Percentage Of Silt Research Articles

The relationships among hydrodynamics, sediment distribution, and chlorophyll in a mesotidal estuary

Tidal bars and tidal flats in shallow, mesotidal estuaries are subject to the action of tidal currents and waves. This complex forcing gives rise to large variations in bottom sediments and related benthic ecosystems. In this research we study the spatial relationships among tidal currents, waves, sediment characteristics, chlorophyll a, and pheophytin in the shallow bars and flats of Plum Island Sound, Massachusetts, USA. Waves and tides have a mild effect on the sand grain size and percent of silt and mud of these shallow deposits. Average wave conditions are more important than extreme events, like Nor’easters, in sediment sorting. The spatial distribution of pheophytin, an indicator of dead microphytobenthos, mimics that of silt and mud whereas chlorophyll a, a proxy for the live microphytobenthos stock, is greatest in sandy areas characterized by intermediate values of tidal velocity.

Nature of the Distributed of the Bed Sediment within Mosul Dam Reservoir, Iraq

Nature of the Distributed of the Bed Sediment within Mosul Dam Reservoir, Iraq Nadhir Al-Ansari Issa E. Issa Sven Knutsson nadhir.alansari@ltu.se issa.elias@ltu.se sven.knutsson@ltu.se Lulea University of Technology, Sweden Abstract Mosul Dam is one of the biggest hydraulic structures in Iraq. It was constructed in 1986 on the Tigris River in the north of Iraq. The initial storage capacity and water surface area of its reservoir reaches 11.11 km3 and 380 km2 respectively at the maximum operation level 330 m a.s.l. The dam was operated in 1986. A total of 56 samples were collected from the bottom of Mosul reservoir covering most of the reservoir area. The results of the analysis of these samples revealed that they were composed of gravel (3.8%), sand (15%), silt (55.5%) and clay (25.7%). The distribution of these sediments indicates that the silt portion represents the highest followed by clay and then sand. However, sand percentages are the highest in the northern zone of the reservoir where the River Tigris enters the reservoir and decreases gradually toward the dam site. In the meantime, silt percentage decreases toward the dam site while the finer fraction (i.e. clay) increases. Keywords.Mosul dam, reservoir sedimentation, sediment size distribution, sediment characteristics. طبيعة توزيع رسابات القعر في خزان سد الموصل - العراق عيسى الياس عيسى نضير الانصاري سفن كنتسون جامعة لوليا / السويد جامعة لوليا / السويد جامعة لوليا / السويد الملخص سد الموصل واحد من اكبر المنشآت الهيدروليكية في العراق كان قد انشىء عام 6891 على نهر دجلة شمال العراق. سعة الخزين الاولية والمساحة السطحية لبحيرته يصل 66.66 كم 3 و 393 كم 2 على التوالي عند اكبر مستوى للتشغيل 333 م فوق مستوى سطح البحر. ست وخمسون عينة من رسوبيات قعر الخزان كانت قد جمعت لتغطي معظم مساحة البحيرة. نتائج التحليل الميكانيكي لهذه الرسوبيات اظهرت ان نسبة الحصى كانت% 3.8 والرمل % 15 والغرين 55.5% والطين % 25.7 . نتائج توزيع الرسوبيات بينت ان نسبة الغرين هي اعلى من الطين والرمل بينما ان نسبة الرمل كانت اعلى في الجزء الشمالي للخزان في المنطقة التي يدخل فيها نهر دجلة الى البحيرة وتقل تدريجيا

Correlation between nematode communities with some environmental parameters in the Dai estuary, Ben Tre

Free living nematode communities were investigated at the same time with some environmental parameters in the Cua Dai estuarine sediment, Ben Tre province. Our results showed that nematode densities, biodiversity and genera richness were quite high. Obtained data of nematode univariate, such as densities, genera richness, biodiversity and feeding type structure was applied statistical method to investigate significant different between sampling stations along the estuary. Remarkably, there is no nematode univariate showed significant correlation with salinity gradient but they did show with pH, coliform, NH4+, PO43-, chlorophyll a and percentage of silt in the sediment.

