The Geometric Features, Shape Factors and Fractal Dimensions of Suspended Particulate Matter in the Scheldt Estuary (Belgium) (original) (raw)
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CATENA, 2003
As many particle-associated contaminants and nutrients are supply controlled, the question of particle source is crucial. In addition, sediment storage has important implications for the delivery and fate of pollutants. Increased accumulations of fine sediment (< 63 Am) in gravel beds not only modify benthic habitat but also increase the retention time of sediment-associated contaminants in these biologically active areas of river systems. It is of overriding importance to determine the origin of the fines and the amount, location and process of storage. There is little doubt that the characteristics of particles can be used to derive this information. There is no general agreement, however, about the characteristics that should be considered in such investigations. Furthermore, scientists have a great demand for simple methods, especially for routine use under dry weather conditions when suspended particle concentrations are low. This investigation shows that, in addition to loss on ignition, the determination of fractal dimension and particle colour also provide a fast and easy approach. After filtering the suspensions through glass microfibre filters, the dried filter residues are scanned by a colour scanner. Particle-bound cations and heavy metals were analysed by atomic absorption spectrometry after material decomposition with nitric acid. Fractal dimensions were obtained from measurement of the digital pictures using standard methods of digital image analysis. The fractal dimension decreases, indicating an increase in the regularity of particle
Fractal dimensions of marine snow determined from image analysis of in situ photographs
Deep Sea Research Part I: Oceanographic Research Papers, 1994
Seventy seven in situ photographs of marine snow ranging in size from 1 to 60 mm wcrc used to calculate one-and two-dimensional fractal dimensions, DI and D 2, in order to characterize aggregate morphology with respect to aggregate perimeter and cross-sectional area. The lowest fractal dimension of D 2 = 1.28 + 0.11 was calculated for marine snow aggregates composed predominantly of a single type of particle (e.g. diatoms or fecal pellets) containing large amounts of miscellaneous debris. Marine snow formed by the aggregation of fecal pellets (D 2 = 1.34 + (I. 16), non-identifiable particles (amorphous, D 2 = 1.63 _+ 0.72), and diatoms (D e = 1.86 _+ 0.131 had increasingly larger fractal dimensions. When combined into a single group, all marine snow aggregates had a fractal dimension of 1.72 _+ /I.117. Larvacean houses, formed originally from a single, nearly spherical particle, were found to have a D 2 value close to the Euclidean value of 2. Based on fractal geometrical relationships, D 2 should have been equal to previous estimates of 113, a three-dimensional fractal dimension. Instead, the D 2 value of 1.72 for the combined group was larger than previous estimates of D 3 of 1.39 and 1.52, probably because of the dominant influence of the diatom aggrcgatcs on thc combined group. Diatom aggregates had the highest fractal dimensions and covered the widest size range of all categories of particles examined.
The use of the fractal dimension to quantify the morphology of irregular‐shaped particles
Sedimentology
For several decades, sedimentologists have had difficulty in obtaining an efficient index of particle form that can be used to specify adequately irregular morphology of sedimentary particles. Mandelbrot has suggested the use of the fractal dimension as a single value estimate of form, in order to characterize morphologically closed loops of an irregular nature. The concept of fractal dimension derives from Richardson's unpublished suggestion that a stable linear relationship appears when the logarithm of the perimeter estimate of an irregular outline is plotted against the logarithm of the unit of measurement (step length). Decreases in step length result in an increase in perimeter by a constant weight (b) for particles whose morphological variations are the same at all measurement scales (self-similarity). The fractal dimension (D) equals 1.0-(b), where b is the slope coefficient of the best-fitting linear regression of the plot. The value of D lies between 1.0 and 2.0, with ...
