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1.
Water is a dominant component of volcanic clouds and has fundamental control on very fine particle deposition. Particle size characteristics of distal tephra-fall (100s km from source volcano) have a higher proportion of very fine particles compared to predictions based on single particle settling rates. In this study, sedimentological analyses of fallout from for the 18 August and 16–17 September 1992 eruptions of Crater Peak, Alaska, are combined with satellite observations, and cloud trajectory and microphysics modeling to investigate meteorological influences on particle sedimentation. Total grain size distributions of tephra fallout were reconstructed for both Crater Peak eruptions and indicate a predominance of fine particles < 125 μm. Polymodal analysis of the deposits has identified a particle subpopulation with mode ~ 15–18 μm involved in particle aggregation. Accounting for the magmatic water source only, calculated ice water content of the 3.7 hour old September 1992 Spurr cloud was ~ 4.5 × 10− 2 g m− 3 (based on an estimated cloud thickness of ~ 1000 m from trajectory modeling). Hydrometeor formation on particles in the volcanic cloud and subsequent sublimation may induce a cloud base instability that leads to rapid bulk (en masse) sedimentation of very fine particles through a mammatus-like mechanism.  相似文献   

2.
Most entrained estuarine sediment mass occurs as flocs. Parameterising flocculation has proven difficult as it is a dynamically active process dependent on a set of complex interactions between the sediment, fluid and the flow. However the natural variability in an estuary makes it difficult to study the factors that influence the behaviour of flocculation in a systematic manner. This paper presents preliminary results from a laboratory study that examined how floc properties of a natural estuarine mud from the Medway (UK), evolved in response to varying levels of suspended sediment concentration and induced turbulent shearing. The experiments utilised the LabSFLOC floc video camera system, in combination with an annular mini-flume to shear the suspended sediment slurries. The flows created in the mini-flume produced average shear stresses, at the floc sampling height, ranging from 0.01 N m−2 to a peak of 1.03 N m−2. Nominal suspended particulate matter concentrations of 100, 600 and 2000 mg l−1 were introduced into the flume. The experimental runs produced individual flocs ranging in size from microflocs of 22.2 μm to macroflocs 583.7 μm in diameter. Average settling velocities ranged from 0.01 to 26.1 mm s−1, whilst floc effective densities varied from 3.5 up to 2000 kg m−3. Low concentration and low shear stress were seen to produce an even distribution of floc mass between the macrofloc (>160 μm) and microfloc (<160 μm) fractions. As both concentration and stress rose, the proportion of macrofloc mass increased, until they represented over 80% of the suspended matter. A maximum average macrofloc settling velocity of 3.3 mm s−1 was attained at a shear stress of 0.45 N m−2. Peak turbulence conditions resulted in deflocculation, limiting the macrofloc fall velocity to only 1.1 mm s−1 and placing over 60% of the mass in the microfloc size range. A statistical analysis of the data suggests that the combined influence of both suspended concentration and turbulent shear controls the settling velocity of the fragile, low density macroflocs.  相似文献   

3.
We examined the occurrence of seasonal hypoxia (O2<2 mg l−1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m−2 d−1), and lower off the Rhône River (<1 g C m−2 d−1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.  相似文献   

4.
The green-lipped mussels Perna viridis were exposed to <500 μm suspended solids (SS) with concentrations of 0 (control), 250, 500, 750 and 1000 mg/L for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significantly higher damages to the cilia on the frontal surface of the gill filaments than that on the abfrontal surface in both demibranchs. Percent ciliary depletion varied with SS concentrations and time. No sign of recovery of the gill filaments was observed after the mussels were transferred to clean seawater. In a second experiment, mussels were exposed to SS with size range from <63, >125–<250 and >250–<500 μm at 600 mg/L, together with a control (0 mg/L) for 14 days, followed by transferring to clean, filtered seawater for 28 days. Results of scanning microscopy showed significant ciliary damages in both the ascending and descending lamellae under the three particle size groups as compared with the control. Percent depletion of frontal cilia was most serious for the >250–<500 μm size group and least for the <63 μm size group. However, percent depletion of abfrontal cilia was most serious for the >125–<250 μm size group and least for the <63 μm size group. No recovery of ciliary damages was observed. The effects of particle size of suspended sediments on the morphological damages of gill filaments in the green-lipped mussels were discussed.  相似文献   