Impacto del uso del suelo agropecuario sobre macroinvertebrados acuáticos en pequeñas quebradas de la cuenca del río La Vieja (Valle del Cauca, Colombia)

The expansion of the agricultural frontier in Colombia has exerted significant pressure on its aquatic ecosystems during the last few decades. In order to determine the impacts of different agricultural land uses on the biotic and abiotic characteristics of first and second order streams of La Vieja river watershed, we evaluated 21 streams located between 1,060 and 1,534 m asl in the municipalities of Alcalá, Ulloa, and Cartago (Valle del Cauca, Colombia). Seven streams were protected by native vegetation buffers, eight had influence of coffee and plantain crops, and six were influenced by cattle ranching. Habitat conditions, channel dimensions, water quality, and aquatic macroinvertebrates were studied in each stream. Streams draining cattle ranching areas had significantly higher dissolved solids, higher phosphorus, higher alkalinity, higher conductivity, and lower dissolved oxygen than those covered by cropland and forests. Coarse substrates and diversity of flow regimes were significantly higher in cropland and protected streams when compared to streams affected by cattle ranching, whereas the percent of silt and slow currents was significantly higher in the latter. A total of 26,777 macroinvertebrates belonging to 17 orders, 72 families and 95 genera were collected. The most abundant groups were Diptera 62.8%, (Chironomidae 49.6%, Ceratopogonidae 6.7%), Mollusca 18.8% (Hydrobiidae 7.2%, Sphaeriidae 9.6%) and Trichoptera 5.7% (Hydropsychidae 3.7%). The Ephemeroptera, Trichoptera, and Plecoptera orders, known for their low tolerance to habitat perturbation, had high abundance in cropland and forested streams, whereas Diptera and Mollusca were more abundant in those impacted by cattle ranching. Results indicate that streams draining forests and croplands have better physical and biological conditions than those draining pastures, and highlight the need to implement protective measures to restore the latter.

Limnocytherina axalapasco, un nuevo ostrácodo dulceacuícola (Podocopida: Limnocytheridae) de lagos de cráter mexicanos

Limnocytherina is a genus conformed by 12 species; its distribution in the American continent is known to be exclusively on the North (neartics), but little is reported about its distribution from Mexico (transition zone) and Central America (Neotropics). Different sampling campaigns were undertaken in three crater lakes from the Axalapascos region in east-central Mexico, during 2008, 2009 and 2011. As a product of these campaings, the new species of Limnocytherina axalapasco was found, which displays some intraspecific variability among populations. In this study, we described the taxonomy, the habitat, the ecological preferences and the larval development of this new species. A total of 10 sediment samples (8 littoral, 2 deepest point) were collected from lakes Alchichica, La Preciosa and Quechulac. We found that L. axalapasco is closely related to two North American species: L. posterolimba and L. itasca as well as one Central American species L. royi comb. nov. With the inclusion of L. axalapasco and L. royi to the genus, the distribution of Limnocytherina is extended to Central America. The four most important distinguishing characters of this new species are: 1) valve surface and margins covered with small, spine-like projections; 2) most of the A1 setae with a highly developed setule at distal part, producing a bifurcate appearance; 3) the upper ramus on the hemipenis is elongated, and by far overpasses dorsal/distal margins, distal lobe is triangular and short, while the hook-like process is prominent, outward orientated, and overpassing the tip of the distal lobe; 4) the UR is moderately developed with seta f3 elongated and setae f1 and f2 short. Considering its ecological characteristics and larval development, L. axalapasco was preferably found in alkaline waters dominated by Cl(-) or HCO3(-) and Na+ or Mg2+, temperatures ranging between 19.1 to 20.3 degrees C, and dissolved oxygen concentrations from 5 to 6.5 mg/L. This species was abundant in deeper (approximately 64m) areas of the saline Alchichica lake, where surface water displayed conductivity values of up to 2 250 microS/cm, and the sand with low percentage of silt resulted the preferred substrate. Along with the description of L. axalapasco, we provide additional information on the hemipenis of L. itasca, L. royi and L. sanctipatricii, and we discuss on the Limnocytherina-type of hemipenis.

