The Naf River estuary is one of the most productive ecological ecosystems in Bangladesh providing an important feeding area for fishes and other aquatic animals. However, detailed information on macrobenthic communities is rarely available in this area. Our study focused on the seasonal and spatial variability of macrobenthic community structure with their biomass and functional guilds. In total, fortyseven taxa were identified under nine major groups and seven functional feeding guilds. Among macrobenthic taxa, Polychaeta was dominant in both seasons and all over the estuary contributing 60% of total benthos.The mean abundance of macrobenthos was higher during the pre-monsoon(2 972±1 994 inds./m~2) than the monsoon(1 572±361 inds./m~2) being maximum at mid-estuary region and minimum at upper and lower estuary regions. Of the diversity indices, density(P=0.01, P=0.003), the number of taxa(P=0.000 3,P=0.000 9) and Margalefs Index(P=0.000 4, P=0.001) of macrobenthos were significantly different among stations and seasons. Shannon-Weiner index(P=0.009, P=0.12) and Pielou's evenness index(P=0.03,P=0.14) had significant differences among stations but not among seasons. Cluster analysis suggested that distribution of macrofauna is strongly influenced by seasonality. Among the feeding types, carnivorous were found to be dominant at the upper and mid-estuarine regions, and herbivorous were at lower estuarine regions. Carnivorous species were dominant in both seasons. Omnivores have shown to be the most abundant feeding type in terms of biomass though herbivores were highest during monsoon. 相似文献
The trophic structure of a community is used to infer ecosystem functioning(e.g. energy transfer and nutrient cycling). Here the trophic structure of the benthic infaunal and epifaunal communities in the Brunei Estuary are characterized, and their distribution along an estuarine pH gradient is analyzed using univariate and multivariate techniques. This analysis revealed that surface deposit feeders(e.g., polychaetes) were numerically dominant within the infaunal communities whereas in the epifaunal communities filter feeders(e.g., bivalves) were highly abundant. Species richness for almost all trophic groups increased toward the lower estuary, except for omnivores in the epifaunal communities, which decreased markedly. Both Analysis of Variance(ANOVA) and Analysis of Similarities(ANOSIM) detected significant differences in the density of respective trophic groups among stations. Within infaunal communities, both Biological and Environmental procedure(BIO-ENV) and Canonical Correspondence Analysis(CCA) showed that trophic shifts were associated with environmental gradients. Surface-deposit feeders and omnivores were the most abundant macrobenthos of the upper estuary characterized by low salinity, low pH, and a higher percentage of mud particles. The proportion of filter feeders and carnivores increased with salinity/pH and sand. A more uniform distribution of trophic structure was found in the lower estuary, with high salinity and pH over sandy habitat. In contrast, within epifaunal trophic groups,the percentage of surface deposit feeders and omnivores declined, but filter feeders remarkably increased toward the sea. The proportion of carnivores remained similar at all stations. Non-Metric Multidimensional Scaling(nMDS) ordination for epifaunal trophic groups clearly demarcated higher salinity/pH stations from lower salinity/pH stations, suggesting different trophic compositions along the estuarine pH gradient. 相似文献
The head-on collision between positron acoustic solitary waves (PASWs) as well as the production of rogue waves (RWs) in homogeneous and PASWs in inhomogeneous unmagnetized plasma systems are investigated deriving the nonlinear evolution equations. The plasmas are composed of immobile positive ions, mobile cold and hot positrons, and hot electrons, where the hot positrons and hot electrons are assumed to follow the Kappa distributions. The evolution equations are derived using the appropriate coordinate transformation and the reductive perturbation technique. The effects of concentrations, kappa parameters of hot electrons and positrons, and temperature ratios on the characteristics of PASWs and RWs are examined. It is found that the kappa parameters and temperature ratios significantly modify phase shifts after head-on collisions and RWs in homogeneous as well as PASWs in inhomogeneous plasmas. The amplitudes of the PASWs in inhomogeneous plasmas are diminished with increasing kappa parameters, concentration and temperature ratios. Further, the amplitudes of RWs are reduced with increasing charged particles concentration, while it enhances with increasing kappa- and temperature parameters. Besides, the compressive and rarefactive solitons are produced at critical densities from KdV equation for hot and cold positrons, while the compressive solitons are only produced from mKdV equation for both in homogeneous and inhomogeneous plasmas. 相似文献
