Physical Mixing Processes Research Articles

Soil organic carbon dynamics: variability with depth in forested and deforested soils under pasture in Costa Rica

Dynamics of soil organic carbon (SOC) inchronosequences of soils below forests that had beenreplaced by grazed pastures 3–25 years ago, wereinvestigated for two contrasting soil types (AndicHumitropept and Eutric Hapludand) in the Atlantic Zoneof Costa Rica. By forest clearing and subsequentestablishment of pastures, photosynthesis changes froma C-3 to a C-4 pathway. The accompanying changes inC-input and its δ13C and 14Csignals, were used to quantify SOC dynamics. C-input from rootturnover at a pasture site was measured by sequentialharvesting and 14C-pulse labelling. With aspatial resolution of 5 cm, data on total SOC,δ13C and δ14C of soil profileswere interpreted with a model that distinguishes threepools of SOC: ‘active’ C, ‘slow’ C and ‘passive’ C,each with a 1-st order decomposition rate(ka, ks and kp). The modelincludes carbon isotope fractionation and depth-dependentdecomposition rates. Transport of C between soillayers was described as a diffusion process, whichaccounts for physical and biotic mixing processes. Calibrated diffusion coefficients were 0.42 cm2yr-1 for the Humitropept and 3.97 cm2yr-1 for the Hapludand chronosequence.Diffusional transport alone was insufficient foroptimal simulation; it had to be augmented bydepth-dependent decomposition rates to explain thedynamics of SOC, δ13C andδ14C. Decomposition rates decreasedstrongly with depth. Upon increased diffusion,differences between calibrated decomposition rates ofSOC fractions between surface soils and subsoilsdiminished, but the concept of depth-dependentdecomposition had to be retained, to obtain smallresiduals between observed and simulated data. At areference depth of 15–20 cm ks was 90 yr-1in the Humitropept and 146 yr-1 in the Hapludand.Slow C contributed most to total organic C in surfacesoils, whereas passive C contributed most below 40 cmdepth. After 18–25 years of pasture, net loss of C was2180 g C m-2 for the Hapludand and 150 g m-2for the Humitropept soil.

The construction of extreme compositions

If a geochemical compositional datasetX (n×p)is a realization of a physical mixing process, then each of its sample (row) vectors will approximately be a convex combination (mixture) of a fixed set of (l×p)extreme compositions termed endmembers. The kpoints in p-space corresponding to a specified set of k (k<p)linearly independent endmember estimates associated with a p-variate (n×p)compositional datasetX,define the vertices of a (k−1)dimensional simplexH.The nestimated mixturesX′ (n×p)which together account for the systematic variation in the datasetX,should each be convex combinations of the kfixed endmember estimates. Accordingly,the npoints in p-space which represent these mixtures should be interior points of the simplexH.Otherwise, for each sample point which lies outsideH,at least one of the mixture coefficients (endmember contributions) will be negative. The purpose of this paper is to describe procedures for expandingHin the situation that its vertices are not a set of extreme points for the set which represents the mixtures.

Nitrogen cycling in sediments with different organic loading

Sediment cores from an intertidal marine area were experimentally loaded with different amounts of organic material, in order to investigate regulation of the processes in the nitrogen cycle, and the fate of the inorganic and organic N (NH4+, No3-, DON, and urea) released through mneraliza- tion. The particulate orgamc matenal was added in amounts of 30 g dw m-2 (30MIX) and 100 g dw m2 (100MIX) mixed into the sediment, and as 100 g dw rrr2 (100SURF) to the sediment top layer. Processes and pools were related to unamended controls (CTRL). The cores were incubated with low (0 to 2 pM) in the overlymg water, and measurements were made after 1 wk. Total sediment respiration rate measured as 0; uptake and CO; release were 10 and 18 mmol m2 dl, respectively, in the CTRL, grad- ually increasmg m the order of treatments (30MIX, IOOMIX, 100SURF) to 62 and 64 mrnol m-' d-' respectively in 100SURF. Higher loading resulted in increasing effluxes of NH4+. DON effluxes were quantitatively significant only from the 100SURF sediment cores. There was an accumulation of dis- solved N-species in the sediment amounting to 12 to 28 % of the loading, with most increase where the organic matter was mixed into the sediment. Nitrification and denitrification rates were highest in the moderately loaded sediment (30MIX), followed by control cores and the 100MIX. There was no nitrifi- cation or denitrification activity in the 100SURF cores, and NO3 was completely absent in this sedi- ment. Dissimilative No3 reduction to NH4+ was insignificant in all treatments. The experiment showed that moderate loading increased N removal through denitrification, while high loading decreased denitrification. The marked differences in the fate of nitrogen, due to organic matter distribution, demonstrated the importance of bioturbation and other physical mixing processes. a Dissolved organic nitrogen . Organic loading