Fractal dimensions of suspended solids in streams: comparison of sampling and analysis techniques
Hydrological Processes, 1999
Fluvial suspended sediment typically consists of a variety of complex, composite particles referred to as¯ocs. Floc characteristics are determined by factors such as the source, size and geochemical properties of the primary particles, chemical and biological coagulation processes in the water column and shear stress and turbulence levels in the stream. Studies of¯oc morphology have used two contrasting methods of sampling and analysis. In the ®rst method, particles settle on a microscope slide and are observed from below using an inverted microscope. The second method uses ®ltration at no or low vacuum and particles deposited on the ®lter are observed with a microscope. Floc morphology can be quanti®ed using fractal dimensions. The aims of the present study were to examine the eect of the two sampling methods on the fractal dimensions of particle populations, and to evaluate for each method how well the fractal dimensions at the various sampling sites re¯ect basin conditions. Suspended solids were collected in triplicate on inverted microscope slides and on 0 . 45 mm Millipore HA ®lters in two southern Ontario streams with contrasting riparian zones during a minor runo event resulting from the melt of a freshly fallen snowpack. An image analysis system was used to determine area, longest axis and perimeter of particles. The morphology of the particle population of each sample was characterized using four fractal dimensions (D, D 1 , D 2 and D K ). Systematic dierences in fractal dimensions obtained with the two methods were observed. For the settling method, outlines of larger particles were frequently blurred because of the distance between the focal plane (the top of the inverted microscope slides) and the plane of the particle outline. In this method, the blurring of large particles can cause an increase in the projected area and length of the particle. The eect on the particle perimeter is unpredictable because it depends on the amount of detail lost through blurring and its eect on the apparent increase in particle size. Because of blurring, D and D 1 tend to be systematically lower for the settling method, whereas the net eect on D 2 is unpredictable. Particle size distributions derived from settling are typically coarser because small, low density particles may remain in the water column and all particles may not deposit on the slides. This loss of ®nes results in systematically lower D K values for the settling method compared with the ®ltration method. Fractal dimensions and particle size distributions obtained with the ®ltration method were sensitive to and clearly indicated dierences between drainage basins and between sites within each basin. These dierences were explained by basin characteristics and conditions. Fractal dimensions and particle size distributions obtained with the settling method were less sensitive to drainage basin characteristics and conditions, which limits their usefulness as process indicators.
Hydrobiologia, 2001
The chlorophyll a content and two operational fractions of carbohydrate (water extractable and EDTA extractable) were measured every three months during one year along transects on a tidal flat in the Ems-Dollard estuary (The Netherlands). Chlorophyll a was used as an indicator of microphytobenthos biomass, which was composed predominantly of epipelic diatoms. Both carbohydrate fractions correlated significantly with chlorophyll a. EDTA extractable carbohydrates were more resistant towards degradation than the water extractable fraction. During most of the year, concentrations of chlorophyll a and carbohydrates were low, but in June, high concentrations of up to 90 μg chlorophyll a/g sediment were found in a narrow zone running parallel to the channel. Maximum concentrations of water extractable carbohydrates and EDTA extractable carbohydrates ranged between 800–1200 and 600–800 μg/g sediment, respectively. The mud content was high (± 90%) at the margin of the tidal flat. This was not limited to the growth season of the diatoms, but was observed throughout the year. This indicated that the high mud content at the mudflat margin was mainly caused by hydrodynamic factors, and not by biostabilization. In June, exceptionally high diatom densities were found in sediment with a high mud content. There was only minor evidence that biostabilization by epipelic diatoms lead to a further increase in the mud content of the sediment.
The aim of the present study was to analyze if small-scale spatial variation of benthic diatom assemblages has consequences for biomonitoring. Benthic diatom samples were collected at one sampling site in a mesotrophic stream in Middle-Sweden from stone and plant (macrophytes and mosses) substrate. Our results showed that spatial variation of both the diatom species composition and the calculated bioindices were similar on both small (distance of centimeter) and medium (distance of decimeters) scales. Spatial variation was also similar on both studied substrates. This implies that it does not matter if a small or a larger area is sampled for biomonitoring as long as no major environmental factors impact certain sites systematically. Diatom assemblages and indices were signifi cant-ly different between substrates. Spatial variation did not contribute much to this variation, and variation on a slide was unimportant. These results confi rm earlier fi ndings that small-scale spatial variation is not a problem when using diatoms to detect anthropogenic impacts to a stream or lake.
