Fine Sediment Inputs Research Articles

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and aPteronarcysstonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.

Stratigraphic units of the Apulian Tavoliere plain (Southern Italy): Chronology, correlation with marine isotope stages and implications regarding vertical movements

The geologic study of the Apulian Tavoliere plain (Apulia region, southern Italy) is extremely difficult due to the scarcity of outcrops and fauna that could be used for dating. The survey in progress of the 1:50,000 scale geological sheet no. 409 “Zapponeta” (including the coastal zone of the Apulian Tavoliere) has prompted us to tackle this problem by using a large set of borehole data and the AAR dating method applied to ostracod shells, which are capable of colonizing all types of environment as long as there is water. This alternative approach has allowed us to recognise nine stratigraphic units or synthems and, for the first time in this area, to date them, and to find a correlation between them and the cycles of sea level variation. The recognised stratigraphic units are: the Coppa Nevigata sands (NEA; middle Pleistocene: MIS 17–16), a rgille subappennine unit (ASP; middle Pleistocene: MIS 15–13), the Coppa Nevigata synthem (NVI; middle Pleistocene: MIS 11), the Amendola subsynthem (MLM 1; middle Pleistocene: MIS 11), an undifferentiated continental unit (UCI; middle Pleistocene: MIS 8–7), the Foggia synthem (TGF; middle–late Pleistocene: MIS 6), the Carapelle and Cervaro streams synthem (RPL; late Pleistocene: MIS 5–3), and the Inacquata farm synthem (NAQ; Holocene). Within the RPL unit, a buried Cladocora caespitosa bioherm referable to MIS 5.5, lacking in warm fauna, and in which the coral is embedded in clay has been found in some boreholes. This is the first finding of Tyrrhenian deposits with C. caespitosa along the Italian Adriatic coast; the presence of this coral in clayey sediments, a very uncommon occurrence, strengthens the hypothesis that the major fossil reefs grew in coastal waters that were characterised by alluvial inputs of fine sediments, higher turbidity, and higher temperature than today. In addition, on the basis of the current evidence, some consideration about the fauna of the MIS 5.5 layer allows us to hypothesise that the Adriatic Sea underwent a more moderate warming compared to that of the Ionian and Tyrrhenian seas. Instead, the finding in the NVI unit of a tropical lagoonal deposit with stromatolites referred to MIS 11 proves that the warming in this stage was undoubtedly greater than that of MIS 5.5. The MM4 borehole, which goes through the MIS 5 layers of the RPL unit, made it possible to recognise two marine phases during MIS 5: the first is referable to the MIS 5.5–5.3 interval, and the second to MIS 5.1. MIS 5.2 is marked by land emersion, whereas no evidence of land emersion between MIS 5.5 and 5.3 has been found. Also for the first time in this area, uplifting and subsiding areas have been recognised and the vertical movements assessed. In general, the data suggest that the Garganic Apulian foreland and the Amendola highland experienced an uplift, while the central-southern part of the study area, belonging to the Apulian Tavoliere plain, suffered a subsidence with rates increasing from north–northwest to south–southeast. In particular, the finding of the MIS 5.5 buried layer with C. caespitosa has allowed us to fill a gap in the data regarding the recent tectonic movements along the Adriatic coast ( Ferranti et al., 2006). This feature proves that there has been a recent subsidence event since MIS 5.5 in the coastal area of the Apulian Tavoliere plain.

Multiple stressors in agricultural streams: interactions among sediment addition, nutrient enrichment and water abstraction

