Resuspension Conditions Research Articles

Wind‐induced resuspension in a shallow lake from Medium Resolution Imaging Spectrometer (MERIS) full‐resolution reflectances

A lack of empirical evidence impedes assessment of the spatial and temporal extent of critical conditions for recurring high turbidity in large wind‐exposed shallow lakes. Here spatiotemporal variation in total suspended matter (TSM) concentration was captured by processing 30 Envisat Medium Resolution Imaging Spectrometer (MERIS) images of a shallow lake (Markermeer) with a spectral matching algorithm. The TSM maps showed elevated downwind concentrations for moderate winds (from 4 to 9 m s−1), which occur 68% of the time. Regressions confirmed the relationship between hourly averaged wind speed and TSM. To explore critical conditions for resuspension, wind speed, linear fetch, and water depth were combined in a spatial model based on simplified linear wave equations. Remotely sensed TSM patterns matched predicted areas of resuspension from these wave equations. On average, over 70% of cells were true positive or negative, with elevated TSM matching the predicted resuspending bottom area and background TSM matching no resuspension. Images acquired during moderate winds register local resuspension. This implies that under these conditions, a critical shear stress threshold for resuspension is passed, followed by upward mixing over the few meters of water column. Images acquired during low wind speeds (≤3 m s−1) either do not show a TSM pattern or display settling because it takes several hours of low wind before all particles are removed from the visible top layer. Because of the good spatial matching, the resuspension model can also be used for future verification of the retrieval capacity of the spectral matching algorithm.

Distribution of bioavailable phosphorus between overlying water and SPM under abrupt expansion condition

Experiments on Phosphorus (P) fraction characteristics in sediment resuspension were performed under adequate hydrodynamic conditions. It is found that the concentration of Suspended Particulate Matter (SPM) in the eddy current region exhibits the “Matthew effect”. Velocity is an impact factor of the Equilibrium Phosphate Concentration (EPC), which is related to other hydraulic conditions. Overall bioavailable dissolved P in the SPM causes migration to overlying water and sediment, eventually being converted into a chemical speciation of P. Conditions of resuspension promote Al-P of SPM that migrated to the sediment and water. Concentrations of Al-P in SPM are reduced. P is released from SPM to water bodies, mainly through conversion into particulate P and dissolved total P. Meanwhile, exchange between SPM and sediments occur mainly through Ca-P migration. Al-P and BD-P possess similar geochemical characteristics or source. Ca-P and Al-P exhibit a negative correlation between migration and conversion.

Contributions of phosphorus on sedimentary phosphorus bioavailability under sediment resuspension conditions

The contributions of phosphorus (P) to the bioavailability of sedimentary P under sediment resuspension conditions were evaluated using water soluble P, readily desorbable P, algal available P, and NaHCO 3-extractable P. Sediment resuspension resulted in a more efficient removal of dissolved inorganic P from the overlying water. As such, the incorporation of P into the sediments was greater under this condition compared with the static condition. The amount of bioavailable P extracted by each method and the proportion of each to the total P (Tot-P) in both sediments with and without resuspension varied. The contributions of input P to P bioavailability under sediment resuspension conditions were considerably lower than those under static conditions. A possible explanation for this is the strong oxidation and the higher contact probability between P and P reactants, such as crystalline iron, aluminum oxides, and organic matter, as a result of sediment resuspension. The amounts of bioavailable P and their proportions to Tot-P in sediments of Lough Erne (deep) and Lake Taihu (shallow) can verify these hypotheses.

Sedimentary phosphorus fractions and bioavailability as influenced by repeated sediment resuspension

Concentrations of phosphorus (P) fractions and changes in their bioavailability in the sediments as influenced by repeated resuspension were determined by sequential fractionation in laboratory experiments. The water and sediment samples used were taken from the campus canal. Sequential fractionation indicated that the concentrations of the iron bound P (BD–P) were predominant, consisting of over 50% of total P (Tot-P) in the sediments that did and did not undergo resuspension. BD–P mobility was reduced due to resuspension resulting from the decline of the proportion ratio of non-occluded Fe–P and occluded Fe–P from 0.53 to 0.29. Therefore, under sediment resuspension conditions, using the sum of loosely sorbed P (NH 4Cl–P), BD–P, aluminium bound P (Al–P), and organic-P (NaOH–nrP) to estimate bio-available P (BAP) might be problematic. However, BAP could be accurately estimated by the sum of NH 4Cl–P, % BD–P (bio-available, non-occluded Fe–P), and NaOH–nrP. By this estimation, the amount of BAP in the sediments as influenced by repeated resuspension decreased by about 10% of Tot-P, compared with the initial state (raw sediments). The results suggest that repeated resuspension could accelerate the transformation of P from mobile fractions to refractory fractions, which can be attributed to the increase of occluded Fe–P, Al–P, and calcium bound P (HCl–P).

