Erosion Threshold Research Articles

Structure and Dynamics of Exopolymers in an Intertidal Diatom Biofilm

Diatom biofilms growing at the surface of the intertidal mudflat of Marennes Oléron, France, were incubated for 48 h in the laboratory under simulated conditions of high- and low tide (immersed and emersed in seawater) and day and night (illuminated or dark conditions). The biofilms were subsequently sampled using the cryolander technique, without disturbing the structure. The samples were kept in liquid nitrogen until they were transferred to the cooled stage of a field-emission cryo-scanning electron microscope, which was used to study the structural relationships between the sediment particles, the diatoms and the different types of extracellular polymeric substances (EPS) produced by these organisms. The diatoms were most abundant at the sediment surface when incubated in the light under emersed conditions. In the dark or when immersed, the diatoms migrated into the sediment. In the light, the diatoms were coated with EPS, while this was not the case when incubated in the dark. When immersed, the sediment surface appeared smooth as the result of the deposition of mud. Under emersed conditions, the coarser silt grains were prominently present. These grains were wrapped with organic matter and bound together through threads of EPS. This was the case both in light and in dark incubated sediment. It is proposed that this latter type of EPS contributes to the increased erosion threshold of intertidal mudflats colonized by biofilms of diatoms.

Simulation of the mineral dust emission frequencies from desert areas of China and Mongolia using an aerodynamic roughness length map derived from the POLDER/ADEOS 1 surface products

To determine the location of the potential dust source regions and to evaluate the dust emission frequencies over the arid and semiarid areas of China and Mongolia (35.5°N–47°N; 73°E–125°E), we established a map of Z0 from the composition of protrusion coefficient (PC) derived from the POLDER‐1 bidirectional reflectance distribution function (BRDF). Using a ° × ° resolution Z0 data set, we derived a map of the 10‐m erosion threshold wind velocity for the Chinese and Mongolian deserts. The retrieved erosion thresholds range from 7 m s−1 in the sandy deserts (Taklimakan, Badain Jaran, and Tengger deserts) to up to 20 m s−1 in the Gobi desert. They are comparable to the minimum wind velocity measured in meteorological stations during dust storms in the Taklimakan (6–8 m s−1) and in the Gobi desert (11–20 m s−1). These erosion thresholds were combined with surface wind fields, soil moisture, and snow cover to simulate the dust emission frequencies of the eastern Asian deserts over 3 years (1997–1999). The simulations point out the Taklimakan desert and the deserts of northern China as the most frequent sources of dust emissions. The simulated seasonal cycle is characterized by a maximum in late spring and a minimum in late autumn and winter. The comparison with climatologic studies of dust storms derived from synoptic observations confirms the importance of these two source areas and the reliability of the simulated seasonal cycle. It reveals an underestimation of the dust emission frequency in the Gobi desert, because of a low frequency of high wind velocities. Our results also suggest that soil moisture and snow cover are not the main factors controlling the seasonal cycle or the interannual variability of the dust emission frequencies. We finally compared the simulated dust event frequencies to occurrences of Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol Index (AAI) higher than 0.7 over the Taklimakan desert, where mineral dust is expected to be the dominant absorbing aerosol. A very good agreement is obtained between the simulated frequencies and the TOMS AAI frequencies on monthly and seasonal timescales.

Implications of bank failures and fluvial erosion for gully development: Field observations and modeling

Gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop. Field observations attribute the headward growth and widening of many gully systems to gravitational mass‐wasting processes of oversteepened sidewalls. Soil saturation, groundwater sapping, and tension crack development contribute to the instability. Recent landscape evolution models treat such mass failures as slope‐dependent continuous sediment transport processes, sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient. In this study we first present an explicit physically based theory for the stability analysis of gully heads and walls. The theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall, with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face. Then, we test the theory against field data collected in our field site in Colorado and against other published data. Second, the theory is implemented in a one‐dimensional hillslope profile development model and the three‐dimensional channel‐hillslope integrated landscape development (CHILD) to study the effects of soil cohesion, erosion thresholds, and stochastic climate on the tempo of gully development and morphology. Preliminary results indicate that wider and shallower gullies develop and integrate, forming wide valleys, when soil cohesion is small. As soil cohesion increases, erosion slows down, gullies become deeper with vertical walls, and episodic mass failures occur. Differences in storm intensity‐duration characteristics and erosion thresholds are predicted to have a significant impact on gully development. Vertical gully walls develop rapidly, and gullies enlarge by slab failures in a climate characterized by high‐intensity, short‐duration storm pulses. However, under low‐intensity, long‐duration storms, gullies quickly stabilize, and vertical walls are eliminated and rounded, forming diffusion‐dominated hilltops. Erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction. Lowering the erosion threshold enhances gully widening by slab failures. Gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls.

A model of sediment transport under the influence of surface bioturbation: generalisation to the facultative suspension-feeder Scrobicularia plana

Flume experiments were designed to study how sediment erodibility was modified by the facultative suspension-feeder Scrobicularia plana living at different densities in sediment beds of various degrees of compaction. Two separate erosion phases were identified from the resuspension kinetics: (1) erosion of an unconsolidated surficial layer (i.e. fluff layer) and (2) the subsequent bed erosion. S. plana were found to influence both erosion phases: (1) erosion rates of the fluff layer were controlled by bioturbation activities, the extents of which were influenced by bivalve density and the degree of compaction of the sediments, and (2) critical thresholds of the subsequent bed erosion decreased as clam density increased, likely due to depressions in the sediment bed that were created by siphon activity. A 1-dimensional vertical model was modified to incorporate the effects of S. plana on fluff layer formation through pseudofaeces production. The modifications to the model and results are compared to a similar approach for Hydrobia ulvae published earlier, and are discussed with respect to the definition of functional groups of bioturbators in sediment transport models. By discussing effects of spatial grouping and analogies to other organisms, this study shows how the influence of one species on sediment resuspension may be generalised and used for model descrip- tions of other species of the same behavioural type. Model developments on pseudofaeces production show that there are asymmetric effects generated over time and by bivalve density for S. plana, which do not occur for H. ulvae.

Biologically induced differences in erodibility and aggregation of subtidal and intertidal sediments: a possible cause for seasonal changes in sediment deposition

This study was carried out to describe the difference in erodibility and aggregation in a tidal basin including both subtidal and intertidal study sites and to use these results to explain the shifting erosion/deposition cycles at the sites. Erosion thresholds, erosion rates and settling velocities of the eroded material were measured at a mudflat transect and at sediment cores taken from a nearby tidal channel during surveys made in May 2000 and March 2002. Surface samples were analysed for grain-size, chl. a content, faecal pellet content, dry bulk density and organic content. Additionally, surface samples were taken at eight occasions in the period January 2002 to May 2003 from shallow tidal channels in the area. These samples were analysed for mud content and showed that major shifts in sediment distribution occurred in the period. The erodibility of the mudflat was generally high due to pelletization by the mudsnail Hydrobia ulvae but close to the salt marsh much lower erodibility was found, probably due to stabilisation by microphytobenthos. In contrast, the erodibility of the channel bed seemed to be very little influenced by biological activity and the relatively low erodibility found here was caused by physical characteristics of the sediment. The sediment eroded from the mudflat was generally strongly pelletized and showed high settling velocities whereas less aggregation and lower settling velocities were found for the channel bed sediments. Temporal variations of the mudflat stability and hydrodynamics resulted in temporal variations of deposition and erosion and the changing stability at the mudflat is likely to be one of the main reasons for a general transport of fine-grained sediment from the mudflat to the channel in the cold seasons and vice versa during the rest of the year.