Laboratory studies of oxic and anoxic degradation of chlorophyll- a in Long Island Sound sediments

Laboratory incubations of Long Island Sound (LIS) sediments show that chlorophyll-a (Chl a) degradation generally follows first-order kinetics, but that the rates and mechanisms of degradation differ under oxic and anoxic conditions. Chl-a in fresh sediment can be operationally defined as being present in three pools: bound Chl- a (nonextractable by acetone), free (acetone extractable) anoxically degradable Chl-a, and free anoxically stable Chl-a. A major pathway of degradation is initial release of bound Chl-a into the free pool. The release rate constants of bound Chl-a into free Chl-a are remarkably faster (> 10×) under oxic (~4 d −1) compared to anoxic (~0.1 d −1) conditions, and a much greater fraction of total Chl-a degrades under oxic conditions. However, the apparent degradation rate constants of free Chl-a are two to three times higher in the absence of oxygen in sediments with high bound Chla (~0.2−0.3 d −1 for anoxic; ~0.1 d −1 for oxic) and about seven times higher in sediments without bound Chl-a (~0.3 d −1 for anoxic; ~0.04 d −1 for oxic). The differences between anoxic and oxic decomposition in incubation experiments, together with naturally observed concentration profiles which reach low levels of Chl-a, imply that Chl-a in natural sediments degrades during the oscillation between oxic and anoxic conditions caused by physical and biological mixing processes. Oscillation experiments (oxic vs. anoxic and anoxic vs. oxic) suggest that the activities of aerobic organisms may be an important factor affecting Chl- a degradation. Examination of effects of meiofauna on Chl- a degradation under oxic conditions implies that microorganisms may play a stronger role in Chl- a degradation than meiofauna (> 0.125 mm). The relative temperature independence of anoxic degradation and temperature dependence of oxic degradation suggest that anoxic degradation may be controlled largely by chemical factors, while oxic degradation may be more strongly controlled by biophysical and biochemical processes.

The Resolution of a Compositional Data Set into Mixtures of Fixed Source Compositions

SUMMARY If a compositional data set is a realization of a physical mixing process, then within tolerable errors its observation vectors should be convex combinations of a fixed set of extreme source compositions called end-members. Given a compositional data set X (n x p) consisting of the measurements on p variables for each of n specimens, but without any prior knowledge on the formation of X, this paper illustrates a solution to the problem of estimating the number and composition of such endmembers. These estimated compositions, in turn, form an algebraical basis for a mixing process which should account for each of the n observation vectors of X.

An evaluation of boundary conditions for one‐dimensional solute transport: 2. Column experiments

As the result of a theoretical comparison of analytical models for one‐dimensional solute transport (Novakowski, this issue), it has been found that to reconcile the substantial differences observed between the models under conditions of large dispersion, a physical modeling study of the processes of solute transport in the vicinity of boundaries must be undertaken. The physical modeling is conducted using columns ranging in diameter from 76 to 352 mm and length from 300 to 400 mm. Geological materials of either large or small coefficient of dispersion are employed as packing for the columns. Reservoirs of finite volume are located at the inlet and outlet boundaries of each column. Using a conservative fluorescent tracer, experiments are conducted to investigate the use of macroscopic continuity in concentration at the boundaries, and the use of the flux‐averaged transformation for this boundary value problem. Concentration of the tracer was determined noninvasively from both the inlet and outlet reservoirs and, for some experiments, resident concentration was determined from within the interior of the column by excavation. Results of the experiments conducted using different volumes of the outlet reservoir show that the analytical model for flux concentration accounting for macroscopic continuity in concentration at the boundaries only poorly simulates the physical mixing process in the outlet reservoir. In addition, the results of the experiments conducted in which resident concentrations were determined from the interior of the column show that the concept of macroscopic continuity is not supported by physical evidence at either the inlet or outlet boundary. Thus, the analytical model in which concentration at the boundaries is macroscopically discontinuous best simulates the solute transport processes for this boundary value problem. Unfortunately, the solutions for resident and flux concentration with these boundary conditions are identical and further distinction between these models cannot be undertaken. Analytical inversion of the Laplace domain solution is also presented.

