Arthur R. M. Nowell

Effects of biological activity on the entrainment of marine sediments

Nowell, A.R.M., Jumars, P.A. and Eckman, J.E., 1981. Effects of biological activity on the entrainment of marine sediments. Mar. Geol., 42: 133-153. The effects of animal tracks and fecal pellet production on the critical entrainment velocity of marine sediments were examined experimentally. Laboratory measurements in a free-surface, seawater flume were made using three sediment sizes. Effects of three species of polychaetes and two species of bivalves were tested. Boundary shear velocity was calculated from the mean velocity profile in the logarithmic region of the boundary layer. Measurements were made with a hot film anemometer. Tracking doubled the boundary roughness and decreased the critical entrainment velocity by 20%. Ambient or “free” sediments were more easily entrained than fecal mounds, which were restrained from movement by mucous adhesion between the fecal coils. Isolated pellets, such as those egested by Amphicteis scaphobranchiata, transported readily as bedload over a cohesive sediment surface.

Mean flow and turbulence scaling in a tidal boundary layer

Abstract Calculations based on data from 10 triplets of orthogonally oriented ducted impeller current meters vertically spaced near the bottom of an estuary in Puget Sound show that: (1) 10 min averaging reduces the uncertainties in the value of the various means to±3.6% for the velocity,± 8.8% for the friction velocity,±50% for the extrapolated zero-velocity height (roughness length), and±40% for the Reynolds stress (95% confidence); (2) because of the magnitude of the lowfrequency (tidal) variability, lengthening of the averaging period does not reduce the uncertainties; and (3) a quasi-stationary model suffices to relate the Reynolds stress to the mean velocity profile within the above uncertainties. Including the dynamic effects of temporal variability in the model does not produce a measurably better prediction.

Induction of suspension feeding in spionid polychaetes by high particulate fluxes

The feeding behavior of three species of spionid polychaetes varied with water velocity. At moderate flows the worms ceased deposit feeding, formed their feeding tentacles into helices, and lifted them into the water column to capture material in suspension. This behavior was apparently a response to increased flux of suspended matter at high flows rather than to flow velocity alone. Organisms capable of switching their feeding behavior may be common in dynamically variable benthic environments.

Effects of bacterial exopolymer adhesion on the entrainment of sand

Abstract Flow velocity required to erode a bed of acid‐washed sand is increased by intergranular adhesion resulting from growth of the benthic marine bacterium Al‐teromonas atlantica. In general, we find that either pure exopolymer alone or exopolymer generated during in situ growth increases erosion resistance of fine quartz sand. Moreover, the degree of erosion resistance increases in proportion to the concentration of exopolymer‐component uronic acids, which in turn is dependent on relative nitrogen content of peptone‐based growth media. Specifically, we observe that approximately 100 nmol of exopolymer or 1.5 nmol of component uronic acids generated by in situ bacterial growth under nitrogen‐rich conditions per gram of dry sediment can effectively double seawater‐flume flow velocity required for initiation of transport of otherwise noncohesive, 125–177 μm quartz grains. This maximal effect corresponds to an estimated adhesive force that exceeds submerged particle weight by an order of magnitude and ex...

Effects of benthos on sediment transport: difficulties with functional grouping

Abstract No consistent functional grouping of organisms as stabilizers vs destabilizers, respectively decreasing or enhancing erodibility, is possible. Benthic organisms can affect erodibility in particular—and sediment transport in general—via alternation (1) of fluid momentum impinging on the bed, (2) of particle exposure to the flow, (3) of adhesion between particles, and (4) of particle momentum. The net effects of a species or individual on erosion and deposition thresholds or on transport rates are not in general predictable from extant data. Furthermore, they depend upon the context of flow conditions, bed configuration, and community composition into which the organism is set. Separation of organism effects into these four categories does, however, allow their explicit incorporation into DuBoys-type and stochastic sediment dynamic models already in use and thus permits the specification of parameters whose measurement will enhance predictability of sediment transport modes and rates in natural, organism-influenced, marine settings. If the variable of prime concern is the total amount of sediment transported, rather than the frequency of transport events or the spatial pattern of erosion and eposition, and if most transport occurs in rare but intense bouts (e.g., winter storms on boreal continental shelves), then it may be possible to ignore organism effects without major sacrifices in accuracy or precision. Under high transport rates, suspended load effects override organism-produced bottom roughness, abrasion removes adhesives from transporting grains, and transport rates (normalized per unit width of the channel or bed) exceed feeding and pelletization rates. Moreover, at high rates most material transports as suspended load, effectively out of reach of the benthos. The transport rates at which organism effects are overridden, however, remain to be determined. For lower transport rates, foraging theory promises to provide insights into organism effects.