Numerical investigation of erosion threshold velocity in a pipe with sudden contraction

This paper deals with erosion prediction in a pipe with sudden contraction for the special case of two-phase (liquid and solid) turbulent flow with low particle concentration. The pipe axis was considered vertical and the flow was either in direction of gravity (downflow) or against it (upflow). The mathematical models for the calculations of the fluid velocity field and the motion of the solid particles have been established and an erosion model was used to predict the erosion rate. The fluid velocity (continuous phase) model was based on the time-averaged governing equations of 3-D turbulent flow and the particle-tracking model (discrete phase) was based on the solution of the governing equation of each particle motion taking into consideration the effect of particle rebound behavior. The effects of flow velocity and particle size were investigated for one contraction geometry considering water flow in a steel pipe. The results showed the strong dependence of erosion on both particle size and flow velocity but with little dependence on the direction of flow. The effect of flow direction was found to be significant only for large particle size and moderate flow velocity. The erosion critical area was found to be the inner surface of the tube sheet (connecting the two pipes) in the region close to the small pipe. The results also indicated the presence of a threshold velocity below which erosion is insignificant for all particle sizes.

The stability of tidal flats in Venice Lagoon—the results of in-situ measurements using two benthic, annular flumes

Seabed properties in Venice Lagoon were examined in situ in two multidisciplinary field campaigns. The purpose of this study was to understand the mechanisms controlling the stability of bed types. Two benthic annular flumes (Sea Carousel and Mini Flume) were deployed simultaneously from a floating pontoon at 24 sites during summer (1998), which were considered representative of the range in bed/habitat types. As well, bottom sampling and coring, water-column monitoring and benthic habitat analyses were carried out. All but three sites were on cohesive sediments. Bed types included bare shelly mudflats and regions colonised by the seagrasses Cymodocea nodosa and Zostera noltii, by filamentous cyanobacteria, and by patches of the macrophytes Ulva rigida and Chaetomorpha sp. A subset (13) of these sites was visited during the subsequent winter to evaluate seasonal changes. Six of the sites were intertidal, the remainder were in the sublittoral zone. Water temperature varied between 5 and 30 °C, and salinity varied between 20 and 38 psu. In the absence of waves, turbidity was generally low (<10 mg/l) and was composed of high amounts of organic matter (25–50%). This indicates that the tidal flows were not competent to support estuarine sediment. Higher levels of turbidity were measured during wind events or boat passage as a result of resuspension from the bed. Bed (saturated) density was, on average, 1770 kg/m 3, which was extremely high for estuarine sediments. Sea Carousel and Mini Flume provided comparable results, despite large differences in instrument footprints. Trends from the two instruments were similar and showed that summertime bed strength exceeded the winter by up to five times. Mean summer erosion thresholds for Sea Carousel and Mini Flume were 1.10 and 0.82 Pa, respectively, whereas during winter, they were 0.69 and 0.74 Pa. The northern lagoon had the most resistant tidal flats due to the stabilizing effect of filamentous cyanobacteria (Biostabilization Index: BI=244%), microphytobenthos (BI=153%) and Z. noltii (BI=206%). The stabilizing effects of C. nodosa (BI=74%), U. rigida (BI=115%) and shell debris (BI=115%) were intermediate, while bare sublittoral mud beds were the least resistant (BI=58%). Summer erosion rates (as a function of applied stress) were lower than winter ones, probably due to water temperature changes. The algorithm E= χτ s β , yielded good results and indicated that erosion rates in Venice Lagoon were high, notwithstanding the high erosion thresholds. The mean summertime friction coefficient was φ=62° and was highest in the central lagoon. The wintertime φ=69° showed that there was no seasonal fluctuation in bed stability. Mass settling in Venice Lagoon was a strong function of suspended sediment concentration ( S) and a decay constant ( k) of the exponential function S( t)= S o e − kt ; it was found to be in continuity with examples from other locations worldwide (and therefore normal).