Covariate selection with iterative principal component analysis for predicting physical soil properties

Local and regional soil data can be improved by coupling new digital soil mapping techniques with high resolution remote sensing products to quantify both spatial and absolute variation of soil properties. The objective of this research was to advance data-driven digital soil mapping techniques for the prediction of soil physical properties at high spatial resolution using auxiliary data in a semiarid ecosystem in southeastern Arizona, USA. An iterative principal component analysis (iPCA) data reduction routine of reflectance and elevation covariate layers was combined with a conditioned Latin Hypercube field sample design to effectively capture the variability of soil properties across the 6250ha study area. We sampled 52 field sites by genetic horizon to a 30cm depth and determined particle size distribution, percent coarse fragments, Munsell color, and loss on ignition. Comparison of prediction models of surface soil horizons using ordinary kriging and regression kriging indicated that ordinary kriging had greater predictive power; however, regression kriging using principal components of covariate data more effectively captured the spatial patterns of soil property–landscape relationships. Percent silt and soil redness rating had the smallest normalized mean square error and the largest correlation between observed and predicted values, whereas soil coarse fragments were the most difficult to predict. This research demonstrates the efficacy of coupling data reduction, sample design, and geostatistical techniques for effective spatial prediction of soil physical properties in a semiarid ecosystem. The approach applied here is flexible and data-driven, allows incorporation of wide variety of numerically continuous covariates, and provides accurate quantitative prediction of individual soil properties for improved land management decisions and ecosystem and hydrologic models.

Determining and Mapping Soil Erodibility Factor (Case Study: Yamchi Watershed in Northwest of Iran)

Among Universal Soil Erosion Equation (USLE) factors (R, K, L, S and P), Soil Erodibility Factor (K) is one of the most important and key factors which determines soil particles resistance to be detachment by water erosion (rainfall and/or runoff) forces. In fact, K factor is the rate of soil loss per rainfall erosion index unit and affected by 6 parameters including soil primary particles (silt, sand and clay), organic matter content and also permeability and structure of soil. The USLE nomograph is one of the most rapid and common methods for calculating K factor based on mentioned parameters. In this study, 38 samples of surface soil (0 - 15 cm) were collected from Yamchi watershed and the percentage of silt, sand, clay and organic matter content were determined in soil laboratory. Also textures of soil samples were determined to choice soil permeability and structure class codes based on United States Department of Agriculture (USDA) published information. Using USLE nomograph equation, K factor was calculated for each soil sample and based on kriging interpolation method, soil erodibility factor (K) map was constructed for entire study area which average soil erodibility factor and average standard error of interpolated map were 0.442 and 0.0076 t·ha·h·ha-1·Mj-1·mm-1, respectively.

Soil Chemical Analysis of Secondary Forest 30 Years after Logging Activities at Krau Wildlife Reserve, Pahang, Malaysia

Lack of management in secondary forest is significantly reducing the recovery process of nutrients especially on the top soil. Based on these facts, the soil texture and soil nutrients in secondary forest are important to be studied. The total of 50 soil samples was collected from different plots at 0-15cm in depth and were analysed in the laboratory. The result shows that the soil of Krau Wildlife Reserve (KWR) is predominated by sandy loam type, soil pH range from strong acidic to slight acidic whereas the compaction range from moderate to serve. The mean of total carbon (C), Nitrogen (N) and CN ratio and phosphorus (P) are lower; however, the Exchangeable Cation (EC) are higher than what other studies found in secondary forest in peninsular Malaysia. Statistical correlation analysis found that the different percentage of sand, silt and clay particles significantly influenced the soil nutrients. Following the criteria of tropical soil quality index, the soil nutrients of KWR 30 years after logging activities is still low.

Microwave Scattering for Soil Texture at X-Band and Its Retrieval Using Genetic Algorithm

In the present paper, we have studied the effect of soil textures, i.e., sand, silt and clay on microwave scattering at X-band (10 GHz) at various incidence angles and like polarizations (i.e., Horizontal-Horizontal; HH-, Vertical-Vertical; VV-). We have proposed a retrieval technique based on Genetic Algorithm (GA) to retrieve soil texture. For this purpose, ten types of soil mixtures having different percentage of sand, silt and clay have been analyzed. The observations were carried out by ingeniously assembled X-band scatterometer. A good agreement has been noticed between estimated and observed soil texture. Study infers that soil texture is quite sensitive to radar scattering and it is possible to retrieve soil texture with radar/scatterometer data with good accuracy and this type of retrieved results can be helpful to predict soil strength as well as soil erosion of the particular area.

Modeling root length density of field grown potatoes under different irrigation strategies and soil textures using artificial neural networks

Root length density (RLD) is a highly wanted parameter for use in crop growth modeling but difficult to measure under field conditions. Therefore, artificial neural networks (ANNs) were implemented to predict the RLD of field grown potatoes that were subject to three irrigation strategies and three soil textures with different soil water status and soil densities. The objectives of the study were to test whether soil textural information, soil water status, and soil density might be used by ANN to simulate RLD at harvest. In the study 63 data pairs were divided into data sets of training (80% of the data) and testing (20% of the data). A feed forward three-layer perceptron network and the sigmoid, hyperbolic tangent, and linear transfer functions were used for the ANN modeling. The RLDs (target variable) in different soil layers were predicted by nine ANNs representing combinations (models) of the eight input variables: soil layer intervals (D), percentages of sand (Sa), silt (Si), and clay (Cl), bulk density of soil layers (Bd), weighted soil moisture deficit during the irrigation strategies period (SMD), geometric mean particle size diameter (dg), and geometric standard deviation (σg). The results of the study showed that all the nine ANN models predicted the target RLD values satisfactorily with a correlation coefficient R2>0.98. The simplest and most complex ANN architectures were 3:2:1 and 5:5:1 consisting of D, SMD, dg, and D, Bd, SMD, σg, dg as the input variables, respectively. Low values of normalized root mean square error (NRMSE) (min=0.101, max=0.227) and mean absolute error (MAE) (min=0.345cmcm−3, max=0.79cmcm−3) proved the high capability of the ANN to predict RLD. The RLD prediction was more accurate in the top soil layers than in the deeper layers; this discrepancy could be possibly attributed to the non-homogenous root distribution in the three soil textures, soil pore structure, and nutrient availability. Results also implied that ANN is a strong modeling tool for simulating the RLD in small data sets. Conclusively, ANN is a powerful tool to predict RLD under a range of soil physical conditions with a high degree of accuracy and may be used in crop growth modeling.

Earthworm community composition, seasonal population structure, and casting activity on Kentucky golf courses

Earthworms can be troublesome pests on golf courses when their soil-rich casts become abundant enough to disrupt the maintenance, aesthetics, and playability of putting greens and fairways. Management of the problem is hindered by lack of knowledge of earthworm community structure on North American golf courses. We surveyed communities of endogeic earthworms inhabiting golf course fairways and putting greens in central Kentucky and tracked the seasonal population structure and casting activity of the predominant species. Seven earthworm species, six of them non-native, were identified in varying proportions from fairways of the different golf courses. Aporrectodea trapezoides (Dugès) dominated, although Allolobophora chlorotica (Savigny), Diplocardia singularis (Ude), and Amynthas sp. also were abundant on some courses. Soil characteristics (pH, percentages of sand, silt, clay, and organic matter) at fairway sample sites were not good predictors of overall earthworm density or proportionate abundance of particular species. Ap. trapezoides was found in the upper 21cm of topsoil throughout the year except when frozen ground precluded sampling. The Ap. trapezoides population consisted mainly of adults and cocoons in late autumn and winter, and juveniles in summer. Casting by Ap. trapezoides, which also occurs on soil-based greens, was greatest in late autumn and early winter, with a secondary peak in early spring. Amynthas hupeiensis, an east Asian megascolecid earthworm, was the only species found damaging sand-based greens. Prolific casting by Am. hupeiensis continued during summer after other species’ casting activity on fairways had waned. Am. hupeiensis is established along river banks in Kentucky and neighboring states, and is used as fish bait. We hypothesize that it may be introduced onto golf courses as cocoons in river sand used for course renovations, or from bait discarded by anglers fishing in golf course ponds.