A new spectral method for calculation of the time-varying area of a laminar flame in homogeneous turbulence

A novel approach to flame area simulation is presented, based on the physical processes of chaotic mixing. The “wrinkled” flame front is represented by a length-scale distribution of distortions of the surface, bounded by the chamber or flame kernel size, and the laminar flame thickness. The turbulent fluid in which the flame propagates is modelled as a length scale spectrum of vortices, with the contribution of each scale to the total turbulent kinetic energy being obtained from a prescribed energy spectrum function. Vortex-flame interaction is modelled by: (a) flame area generation by the vortices at their length scale, (b) stretching and folding by generation of flame area at larger scales. The effect of flame propagation on the radius of curvature of the flame constitutes the third mechanism for altering the “wrinkle” spectrum. The resulting integro-differential equation is solved numerically by a time-marching procedure to yield the evolution of surface area and asymptotic (equilibrium) limit for given conditions of turbulence and prescribed laminar flame speed. Good agreement with experimental results is obtained for the fractal dimension of passive scalar and flame surfaces, and also with turbulent flame speed data over a wide range of conditions. The model also makes predictions of flame-speed trends in the near-wall region in agreement with observed behaviour, and also shows that there is a weak dependence of turbulent flame speed on S l additional to that on u′ /S l used in the Bradley correlations. The effects of turbulent flame stretch and vortices smaller than the laminar flame thickness are not modelled, leading to reduced agreement in the high u′ /S l limit. The physical nature of the modelling means that these, and other phenomena such as holes in the flame front caused by rapid local stretching, can be easily included in future work.

Time scales of physical and chemical processes in chemically reactive plumes

The rate of non-linear chemical reactions in a dispersing plume, such as the formation of NO2, is discussed on the basis of various models and measurements of the oxidation rate of NO. Results of calculations with four different reactive plume models are compared with a set of plume measuremeent and discussed in terms of the ration of the time scales of physical (Tp) and chemical processes (Tc), the Damköhler number:Tp/Tc. In modelling a fast non-linear chemical reaction an interaction between physical processes of dispersion and mixing and chemical reactions can be expected. Models may differ in that they assume either chemical equilibrium (Tp/Tc⪢1) or no chemical equilibbrium (Tp/Tc⪡1).From our study it is concluded that for the first 5 km from a stack concentrations in the plume change more rapidly by dilution due to dispersion of the plume than by chemical reactions. Farther off it is the other way around and the chemistry is fast enough to achieve chemical equilibrum in the plume parcels.Conversion of NO is then inhibited by the mixing rate of the plume with its surrounding air. To incorporate the mixing rate of the plume correctly into a model, in principle the concentration distribution in a momentary plume has to be used. If a non-linear chemical reaction is modelled it is therefore important to establish the averaging times on which the dispersion parameters in the model are based. The contribution of plume meandering to dispersion, which is especially important in the daytime, can be eliminated by taking shorter averaging times. This leads to a marked improvement of modelling results. Further research is required to investigate how the contribution of fluctuations in the concentrations around the mean has to be incorporated into a model of a chemically reactive plume.