Transport of juvenile clams: effects of species and sediment grain size

Erosion and transport of juvenile benthic invertebrates, including bivalves, have the potential to alter patterns of distribution and abundance during the early post-settlement period. However, the factors influencing rates of postlarval dispersal are not well understood. Both hydrodynamics and behaviour (e.g. burrowing) are likely to play a role in determining patterns of transport of juvenile bivalves. To determine the relationship between sediment transport and bivalve dispersal, experiments were conducted in a racetrack flume to examine the effect of grain size, flow, and clam size on rates of erosion of two species of juvenile clams ( Mya arenaria and Mercenaria mercenaria). Results of the experiments were compared to predictions of erosion thresholds based on the physical characteristics of the sediment and clams. Erosion of Mercenaria was greater than Mya, the opposite of predictions based on Mercenaria's greater density, indicating the importance of burrowing behaviour. In most cases, erosion also was greater in the finer sand, in contrast to the predicted similarity of erosion thresholds of the two sediments. However, clam erosion did increase with increasing shear velocity and decrease with clam size, as expected. The results of this study indicate that both hydrodynamics and behaviour play roles in the transport of these two species of juvenile bivalves and that their vulnerability to passive erosion cannot be predicted solely from knowledge of sediment transport.

Extracellular polymeric substances: quantification and use in erosion experiments

Extracellular polymeric substances (EPS) is a generic term often applied to high molecular weight polymers implicated in the biostabilisation of natural sediments. Quantitative analysis of in situ EPS production rates and sediment contents has usually involved extraction of EPS in saline media prior to precipitation in alcohol and quantification against a glucose standard (phenol–sulphuric acid assay). Extracted and synthetic EPS has also been used to create engineered sediments for erosion experiments. This study investigated two steps in the EPS extraction procedure, saline extraction and alcohol precipitation. Comparisons of the effects of different extracted polymers were made in sediment erosion experiments using engineered sediments. Sediment EPS content decreased as the salinity of the extractant increased, with highest values obtained for extraction in fresh water. Potential errors were observed in the quantification of the soluble colloidal polymer fraction when divided into EPS and lower molecular weight polymers (LMW) as used in many studies. In erosion studies, 15mgkg−1 of alcohol (IMS) extracted EPS polymer (in 5gkg−1 IMS precipitate, equivalent to approximately 5gsaltkg−1 sediment dry weight) decreased the erosion threshold of cohesive sediments whereas 30mgkg−1 (in 10gkg−1 IMS precipitate, approximately 10gsaltkg−1 sediment dry weight) had no effect compared to controls. This could be due to the influence of EPS on water content: low levels of EPS did not bind but prevented desiccation, lowering sediment stability against controls. At higher EPS content, binding effects balanced water content effects. Salt alone (at 10gkg−1) slightly increased the erosion threshold after a 6-h desiccation period. In comparison, carbohydrates produced without alcohol precipitation (rotary evaporation) increased the erosion threshold at both 0.5 and 1.0g EPSkg−1 dry weight of sediment. It was concluded that the role of microphytobenthic polymers in biostabilisation of sediments is best determined through the study of natural intact sediment samples.

Observations of coastal sediment erosion using in-line holography

This paper demonstrates how in-line particle field holography may be used to study erosion processes in coastal sediments, in particular to visualize the morphologies of eroded sediment particles and to investigate how organic matter affects the floc size and erosion threshold in incipient sediment erosion. In-line holography produces 3D images and provides higher resolution over larger recording volume compared with other imaging techniques. The study examines the dependence of the observed shearing stress, particle size distributions and morphologies of particles on the duration of desiccation of the sediment bed, and on the concentration of extra-cellular polymeric substance. Both artificial sediment (produced from cleaned sediments by the addition of natural extra-cellular polymeric substance) and intact sediment were used. The required hydrodynamic shear stress to produce erosion on the sediment beds was generated using a cohesive strength meter (CSM). Characterization of the eroded particle properties was carried out using 2D image analysis.