Anisotropic statistical scaling of vadose zone hydraulic property estimates near Maricopa, Arizona

[1] Fluid flow and mass transport in the vadose zone are strongly influenced by spatial variability in soil hydraulic properties. It has become common to characterize spatial variability geostatistically and to solve corresponding flow and/or transport problems stochastically. The typical (though not only) approach is to treat log saturated hydraulic conductivity, Y = log10 Ks, and perhaps other medium properties as statistically homogeneous, isotropic, or anisotropic multivariate-Gaussian random fields with unique variance and autocorrelation scale. A growing body of literature suggests that Y as well as many other variables may possess heavy-tailed, non-Gaussian distributions, and/or scale-dependent statistical parameters representing a multiscale, hierarchical structure. Elsewhere, we have demonstrated that these important phenomena are difficult to detect, and are not fully tractable, with standard geostatistical methods. They are however detectable and tractable with a novel geostatistical method of analysis which treats such variables as samples from sub-Gaussian random fields subordinated to truncated fractional Brownian motion (tfBm) or truncated fractional Gaussian noise (tfGn). Each such field is a mixture of Gaussian components having random variances. The purpose of this paper is to explore the extent to which hydraulic parameters of unsaturated soils exhibit heavy-tailed distributions and statistical scaling of the above type. As a test bed we have selected an experimental site near Maricopa, Arizona, at which soil texture data have been collected in boreholes and trenches to a depth of 15 m over an area of 3600 m2. Since hydraulic parameters are difficult to measure to such depths we estimate them using a neural network pedotransfer model. Input data include percent sand, silt, and clay. Outputs include estimates of saturated and residual volumetric water content, saturated hydraulic conductivity, and (due to their wide applicability) parameters of the van Genuchten-Mualem constitutive model of unsaturated soil property variations with capillary pressure head. We find indeed that vertical and horizontal increments of our hydraulic parameter estimates exhibit heavy-tailed frequency distributions and statistical scaling which conform closely to our novel geostatistical framework. Its generality and applicability to the Maricopa site has far reaching implications vis-à-vis the geostatistical characterization of vadose zone properties and the stochastic modeling of flow and transport in unsaturated media at other sites.

세립토의 회복탄성계수(Mr)에 대한 지반물성치의 영향

The objective of this study was to identify the effect of engineering properties on the resilient modulus () of cohesive soils as subgrade. Eight representative cohesive soils representing A-6, and A-7-6 soil types collected from road construction sites, were tested in the laboratory to determine their basic engineering properties. The laboratory tests for the engineering properties were Atterberg limits test, sieve analysis, hydrometer test, Standard Proctor compaction test, and unconfined compressive strength test. Resilient modulus test and unconfined compressive strength test were conducted on unsaturated cohesive soils at three different moisture contents (dry of optimum moisture content, optimum moisture content, and wet of optimum moisture content). The increase in moisture content considerably affected the decrease in the resilient modulus. The resilient modulus increased with an increase in maximum unconfined compressive strength, percent of clay, percent of silt and clay, liquid limit and plasticity index. The resilient modulus decreased with an increase in percent of sand.

오염 준설퇴적토의 입자분리기술에 관한 연구

As sediment contamination problems have recently been raised in Korea, the need for technologies to remove contaminants in sediments has increased. Contaminated sediments in Korea has been annually dredged and treated using processes of coagulation/flocculation, sedimentation on barges, dewatered and dried at prepared site, and then disposed at a landfill site, which is very costly, and only a limited landfill space available in Korea. Contaminants in media containing a high percentage of silt and clay sized particles, typically, are strongly adsorbed on the particles and difficult to remove. Particle separation processes that separate the fine clay and silt particles from the coarser sand and gravel and concentrate the contaminants into a smaller volume of sediment that can be further treated of disposed of, are very effective in the post step processes. In this study are to test the feasibility of treating dredged sediments using a hydrocyclone process, and to estimate design parameters for a pilot scale test. A hydrocyclone was operated to separate larger particles from the sediments. It was found that the particle separation was greatly affected by the solid contents and inlet pressure in the hydrocyclone.