1. Many ecosystems are influenced simultaneously by multiple stressors, and the consequences of stressors are often unpredictable on the basis of knowledge of single effects. Agriculture affects streams world-wide via nutrient enrichment, elevated fine sediment and water abstraction for irrigation, but the combined impacts of these stressors are unknown. 2. We manipulated all three stressors simultaneously in an 18-day experiment and determined their individual and pair-wise combined effects on benthic invertebrates, algal biomass and leaf decay. We added nutrients (phosphorus plus nitrogen) and/or fine sediment (grain size 0·2 mm) to 18 experimental stream channels (dimensions 250 × 15 × 15 cm) supplied with water from a nearby stream. Three sediment and three nutrient treatments (high, intermediate, natural) were applied to each of six channels while flow was reduced by 80% in half the channels. Invertebrates (composition, abundance) and algae (chlorophyll a) were assayed using ceramic tile substrata and leaf decay was assayed using bundled leaves of a native shrub. Invertebrates colonizing leaf packs were also sampled. 3. Effects of sediment addition and flow reduction on biological response parameters were twice as common as nutrient enrichment effects. Nutrient enrichment increased total invertebrate abundance on tiles, algal biomass accrual and leaf decay rates, whereas both sediment addition (at the highest level) and flow reduction had mostly negative effects (e.g. reduced algal biomass, invertebrate abundance and/or taxonomic richness). 4. Stressors interacted often, and interactions between sediment and flow were particularly common. The negative impact of added sediment on aquatic biota was stronger at reduced flow, especially on tile substrata that were more exposed to the current than leaf-pack substrata. 5. Synthesis and applications. Our key findings imply that abstracting water from a stream already subjected to high fine sediment inputs may have far worse effects on the invertebrate fauna than abstraction from a similar stream with lower sediment levels. Aquatic resource managers should be aware of this important interaction between multiple stressors.

SEPAL – a simple GIS-based tool to estimate sediment pathways in lowland catchments

Abstract. Even though soil loss in the lowlands imposes not as much a restriction on land use and agricultural productivity as in erosion affected mountainous areas, the input of fine sediment into the rivers and streams is a concern due to water quality issues and substrate siltation. Drains, river banks and agricultural fields are the three main sources of fine sediment in lowland regions. For a successful implementation of measures to decrease sediment input a well-founded knowledge of the individual entry pathways is essential. To assess the importance of possible entry pathways, a GIS based methodology (SEPAL) has been established combining the ABAG, a river bank erosion formula and a regression approach to include the contributions of drains. SEPAL has been applied on a study catchment in Northern Germany. The results show that 15% of the sediment input into the river comes from agricultural drains, 71% from river banks and 14% from adjacent fields. A comparison of the results with field-mapping and -sampling shows that the approach is plausible. The calculated total annual sediment input is 616 t yr−1, while the measured suspended sediment load is 636 t yr−1. It can be concluded that the methodology is suitable for estimating sediment entry pathways and annual sediment loads in lowland catchments as a base for modelling projects and further investigations. However, further work is necessary for gaining sound knowledge about uncertainties and especially about the processes forcing sediment input from drains.

Mineralogy and grain size variations along two carbonate margin-to-basin transects (Pedro Bank, Northern Nicaragua Rise)

The analysis of nine periplatform cores in the surrounding of the Pedro Bank carbonate platform showed the presence of characteristic depositional environments during the last 300 ky. This subdivision is based on mineralogical and grain size variations in space and time. Along the leeward, downcurrent transect the fine sediment fraction (< 63 μm) dominates the periplatform sediments during interglacial highstands in sea level, and no spatial variations in the fine sediment concentration could be observed within 100 km offbank distance. The sediments are mainly of neritic origin preferentially fine aragonite needles. With increasing distance from the margin (> 40 km) aragonite is still the most abundant mineral, but the percentage decreases, while the pelagic carbonate mineral content (LMC) increases. Within the subordinate coarse-fraction classes (> 63 μm) the very fine sand-fraction dominates at proximal sites (< 20 km) as a result of the influence of fine neritic sediments shed offbank. More distal sites (> 20 km) show a more bimodal distribution pattern in the coarse grain sizes with maximum percentages within the very fine and medium sand-fraction, indicating a mixed neritic/pelagic signal. During glacial lowstands in sea level a twofold division in the spatial distribution of the periplatform sediments is evident. A proximal environment (< 28 km) with enhanced coarse-fraction percentages vs. a distal environment (> 28 km) showing a strong dominance of the fine fraction (> 90%). The increased coarse-fraction percentage during glacials at proximal sites results from various interacting processes: (1) lower input of fine neritic sediments, (2) increased current winnowing, and (3) redepositional processes at the upper slope during lowered sea level, and the export of this material to “proximal basinal” sites (< 28 km). At upcurrent sites the mineralogy displays a similar spatial evolution as seen along the downcurrent margin, but with overall reduced percentages. This is the result of the lower export potential of the platform against the main direction of the Caribbean Current, the most important factor for neritic sediment dispersal in the study area. The coarse-fraction content is slightly higher during interglacials compared to the downcurrent margin. During glacials a similar reduction in the percentage of the coarse-fraction is evident in a spatial context. This also substantiates the reduced sediment export and redeposition potential at the upcurrent margin.

