meters within the same sediment type. Velocity was generally nondispersive in sands, while highly variable silt velocities prevented any meaningful dispersion estimates from being determined. The attenuation coefficient was proportional to frequency for 75% of the experimental sites. The measured compressional wave properties were compared to predictions from the Grain-Shearing model. For the sandy sites, the phase velocities predicted by the Grain Shearing model exceed those measured, while predicted phase velocities agreed with measured group velocities at specific locations for the silty sites. For both silts and sands predicted dispersions are comparable to the intrinsic errors in group velocity and hence undetectable. The attenuation coefficients predicted by the Grain Shearing model adequately describe the measured attenuation coefficients, within the observed variability.

Contrasting dynamics of ciliate communities in sandy and silty sediments of an estuarine intertidal flat

During a 1-year study of the ciliate faunas of a silty and a sandy site on an intertidal flat in the Westerschelde estuary, a total number of 107 taxa were recorded belonging to at least 52 genera and 15 orders. Our results suggest that physical properties of the sediment were more important in regulating ciliate abundance, diversity and community composition than food availability, predation, temperature or oxygen concentration. Ciliate abundance and diversity were positively related to sediment grain size and the ciliate community of silty sediments was found to be a subset of that of sandy sediments. At the sandy site, where the sediment composition was stable, seasonal changes in the ciliate community were related to changes in food availability and/or temperature. At both the sandy and silty sites, a clear vertical gradient in the ciliate community was observed that appeared to be linked to gradients in food availability and oxygen concentration. These vertical gradients in ciliate community composition, however, were less steep than the measured oxygen gradients, probably due to the presence of oxic microniches in the anoxic zone.

Role of physical and biological processes in sediment dynamics of a tidal flat in Westerschelde Estuary, SW Netherlands

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 274:41-56 (2004) - doi:10.3354/meps274041 Role of physical and biological processes in sediment dynamics of a tidal flat in Westerschelde Estuary, SW Netherlands J. Widdows1,*, A. Blauw2, C. H. R. Heip3, P. M. J. Herman3, C. H. Lucas4, J. J. Middelburg3, S. Schmidt5, M. D. Brinsley1, F. Twisk6, H. Verbeek6 1Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth PL1 3DH, UK 2WL | Delft Hydraulics, PO Box 177, 2600 MH Delft, The Netherlands 3Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-KNAW), Korringaweg 7, PO Box 140, 4401 AC Yerseke, The Netherlands 4School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK 5Laboratoire des Sciences du Climat et de l¹Environnement (CEA-CNRS), Domaine du CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France 6National Institute for Coastal and Marine Management, Grenadresweg 31, PO Box 8039, 4330 EA Middelburg, The Netherlands *Email: j.widdows@pml.ac.uk ABSTRACT: This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr-1, based on 210Pb and 137Cs profiles. 7Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm2 yr-1. The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr-1, but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr-1). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including Œbio-stabilisation¹ by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of Œbio-destabilisation¹ by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s-1, and accompanied by a 30- to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites. KEY WORDS: Sediment dynamics · Erosion · Sedimentation · Mixing · Microphytobenthos · Biostabilisation · Bioturbation · Modelling Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 274. Online publication date: June 24, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

In Situ Estimate of Shear Wave Velocity Using Borehole Spectral Analysis of Surface Waves Tool

In situ field testing has been performed over the past several years at a silty sand site in Austin, Tex. using the borehole spectral analysis of surface waves (SASW) tool to develop the technique and assess the validity of the method. The borehole SASW tool is an inflatable pressuremeterlike device that allows surface wave measurements to be performed along the wall of an uncased borehole while varying the in situ states of stress. Field results demonstrate the applicability of borehole SASW testing as a method to characterize soil sites and provide information about in situ shear wave velocity and the relationship between shear wave velocity and state of stress. Results from a borehole SASW test conducted at the Austin site are presented herein to demonstrate the applicability and validity of the method.