The effects of clam fishing on the properties of surface sediments in the lagoon of Venice, Italy

Abstract. Harvesting of clams(Tapes philippinarum) has important socio-economic and environmental implications for the Venice lagoon area, Italy. Clam harvesting disrupts the structure of benthic communities but the effects upon sediment stability and surface structure remain unclear. The effect of clam fishing on the sediment properties of the lagoon bed was investigated at two different sites, a heavily fished site (San Angelo) and an infrequently fished site (San Giaccomo). Both sites were assessed for immediate impacts of fishing, using indicators of biogenic sediment stabilisation. Samples were taken at three points along three 100 m linear transects at each site prior to and post fishing. Paired samples were also taken parallel to each transect at a distance of 5m, to allow for temporal variation. Sediment stability, measured with a cohesive strength meter (CSM), was significantly higher at the less impacted site (F1,34 = 6.23, p &lt; 0.018), was correlated with indicators of biogenic sediment stabilisation and decreased by approximately 50% following the trawling event. Concomitant decreases in chlorophyll a (chl a), colloidal-S carbohydrate and dry bulk density were observed on the transect after fishing but not adjacent to the fishing path. At the heavily impacted site, clam fishing by trawling had, in general, no significant effect on the biological and physical properties (although chl a did decrease significantly after fishing). The lack of a significant impact from fishing at the impacted site was attributed to the higher frequency of fishing occurring in this area. Hence, frequent fishing of the lagoon prevents establishment of biotic communities, preventing biostabilisation and thus reduces the stability of the surface sediment. Keywords: clam harvesting, erosion threshold, microphytobenthos, sediment, stability

The Influences of Biological Activity and Consolidation Time on Laboratory Cohesive Beds

ABSTRACT A small annular laboratory flume (Mini Flume) was used to investigate the changes in erosion threshold and erosion rate on natural mud taken from an intertidal mud flat surface in Southampton Water, U.K. Mud samples were wet sieved at 2 mm to remove shells and large macrofauna and macroflora and then mixed into a turbid slurry with local estuarine water. A mud bed was formed in the flume by letting the sediment deposit in still water at room temperature (20°C). The bed was subjected to steady flows of increasing magnitude in a series of steps. The flume was subjected to illumination on a dark-light cycle of 12 hours to simulate the solar cycle. Subsequent erosion experiments were carried out after periods of bed consolidation of 2, 6, 12, 24, 48, and 96 hours in order to examine changes in bed stability with time. This series of experiments was considered the baseline situation in which algal and bacterial binding were present. At the end of each experiment, the bed was fully resuspended, mixed for a period of 5 minutes, and then allowed to redeposit. A second series of experiments was undertaken using the same sediment, but in this case the bed was continuously held in darkness (no photosynthesis of microphytobenthos). A third series of experiments was carried out in darkness, but antibiotics were added to the flume to remove the effects of bacterial activity. A final series of experiments was carried out with the addition of 50 ml l-1 of sodium hypochlorite to remove all biological activity in order to examine the effects of electrochemical binding (the control). The results reveal that under the normal conditions of light (day-night) an increase in sediment stability (stabilization index, BI) of circa 3 was found over that of electrochemical bonding. The bed treated with antibiotics suffered an overall stabilization of 1.5 relative to the control.