Vertical profiles of pelagic communities in the vicinity of the Azores Front and their implications to deep ocean ecology

Three early wintertime and five early summertime stations were occupied in the vicinity of the Azores Front to the south-west of the islands. At each station the surface 1100–1400m of the water column were systematically sampled both by day and by night, using the Institute of Oceanographic Sciences' RMTI+8M system. The biological sampling was set in the context of an extensive study of the physical oceanography of the frontal region. It included one station in the core of an eddy which was seen to spawn from a meander during the course of the physical observations. The biological communities on either side of the front were almost indistinguishable in terms of their specific composition. However, there were substantial differences in the sizes of the standing-crops, the vertical distributions of the major taxonomic groups and the ranges of the diel vertical migrations being carried out by these groups. The waters on both sides of the front were oligotrophic. However, both phytoplankton standing-crop, as indicated by chlorophyll concentrations, and measures of productivity suggested that the water to the north (Eastern Atlantic Water), which was slightly cooler and with a shallower and steeper nutricline, was a little more productive than the water to the south (Western Atlantic Water). Associated with this trend in phytoplankton crop and productivity were quite sharp declines in the standing-crops of both plankton and micronekton across the front, moving from EAW to WAW. There was no evidence of enhanced productivity within the front itself, but there was an increase in the micronektonic standing-crop. Between the stations, there were consistent trends in the ratios of the standing-crops of plankton and micronekton, which were sampled simultaneously. By day, the planktonic biomasses exceeded the micronektonic biomasses by up to an order of magnitude in the wind-mixed layer. The ratios declined with depth, approaching unity at 200m during the wintertime, but remaining quite high to depths of 800m during the summer. At night, the ratios tended to be lower as a result of diel vertical migration, but except in one or two of the deepest samples the planktonic biomass always exceeded that of the micronekton. The total biomass profiles were very similar in shape between the stations, although there were quite large differences in the depths of the quartiles. There were substantial differences between the day and the night profiles. These could be attributed mostly to the effects of diel vertical migrations, but must also have included unquantifiable effects as a result of variations in net avoidance. The subdivision of the catches into the major taxonomic groups showed that the degree of avoidance varied substantially between them; euphausiids were the most effective daytime avoiders, but decapod crustaceans and fish showed a suprisingly small tendency to avoid. The ranges and the numbers of each group undertaking vertical migrations varied markedly between the stations, and these differences were expressed in the biomass estimates. Attempts have been made to quantify the movement of biomass in and out of the surface 200m and 500m resulting from these migrations. There were substantial changes in migratory behaviour across the front, even though there were no detectable changes in the light profiles. The data also imply that the micronektonic migrants play an important role in the active transport of organic substances down from the euphotic zone to mesopelagic depths (>500m), either in their gut contents or directly as a result of predation at depth. In addition, migrating plankton seem likely to be important in redistributing nutrients about the nutricline in these oligotrophic regions where the water column stability limits physical vertical mixing processes. Preliminary estimates suggest that their influence may be sufficiently large to alter the ratio between “new” and “old” production.

Nutrients in seawater and interstitial water of the sediments in the Huanghe River estuarine area (November)

The investigation shows that the concentrations of nutrients are high in estuarine and coastal waters and low in offshore waters. The concentration of nitrate in estuaries is controlled through a physical mixing process and is also affected by biotic activity. The annual transport of total inorganic nitrogen and dissolved phosphate-phosphorus from the Huanghe River water to the sea is about 8.45 ×104 and 1.45×103 tons respectively. The distributions of inorganic nitrogen and silicate in interstitial water of surface sediments are similar to those in surface and bottom seawater. Their contents in interstitial water are 227–552 μmol/l (average375) for ammonia, 0.31–9.0 μmol/l (average 1.6) for nitrite, 0–41 μmol/l (average6.0) for nitrate, and 41–139 μmol/l (average 77) for silicate. The average concentrations of phosphate in the surveyed area are 0.64 μmol/l for seawater and 1.2 μmol/l for interstitial water. A cycle of phosphate in the estuary is also suggested in this paper.

The Lake Ontario Life Support System

Primary productivity provides most of the energy to support aquatic food chains. The rate is not only influenced by available solar radiation but also by temperature, availability of phosphorus, and the influence of physical mixing processes. The special features of Lake Ontario such as changes in phosphorus concentration, calcium carbonate precipitation, and silica deficiency on primary productivity, concentration of particulate carbon, and chlorophyll are discussed. Our lack of understanding of food chain and nutrient regeneration processes is illustrated through our failure to balance carbon production with losses through zooplankton grazing and sedimentation. It was demonstrated, however, that bacteria are not responsible for nutrient regeneration through "mineralization" but nutrients are effectively recycled in the water column at the second and third trophic levels.