Assessment of suspended sediment concentration and load from a large Himalayan glacier

An assessment of suspended sediment concentration (SSC), load, yield and erosion rate has been undertaken for the Gangotri Glacier drainage basin (nearly 50% glaciated) located in the Garhwal Himalayas. Data were collected for four ablation seasons (2008–2011). Mean monthly SSCs, for May, June, July, August and September during the study period was 1,011, 1,384, 1,916, 1,675 and 567 ppm, respectively, indicating highest SSC in July, followed by August. For the entire melt season, the mean daily SSC was computed to be 1,320 ppm. Similar trends were also found for the sediment load and about 67% of the total suspended sediment load of the melt period was transported during the months of July and August. Sediment yield for the study basin was computed to be about 2,863 tonnes km–2yr–1. For the entire ablation period, the erosion from the Gangotri Glacier basin is estimated to be about 1.0 mm. There was a poor relationship between SSC and discharge and hysteresis effect was prominent in the melt stream. The average percentages of clay, silt and sand were found to be 3, 67 and 30%, respectively, which suggest maximum content of silt followed by sand.

The impact of poultry litter application on sediment chemistry of the Broadkill River estuary system, Delaware

This project examined the impact of long-term poultry litter application on the chemical signatures of As, Cu, Zn, and P in stream sediments of the Broadkill River watershed within the Delmarva Peninsula, a region of intense poultry production. Thirty-seven sediment samples were collected from Broadkill River drainage systems and analyzed for litter-derived elements (As, Cu, Zn, P) and basic soil parameters such as particle size distribution, organic matter, and soluble salts. Results showed that concentrations of elements in stream sediments are approximately log-normally distributed. Spatial variability in concentrations of elements was evident, with most elements increasing in concentration and enrichment from upgradient headwaters to downgradient reaches draining predominantly agricultural areas. Results of correlation analyses showed positive significant correlation among elements; elements were also positively correlated with percent clay and silt in the sediment. Using GIS maps with overlays of hydrology and land use activities, statistical correlations between As, Cu, Zn, and P enrichment factors and land use were examined. Results showed statistically significant relationships between As, Mn, and Zn enrichment factors and residential areas within the watershed, but did not show a statistically significant relationship between element enrichment factors and agricultural land use. Factors that complicate this type of landscape-scale study include the presence of poultry processing plants, impoundments, changes in land use over time, and the influence of tides, all of which can have direct and indirect influences on element mobility.

Sedimentary characteristics and status of water quality in Polwatta river and Weligama bay in Sri Lanka

The Weligama Bay is a one of the rocky shorelines in the coastline of Sri Lanka and Polwatta River is the major river which enters to south part of the Bay. The study objectives were to find out the sedimentary characteristic, and water quality status in Polwatta River and Weligama bay. Results indicated that, mean water temperature, pH, NO 3 - -N, PO 4 -3 , and NH 4 - -N, in Polwatta River and Weligama Bay are 28.2 0 C ± 1.8, 6.7 ± 0.3, 0.11 ± 0.04 mg/l, 0.73 ± 0.1 mg/l, 0.46 ± 0.1 mg/l and 28.5 ± 1.0 0 C, 7.57 ± 0.1, 0.51 ± 0.04 mg/l, 0.40 ± 0.1mg/l and 0.20 ± 0.1 mg/l in respectively. The, mean suspended sediment concentrations were recorded in Polwatta River was 241.1 ± 150 mg/l. However, mean gravel, sand and silt percentages in the Weligama Bay are 1.42 ± 1.4, 96.33 ± 0.5 and 2.25 ± 0.3 in respectively. The study of current patterns shows the direction is west to east of the Bay and surface current is 12 cm/s. Results reveal that, nutrients in the Polwatta River and Weligama Bay are within the acceptable limits published by Sri Lanka Standard Institute and draft CEA ambient water quality standard. Also, high concentration of suspended sediment load coming through the Polwatta River during south-west monsoon period and part of that remain in the estuary and other discharge to the Bay. Also, due to currents sediment traverse to southern part of the coast in Weligama. DOI: http://dx.doi.org/10.4038/jepsl.v2i1.5745 Journal of Environmental Professionals Sri Lanka Vol.2(1) 2013 38-51