Modeling of the interactions between forest vegetation, disturbances, and sediment yields

The controls of forest vegetation, wildfires, and harvest vegetation disturbances on the frequency and magnitude of sediment delivery from a small watershed (∼3.9 km2) in the Idaho batholith are investigated through numerical modeling. The model simulates soil development based on continuous bedrock weathering and the divergence of diffusive sediment transport on hillslopes. Soil removal is due to episodic gully erosion, shallow landsliding, and debris flow generation. In the model, forest vegetation provides root cohesion and surface resistance to channel initiation. Forest fires and harvests reduce the vegetation. Vegetation loss leaves the land susceptible to erosion and landsliding until the vegetation cover reestablishes in time. Simulation results compare well with field observations of event sediment yields and long‐term averages over ∼10,000 years. When vegetation is not disturbed by wildfires over thousands of years, sediment delivery is modeled to be less frequent but with larger event magnitudes. Increased values of root cohesion (representing denser forests) lead to higher event magnitudes. Wildfires appear to control the timing of sediment delivery. Compared to undisturbed forests, erosion is concentrated during the periods with low erosion thresholds, often called accelerated erosion periods, following wildfires. Our modeling suggests that drainage density is inversely proportional to root cohesion and that reduced forest cover due to wildfires increases the drainage density. We compare the sediment yields under anthropogenic (harvest) and natural (wildfire) disturbances. Disturbances due to forest harvesting appear to increase the frequency of sediment delivery; however, the sediment delivery following wildfires seems to be more severe. These modeling‐based findings have implications for engineering design and environmental management, where sediment inputs to streams and the fluctuations and episodicity of these inputs are of concern.

Drainage basin sensitivity to tectonic and climatic forcing: implications of a stochastic model for the role of entrainment and erosion thresholds

AbstractLong‐term average rates of channel erosion and sediment transport depend on the frequency–magnitude characteristics of flood flows that exceed an erosion threshold. Using a Poisson model for rainfall and runoff, analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold. Solutions are derived and numerically tested for three erosion/transport formulas: the Howard–Kerby shear‐stress incision model, the Bridge–Dominic sediment transport model, and a generic shear‐stress sediment transport model. Results imply that non‐linearity resulting from threshold effects can have a first‐order impact on topography and patterns of dynamic response to tectonic and climate forcing. This non‐linearity becomes significant when fewer than about half of flood events are capable of detaching rock or sediment. Predicted morphology and uplift‐gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope‐linear or shear‐linear erosion laws. These results imply that particle detachment thresholds are not details that can be conveniently ignored in long‐term landscape evolution models. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Effects of diet on sinking rates and erosion thresholds of mussel Perna canaliculus biodeposits

Suspension-feeding bivalves produce biodeposits (faeces and pseudofaeces) that have much higher sinking velocities than their constituent particles. Consequently, they cause sedimen- tation and redistribution of material that might otherwise not be deposited, enhancing the ben- thic-pelagic coupling of nearshore ecosystems. We quantified the dispersal characteristics (sinking velocity and erosion threshold) of biodeposits produced by the mussel Perna canaliculus, a species with high natural abundance that is also intensively cultured in New Zealand. We examined biode- posits produced by mussels of a wide size range (27 to 114 mm shell length) fed 3 diets (natural (N), algae (A) and silt (S) dominated) in the laboratory. Larger mussels produced bigger biodeposits that sank more quickly, and highly significant relationships between biodeposit size (faecal pellet width and pseudofaeces area) and sinking velocity were derived for each diet (r 2 = 0.43 to 0.79). Biodeposit sinking velocities also varied significantly with diet. Mean sinking velocities of faecal pellets pro- duced on the A diet (0.67 cm s -1 ) were approximately 4× lower than those of pellets produced on the N and S diets (2.46 to 2.71 cm s -1 respectively). Pseudofaeces produced on the N and A diets settled 50 to 70% slower than faeces produced on the same diet, but sinking velocities of S diet faeces and pseudofaeces were similar. In situ estimates of biodeposit sinking velocities measured in the Firth of Thames, New Zealand, were comparable to those produced on the N diet. Erosion thresholds were also affected by diet but not by mussel size. Ninety percent of faecal pellets produced on the A diet eroded at a bed shear velocity (u*) of 0.28 cm s -1 , half that required to erode the same percentage of faecal pellets produced on the N and S diets. Pseudofaeces generally eroded over a similar but wider range of u * , and the differences between the 3 diets were less distinctive than for faecal pellets. These results emphasise that mussel biodeposit dispersal (and hence the flux to the benthos) depends on the available diet and mussel size and thus could differ significantly between locations, seasons and the size structure of the population.