5.
An instrument system for the investigation of particle fluxes   总被引：1，自引：0，他引：1  
We present the rationale, design, and use of an instrument system to measure the variability of vertical and horizontal particle fluxes. The system features a new sequentially sampling sediment trap which collects and seals 10 separate samples during a single deployment. Horizontal particle fluxes are simultaneously monitored with a beam transmissometer interfaced to a standard Aanderaa current meter. Results from a 10-week deployment of instruments at several depths in a deep fjord estuary indicate that the trapping rate increases from 0.5g m−2 day−1 at 20 m to 150g m−2 day−1 at 200 m (5 m above bottom) because of frequent erosion in the deep waters. Periodic flushing of the deep water by intrusions of marine water over the seaward sill markedly enhances erosion and causes an up-estuary particle transport comparable to the vertical particle flux originating at the surface.  相似文献   

6.
In surface sediments, a statistically significant increase was measured in the concentration of copper and cadmium along all transects, and of zinc along only the most southern (Gazi) transect, radiating into the Indian Ocean perpendicular to the Kenyan coast. Mean copper and cadmium increased from 5 to 30 μg g−1 dry wt and from 0.01 to 0.34 μg g−1 dry wt in shallow coastal (±20 m depth) to deep-sea stations (±2000 m depth), respectively. These gradients were found both during the south-east monsoon and north-east monsoon period. The shallow estuarine zone of the Sabaki river mouth showed significantly enhanced levels of total organic carbon and nitrogen and all metals analysed, except cadmium.In crustaceans, the concentration of copper and particularly cadmium was significantly above baseline levels, varying from 45 to 90 μg g−1 dry wt and 1.0 to 8.5 μg g−1 dry wt, respectively. Zinc levels (49–102 μg g−1 dry wt) were at about baseline levels or a little elevated. On the contrary, lead showed very low concentrations, varying from 0.1 to 0.6 μg g−1 dry wt. Other species generally showed the same pattern.  相似文献   

7.
Flocculation has an important impact on particle trapping in estuarine turbidity maximum (ETM) through associated increases in particle settling velocity. To quantify the importance of the flocculation processes, a size-resolved flocculation model is implemented into an ocean circulation model to simulate fine-grained particle trapping in an ETM. The model resolves the particle size from robust small flocs, about 30 μm, to very large flocs, over 1000 μm. An idealized two-dimensional model study is performed to simulate along-channel variations of suspended sediment concentrations driven by gravitational circulation and tidal currents. The results indicate that the flocculation processes play a key role in generating strong tidal asymmetrical variations in suspended sediment concentration and particle trapping. Comparison with observations suggests that the flocculation model produces realistic characteristics of an ETM.  相似文献   

8.
Freshwater flocculation of suspended sediments in the Yangtze River, China   总被引：3，自引：1，他引：2  
Leicheng Guo  Qing He 《Ocean Dynamics》2011,61(2-3):371-386
This study focuses on suspended sediments and in situ flocculation in the Yangtze River, with the goal of improving our understanding of the relationship between freshwater and estuarine flocculation. A field survey with state-of-the-art instruments was carried out in January 2008 in the reach from downstream of the Three Gorges Dam to the estuary. The data show that in situ floc mean diameters range from 22 to 182???m in the river, whereas the median dispersed grain sizes are 4.4?C11.4???m. This demonstrates that flocculation is an important process during the transport of suspended sediments along the river. The flocculation characteristics, suspended sediment concentration and dispersed grain sizes all vary longitudinally in the main stream of the Yangtze River. Biochemical factors are likely be more significant in the freshwater flocculation than in the estuary, where hydrodynamics and biochemical factors are both important. Flocculation is found in the freshwater river, in the estuary and in coastal waters, which indicates that dynamic break-up/reflocculation processes take place during the suspended sediment transport. The freshwater flocs may behave as parent flocs to the estuarine flocculation. This study enhances our understanding of flocculation from estuarine and coastal areas to fresh river systems and provides insights into the effects of input of riverine flocs to the estuarine flocculation and into the sources and fate of flocs.  相似文献   