Stream Communities Along a Catchment Land-Use Gradient: Subsidy-Stress Responses to Pastoral Development

When native grassland catchments are converted to pasture, the main effects on stream physicochemistry are usually related to increased nutrient concentrations and fine-sediment input. We predicted that increasing nutrient concentrations would produce a subsidy-stress response (where several ecological metrics first increase and then decrease at higher concentrations) and that increasing sediment cover of the streambed would produce a linear decline in stream health. We predicted that the net effect of agricultural development, estimated as percentage pastoral land cover, would have a nonlinear subsidy-stress or threshold pattern. In our suite of 21 New Zealand streams, epilithic algal biomass and invertebrate density and biomass were higher in catchments with a higher proportion of pastoral land cover, responding mainly to increased nutrient concentration. Invertebrate species richness had a linear, negative relationship with fine-sediment cover but was unrelated to nutrients or pastoral land cover. In accord with our predictions, several invertebrate stream health metrics (Ephemeroptera-Plecoptera-Trichoptera density and richness, New Zealand Macroinvertebrate Community Index, and percent abundance of noninsect taxa) had nonlinear relationships with pastoral land cover and nutrients. Most invertebrate health metrics usually had linear negative relationships with fine-sediment cover. In this region, stream health, as indicated by macroinvertebrates, primarily followed a subsidy-stress pattern with increasing pastoral development; management of these streams should focus on limiting development beyond the point where negative effects are seen.

Dispersion of gold in stream sediments in the Sungai Kuli region, Sabah, Malaysia

The distribution of Au in a Au-rich tributary of the Sungai Kuamut, Sabah, Malaysia, was studied initially by the authors in 1999 prior to disturbance of the tropical rain forest by logging. That study found: (i) a strong but extremely erratic Au anomaly with median values of 230 to 690 ppb in the fine and medium sand sized fractions from both cobble and gravel sites; and (ii) Au values typically &lt;15 ppb in the −53 μm fraction. It was suggested that increased input of fine sediment into the stream after logging might substantially dilute and modify the Au anomaly. Logging of the drainage basin in 2000–2001 provided an opportunity to test this possibility. Results from sampling at gravel sites in 2001 show no dilution of Au values after logging. This is attributed to the occurrence of rainfall and flood events that counteracted dilution of the anomaly by releasing deeply buried Au and/or flushing excess fine sediment from the stream channel. It is concluded that the strength of Au anomalies in stream sediments is likely to fluctuate as a result of the interactions of rainfall events, stream discharge, and the effects of land disturbance on erosion.

Calcimicrobial cap rocks from the basal Triassic units: western Taurus occurrences (SW Turkey)

A post-extinction calcimicrobial cap rock occurs above the giant Permian skeletal carbonate platform exposed in the western Taurus Mountains (southern Turkey). It was formed during the main step of a very rapid and large-scale platform flooding (Earliest Triassic) and has been found also in other Tethyan localities. This calcimicrobial cap rock, 20 to 40 m thick, consists of thrombolitic and stromatolitic build-ups at the base and most oolitic grainstone in the upper part. It was terminated by a sudden input of fine terrigenous sediments. The domal, columnar and conical stromatolites are ‘anachronistic’ deposits as are the abundant botryoidal and fanning aragonite crystal pseudomorphs. This again shows the uniqueness of the Earliest Triassic period and indicates a delayed biotic recovery. To cite this article: A. Baud et al., C. R. Palevol 4 (2005) .