Enhanced deposition to pits: a local food source for benthos

Particle deposition experiments using mimics of biogenous negative relief (“pits”) and low-excess-density particles in a small annular flume indicate a significantly enhanced deposition rate (number of particles per time) compared to smooth, flat patches of the same diameter. This study included flow visualizations as well as observations of particle residence times, particle concentrations in the pits, and particle fluxes to the pits from the main flow. Experimental conditions of particle concentration, shear velocity, and particle settling velocity mimicked the dynamic characteristics (low excess density and large size) of organic-rich floes and flow conditions in the subtidal and deep sea where biogenous pits are common features. Results suggest that pits provide benthic organisms an important capture mechanism for such floes. Flow visualizations concur qualitatively with previously reported results for twodimensional cavity flow, with unique features due to the conical shape of the pits. When the Rouse number (settling velocity/shear velocity) was much less than 1, pit deposition rate increased with increasing pit aspect ratio (AR = depth/diameter; ranging from 0.25 to 2) and always exceeded deposition to a flat patch of comparable diameter. For the single aspect ratio tested (AR = 0.5) under conditions of increasing turbulence, deposition to the pit increased under transitional flow, but then decreased to near zero when conditions reached fully rough flow. Relative enhancement of deposition to this pit decreased with increased ambient bed roughness since grave1 beds also effectively collect particles. Particle concentration inside pits decreased weakly with pit aspect ratio but greatly increased with increasing roughness Reynolds number. Particle residence time increased somewhat with pit aspect ratio but decreased significantly with increasing roughness Reynolds number. Particle flux into pits from the main flow increased with both increasing aspect ratio and increasing roughness Reynolds number. Enhancement of food supply to pit inhabitants thus depends on the flow regime.

Predicting erosion resistance of muds

Abstract When measured or apparent yield stress of cohesive muds is assumed to reflect average particle-particle bond strength in network structures, critical bed shear stress required for grain-by-grain entrainment can be predicted from an analysis of the forces acting on component grains of geometrically-flat mud beds. Erosion resistance is then defined in terms of cohesive yield stress as well as nominal particle size, shape, relative density, and packing geometry. Predicted values of critical bed shear stress compare favorably with existing data from selected studies in which appropriate yield stress and hydrodynamic parameters have been reported.

Spectral Scaling in a Tidal Boundary Layer

Abstract The simple scaling of a tidal bottom boundary layer by the shear velocity, u*, and the wall to the wall describes well the mean Bow field. To test the full extent of this scaling measurements were made of the turbulence spectra in a natural tidal flow and a steady canal flow. The scaling of the turbulence spectra by the distance to the wall works only when the ratio of spectral rate to the mean shear is large. Under these conditions estimates of shear velocity u*, based on the dissipation derived from the magnitude of the inertial range of the spectra were found to agree to within 10 percent with estimates of the shear velocity from the mean velocity profiles and Reynolds stress. Within the 10 percent error bounds no effects ascribable to time dependence in the tidal spectra discerned, and hence the simple scaling of the momentum field by u* and z may be used for higher-order moments.