9.
Lake Koronia, a Ramsar site, is shallow, polymictic, hypertrophic and until recently was aerially the fourth largest lake in Greece. Although exceeding 5 m in the past, lake depth has declined progressively from 3.8 m in 1980 to < 1 m in 1997, reducing surface area and water volume by 50% and 80%, respectively. Specific conductivityincreased from 1300 μS cm−1 in 1977 to >6000 μS cm−1 in 1991. Increased phosphate concentrations from the late 1970's (8–45 μg L−1) to the late 1990's (100–1000 μg L−1) document that the previously eutrophic system with a limited littoral zone switched to hypertrophy dominated by massive cyanobacteria blooms. Oxygen saturation of the water column increased progressively from about 80% in 1983 to full saturation about 1993, after which it decreased progressively to only 20% saturation in 1997. In spite of cyanobacteria dominance, community metabolism of the lake switched from progressively increasing autotrophy to rapidly advancing heterotrophy associated with progressive water-level reduction leading to fish extirpation in the lake.  相似文献   

10.
PCB (polychlorinated biphenyls) contamination and its relationship to SPM (suspended particulate material) have been studied in the Seine Estuary, which is heavily polluted by these persistent and hydrophobic man-made chemicals. Two sampling cruises have been performed during different freshwater discharge conditions. PCB and SPM concentrations, as well as grain-size distributions in the particulate material have been determined. Water samples have been collected at fixed positions during a tidal cycle, and along transects within the estuary.PCB concentrations vary from 2 ng 1−1 in the marine zone to 250 ng 1−1 within the estuary, and reach 1.3 × 103 ng 1−1 in the turbidity maximum zone. During a tidal cycle, low PCB concentrations are observed at high water, and are in the same range in February as in July. High PCB contamination is observed at low water, but PCB concentrations are about five times higher in February. SPM and PCB variations are well correlated in both periods of observations. Higher PCB contamination during February is explained by higher SPM inputs, mainly due to particles of riverine origin. The transport of PCB within the estuary depends on the quantity and the grainsize composition of suspended material, which varies according to freshwater discharge and tidal amplitude.  相似文献   

11.
Field measurements have been carried out to obtain more quantitative knowledge on the longitudinal distribution of the fine-grained sediment dynamics in the Ems Estuary. Both the short-term (time scale < tidal period) and the long-term (tidally averaged) fine-grained sediment transports have been investigated. It is shown that the short-term erosion/sedimentation cycles are most characteristic and are the building blocks for the ultimate long-term transports. Herein, the macroflocs play a key role. The macroflocs had sizes in the range of 200 to 700?μm, sometimes more than 1?mm, and survived high current velocities. The settling velocities were in the range of 0.5 to 8?mm/s. Turbulence intensities and suspended sediment concentration are of utmost importance for the settling properties of the sediment, but the “flocculation ability??of the suspended sediment is shown to be of equal importance. The suspended sediment concentration is only important at low levels of turbulence. At high levels of turbulence, the limiting properties of turbulence dominate. It is demonstrated that the surface properties of the suspended sediment particles are of prime importance for the flocculation ability of the water/sediment mixture, as do the sizes of the resuspended microflocs. Special attention is given to the longitudinal distribution of the sizes and settling properties of the macroflocs as well as of the sediment transports. The flocculation ability was shown to vary almost one order of magnitude along the estuary. These higher flocculation abilities in the direction of higher salinities are responsible for the increase of the sizes of the macroflocs in the seaward direction. Therefore, notwithstanding the low suspended sediment concentrations at the seaward boundary of the Ems Estuary, large macroflocs and corresponding high settling velocities were observed in this area. It explains that, in the Ems Estuary, no distinct effect was determined of the suspended sediment concentration on the settling properties of the suspended macroflocs.  相似文献   

12.
Concentrations of dissolved226Ra in Winyah Bay, South Carolina, and in the adjacent Atlantic Ocean are augmented by the desorption of radium from sediments in the low-salinity area of the estuary and diffusion from bottom sediments. Desorption of226Ra is reflected by lower concentrations in suspended sediments from higher-salinity regions of the estuary. Bottom sediments from the high-salinity region have lower226Ra/230Th activity ratios than those from the low-salinity end.The shape of the dissolved226Ra vs. salinity profile is influenced by the river discharge. During average-discharge conditions, desorption of226Ra from suspended and bottom sediments increases the dissolved226Ra concentrations by a factor of 3.5 as the water passes through Winyah Bay. High river discharge produces an initial increase of dissolved226Ra by a factor of 2 to 3 and apparently reflects only desorption from suspended sediments. By driving the salt wedge down the estuary and reducing the zone of contact of salt water with bottom sediments, the high-flow conditions sharply reduce the flux of226Ra from bottom sediments.  相似文献   