Effects of fine sediment inputs from a logging road on stream insect communities:a large-scale experimental approach in a Canadian headwater stream

A forest headwater stream was manipulated (logging road-crossing amended) to induce fine sediment inputs. Benthic inorganic sediment concentrations .particles 1.5–250 μm increased from a 2-year pre-disturbance average of about 800 g m–2 to over 5000 g m–2 that persisted for 3 years. Aquatic insect communities were examined over the 5-year study period in the manipulated and nearby reference streams. Overall, the effects of the fine sediment increases on aquatic insect communities were minimal. There were no significant effects of sedimentation on total aquatic insect abundance or biomass. An index of multivariate dispersion gave no evidence of community stress at the manipulated site. Multivariate ordination plots and time trends among univariate community metrics indicated only subtle changes in community structure. Among the univariate metrics (16 time series analyses in total), six gave evidence of a sediment impact on aquatic insect communities. Of those, the clearest indications of an effect were small reductions in diversity and richness of spring communities. These resulted from a significant decline in the proportion of spring shredders, accompanied by a significant increase in the percent Chironomidae. This large-scale experimental approach integrated the realism of a whole-stream study with the control of a manipulative study by including pre-manipulation measurements and excluding other confounding catchment disturbances. In this regard, it may provide a more realistic measure of benthic community level responses to sedimentation in streams at a magnitude associated with logging activity than many previous studies.

Intergranular flow velocity through salmonid redds: sensitivity to fines infiltration from low intensity sediment transport events

This paper presents results from a novel technique allowing continuous monitoring through multiple storm events of interstitial flow in salmonid redds. Previous studies have shown that long-term increases in fine sediment inputs into rivers can silt up spawning beds, reduce intergravel flow and threaten egg survival. Not enough is known, however, about the temporal and spatial scales of the physical processes affecting spawning habitat. The short-term sensitivity of intergravel flow through salmon nests to low-intensity sediment transport events has not been documented. Furthermore, it is unclear if the egg pocket flow vital to incubation is principally controlled by the hydraulic conductivity of the redd patch or by that, generally lower, of the ambient riffle substrate. The purpose of this study was to determine if individual runoff events could affect intergravel flow in salmon nests and to investigate the sensitivity of interstitial flow to the fines content and conductivity of the redd patch. During the summer and autumn of 2001, a new intergravel velocity sensor based on the hot wire principle made it possible to continuously monitor, over five months, interstitial velocities in artificial redds in four tributaries of the Cascapedia River, Quebec. Fifteen low and moderate intensity runoff events (up to 50% bankfull) were monitored. Data were obtained for each storm on suspended sediment transport as well as sand infiltration rates in sediment collectors emplaced in redd zones. It was found that redd interstitial velocities were reduced whenever a runoff event deposited more than 7 kg/m2 of sands in infiltration traps. In addition, redd interstitial velocities were reduced four out of the five times that the event-integrated suspended sediment dose exceeded 7 mg l−1 day (dose is defined as the area under the concentration time curve). In the study conditions, where ambient riffle sediment has relatively moderate permeability and localized groundwater upwelling is negligible, our data suggest that significant intergravel flow (0.1–0.6 mm/s) can be triggered through 2 m long redd patches, in response to the redd-scale water surface gradient and the relatively higher conductivity of the redd patch, after spawner activity. Copyright © 2005 John Wiley & Sons, Ltd.