Stochastic Environmental Research and Risk Assessment - Pressure fluctuations are among the favorite topics for hydraulic engineers due to their critical role in the design and safe operation of... 相似文献
The present study deals with the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts, Bangladesh with the lithological study of the boreholes sediments. Major lithofacies of the studied boreholes are clay, silty clay, sandy clay, fine to coarse grained sand, gravels and sands with(fragmentary) ironstones. The ironstones contain major oxides, Fe_2 O_3*(*total Fe)(avg. 66.6 wt%), SiO_2(avg. 15.3 wt%), Al_2 O_3(avg. 4.0 wt%), MnO(avg. 7.7 wt%), and CaO(avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe_2 O_3* along with Al_2 O_3 and MnO indicate the ironstone as goethite and siderite, which is also validated by XRD data. A comparatively higher percentage of SiO_2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks. These ironstones also have significant amounts of MnO(avg. 7.7 wt%) suggesting their depositional environments under oxygenated condition. Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter. Iron concretion was mostly formed by bacterial build up in swamps and marshes, and was subsequently embedded in clayey mud.Within the coastal environments, the water table fluctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone. 相似文献
In the present paper, the effects of free convection currents and the viscous dissipation on the unsteady flow of an electrically conducting and viscous incompressible fluid around an uniformly accelerated vertical porous plate subjected to a suction or injection velocity inversely proportional to the square root of time, in presence of a transverse magnetic field, have been investigated. Analytical solutions for the velocity and the temperature distributions, the skin-friction and the rate of heat transfer are obtained for small magnetic parameterM. During the course of discussion the effects of the Grashof number Gr, the Eckert number Ec, the suction/injection parametera have been considered for unit value of the Prandtl number Pr.Nomenclature
a
suction/injection parameter
-
Cp
specific heat at constant pressure
-
B0
magnetic induction
-
g
acceleration due to gravity
- Gr
Grashof number (g(Tw
–T
)/U03
)
-
K
thermal conductivity
-
M
magnetic field parameter (B02
/U02
)
- Pr
Prandtl number (Cp/K)
-
T
temperature of the fluid near the plate
-
Tw
temperature of the plate
-
T
temperature of the fluid at infinity
-
t
time
-
t
dimensionless time (tU02
/)
-
u
velocity of the fluid
-
u
non-dimensional velocity (u/U0)
-
U
velocity of the plate
-
U
dimensionless velocity of the plate (U/U0)
-
U0
reference velocity
-
v0
suction velocity
-
v0
non-dimensional suction velocity (v0/U0)=at–1/2
- Ec
Eckert number ((U0)2/3/Cp(T
w
–T
))
-
T
dimensionless temperature of the fluid near the plate ((T–T
)/(Tw–T
))
-
x, y
coordinates along and normal to the plate
-
x, y
dimensionless coordinates (y=yU0/)
-
kinematic viscosity
-
coefficient of volume expansion
-
electric conductivity of the fluid
-
y/2t1/2
-
density of the fluid
-
skin-friction
-
dimensionless skin-friction
-
q
rate of heat transfer
-
q
non-dimensional rate of heat transfer
-
coefficient of viscosity
- e
magnetic permeability
On leave of absence from Department of Mathematics, University of Dhaka, Bangladesh 相似文献
Natural Hazards - Drought is a natural calamity frequently occurs at Barind Tract in northwest Bangladesh and affects both the human and natural life. An initiative has been taken... 相似文献
The present study deals with the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts, Bangladesh with the lithological study of the boreholes sediments. Major lithofacies of the studied boreholes are clay, silty clay, sandy clay, fine to coarse grained sand, gravels and sands with (fragmentary) ironstones. The ironstones contain major oxides, Fe2O3* (* total Fe) (avg. 66.6 wt%), SiO2 (avg. 15.3 wt%), Al2O3 (avg. 4.0 wt%), MnO (avg. 7.7 wt%), and CaO (avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe2O3* along with Al2O3 and MnO indicate the ironstone as goethite and siderite, which is also validated by XRD data. A comparatively higher percentage of SiO2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks. These ironstones also have significant amounts of MnO (avg. 7.7 wt%) suggesting their depositional environments under oxygenated condition. Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter. Iron concretion was mostly formed by bacterial build up in swamps and marshes, and was subsequently embedded in clayey mud. Within the coastal environments, the water table fluctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone.