John P. Crimaldi

A DELICATE BALANCE: ECOHYDROLOGICAL FEEDBACKS GOVERNING LANDSCAPE MORPHOLOGY IN A LOTIC PEATLAND

The Everglades ridge and slough landscape is characterized by elevated sawgrass ridges regularly interspersed among lower and more open sloughs that are aligned parallel to the historic flow direction. Landscape degradation, characterized by topographic flattening, has coincided with a century of drainage, levee construction, nutrient enrichment, and flow reductions. Here we develop a conceptual model of Everglades landscape dynamics based on a literature synthesis and supported by the numerical model PeatAccrete. We propose that two feedback mechanisms govern landscape characteristics. The first, simulated with PeatAccrete, involves differential peat accretion governed by water level and phosphorus concentration, leading to the attainment of an equilibrium ridge elevation relative to slough. Differential peat accretion, however, cannot produce a characteristic ridge width or landscape wavelength. Instead, we propose that feedback between channel morphology and sediment mass transfer controls lateral and longitudinal topographic features, consistent with processes in anabranching rivers. This sediment transport feedback was critical in pattern initiation and evolution, and sediment redistribution from slough to ridge provides a plausible mechanism for preventing gradual ridge expansion. However, PeatAccrete model results show that, in the absence of sediment transport, ridges expand only on the order of meters per century. This result suggests that a combination of factors has driven the widespread disappearance of sloughs over the past century, including altered vertical peat accretion rates that lead to slough infilling. Sensitivity tests indicated that changes in duration and depth of surface water inundation, phosphorus supply, and redox potential have altered differential peat accretion rates in a way that favors topographic flattening. These factors are relatively well defined compared with the role of sediment transport, which requires further quantification. Because both positive and negative feedback processes interact in the Everglades, the trajectory of landscape evolution in time will depend upon current conditions, with areas of remnant ridge and slough topography being more likely than areas of degraded topography to respond to changes in water management in ways that enhance landscape heterogeneity over human timescales. Dual feedbacks between peat accretion and sediment transport are likely important controls on landscape evolution in low-gradient peatlands worldwide with pulsed, unidirectional flow.

The relationship between mean and instantaneous structure in turbulent passive scalar plumes

Laboratory investigations of a turbulent scalar plume are performed to investigate the relationship between instantaneous scalar structure and the resulting mean scalar statistics. A planar laser-induced fluorescence technique is used to image two-dimensional instantaneous spatial plume structure at various locations and in three orthogonal planes. Long image sequences are used to calculate time-averaged scalar statistics (concentration mean, variance and intermittency), and the relationship between these statistics and the observed instantaneous scalar structure is discussed. We present both snapshots and animations of instantaneous scalar structure at various locations within the boundary layer. As with all boundary layer phenomena, the structural variation is greatest in the vertical direction (normal to the bed). The existence of a persistent, relatively uniform layer of dye within the viscous sublayer is identified. In this layer, instantaneous concentrations are moderate, but the persistence of the dye produces a relatively high mean concentration. Above this layer, stronger fluctuations and higher peak concentrations are present, but lower values of the intermittency produce lower mean concentrations. It is argued that a combination of three time-averaged statistics (mean, variance and intermittency) is required to deduce meaningful information about the nature of the instantaneous scalar structure.

Predicting organic floc transport dynamics in shallow aquatic ecosystems: Insights from the field, the laboratory, and numerical modeling

Received 13 June 2008; revised 10 September 2008; accepted 15 October 2008; published 14 January 2009. [1] Transport of particulate organic material can impact watershed sediment and nutrient budgets and can alter the geomorphologic evolution of shallow aquatic environments. Prediction of organic aggregate (‘‘floc’’) transport in these environments requires knowledge of how hydraulics and biota affect the entrainment, settling, and aggregation of particles. This study evaluated the aggregation and field transport dynamics of organic floc from a low-gradient floodplain wetland with flow-parallel ridges and sloughs in the Florida Everglades. Floc dynamics were evaluated in a rotating annular flume and in situ in the field. Under present managed conditions in the Everglades, floc is not entrained by mean flows but is suspended via biological production in the water column and bioturbation. Aggregation was a significant process affecting Everglades floc at high flume flow velocities (7.0 cm s �1 ) and during recovery from high flow; disaggregation was not significant for the tested flows. During moderate flows when floc dynamics are hydrodynamically controlled, it is possible to model floc transport using a single ‘‘operative floc diameter’’ that accurately predicts fluxes downstream and to the bed. In contrast, during high flows and recovery from high flows, aggregation dynamics should be simulated. When entrained by flow in open-water sloughs, Everglades floc will be transported downstream in multiple deposition and reentrainment events but will undergo net settling when transported onto ridges of emergent vegetation. We hypothesize that net transport of material from open to vegetated areas during high flows is critical for forming and maintaining distinctive topographic patterning in the Everglades and other low-gradient floodplains.

Reaction enhancement of isolated scalars by vortex stirring

The effect of vortex stirring on reaction rate enhancement is investigated for two reactive scalars initially separated by a third nonreactive scalar. The presence of the intervening scalar precludes reactions at early times. Vortex stirring accelerates the coalescence of the reactive scalars relative to pure diffusion and enhances the resulting reaction rates. Analytical and numerical results for reactive stirring by a single point vortex are shown for a range of Peclet (Pe) and Damkohler (Da) numbers. At low Da and high Pe, nondimensional reaction rates grow as Pe1/3, and peak reaction times decrease as Pe−2/3. Reaction rates scale linearly with Da for slow reactions, but this scaling breaks down for fast reactions due to reactant depletion. The stirring-induced reaction rate enhancement is shown to be relatively insensitive to initial placement of scalars within the point vortex. The study provides mechanistic insights into more general mixing and reaction problems involving initially isolated scalars.

The role of structured stirring and mixing on gamete dispersal and aggregation in broadcast spawning

Summary Broadcast-spawning benthic invertebrates synchronously release sperm and eggs from separate locations into the surrounding flow, whereupon the process depends on structured stirring by the flow field (at large scales), and sperm motility and taxis (at small scales) to bring the gametes together. The details of the relevant physical and biological aspects of the problem that result in successful and efficient fertilization are not well understood. This review paper includes relevant work from both the physical and biological communities to synthesize a more complete understanding of the processes that govern fertilization success; the focus is on the role of structured stirring on the dispersal and aggregation of gametes. The review also includes a summary of current trends and approaches for numerical and experimental simulations of broadcast spawning.

The Physics of Broadcast Spawning in Benthic Invertebrates

Most benthic invertebrates broadcast their gametes into the sea, whereupon successful fertilization relies on the complex interaction between the physics of the surrounding fluid flow and the biological properties and behavior of eggs and sperm. We present a holistic overview of the impact of instantaneous flow processes on fertilization across a range of scales. At large scales, transport and stirring by the flow control the distribution of gametes. Although mean dilution of gametes by turbulence is deleterious to fertilization, a variety of instantaneous flow phenomena can aggregate gametes before dilution occurs. We argue that these instantaneous flow processes are key to fertilization efficiency. At small scales, sperm motility and taxis enhance contact rates between sperm and chemoattractant-releasing eggs. We argue that sperm motility is a biological adaptation that replaces molecular diffusion in conventional mixing processes and enables gametes to bridge the gap that remains after aggregation by the flow.

A mixing-length formulation for the turbulent Prandtl number in wall-bounded flows with bed roughness and elevated scalar sources

Turbulent Prandtl number distributions are measured in a laboratory boundary layer flow with bed roughness, active blowing and sucking, and scalar injection near the bed. The distributions are significantly larger than unity, even at large distances from the wall, in apparent conflict with the Reynolds analogy. An analytical model is developed for the turbulent Prandtl number, formulated as the ratio of momentum and scalar mixing length distributions. The model is successful at predicting the measured turbulent Prandtl number behavior. Large deviations from unity are shown in this case to be consistent with measurable differences in the origins of the momentum and scalar mixing length distributions. Furthermore, these deviations are shown to be consistent with the Reynolds analogy when the definition of the turbulent Prandtl number is modified to include the effect of separate mixing length origin locations. The results indicate that the turbulent Prandtl number for flows over complex boundaries can be mo...

Reaction enhancement of initially distant scalars by Lagrangian coherent structures

Turbulent fluid flows have long been recognized as a superior means of diluting initial concentrations of scalars due to rapid stirring. Conversely, experiments have shown that the structures responsible for this rapid dilution can also aggregate initially distant reactive scalars and thereby greatly enhance reaction rates. Indeed, chaotic flows not only enhance dilution by shearing and stretching but also organize initially distant scalars along transiently attracting regions in the flow. To show the robustness of this phenomenon, a hierarchical set of three numerical flows is used: the periodic wake downstream of a stationary cylinder, a chaotic double gyre flow, and a chaotic, aperiodic flow consisting of interacting Taylor vortices. We demonstrate that Lagrangian coherent structures (LCS), as identified by ridges in finite time Lyapunov exponents, are directly responsible for this coalescence of reactive scalar filaments. When highly concentrated filaments coalesce, reaction rates can be orders of mag...

Reaction of Initially Distant Scalars in a Cylinder Wake

We used analytical and numerical techniques to investigate the effect of a quasi-steady-state laminar wake behind a circular cylinder on the second-order reaction between two initially distant scalars. The scalars are released continuously from locations upstream of the cylinder, and are separated from each other by a lateral distance that initially impedes the reaction. By comparing the laterally integrated reaction evolution for cases with and without the cylinder wake, the direct effect of the wake on reaction enhancement is determined. We compute the reaction for a range of reaction speeds, scalar diffusivities, and scalar release geometries. The presence of the cylinder wake generates significant reaction enhancement for all cases; the ratio of the reaction with and without the cylinder increases with the initial scalar separation distance. We identify the mechanism for the reaction enhancement, and demonstrate that the reaction rate in the cylinder wake can be predicted by a simple analytical model ...

Hydrodynamic shear removal of the nuisance stalk‐forming diatom Didymosphenia geminata

The removal of benthic algae during periods of high flow is critical in maintaining the biodiversity of stream ecosystems. Here we determine a shear removal function for the nuisance, stalk-forming benthic diatom Didymosphenia geminata by using samples collected from Rocky Mountain streams subjected to increasing bed shear stress in a laboratory flow chamber experiment. A linear shear removal function was observed to apply over the range of the shear stress obtained in the flow chamber. The overall removal of biomass was low. Less than 25% of the biomass was removed at a shear stress similar to that which would result in widespread bed disturbance in the stream. These results support the hypothesis that physical abrasion during periods of bed disturbance, rather than simply elevated shear stress, is the primary control on the removal of benthic algae such as D. geminata that are well adapted to the high-shear environments of mountain streams. The results also indicate that the shear removal function generally decreases with increasing biomass and mat thickness, the potential consequence of a positive feedback with near-bed hydrodynamics as the mats develop in the streams. The shear removal function was also influenced by the health and condition of the mats. Greater biomass removal was observed for mats in poorer condition. These mats had higher sediment content and were collected at the end of the growing season and from sites at lower elevations.