Encounter rate by turbulent shear of particles similar in diameter to the Kolmogorov scale

To clarify the rate at which particles similar in size to the smallest eddies in a turbulent fluid encounter one another via turbulent shear, 3-D video motion analysis was used to make direct measurements of relative velocities between closely spaced, near-neutrally buoyant, 700-μm mean diameter, polystyrene latex spheres suspended in an oscillating-grid turbulence tank . Smallest eddy size, termed the Kolmogorov scale, X, was estimated as (v 3/e)°u where v is fluid viscosity and a is the dissipation rate of turbulent kinetic energy . For runs made in water, the effective particle diameter examined was z 3-6 times larger than X . To measure relative velocities for particles just smaller than the Kolmogorov scale, the viscosity of the suspending fluid was increased = 25 times by the addition of Methocel, a commercially available, methyl cellulose synthetic gum used for fluid thickening . For runs made in Methocel, effective sphere diameter was = 0.2-0.5 times the Kolmogorov scale . Turbulent kinetic energy dissipation rate was estimated by traversing the measuring volume of a laser-Doppler velocimeter fiberoptic probe through the fluid at speeds high relative to the fluctuating fluid velocities in the tank . Resulting time series were used in analogy with instantaneous spatial series to calculate root-mean-square fluctuating velocities and integral length scales of turbulence, which in turn served as input for calculation of e . By examining the relationship between Reynolds number based on relative velocity between particles and particle separation distance relative to A, two competing hypotheses were tested . The first, that turbulent eddying motions control relative velocity between closely spaced particles, was accepted for particles both slightly larger and slightly smaller than the Kolmogorov scale (0 .05 < p < 0.10). The second, that viscous forces control relative velocity between particles, was strongly rejected in both cases (p = 0.004). The finding contradicts earlier assumptions and assertions that viscosity dominates small-scale particle interactions for sizes near the Kolmogorov scale, and it indicates that relative velocities between particles are greater than previously thought . Relative to biological mechanisms of particle encounter, turbulence therefore plays a role greater than is presently assumed in effecting encounter among particles and also between particles and organisms .

A SIMPLE MODEL OF FLOW-SEDIMENT-ORGANISM INTERACTION

Abstract Previous models of organism-sediment interaction have been limited primarily to eddy diffusion-type mixing. While such models are useful in interpreting some stratigraphic records, they cannot be expected to provide mechanistic insights into the diversity of interactions among sedimentary processes, organism activities, and boundary layer flows. To provide a more general formulation capable of dealing with all three types of processes in concert, a simple, discrete-time Markov model is generated. This version of the model deals explicitly only with particulate and not liquid phases. The initial formulation is a two-compartment, ergodic model of deposit feeding. Particle selection by deposit feeders results in incorporation of the selected particles into fecal pellets, and fecal pellet disaggregation completes the cycle. Particles which are more strongly selected or which are incorporated into more robust pellets thus spend more time and reach higher relative concentration in the fecal-pellet compartment. The next level of complexity is achieved by adding burial from the “free” sediment (non-fecal-pellet) compartment. This model predicts that particles which are more strongly selected or incorporated into more robust fecal pellets will have greater residence times in surficial sediments (both pellets and “free” sediments) before they become part of the stratigraphic record, and will therefore achieve higher concentrations in surficial sediments than will less preferred particles. The full flow-sediment-organism system is reached by adding the potential for lateral advection of particles. The probability of such advection is allowed to differ between pellets and “free” sediments. Increasing selection and robustness of fecal pellets then can either increase or decrease relative concentration and residence time of a given particle type in surficial sediments. The model thus provides a means of explaining differences in stratigraphic patterns among materials deposited simultaneously. It also suggests that deposit-feeder selection may increase the average age (i.e., time since initial deposition) of surficial sediments by maintaining preferred particle types near the surface. Selection may likewise control the texture of surficial sediments-in a manner which is easily confused with purely physical sorting. Parameters which require careful measurement in testing these simple predictions are selectivity by deposit feeders (probability of selection), rate of fecal-pellet breakdown under realistically simulated conditions (probability of pelletal disaggregation), differential erodibilities of pellets versus “free” sediments (relative probabilities of lateral advection), and rates of burial of “free” sediments versus pelletized sediments (probabilities of burial).