Sediment resuspension and erosion by vortex rings

Particle resuspension and erosion induced by a vortex ring interacting with a sediment layer was investigated experimentally using flow visualization (particle image velocimetry), high-speed video, and a recently developed light attenuation method for measuring displacements in bed level. Near-spherical sediment particles were used throughout with relative densities of 1.2–7 and diameters (d) ranging between 90 and 1600 μm. Attention was focused on initially smooth, horizontal bedforms with the vortex ring aligned to approach the bed vertically. Interaction characteristics were investigated in terms of the dimensionless Shields parameter, defined using the vortex-ring propagation speed. The critical conditions for resuspension (whereby particles are only just resuspended) were determined as a function of particle Reynolds number (based on the particle settling velocity and d). The effects of viscous damping were found to be significant for d/δ<15, where δ denotes the viscous sublayer thickness. Measurements of bed deformation were obtained during the interaction period, for a range of impact conditions. The (azimuthal) mean crater profile is shown to be generally self-similar during the interaction period, except for the most energetic impacts and larger sediment types. Loss of similarity occurs when the local bed slope approaches the repose limit, leading to collapse. Erosion, deposition, and resuspension volumes are analyzed as a function interaction time, impact condition, and sediment size.

Assessment of the deposition of polluted sediments transferred by a catastrophic flood and related changes in groundwater quality

Summary In the second half of the 20th century, industry and agriculture in the mountainous part of the Czech reaches of the Elbe caused much water pollution and a high suspended load. A reservoir with a total volume of 92,00,000 m 3 was built in 1914 to close the mountainous part of the watershed and to prevent floods endangering the cities downstream. Approximately 20% of its volume has now been filled with polluted sediments. These could potentially be transported downstream and settled in the flood plain, and could thus also influence the quality of the ground water. This paper sets out to model such an event. The authors carried out a radar investigation to find where the sediments were located and to compute the amounts of sediments. Samples were taken for quality analyses. The re-suspension conditions in the reservoir were computed using hydraulic methods, and a hydrograph of a catastrophic 500-year flood ( Q 500 ) was constructed by stochastic hydrology. The upgraded FAST 2D dynamic flow model was used to compute a map of the sediments settled in the flood plain, and the movement of the pollution below the ground was then described using the MT3DMS model. The principal result is a simulated prediction of the concentration of zinc (as a general representative of heavy metals) in different aquifer layers after 10 and 20 years. The results do not show a severe risk according to the Czech national guideline for remediation.

Photochemical changes in chemical markers of sedimentary organic matter source and age

Several chemical markers of organic matter source and age are shown to be susceptible to light-induced alteration. To test for the photochemical lability of markers previously employed for sediments from the Louisiana coastal zone, we subjected sediments under resuspension conditions to simulated sunlight, and monitored changes in C:N and Br:OC ratios, δ 13C, Δ 14C, and lignin composition. Markers of terrigenous origin (high C:N, lignin) decreased and δ 13C became enriched in sediments containing primarily terrigenous organic matter, while a marker of marine organic matter (Br:OC) decreased in samples containing significant contributions from this source. Preferential loss of 14C from all sediments indicated enhanced photochemical lability of organic matter of relatively recent origin, consistent with the changes in chemical markers. Most, but not all, experimental alterations are consistent with field distributions of these markers. Relatively small experimental changes in the markers in combination with confounding processes in the environment, however, prevent these parallel trends from providing any more than a consistency test for the importance of photochemical reactions in this region.