13.
Natural concentrations of 238U and δ234U values were determined in estuarine surface waters and pore waters of the Amazon and Fly (Papua New Guinea) Rivers to investigate U transport phenomena across river-dominated land–sea margins. Discharge from large, tropical rivers is a major source of dissolved and solid materials transported to the oceans, and are important in defining not only oceanic mass budgets, but also terrestrial weathering rates.On the Amazon shelf, salinity-property plots of dissolved organic carbon, pH and total suspended matter revealed two vastly contrasting water masses that were energetically mixed. In this mixing zone, the distribution of uranium was highly non-conservative and exhibited extensive removal from the water column. Uranium removal was most pronounced within a salinity range of 0–16.6, and likely the result of scavenging and flocculation reactions with inorganic (i.e., Fe/Mn oxides) and organic colloids/particles. Removal of uranium may also be closely coupled to exchange and resuspension processes at the sediment/water interface. An inner-shelf pore water profile indicated the following diagenetic processes: extensive (1 m) zones of Fe(III)—and, to a lesser degree, Mn(IV)—reduction in the absence of significant S(II) concentrations appeared to facilitate the formation of various authigenic minerals (e.g., siderite, rhodocrosite and uraninite). The pore water dissolved 238U profile co-varied closely with Mn(II). Isotopic variations as evidenced in δ234U pore waters values from this site revealed information on the origin and history of particulate uranium. Only after a depth of about 1 m did the δ234U value approach unity (secular equilibrium), denoting a residual lattice bound uranium complex that is likely an upper-drainage basin weathering product. This suggests that the enriched δ234U values represent a riverine surface complexation product that is actively involved in Mn–Fe diagenetic cycles and surface complexation reactions.In the Fly River estuary, 238U appears to exhibit a reasonably conservative distribution as a function of salinity. The absence of observed U removal does not necessarily imply non-reactivity, but instead may record an integration of concurrent U removal and release processes. There is not a linear correlation between δ234U vs. 1/238U that would imply simple two component mixing. It is likely that resuspension of bottom sediments, prolonged residence times in the lower reaches of the Fly River, and energetic particle–colloid interactions contribute to the observed estuarine U distribution. The supply of uranium discharged from humid, tropical river systems to the sea appears to be foremost influenced by particle/water interactions that are ultimately governed by the particular physiographic and hydrologic characteristics of an estuary.  相似文献   

14.
Z. Shi  H. J. Zhou 《水文研究》2004,18(15):2877-2892
Theoretical and experimental studies were undertaken to gain insight into physical parameters controlling the flocculation and settling properties of mud flocs in the Changjiang Estuary, China. The Rouse equation is applied to vertical profiles of suspended sediment concentration to determine the bulk mean settling velocity (ws) of sediment suspended in the Changjiang Estuary. Both in situ point‐sampled and acoustically measured profiles of suspended mud concentrations were fit selectively. The calculated settling velocities ws mainly ranged from 0·4 to 4·1 mm s?1 for the point‐sampled data set, and from 1·0 to 3·0 mm s?1 for the acoustically measured data set. Furthermore, the settling velocities of mud flocs increased with mean concentration (C?) of mud flocs in suspension and were proportional to increasing bottom shear stress (τb) of tidal flow. The best equation for the field settling velocity of mud flocs in the Changjiang Estuary can be expressed by the power law: ws = mC?n (m, 1·14–2·37; n, 0·84–1·03). It is suggested that C? and τb were the dominant physical parameters controlling the flocculation and ws of mud flocs in suspension. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

15.
To detect temporal changes and the origin of the refractory dissolved organic matter in the Upper Rhône River, UV light absorbance (A) at 285 nm and quantitative dissolved organic carbon (DOC) measurements were carried out. Data from 63 visits to the main channel over a period of two years and from visits to different waterbodies in the alluvial plain before and after a flood are presented. There was a good correlation between A (0.019–0.160) and the DOC content (1.40–9.81 mg/L) for the waterbodies, but not for the river axis with lower A (0.013–0.044) and DOC content (1.13–2.20 mg/L). Due to this good correlation, the DOC content could be quantified for the waterbodies by absorbance measurements only. For the river water this indirect determination of the DOC content was not possible. However, the A/DOC ratio showed changes in the composition of DOC of river water and provided indications about the origin of the dissolved organic matter in the Upper Rhône River.  相似文献   