Biological and physical processes structuring deep-sea surface sediments in the Scotia and Weddell Seas, Antarctica

Abstract Five deep-sea transects were sampled in the Scotia Sea and Weddell Sea from the Drake Passage to the South Sandwich Islands, Antarctica, with depths ranging from 767 to 6333 m. A combination of sediment surface and sediment profile imaging was used to characterize the physical and biological processes structuring the benthic boundary layer and surface sediments from the sediment–water interface (SWI) to as much as 17 cm below the SWI. The factors structuring surface sediments were related to distance from Antarctic landmasses, which are the main source of sediment, and deposition of water-column or sea-ice primary productivity. Stations near the continental shelf and slope tended to have coarse (granule to cobble) sediments derived from melting sea-ice. Input of fine sediments was estimated to be low at these stations because many of the coarse sediment grains appeared to be manganese coated. At the deepest stations in the Ona Basin, South Shetland Trench, and South Sandwich Trench, sediment appeared to be primarily hemipelagite silt-clay but were varved with as many as four distinct color laminations. These layers varied in thickness fom 0.6 to 10.6 cm and may have been related to turbidite flows. While geology and physical processes that deliver sediment to the bottom were the primary factors dictating substratum characteristics, biological processes were, to varying degrees, important in structuring the SWI and sediment fabric. Biological processes, primarily in the form of bioturbation, were most prominent at fine-grained stations but were also obvious even on rock substratum on top of the Shackleton Fracture Zone. There were gradients between the degree of bioturbation and type of biogenic structure present with depth. At depths 75% of the sediment bioturbated) the structure of surficial sediments at 5 of 13 stations. At depths >2000 m, biological processes were overall less diverse but dominated at 8 of 11 stations.

Stream regulation and nitrogen dynamics in sediment interstices: comparison of natural and straightened sectors of a third‐order stream

AbstractNitrogen dynamics were studied in the interstitial environment (i.e. hyporheic zone) of a sandy‐bottom stream in a rural landscape. A third‐order stream in Brittany (France) was studied at 11 stations (riffles) to evaluate spatial patterns of water exchange between surface and interstitial habitats. More intensive sampling was conducted in three riffles selected according to their hydrological characteristics. Chemical characteristics (especially nitrogen) and microbial denitrification were studied at 12.5 and 25 cm depth upstream, and 25 cm depth downstream of each riffle. This study confirms that the interstitial habitat of a N‐rich stream acts as a sink for the nitrate‐nitrogen. Experimental manipulation of sediment cores indicates that denitrification is limited by carbon in surface (i.e. benthic) and by nitrate in hyporheic sediments. River regulation increases inputs of fine sediments, modifies river channel location, and generates changes in the spatial patterns of biogeochemical processes, water origins, and hydrologic exchanges. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Predicting fine sediment dynamics along a pool-riffle mountain channel

Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches. Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of ∼7000 m 3 of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations. The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field. A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment.

Local and Landscape-Scale Predictors of Salamander Abundance in New Hampshire Headwater Streams.

We investigated the response of Gyrinophilus porphyriticus ( Plethodontidae), the spring salamander, to timber harvest in headwater streams in New Hampshire ( U.S.A.). We conducted cover-controlled, whole-stream surveys for G. porphyriticus in 25 first-order streams in drainages encompassing a range of timber-harvest histories. Streams in two different landscape configurations were chosen, isolated, or paired, based on the presence or absence of a confluent first-order stream-a potential source of immigrants. We analyzed salamander abundance with respect to logging history, presence of the brook trout ( Salvelinus fontinalis), salamander population connectivity, and other measures of physical habitat and aquatic conditions. In stepwise multiple-regression analysis, the best multivariate model of G. porphyriticus abundance included fish presence/absence; effect category, a combined function of years since harvest and substrate embeddedness by fine sediment; and landscape configuration. These three factors explained 74% of the variation in G. porphyriticus abundance. G. porphyriticus was less abundant in streams where fish were present, likely because of predation by S. fontinalis or asymmetric competition between these species. Abundance also declined with increasing substrate embeddedness and/or decreasing years since harvest, indicating an effect of fine sediment inputs and harvest history on this species. Finally, G. porphyriticus was less abundant in isolated streams than in paired streams. This result suggests that landscape-scale population connectivity may buffer this species from the negative effects of local (i.e., stream-scale) habitat perturbation. We hope this work will encourage others to develop multiscale models of land-use effects on stream biota and thereby ultimately increase the range and effectiveness of conservation strategies for these species.

Modelling input of fine granular sediment into drainage systems via gully-pots

An existing model of sediment wash-off from paved surfaces has been extended to predict fine granular sediment inputs into drainage systems via roadside gully-pots. The model comprises three separate components: (1) a model of sediment accumulation on paved surfaces; (2) a model of sediment wash-off from these surfaces; and (3) a model of sediment deposition and erosion in gully-pots. The first two model components have been previously successfully tested, whereas the third component of the model is presented in this paper for the first time. The model was applied to two small catchments located in Dundee, Scotland, where data on flow rates and sediment concentrations were collected at the inlets into the main drainage systems from the road gullies. It was found necessary to calibrate only the build-up component of the model to achieve good agreement between the simulated and measured data. It was concluded that only two calibration parameters are required to model the input of fine granular sediment into storm drainage systems via gully-pots.

Fine Sediment and Macroinvertebrate Assemblages in Appalachian Streams: A Field Experiment with Biomonitoring Applications

Forest management operations may result in above-ambient inputs of fine sediments to streams with consequences for stream biota. A field experiment was conducted at the Fernow Experimental Forest in West Virginia to examine the effects of fine sediment on macroinvertebrate assemblages. Fine sediment (<2 mm) was varied between 0 and 30% in 5% increments in otherwise natural substratum mixtures in 0.03-m² trays deployed at each of 3 sites along Elklick Run, a 4th-order stream, and allowed to colonize for 5 wk. Macroinvertebrate density (-, sign indicates response to fine sediment), biomass (-), EPT (Ephemeroptera, Plecoptera, and Trichoptera) taxa richness (-), the proportion of Ephemeroptera composed of Baetidae (+), and the proportion of chironomids composed of Chironominae (-) and of Orthocladiinae (+) were the most sensitive and reliable metrics. Sediment effects on metrics were generally subtle, even when statistically significant. Ordination and the relative abundance of individual taxa indicated that assemblage structure was altered by treatment, but the shift was generally less than the variation in structure among the 3 sites. Percent organic matter and interstitial velocity were quantified, but did not appear to explain observed responses to treatment. Comparison of assemblages colonizing treatment mixtures with assemblages colonizing natural substratum indicated that most aspects of the experiment were realistic. Macroinvertebrate assemblages were also sampled in 15 local streams, which varied in fine-sediment bed loads, to determine if predictions based on the field experiment were supported at a larger spatial scale. Survey results generally agreed qualitatively with experimental results suggesting that some metrics were robust at a regional scale for physically and geologically similar streams.

Seasonal, Neap-spring and Tidal Variation of Cohesive Sediment Concentration in the Scheldt Estuary, Belgium

The cohesive sediment transport in tidal rivers and estuaries is determined by the river discharge, the morphology, the salinity, temperature and particulate gradients, the deposition and erosion processes, the effective settling velocity of mud particles and by the physico-chemical and biological processes. The combination of these processes ensures that mud concentration varies on different time-scales ranging from seconds to seasons. Based on a long-term measurement campaign at Prosperpolder (Scheldt Estuary, Belgium), a detailed analysis of mud concentration, salinity, temperature and freshwater discharge with regard to the various involved time-scales is presented.The measurements at Prosperpolder were carried out between September 1992 and December 1993 at 0·5 and 1·5m from the bottom. The data indicate that the mud concentration is correlated with tides, spring–neap cycles and with seasons. The mud concentration in Prosperpolder is on average higher during flood than during ebb, which can be explained by the flood dominant character of the currents. The tide averaged mud concentration is 1·3–1·7 times higher during a spring tide than during a neap tide. The ratio between the winter and summer concentration is about 2. The seasonal variations are due to the river discharge (transport, shift of the turbidity maximum), temperature (biological activity, climatological factors) and land erosion (terrestrial input of fine sediments).

Cyclic Architecture of a Carbonate Sequence, Early Aptian Shuaiba Formation, Al Huwaisah Field, Oman

Sequence stratigraphy of carbonates is a topic of ongoing controversy. In particular, small-scale shallowing-upward cycles can provide some key information needed for interpretation of carbonate sequences and/or third-order sea level changes. The early Aptian Shuaiba Formation in the Al Huwaisah field consists of about 90 m of shallow-water limestones. Throughout the formation, an overall decreasing influx of fine detritus is notable toward the top. The sequence can be subdivided into a basal unit and an overlying unit. Both units are composed of meter-scale shallowing-upward cycles of different composition, which can be recognized in core and well logs. Fourier analysis of the first principle component of a set of well logs (GR, FDC, CNL, Sonic) revealed an abrupt change in spectral behavior between the two units. Toward the top, the spectra are [open quotes]cleaning upward[close quotes] with an increasing pronunciation of a peak grouping of 1: 2: 5, suggesting a better preservation of orbital variations in the upper unit. Preservation of orbital forcing in shallowing-upward cycles requires rapid rates of sedimentation. In addition, increased shallow-water carbonate production on the platform is indicated by the appearance of reefal organisms. Hence, a higher rate of sedimentation and therefore a faster aggradation of themore » platform is inferred for the upper unit, which could have resulted from an increased rate of relative sea level rise. The sudden facies differentiation on the broad Arabian shelf in the upper part of the early Aptian reflects the development of an intrashelf basin. Changes in rate of relative sea level rise on the Arabian shelf might explain the repeated alternation from an easily correlatable ramp-type sedimentation, with slightly higher input of fine terrigenous sediment (e.g., lower unit of Shuaiba Formation) and a differentiation into platform and intrashelf basin facies due to faster aggradation (e.g., upper unit of Shuaiba Formation).« less

Changes in the benthic invertebrate communities of the thomson river, southeastern Australia, after dam construction

AbstractBenthic macroinvertebrates were quantitatively sampled at four sites, one above (T6) and three (T14A, T16, T21A) downstream of the Thomson Dam, Victoria, Australia, for three summers after completion of dam construction in 1983. Community composition was compared with that recorded previously during dam construction to determine the effects of the main environmental consequences of the dam, viz the input of fine sediment (&lt; 2mm grain size), the temporary release of cold hypolimnetic water during summer, and altered levels of discharge. Unnaturally low summer water temperatures were only recorded at the two sites immediately downstream of the dam (in order downstream T14A and T16). After water temperatures returned to normal summer levels, species richness increased at these sites. Levels of fine sediment increased in the surface layers of the riverbed at T16 but not at T14A. By the time this study began levels at T16 had been reduced almost to pre‐dam levels. At T21A levels approximately tripled after completion of construction. At T14A and T16 marked increases in species richness occurred during this study whereas at T21A species richness did not change, and was lower than that recorded during dam construction. Ordination and classification of samples clearly displayed these changes in the benthic communities downstream of the dam and the lack of change at T6 above the dam. Altered discharge resulted in some loss of habitable area and thus total standing stock at T14A only.

Sedimentology and Geochemistry of Modern Sediments in the Kingston Basin of Lake Ontario

Accumulation of Recent fine sediment is limited to deep water areas in the Kingston basin and thicknesses of more than 50 cm are found only in the St. Lawrence trough; elsewhere, sands and silty sands are largely composed of reworked material. Local sources provide little input of fine sediment to the Kingston basin. Statistical relationships derived from geochemical analyses indicate several forms of association between heavy metals and organic carbon, sulphides, clays, hydrous Fe and Mn oxides, and carbonates. A comparison between Kingston and Niagara sediments shows that the combined effects of riverine and lacustrine erosion have resulted in a generally greater content of carbonates in the Niagara sediments. The content of Fe (which is partly influenced by redox potential) is highest, however, in sediment of the Kingston basin, where higher P values are also associated with an increased clay content. Differences in the concentrations of heavy metals in Lake Ontario, except for Hg, Co, Cu, and Zn, are largely explained by variations in silt and clay contents. Contaminant loadings from the Niagara River are largely responsible for the anomalously high concentrations of Hg and, to a lesser extent, Cu and Zn. Cobalt occurs at higher concentrations in the sediments of eastern Lake Ontario, where its presence is thought to reflect glacial dispersal patterns.