Modeling sediment impact on the transport of fecal bacteria

Sediment storage of fecal bacteria has been observed for a long time. However, limited modeling efforts have been conducted to dynamically and explicitly study the impact of sediment on the transport of fecal bacteria. In this paper, a modeling framework based on the Environmental Fluid Dynamics Code (EFDC) model has been proposed. Free and particle-associated fecal bacteria can be modeled explicitly using the developed model. A linear adsorption relationship is incorporated into the model to obtain a sediment concentration dependent attachment ratio. The deposition and resuspension fluxes of fecal bacteria across the sediment bed–water interface are calculated coupled with the sediment deposition and resuspension flux. The model was applied to hypothetical scenarios to evaluate the sediment impacts under ideal settling and resuspension conditions. A case study was provided to test the resuspension calculation of fecal bacteria under an artificial flooding condition. Using this model, the contributions of fecal bacteria from sediment bed and from the watershed can be modeled separately and explicitly.

Effect of diffusion and resuspension on nutrient release from submerged sediments

Nutrient release from a coarse-sand (CS) and a fine-sand (FS) estuarine sediment into the overlying water column was studied under static (diffusion-controlled) and dynamic (resuspension) conditions. Resuspension increased the amounts of and P released from the sediment cores compared to those released by diffusion alone due to direct release of these nutrients from the readily available pore-water and ion exchange fractions. The average effective and P diffusion coefficients (D e) were found to be similar for both sediments, and did not vary significantly between static and dynamic conditions. Values of D e were typically of the order of 10−5 to 10−6 cm2 s−1. The average daily and P diffusive flux density from CS and FS into the overlying water column of the study area was estimated to be 24.6, 0.02 and 0.39 mg m−2 d−1, and 66.9, 0.017 and 0.002 mg m−2 d−1, respectively. These fluxes suggest that nutrient release from the sediments via diffusion and small-scale resuspension events may represent only a minor source of and P to the water column of the study area.

Resuspension of Sediment‐Associated Escherichia coli in a Natural Stream

In this study, a tracer bacteria was used to investigate the resuspension and persistence of sediment-associated bacteria in a small alluvial stream. The study was conducted in Swan Creek, located within the Grand River watershed of Ontario, Canada. A 1.1-m2 section of the bed was seeded with a strain of Escherichia coli resistant to nalidixic acid (E. coli NAR). The survival, transport, and redistribution of the tracer bacteria within a 1.7-km river section downstream of the source cell was assessed for a 2-mo period following the introduction of the tracer bacteria. This study has illustrated that enteric bacteria can survive in bed sediments for up 6 wk and that inactivation of the tracer bacteria resembled typical first-order decay. Critical conditions for resuspension, as well as resuspension rates, of sediment-associated bacteria were determined for several storm events. The critical shear stress for E. coli NAR resuspension in Swan Creek ranged from 1.5 to 1.7 N m(-2), which is comparable with literature values for critical shear stresses for erosion of cohesive sediments. Bacteria resuspension was primarily limited to the rising limb of storm hydrographs implying that a finite supply of sediment-associated bacteria are available for resuspension during individual storm events. The information presented in this paper will further the development of representative microbial water quality models.

Variability of parameters measured during the resuspension of sediments with a particle entrainment simulator

The release of contaminants from sediments is an important problem facing environmental managers concerned with issues such as maintenance dredging, habitat restoration and dredge spoil placement. While there are laboratory methods to assess the remobilization of contaminants from resuspended sediments, little is known about how their operating characteristics influence a sediments response to resuspension. In this study, a particle entrainment simulator (PES) for resuspending sediments was evaluated by assessing variability among replicates and over operation time (12 h) for three estuarine sediments. These sediments varied in physical and chemical properties as well as the degree of metal contamination. Results showed that under identical resuspension conditions, there was low variability among sediment replicates. Sediment properties and the degree of metal contamination affected the magnitude and variation in water column parameters and contaminant release. Fine-grained sediments affected water column parameters, which stabilized over several hours of resuspension. While metal concentrations in the aqueous and particulate samples varied throughout operation, variation was highest for Cd and Ni. Compared to other methods used to evaluate the effects of sediment resuspension, the PES has operational advantages such as the ability to monitor water column conditions and resuspend sediment at specified energy levels. Overall, the PES provided reproducible conditions of resuspension effects among sediments varying in composition and contamination, demonstrating its value as a tool to evaluate contaminant mobility.