Effects of epibenthic predators in flow: transport and mortality of juveniles of the soft shell clam Mya arenaria

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 279:151-160 (2004) - doi:10.3354/meps279151 Effects of epibenthic predators in flow: transport and mortality of juveniles of the soft shell clam Mya arenaria Heather L. Hunt* Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Rd., New Brunswick, 08901 New Jersey, USA Present address: Biology Department, University of New Brunswick, PO Box 5050, Saint John, New Brunswick, E2L 4L5, Canada *Email: hhunt@unbsj.ca ABSTRACT: During the early post-settlement period, dispersal and predation can drastically alter patterns of distribution and abundance of benthic invertebrates. These 2 processes may interact if predators cause disturbance and alter rates of transport of juvenile clams. In this study, experiments were conducted in an annular flume to examine the effect of 2 epibenthic predators, the seven-spine bay shrimp Crangon septemspinosa and juveniles of the green crab Carcinus maenas, on rates of erosion and mortality of juvenile soft shell clams Mya arenaria. Clams were exposed to 2 flow speeds (u = 9 cm s-1, shear velocity u* = 0.30 cm s-1 and u = 27 cm s-1, u* = 0.97 cm s-1) in the presence and absence of predators. Erosion of sediment and clams, and formation of ripples occurred at the high-flow velocity only in the presence of shrimp. This also occurred when shrimp disturbed the sediment prior to the experimental run, suggesting that shrimp activity changed bottom topography and the erosion threshold of the sediment. Juvenile green crabs caused less disturbance of the sediment and their activities did not result in transport of sediment or clams. Gut content analysis indicated that juvenile green crabs preyed on juvenile M. arenaria. Shrimp preyed on clams in 1 of 2 experiments, probably due to a difference in size of clams between experiments (shell length = 1.0 vs. 3.0 mm). This study demonstrates that, in addition to causing mortality, predators can indirectly affect their prey by causing transport of sediment and associated small benthic invertebrates. KEY WORDS: Predators · Postlarval dispersal · Bivalves · Sediment transport Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 279. Online publication date: September 28, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Bio-dependent bed parameters as a proxy tool for sediment stability in mixed habitat intertidal areas

The stability of cohesive and non-cohesive sediments in a mixed intertidal habitat within the Ria Formosa tidal lagoon, Portugal, was examined during two field campaigns as part of the EU F-ECTS project. The cohesive strength meter Mk III was used to determine critical erosion shear stress ( τ c) within a variety of different intertidal habitats and substrata, including Spartina maritima fields and Zostera noltii beds. The best predictor(s) for τ c were derived from a range of properties measured for the surface sediments (chlorophyll a, colloidal carbohydrate, water, organic content, % fraction <63 μm, and seabed elevation). Pigment biomarkers were used to identify the dominant algal groups within the surface phytobenthic assemblage. Strong, seasonally dependent relationships were found between τ c and habitat type, chl a, colloidal carbohydrate and bed elevation. Typically, critical erosion thresholds decreased seawards, reflecting a change from biostabilisation by cyanobacteria in the upper intertidal areas, to biostabilisation by diatoms on the bare substrata of the channel edges. In the late summer/early autumn, cyanobacteria were the main sediment stabilisers, and colloidal carbohydrate was the best bio-dependent predictor of τ c across the entire field area. In the late winter/early spring, cyanobacterial activity was lower, and sediment stabilisation by Enteromorpha clathrata was important; the best predictor of τ c was bed elevation. The implications and use of proxies for sediment stability are discussed in terms of feedback and sedimentation processes operating across the intertidal area.