Mixing of ambient air in a plume and its effects on the oxidation of NO

Formation of NO 2 in the exhaust plumes of Dutch power plants is studied by both field measurement and modelling. It is argued that models that do not take inhomogeneous mixing into account cannot adequately describe the measuring results, since NO to NO 2 oxidation rates are determined by actual local concentrations. A simple way to introduce inhomogeneous mixing is described and incorporated in a simulation model roughly following that of Carmichael and Peters (1981, Atmospheric Environment 15, 1069–1074). Model calculations are applied to airborne and ground measurements which were carried out under widely varying atmospheric conditions. The results show that, at least at plume height (some hundreds of metres), a significant part of the oxidation process occurs in an inhomogeneous mixed plume. NO 2 NO x ratios are calculated and the contribution of oxygen to the oxidation of NO and the large deviations of photostationary equilibrium are analyzed. Deviations of photo-stationary equilibrium are explained by two causes, i.e. the various time rates at which the physical processes of dispersion and mixing and the chemical processes of molecular reactions in the atmosphere occur; and also by an extra contribution of ozone in unmixed ambient air parcels in the plume.

Behavior of Fe, Mn, Cu and Cd in the duwamish river estuary downstream of a sewage treatment plant

The behavior of dissolved Fe, Mn, Cu and Cd in the Duwamish River downstream of a sewage treatment plant located near Seattle, Washington, was investigated in three distinct zones: (1) immediately downstream of the effluent outfall, (2) further downstream in the freshwater portion of the river and (3) in the estuarine mixing zone. A three-end-member tracer model utilizing salinity and dissolved inorganic nitrogen was developed to distinguish physical mixing processes from biogeochemical reactions. The results of this investigation indicate that Fe was removed from the dissolved phase onto particulates near the plant. Although no additional reactions controlled the distribution of dissolved Fe, Mn and Cu in the freshwater portion of the river, Cd was removed from the dissolved phase. In the estuarine mixing zone, a significant fraction of the dissolved Fe and Cu was removed from solution, while Mn and Cd desorbed from particulates. The chemical forms of the trace metals rather than their sources appear to determine their participation in these reactions.

Eddy-Enhanced Dispersion of Ocean-Dumped Organic Waste at Deep Water Dumpsite - 106

Volatile organic compounds (VOC) discharged into northwestern Atlantic slope waters at a deep ocean dumpsite were dispersed to background levels within 24 h. The main VOC (&gt;85%) of the waste was toluene, with a large mercaptan component. Several trichlorobenzenes were present in trace amounts. Initial dilution from wake dispersion reduced the waste levels by a factor of 1300. Mixing processes and evaporative loss to the atmosphere further reduced surface water concentrations of VOC by a factor of 1000 in 9 h and to background levels within 24 h. The rapid loss of volatiles from the dumpsite waters results from the very volatile nature of the dumped material and physical mixing processes. The presence of the edge of a large, anticyclonic, warm-core Gulf Stream eddy (ring) in the dumpsite area probably increased local mixing processes which resulted in the rapid dispersion of the waste. The presence of the ring water in the dumpsite area could also account for the low background concentrations observed in the dumpsite.

Trace Element Distributions in the Gulf Stream Adjacent to the Southeastern Atlantic Continental Shelf—Influence of Atmospheric and Shelf Water Inputs