PHOSPHORUS FRACTIONATION AND SHORT-TERM MOBILITY IN THE SURFACE SEDIMENT OF A POLYMICTIC SHALLOW LAKE TREATED WITH A LOW DOSE OF ALUM (COURTILLE LAKE, FRANCE)

Lake Courtille was a polymictic eutrophic lake prone to large pH variations and sediment resuspension. Short term P release (96 hr) under laboratory resuspension conditions for two sediment sample types, surface sediments which represent sediment accumulated over several years and trap sediments which represent current year sediment, was studied. The experiments were carried out in oxic conditions at different pH values. According to phosphorus fractionation, Fe-bound-P, Al- bound-P and Organic-bound-P comprised the largest phosphorus pool (80% of Total Phosphorus). Phosphorus release was from the Fe-P, Al-P and organic fractions. These represent 35% of the total phosphorus content at pH 10. At circumneutral pH, in oxic conditions, a lower quantity of P was released (roughly 4% of total P sediment content) than at pH 10. The organic matter mineralization occurring at the end of summer can also influence P release as illustrated by the difference in total phosphorus content between sediment samples of 2000 and of the last five years. This type of P release was observed in Lake Courtille. High soluble reactive phosphorus content was measured in the water column at the end of summer. Aluminium treatment was thus not efficient in preventing P release. Moreover, a higher alum dose was needed to reduce P content in the water column in order to decrease primary productivity and subsequent organic matter sedimentation.

Resuspension by saline and particle‐driven gravity currents

Saline and particle‐driven gravity currents play an important role in resuspension by environmental and industrial flows. We develop a model of resuspension by unsteady buoyancy‐driven flows by bringing together results from the large body of literature dealing with the dynamics of gravity current and resuspension in channel flows. The criterion for resuspending material of density ρp and diameter b is expressed in terms of a modified Shields parameter, Θ = ρfu2/(ρp − ρf)gb, where u is the depth‐averaged velocity within the current and ρf is the bulk fluid density. The critical modified Shields parameter, Θc, which corresponds to the condition when erosion is initiated, is a function of the particle Reynolds number, Rep = bυT/ν, expressed in terms of the terminal fall velocity of the sediment particles υT. Particles characterized by Rep ≫ 1 are resuspended when Θ > Θc ≈ 0.04/cD where cD is the bed drag coefficient. According to this resuspension criterion, a two‐dimensional saline gravity current generated by the release of a volume V per unit width of fluid density ρc in an ambient fluid ρa resuspends material over an erosive distance which scales as . These results are extended to describe resuspension by particle‐driven gravity currents, an analysis complicated by the change of bulk density of the suspension with time as particles sediment from the current. Here the erosive length scale is bounded by the maximum extent of the particle‐driven gravity current. A general form for the vertical mass flux qd is assumed, and the total mass resuspended is calculated in terms of the initial characteristics of the gravity current and shown to be a function of the volume V of dense fluid released, independent of the geometry of release, and independent of gravitational acceleration. Complementary laboratory experiments of resuspension by two‐dimensional saline and particle‐driven gravity currents are presented. These experiments consisted of a lock release of dense fluid running over a layer of particles. The critical conditions for resuspension were determined for different materials, and the variation of the erosive distance with gravity current characteristics was studied. These observations are discussed with reference to the theoretical model.

Numerical model of frazil ice and suspended sediment concentrations and formation of sediment laden ice in the Kara Sea

A one‐dimensional (vertical) numerical model of currents, mixing, frazil ice concentration, and suspended sediment concentration has been developed and applied in the shallow southeastern Kara Sea. The objective of the calculations is to determine whether conditions suitable for turbid ice formation can occur during times of rapid cooling and wind‐ and wave‐induced sediment resuspension. Although the model uses a simplistic approach to ice particles and neglects ice‐sediment interactions, the results for low‐stratification, shallow (∼20‐m) freeze‐up conditions indicate that the coconcentrations of frazil ice and suspended sediment in the water column are similar to observed concentrations of sediment in turbid ice. This suggests that wave‐induced sediment resuspension is a viable mechanism for turbid ice formation, and enrichment mechanisms proposed to explain the high concentrations of sediment in turbid ice relative to sediment concentrations in underlying water may not be necessary in energetic conditions. However, salinity stratification found near the Ob' and Yenisey Rivers damps mixing between ice‐laden surface water and sediment‐laden bottom water and probably limits incorporation of resuspended sediment into turbid ice until prolonged or repeated wind events mix away the stratification. Sensitivity analyses indicate that shallow (≤20 m), unstratified waters with fine bottom sediment (settling speeds of ∼1 mm s−1 or less) and long open water fetches (>25 km) are ideal conditions for resuspension.