16.
Tropical mobile mud belts represent a major class of biogeochemical and diagenetic systems characterized by extensive and frequent physical reworking of fine-grained, organic-rich deposits underlying oxygenated waters. Large regions of the Gulf of Papua, Papua New Guinea deltaic complex are dominated by such conditions. A reworked mud belt lies within the inner shelf between 10 and 20 m depth on a sedimentary clinoform derived from coalescing deltas. Deposits across the topset are typically suboxic, nonsulfidic over the upper 0.5–1 m, and have low to moderate maximum pore water concentrations of dissolved Fe(II) and Mn(II) (100–200, but up to 800 μM). Sediments are reactive, with surficial ΣCO2 production 0.1–0.3 mM d−1 and benthic O2 fluxes 23±15 mmol m−2 d−1 (upper 20 cm). The highest rates occur within inner topset deposits (10–20 m) and near the high accumulation rollover region of the topset–foreset beds (40–50 m). Lower rates are found inshore along intertidal channels—mangrove fringe and within scoured or exposed consolidated deposits of the middle topset region. Remineralization rate patterns are independent of relative dominance by terrestrial or marine carbon in sediments. Dissolved O2 usually penetrates 2–5 mm into surface sediments when macrofaunal burrows are absent. More than 75% of the highly reactive sedimentary Fe(III) pool (350–400 μmol g−1) is typically diagenetically reduced in the upper 0.5 m. Pore water can be measureably depleted at depths >0.5 m, but dissolved H2S generally remains below detection over the upper 1–2 m. As in other deltaic topset regions, concentration gradients often indicate that compared to many marine deposits of similar sediment accumulation rates, relatively refractory Corg is supplied to the SO4 reducing zone. Sedimentary C/S ratios are 4–6 within the suboxic topset regions but decrease to <3 in offshore foreset beds where sulfidic diagenesis dominates. Only 15–20% of the diagenetically reduced Fe(II) is pyritic and a maximum of 10–25% is carbonate, implying that most Fe(II) is associated with authigenic or lithogenic silicates or oxides. The dominance of suboxic, nonsulfidic diagenetic processes reflect coupling between delivery of oxide-rich terrestrial debris, remobilization and reoxidation of deposits, and repetitive entrainment/remineralization of both labile and refractory organics. Distinct sedimentary indicators of reactive, suboxic mobile mud belts within tropical climatic zones are: abundant total highly reactive Fe (ΣFeR )>300 μmol g−1; most reactive Fe is diagenetically reduced (ΣFe(II)/ΣFeR0.7–0.8); the proportion of diagenetically reduced Fe present as pyrite is low (Py–Fe(II)<0.2); C/S 4–8; and Corg/particle surface area <0.4 (mg C m−2). These depositional environments must be most common in tropical climates during high sea stand.  相似文献   

17.
The vertical distribution of Hg and Pb were determined in a sediment core collected from the Sagua estuary (North Cuba) that receives input from the Sagua river, one of the most polluted rivers discharging into the Cuban coastal environment. Depth profiles of metal concentrations were converted to time-based profiles using the 210Pb dating method and confirmed with the 137Cs fallout peak. The mean mass accumulation rate was estimated to be 0.17 ± 0.04 g cm−2 y−1 (mean sediment accumulation rate 0.52 ± 0.13 cm y−1) and the core bottom was estimated to date back about 130 years. The historical sedimentary record showed a strong enrichment of mercury concentrations in the past decades, caused by the incomplete treatment of industrial wastes from a chlor-alkali plant with mercury-cell technology in the Sagua river basin. Lead fluxes to sediments showed a gradual increase from the 1920s to present, which agrees with a population increase in Sagua la Grande City. Fluxes of both metals have increased the past 25 years, with values reaching a maximum of 0.5 and 3.9 μg cm−2 y−1 for Hg and Pb, respectively.  相似文献   