As part of an effort to define processes that control the exchange of trace elements between southeastern Atlantic shelf waters and the Gulf Stream at the shelf boundary we determined the vertical distribution of particulate Al, Fe, Mn, Ni, Cu, Zn, Pb, and Cd and total dissolvable Fe, Ni, Cu, Zn, Pb, and Cd in the Gulf Stream adjacent to the shelf. Simultaneous determination of the particulate forms of Cu, Ni, and Cd and potential carrier phase indicators such as particulate Al, Fe, Mn, and organic C permitted assessment of the relative importance of clay minerals, hydrous oxides, and organic matter in the control of the trace element content of suspended matter. Particulate trace element concentration data were used in conjunction with a first-order removal model to estimate downward vertical fluxes of Fe, Mn, Ni, Cu, Pb, and Cd. Comparison of these flux estimates with atmospheric inputs of these same elements indicated that, with the possible exceptions of Mn and Cu, vertical processes largely control the distribution of trace elements in the Gulf Stream and that these processes are primarily biological in nature. Estimates of "new" production, in concert with an estimated vertical flux of C and the observed nitrate concentration profile, were used to predict a vertical eddy diffusion coefficient of ~0.4 cm2∙s−1 in the highly stratified region of the Gulf Stream. Injection of trace elements from shelf waters into surface Gulf Stream waters, with the possible exceptions of Mn and Cu, are apparently masked by a combination of atmospheric input and large-scale physical mixing processes.

Celtic Sea and Armorican current structure and the vertical distributions of temperature and chlorophyll

Vertical sections of temperature and chlorophyll a across the slopes and shelf of the Celtic Sea in the summer show the characteristic regimes; oceanic, slope, shelf, frontal, and mixed. Increases of surface chlorophyll a are commonly observed along the shelf tidal fronts where the thermocline outcrops at the surface, and also at the shelf-break. The variations in phytoplankton biomass are most readily interpreted in terms of the effects of physical mixing processes due to wind and tide on the availability of inorganic nutrients and light energy. On the shelf, mixing processes, both due to internal waves, inertial currents, and to boundary induced turbulence caused by tidal shear associated with the sea floor, play an important role in determining the observed vertical structures. A numerical model is used to define regions where tidal mixing processes are likely to be relatively important and provides the physical framework for interpreting the temperature and chlorophyll a profiles.

Salt Dispersion in the Hudson Estuary

Abstract The seaward transport of salt by river discharge through an estuary is balanced under steady conditions by landward dispersion effected by various physical mixing processes. Observations of current and salinity in the lower Hudson estuary provide a basis for assessing the relative importance of these different dispersion processes. Wind effects were estimated from two moored current meter records of 1- and 2-months length. There was a significant but weak correlation between wind and currents. Three cross-sectional surveys each lasting 25 h provide estimates of salt dispersion by other processes. Current and salinity data from the surveys were decomposed into various temporal and spatial means and departures from these means. Covariances between the various quantities are interpreted in terms of physical dispersion processes. The largest contributor to salt dispersion in the Hudson is the steady shear of gravitational circulation. Steady shear dispersion varies by a factor of 5 between spring hig...

Spatial patterns in North Sea plankton

Near-surface temperature and in vivo chlorophyll observations were made continuously on 34 lines of 90-km length in the North Sea during April and May 1976. Spectral analysis of these data permit comparisons of the ratio of slopes of these spectra with theories of the relation between phytoplankton growth and physical mixing processes. It is possible to separate periods when the phytoplankton growth is affected mainly by physical dispersion from periods when grazing is the controlling factor.

Free convection in thermally stratified water cooled from above

The thermal and fluid kinetics of the convective layer during cooling from the free surface of a thermally stratified water by convection, latent energy transport, and radiation is studied. Laboratory experiments were performed and a Mach-Zehnder interferometer was used to measure the unsteady temperature distribution in a test cell filled with water which was previously stratified. A simple mathematical model based on a thermal energy balance is developed to predict the thickness and the mean temperature of the layer. The model predictions agreed to within 10% with the data from controlled laboratory experiments. It was found that the numerical solution to the model equations agree better with the experimental data than those based on the closed form analytical solution using the constant average surface heat flux. It was determined that the surface boundary condition and the internal physical processes of mixing and entrainment must be better understood in order to model the dynamics of the mixed layer in natural waterbodies.

On the performance of closed heterogeneous chemical reaction systems: Selectivity of complex chemical networks

We investigate the performance of batch slurry chemical reaction systems in which the departure from the idealization of perfect contacting is important so that multicomponent physicochemical interactions must be taken into account. We construct a reduced state variable description via projection onto Im K and exhibit conditions on the reduced diffusion and chemical operators which define a wide class of stable systems. We investigate the selectivity of chemical networks made up of chemically independent subnetworks which compete for a common catalyst surface and interact via the physical processes of mixing and diffusion.