Mobilization of PAHs and PCBs from In-Place Contaminated Marine Sediments During Simulated Resuspension Events

A particle entrainment simulator was used to experimentally produce representative estuarine resuspension conditions to investigate the resulting transport of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) to the overlying water column. Contaminants were evaluated in bulk sediments, size fractionated sediments, resuspended particulate material and in some cases, dissolved phases during the experiments. The two types of sediments used in the experiments, dredged material and bedded estuarine sediment, represented gradients in contaminant loadings and sediment textural characteristics. For the bedded sediment, resuspension tended to winnow the sediments of finer particles. However, in the case of the more highly contaminated dredge material, non-selective resuspension was most common. Resuspension resulted in up to orders of magnitude higher particle-bound organic contaminant concentrations in the overlying water column. Dissolved phase PAH changes during resuspension were variable and at most, increased by a factor of three. The sifting process resulted in the partitioning of fine and coarse particle contaminant loading. For bedded sediments, accurate predictions of PAH and PCB loadings on resuspended particles were made using the mass of resuspended particles of different sizes and the concentrations of contaminants in the particle pools of the bulk sediment. However, due possibly to contributions from other unmeasured particles (e.g. colloids), predictions were not possible for the dredge material. Thus, knowledge of the redistribution and fate of colloids may be important. The partitioning of PAHs between the dissolved and particulate phases during resuspension events was predicted to within a factor of two from the amount of organic carbon in each of the resuspended samples. These experiments show that contaminant transport is a function of the chemistry and textural characteristics of the bulk sediment and the winnowing action during resuspension events. Existing relationships of resuspension and settling velocity are probably useful in predicting chemical redistribution.

浅い内湾底泥の巻き上げが栄養塩溶出に及ぼす影響

The effect of resuspension on nutrient release from sediments was studied by field observations and laboratory experiments. Field observations were carried out in two shallow estuaries, Matsushima Bay and Shiogama Port. Three laboratory experiments reflecting the conditions of continuous resuspension, intermittent resuspension and a static state were carried out using estuarine sediment cores. In the estuary, a fluid mud layer was formed just above the sediment and an organic surface layer with an ignition loss of over 20% was found at the top of the sediment in the summer, which increased the concentration of suspended solids during resuspension. The resuspension caused by a tidal current began a little before the low tide and continued until the high tide. It was found that the release rate of dissolved inorganic nitrogen under conditions of continuous resuspension was higher than that under a static state, because the release of ammonium nitrogen from pore water in the sediments occurred simultaneously with that from the sediment in the former case. The nitrification rate under conditions of intermittent resuspension was lower than that in the other two systems. On the other hand, phosphate release was not observed under an aerobic static state. It was also recognized that the concentration of phosphate decreased at the beginning of resuspension, and increased thereafter.

Resuspension in the Kattegat: Impact of variation in wind climate and fishery

The recurrence of various agents of fine sediment resuspension in the Kattegat Sea, notably wind-induced wave action and demersal trawling, and their seasonal variation and long-term trends have been estimated. A comparison between the sediment-water interface and the spatial distribution of theoretical wind/wave impact indicated that the sediment is resuspended by further agents at depths below the permanent halocline between 10 and 20 m depth. The climatic deterioration in 1940–1970 had a significant influence on the recurrence of wind-induced resuspension, although this was mostly limited to a 10–30% shortening of the recurrence period. On the other hand, a quantification of the effect of demersal trawling in the same period suggests a total shortening of recurrence at bottoms below the halocline by 75–85% in the autumn and winter, and with one order of magnitude (90%) in spring and summer, compared with the effect of waves only in the 1930s; resuspension caused by near-bottom uni-directional currents was not quantified. At these deeper bottoms, resuspension conditions have approached those prevailing at shallow bottoms above the halocline.