Microphytobenthos, their Extracellular Polymeric Substances, and the Morphogenesis of Intertidal Sediments

Intertidal sediments are important areas that separate the land from the sea and form natural coastal defenses. They are known as highly productive ecosystems, fueling the coastal food web. It is also conceived that microphytobenthos contribute to the stability of intertidal sediments by increasing the erosion threshold and that they are major players in coastal morphodynamics. Depending on the sedimentary composition of intertidal flats, different types of microphytobenthos colonize the sediment surface. Fine sand sediment is often colonized by cyanobacteria, prokaryotic algae, which form dense and rigid microbial mats. Mudflats on the other hand are characterized by the development of thin biofilms of epipelic diatoms. Both groups of phototrophic microorganisms excrete extracellular polymeric substances (EPS), but they do so in different ways and for different reasons. Two operationally defined fractions, water- and EDTA-extractable EPS, have been obtained from intertidal diatom biofilms and from cultures. They differ in composition and their production seems to be under different metabolic control. Water-extractable EPS are considered to be closely associated with the diatoms and are rich in neutral sugars, notably glucose. These EPS show a dynamic relationship with the microphytobenthic biomass. EDTA-extractable EPS are tightly bound to the sediment, probably through bridging by divalent ions. This material is rich in uronic acids and other acid sugars and is weakly related to chlorophyll. These EPS have been conceived to be a major factor in the structuring and diagenesis of coastal sediments and essential for increasing the sediment erosion threshold. However, this relationship is now questioned.

Correction to “Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem”

[1] In the paper “Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem” by Noah P. Snyder, Kelin X. Whipple, Gregory E. Tucker, and Dorothy J. Merritts (Journal of Geophysical Research, 108(B2), 2117, doi:10.1029/2001JB001655, 2003), the authors wish to correct several minor errors in equations contained in the published version. [4] There are several errors in Appendix A. The entire corrected appendix is as follows. A full discussion of the stochastic threshold bedrock channel incision model is given by Tucker [2003]. [6] This research was funded by National Science Foundation grants EAR-9725723 to Whipple and EAR-9725348 to Merritts.

Relating spatial and temporal variability in sediment chlorophylla and carbohydrate distribution with erodibility of a tidal flat

Trends in the spatial distribution of chlorophylla (chla) and colloidal and total carbohydrates on the Molenplaat tidal flat in the Westerschelde estuary, Netherlands, reflected spatial differences in physical properties of the sediment. Results from a Spearman Rank Order Correlation indicated that many of the physical and biological measures covaried. Multiple regression analyses describing the relationship between colloidal carbohydrates and sediment properties resulted in several highly significant equations, although in all cases chla was able to predict colloidal carbohydrate content. Relationships between sediment surface chla and colloidal carbohydrate, and sediment erodibility (i.e., critical erosion threshold, Ucrit, and mass of sediment eroded at a velocity of 30 cm s−1) determined in annular flume experiments were examined. Overall sediment erodibility was lowest (i.e., high thresholds, low mass eroded) for the siltiest sediments in June 1996 when chla and colloidal carbohydrates were high (56.9 μg gDW−1 and 320.6 μg gluc.equ. gDW−1, respectively), and greatest (i.e., low thresholds, high mass eroded) at the sandier sediments in September 1996, when chla and colloidal carbohydrates were low (1.0 μg gDW−1 and 5.7 μg gluc.equ. gDW−1, respectively). When sediments were grouped according to relative silt content, the most significant relationships were found in muddy sand with a finegrained fraction (<63 μm) of 25–50%. Thresholds of erosion increased, while mass of sediment eroded decreased, with increasing chla and colloidal carbohydrate. A similar trend was observed for the sand-muddy sand (63 μm 10–25%). In the sand (63 μm 0–10%), there were no relationships for Ucrit, whereas mass eroded appeared to increase with increasing chla and colloidal carbohydrate. The increased carbohydrate may stick sand grains together, altering the nature of erosion from rolling grains to clumps of resuspension.