Estuarine, Coastal and Shelf Science, 2005
Spatio-temporal changes in taxonomic composition and structure of an epipelic diatom assemblage from an intertidal mudflat on the French Atlantic coast was studied over an annual cycle along a crossshore transect. The assemblage structure was described by estimating both relative abundance and contribution to biovolume of each species. Results showed that the assemblage was numerically dominated by small-sized species (mean relative abundance of 91%). Large species, however, significantly contributed to the total biovolume (mean contribution to biovolume of 49%). A factorial correspondence analysis indicated that the epipelic assemblage was relatively homogeneous along the cross-shore transect but emphasized the seasonal succession of diatom species. In relative abundance, the assemblage structure was characterized by the dominance of the small species Navicula phyllepta throughout the year with a seasonal succession of secondary species, including only one large diatom (Gyrosigma peisonis). In biovolume, small (N. phyllepta and Navicula gregaria) and large species (Pleurosigma angulatum and G. peisonis) alternatively dominated the assemblage. Describing the epipelic assemblage using cell volume emphasized the contribution of large species and revealed that the assemblage contained two diatom fractions, characterized by different biological and physiological behaviours, which may alternatively represent a large proportion of the biomass.
Flocculation affects the size, surface area, density and shape of fine-grained suspended solids in rivers which alters the transport properties of cohesive sediment. Factors controlling the shape of floes include the source, size and geochemical characteristics of primary particles, varying degrees of chemical and biological coagulation in the water column as well as shear stress and turbulence levels in the stream. Floe shape can be quantified using fractal dimensions. This study examines spatial and temporal variability in fractal dimensions of suspended solids in two southern Ontario streams with contrasting riparian zones. Suspended solids were collected in triplicate at upstream and downstream sites in Strawberry Creek and Cedar Creek prior to snowmelt and during snowmelt. An image analysis system was used to determine area, longest axis and perimeter of particle populations. Fractal dimensions of the particle populations on each filter were calculated from the area-perimeter r...
Can J Civil Eng, 2005
Morphology of particle populations of cohesive sediment were examined during settling experiments in an annular flume with different initial sediment concentrations (200 and 350 mg/L) at constant bed shear stress (0.121 N/m 2 ) using fractal dimensions. The area, longest axis, and perimeter of suspended solids were measured with light microscopy and an image-analysis system to determine three fractal dimensions (D, D 1 , D 2 ). The ratio between the initial and steady state (time T = 300 min) sediment concentration was 0.54 for both experimental runs and is a function of bed shear stress, not the initial sediment concentration. The fractal dimension D changed from 1.32 at the start of the experiment to 1.36 at steady state, which represents an increase in shape irregularity of larger particles over time compared with smaller particles. At steady state, D 1 and D 2 were 1.19 and 1.66, respectively. Small increases in D 1 and D 2 over time indicated a change in morphology towards longer and more elongated particles. The D 2 measurements in the present study indicate that differential sedimentation is the predominant flocculation mechanism of cohesive sediments in the flume settling experiments. Fractal dimensions of suspended solids were not significantly different at steady state as a function of initial sediment concentration.
A fractal approach for detecting spatial hierarchy and structure on mussel beds
Marine Biology, 2001
Within beds of blue mussel (Mytilus edulis L.), individuals are aggregated into small patches, which in turn are incorporated into bigger patches, revealing a complex hierarchy of spatial structure. The present study was done to find the different scales of variation in the distribution of mussel biomass, and to describe the spatial heterogeneity on these scales. The three approaches compared for this purpose were fractal analysis, spatial autocorrelation and hierarchical (or nested) analysis of variances (ANOVA). The complexity (i.e. patchiness) of mussel aggregations was described with fractal dimension, calculated with the semivariogram method. Three intertidal mussel beds were studied on the west coast of Sweden. The distribution of wet biomass was studied along transects up to 128 m. The average biomasses of blue mussels on the three mussel beds were 1825±210, 179±21 and 576±66 g per0.1 m2, respectively, and the fractal dimensions of the mussel distribution were 1.726±0.010, 1.842±0.014 and 1.939±0.029 on transects 1–3, respectively. Distributions of mussels revealed multiscaling behaviour. The fractal dimension significantly changed twice on different scales on the first bed (thus showing three scaling regions), the second and third beds revealed two and three scaling regions, respectively. High fractal dimension was followed by significant spatial autocorrelation on smaller scales. The fractal analysis detects the multiple scaling regions of spatial variance even when the spatial structure may not be distinguished significantly by conventional statistical inference. The study shows that the fractal analysis, the spatial autocorrelation analysis and the hierarchical ANOVA give complementary information about the spatial variability in mussel populations.