Longitudinal Variation in the Ohio River Fish Community

AbstractThe Ohio River fish community was sampled three times (June, August, and October) in 2005 by electrofishing near 17 power plants encompassing nearly the entire length of the Ohio River (river kilometer 82–1,527). Six 500‐m zones were electrofished at each plant. Using a generalized additive model, we examined this data set of 306 electrofishing samples to determine how the abundance of 31 taxa varied over the length of the river. We also looked for trends in several measures of community health (catch rate, species richness, index of biotic integrity, and modified index of well‐being). Of the 31 taxa examined, all but three showed longitudinal differences over the course of the river. Based on the patterns for the 31 taxa and 5 community measures, we assigned each taxon or measure to one of seven classes. The largest class included 10 taxa that declined steadily in abundance from upriver to downriver. All community measures followed a similar pattern except that each measure increased near the downstream‐most plant. Conversely, only two taxa (Striped Bass Morone saxatilis and River Carpsucker Carpiodes carpio) showed the opposite trend (i.e., increasing in a downstream direction). Some taxa were restricted to the upper or lower end of the river or to certain reaches along the river. Species that were restricted to the upper river were primarily stream fishes (certain minnows Notropis spp. and several darters Etheostoma spp.). Conversely, fish species that were restricted to the lower river were primarily large‐river fishes as well as species that were typical of the Mississippi River fauna. Site‐specific differences in water temperature, Secchi depth, specific conductance, and dissolved oxygen were generally unimportant in explaining the observed patterns. Substrate type (percent cobble/boulder and percent silt) occasionally was important. The observed longitudinal trends appeared to be primarily in response to natural temperature and habitat differences between the upper and lower sections of the Ohio River.Received March 16, 2012; accepted March 10, 2013

Land leveling and changes in soil properties in paddy fields of Guilan province, Iran

The aim of this study was to evaluate the impacts of land leveling on some of soil properties. Land leveling as an effective strategy for paddy land management is one of the most important programs run by the Ministry of Jihad-Agriculture in Guilan province, northern part of Iran. Precise land leveling improves irrigation application, fertilization efficiency, and uniform soil moisture for germination. Also, it increases yields and cultivable lands and reduces delivery losses. Running land-leveling projects, however, leads to such disadvantages as severe soil alteration and consequently disruptions to the soil physical, chemical, and biological properties. Based on statistical analysis, on the one hand land, leveling led to significant increases (P < 0.01) in bulk density, clay percentage, and EC, and on the other hand, it resulted in significant decreases (P < 0.01) in the soil organic matter, silt percentage, NPK, pH, soil respiration, bacterial community, and microbial biomass carbon. Microscopic images of buried slides indicated that there was a reduction in the soil microorganism communities and diversities as well. With regard to the management of paddy fields, despite beneficial impacts of land leveling, the results indicate that due to malperformance of the project, which is a consequence of the use of inappropriate machinery and/or nonremoval of surface soil and its re-diffusion, most of soil-desirable properties are reduced and are not revitalized even after 4 years.

Potential effects of an invasive bivalve, Nuttallia obscurata, on select sediment attributes within the intertidal region of coastal British Columbia

On the west coast of British Columbia, Canada, the varnish clam, Nuttallia obscurata, is a rapidly spreading invasive species that can reach high densities (i.e. 800individualsm−2). A field survey (Tier I) and an in situ mesocosm density manipulation experiment (Tier II) were applied to determine the potential effects of this invasive bivalve on select sediment attributes within the intertidal zone. In situ experiments involved seeding 1m2 mesocosms with varnish clams at four densities; 0 (control), 200 (low), 500 (mid) and 800 (high). Sediment samples were taken 7 times over a three week period and analyzed for organic matter, ammonium and grain size (gravel, coarse silt, fine silt and silt) at three different depths, 0–3cm (surface), 3–6cm (intermediate) and 6–9cm (bottom). Field surveys demonstrated that at natural densities bivalve distribution was best explained by sediment grain size. Contrary to expectations, the in situ manipulations indicated that at high densities varnish clams did not significantly increase sediment organic matter concentrations. Their ability to deposit feed and re-ingest biodeposits may have prevented accumulations. High densities of varnish clams did however result in significantly higher concentrations of sediment ammonium and percent silt, but mostly within the lower sections of the sediment core. Reworking of the surficial sediments due to intense storm activity likely obscured any differences in determined sediment attributes among the 4 treatments. Nitrogen is a limiting nutrient with ammonium preferentially used by phytoplankton and microphytobenthos. These primary producers form the basis of all marine food webs thus increases in amounts of ammonium to shallow coastal seas as a result of high densities of an invasive bivalve within intertidal zones could result in an increased risk of eutrophication within these sensitive regions.