Carbon standing stock distribution in the cuphotic zone of Lake Kinneret and the immediate fate of primary-produced carbon arc very different during late winter-early spring (with the occurrence of the annual dinollagellate bloom) than they are in late summer (when nanophytoplankton are the dominant primary producers). We used a linear programming model to construct balanced carbon flow charts for these two seasons based on measured primary productivity; on carbon standing stocks of algae, bacteria, flagellates, ciliates, cladocerans, rotifers, and fish; and on data on turnover times, respiration, and grazing rates obtained in 1989. The charts were compiled to fit as closely as possible all obscrvcd and inferred estimates of carbon fluxes while simultaneously ensuring that mass balance and key biological constraints were maintained for each of the 10 compartments representing the principal biota of the Kinneret food web. We used the model to examine the extent to which individual intercompartmental flux rates were free to vary while the mass-balance and biological constraints were enforced. The model was also capable of generating different yet feasible flow-chart scenarios; it thus proved useful in suggesting alternative hypotheses concerning the role of the microbial food web in the euphotic waters of Lake Kinneret.A major aim of both theoretical and applied aquatic research has long been to understand the patterns of flow of carbon and other elcments through the pelagic biota in lakes and seas. Within the last decade it has become increasingly evident that the complex communities of microbial organisms in the pelagic zone in both marine and freshwater environments play a key role in processing and transferring carbon, nitrogen, and phosphorus from primary producers to metazoan plankton and fish (Pomeroy 1974;Azam et al. 1983; Berman 1990). Much debate has been generated concerning the function of microbial food webs as Acknowledgments WC are indebted to B. Azoulai, K. D. Hambright, M. Gophen, G. Nahum, 0. Hadas, U. Pollinghcr, Y. 2. Yacobi, and T. Zohary for data, discussion, and dissent. We are grateful for the comments of two rcvicwcrs that enabled us to improve and revise the original version of the paper.
Abstract. Mixing is the physical process through which solute is spread into a fluid by the stretching and folding of material lines and surfaces. Mixing, as compared to dilution, is important to solute spreading by groundwater because it operates on much shorter timescales than does dilution, and it provides the increased plume boundary area and high local concentration gradients that promote effective solute dilution. In this paper, the mixing process is investigated theoretically for subsurface tracer plume movement, using as heuristic examples both steady and unsteady groundwater flows in a perfectly stratified aquifer whose properties mimic those of the sand aquifer at the Borden site. It is shown that the stretching efficiency, a parameter that characterizes the effectiveness of mixing, is largest at transitions between regions of highly contrasting hydraulic conductivity and, more broadly, that pronounced spatial variability in the hydraulic conductivity is conducive to good mixing because of the periodic resurgences in material line stretching that it causes. Unsteady groundwater flow resulting from a decrease in vertical groundwater flux with time leads to greater rates of material line stretching than do steady flows, whereas little difference from the steady flow case occurs for unsteady groundwater flow under a time-varying horizontal hydraulic head gradient. Overall, pronounced spatial variability in the hydraulic conductivity is the most important contributor to good mixing of a tracer solute plume, but highly effective mixing requires additional physical conditions that induce chaotic solute pathlines. IntroductionSolute plumes moving in aquifers are spread by two important but quite distinct physical processes. One is termed dilution, through which a plume is gradually distributed over an ever increasing volume of an aquifer to produce a reduction in solute concentration. Dilution is driven by local dispersion (i.e., dispersion processes occurring at the Darcy scale) and, secondarily, by complexity in plume shape, which serves to increase the plume boundary area through which solute mass can be transported dispersively. Kitanidis [1994]
Abstract. The perfectly stratified aquifer has often been investigated as a simple, tractable model for exploring new theoretical issues in subsurface hydrology. Adopting this approach, we show that steady groundwater flows in the perfectly stratified aquifer are always confined to a set of nonintersecting permanent surfaces, on which both streamlines and vorticity lines lie. This foliation of the flow domain exists as well for steady groundwater flows in any isotropic, spatially heterogeneous aquifer. In the present model example it is a direct consequence of the existence of a stream function. We then demonstrate that tracer plume advection by steady groundwater flow in a perfectly stratified aquifer is never ergodic, regardless of the initial size of the tracer plume. This nonergodicity, which holds also for tracer advection in any isotropic, spatially heterogenous aquifer, implies that stochastic theories of purely advective tracer plume movement err in assuming ergodic behavior to simplify probabilistic calculations of plume spatial concentration moments. IntroductionNatural aquifers often exhibit stratification, both geologically and in the geostatistical sense, that the correlation length for the hydraulic conductivity is considerably smaller along the vertical direction than it is along horizontal directions (see, e.g., the sand aquifer properties summarized by Rehfeldt and Gelhar [1992]). In the limiting case of horizontal correlation lengths that tend to infinity, a simple model emerges in which the hydraulic conductivity exhibits spatial variability only along the vertical direction. The mathematical tractability of this perfect-stratification model has made it especially convenient for exploring new physical concepts and theoretical approaches to groundwater flow and transport [e.g., Dagan, 1990].Recent modeling studies of the perfectly stratified aquifer have focused on calculations of the transport of a tracer plume that is advected by a random, steady velocity field under a prescribed gradient of hydraulic head without boundary effects. The emphasis in these studies has been on preasymptotic transport, an effect of incomplete sampling of the groundwater velocity field by the moving spatial points with which a tracer plume is associated mathematically
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