Flocculation and de-flocculation of suspended matter in estuaries

Flocculation and de-flocculation have been observed in estuaries and the processes involved are discussed. Salt flocculation plays a minor role, if any. Flocs are mainly formed by a combination of two groups of processes: those bringing particles together, and those keeping them together. The first group is related to the flow conditions in the water and to organisms that clog particles into faeces or pseudofaeces or attach them to mucus. The second is related to the formation of sticky organic matter by a variety of organisms. Two main types of flocs have been observed: microflocs with a diameter up to 125 μm, and macroflocs with a maximum size of 3–4 mm. Microflocs consist of mineral particles and organic matter. Their formation is strongly related to the origin of the organic matter, to the organisms producing it, and to conditions of (temporary) deposition and resuspension. Mucopolysaccharides, produced by bacteria, algae and higher plants and mobilized from the suspended matter at low salinity, are important in glueing particles together. The microflocs, together with single mineral particles, are the basic building units of macroflocs. Macroflocs are fragile and formed in the water under conditions of viscous flow. Their maximum size is determined by turbulent shear, the size of the smallest turbulent whirls on the Kolmogorov scale being of the same order as the maximum floc size. They are easily destroyed during sampling. In estuaries the microflocs of fluvial origin are broken up by organisms consuming the organic matter in the microflocs, and reflocculated into new microflocs with newly formed organic matter of estuarine origin. The size of the microflocs, as seen under a microscope or determined by size analysis, is likely to be determined mainly by the binding strength of the organic matter in relation to the total mass of the floc and the physical disturbance produced during sampling and size analysis.

Distribution and particle size of suspended matter in the Southern Bight of the North sea and the Eastern channel

The distribution of suspended matter content in the Southern Bight is related to the resultant current pattern. Suspended matter is concentrated in the coastal waters and high concentrations occur where resultant currents of different directions meet near to the large gyres off the Belgian-Dutch coast and off East Anglia. Suspended matter is chiefly supplied to the Southern Bight from the Channel through Strait Dover-Calais (at least 8 to 11.5 × 10 6, probably 11.5 to 15 × 10 6 tons· a −1. Other sources are the rivers that enter the Southern Bight ( ca 2 × 10 6 tons·a −1), coastal erosion ( ca 0.3 × 10 6 tons·a −1), erosion of the sea floor (in the order of several million tons·a −1) and the atmosphere ( ca 0.5 × 10 6 tons·a −1). Supply from primary production is small (less than 1 × 10 6 tons·a −1). Total supply of suspended matter to the Southern Bight is 17.5 to 21.5 × 10 6 tons·a −1. The concentration of suspended matter in the coastal waters is probably related to a shoreward displacement of bottom water. Particle size determinations were made (with a Coulter Counter) during the winter in the absence of living plankton. Measurements were made directly after sampling or within several hours. In the Belgian-Dutch Coastal Water particle size is somewhat finer than in Strait Dover-Calais and in the central part of the Southern Bight (the “Channel Water”). In the English Coastal Water particle size is more variable. The bulk of the suspended particles has a diameter of 5 to 20 μm but there is a variable admixture of particles of 1 to 3 μm. Most size distributions are a mixture of two log-normal distributions. In the Belgian-Dutch coastal water, in the eastern Channel and in Strait Dover-Calais regularly approximately log-normal distributions are present. The size distributions reflect the multiple origin of the suspended material and the variable conditions of deposition, resuspension and aggregation.

On the nature of the ribonucleic acid synthesized in the presence of chloramphenicol

1. 1. Two of the “abnormal” properties of chloramphenicol-RNA can be readily explained by the fact that some of the protein-free chloramphenicol-RNA is easily removed from the high-speed particles. 2. 2. A third property, degradation of the chloramphenicol-RNA following chloramphenicol removal, was found to be dependent on the conditions of resuspension of the cells rather than on an inherent instability of the RNA. 3. 3. The unstable chloramphenicol-RNA can be converted to a stable form in the presence of amino acids. Evidence for a similar conversion of an unstable RNA formed in the absence of chloramphenicol was found. 4. 4. The conversion appears to involve the synthesis of a special protein fraction. This protein, formed in the complete absence of detectable RNA metabolism, is preferentially synthesized and found in the high-speed pellet. The acid-soluble proteins are also rapidly synthesized under these conditions (following RNA accumulation in the presence of chloramphenicol). 5. 5. An hypothesis involving a three-step pathway for RNA synthesis, blocked at the first step by chloramphenicol, is consistent with many of the observations involving chloramphenicol inhibition.