18.
Surface waters collected along the salinity gradient of the Hudson River estuary in four cruises between 1995 and 1997 were size-fractionated into particulate (>0.45 μm), ‘dissolved' (<0.45 μm), colloidal (10 kDa, 0.45 μm) and low molecular weight (<10 kDa) phases. Dissolved Cd concentrations (range: 0.11–1.19 nM) in surface waters of the estuary appear to have decreased fourfold (from an average of 2.36 to 0.61 nM) over a 23-year period, since the initial analysis of samples collected in the 1970s by Klinkhammer and Bender [Estuar. Coastal Shelf Sci. 12 (1981) 629–643]. This interannual decline reflects improvement in sewage treatment and the elimination of industrial Cd sources to the Hudson River estuary. In contrast, dissolved Mn levels (range: 0.033–1.46 μM) have remained relatively constant over the same period of time, suggesting that anthropogenic sources have very limited impact on Mn concentrations in the estuary. The concentrations of both Cd and Mn appeared to strongly depend on the season and/or river discharge. The highest concentrations were detected under low freshwater discharge, implying that limited hydraulic flushing allows a build-up of metals in the water column. Although the decline in Cd levels within the estuary reflects a reduction in the magnitude of anthropogenic inputs, mass balance estimates indicated that current sources of Cd to the estuary include sewage discharges (in the lower estuary around Manhattan) and diagenetic remobilization from industrial Cd deposited in sediments nearly 2 decades ago (in the upper estuary near Foundry Cove). Moreover, under low river discharge, the sources considered in our model (sewage, riverine input, atmospheric deposition, and benthic fluxes) could account for no more than 60% of the Cd exported from the lower estuary to the ocean. This suggests that undefined sources such as ground water and inputs from other watersheds (e.g., Long Island Sound and Newark Bay) may potentially influence the water quality of the New York Harbor. The size-fractionated metal concentrations indicated that most of the traditionally defined ‘dissolved' Cd and Mn consisted of <10 kDa molecular weight species. High molecular weight colloidal species of Mn accounted for about 50% of the dissolved fraction at the riverine end-member and <5% at intermediate and high salinities. Colloidal Cd accounted for <6% of the dissolved phase throughout the estuary. Unlike the non-conservative excess (relative to ideal dilution of river water and seawater) of dissolved Mn observed along the estuary, high molecular weight colloidal Mn appeared to be removed at the head of the estuary. The small contribution of colloidal Cd and Mn to the ‘dissolved' phase suggests that remobilization from suspended particulate phases and/or from sediments occurs through the formation of small molecular weight species.  相似文献   

19.
In the present work, we built a mathematical model of polychlorinated biphenyl (PCB) bioaccumulation in Perna viridis, namely, a one-compartment model with a time dependent incorporation rate R (μg g−1 lipid per ppb water per day), with positive substrate cooperativity as the underlying physical mechanism. The temporal change of the PCB concentration Q (μg g−1 lipid) in the soft tissues of the mussel depends on the competition of the input rate RW and the output rate kQ, where W is the concentration of PCB in water (ppb water) and k is the elimination rate (per day). From our experimental data, k=0.181±0.017 d−1. The critical concentration in water Wc for positive substrate cooperativity was found to be 2.4 ppb. Below Wc, R is a constant. For a water concentration of 0.5 ppb Aroclor 1254, R=24.0±2.4 μg g−1 lipid ppb−1 d−1. Above Wc, positive substrate cooperativity comes into effect and R becomes a function of time and dependent on the concentration Q in a form R=γQ/(Q+δ). This is the case for a water concentration of 5 ppb Aroclor 1254, where γ=15.1 μg g−1 lipid ppb−1 d1 and δ≈200 μg g−1 lipid. From this model, the uptake is exponentially increasing when the PCB concentration in the mussel is small compared to 200 μg g−1 lipid, and hyperbolically increasing when the concentration is large compared to 200 μg g−1 lipid, which are consistent with the experimental data. The model is useful for understanding the true processes taking place during the bioaccumulation and for risk assessment with higher confidence. Future experimental data which challenge the present model are anticipated and in fact desirable for improvement and perfection of the model.  相似文献   

20.
Profiles of velocity turbulence in Monterey Canyon, made with a recently developed expendable probe, show the existence of a very turbulent bottom boundary layer. The turbulent flow is up to 170 m thick and has peak microscale shears of 1 m s−1 per meter. The rate of dissipation of kinetic energy, based on the observed shear variance, averaged over the depth of the turbulent boundary layer ranged from 70 to 500 × 10−6W m−3. Temperature measurements indicate that the flow was up canyon at a time of low tide. The upper bound for the vertical eddy viscosity is estimated to be17 × 10−4m2s−1 and for the vertical eddy diffusivity is estimated to be 15 × 10−4m2s−1. The large vertical scale and the intensity of the observed boundary layer suggest that the flow in Monterey Canyon may be important for the renewal and circulation of water over the continental shelf in the bay area.  相似文献   

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