Physical Mixing Processes Research Articles

한국 서해 경기만 연안역에서 수질환경의 시.공간적 변화 특성과 조절 요인

Temporal (seasonal) and spatial distributions and variations of various physico-chemical factors (salinity, temperature, pH, DO, COD, SPM, POC, silicate, DIP, DIN) in surface and bottom waters were studied in the coastal environment with typical macro-tidal range and monsoonal weather condition, Gyeonggi Bay, west coast of Korea. Spacial distribution patterns of these factors were generally similar to each other, and appeared to be inversely related to the distribution pattern of salinity, suggesting that water quality of the study area was primarily controlled by the physical mixing process of Han-River freshwater with nearby coastal seawater. During flooding season, silicate- and nitrogen-rich Han River water directly flowed into offshore as far as <TEX>$20\sim30\;km$</TEX> from the river mouth, probably causing serious environmental problems such as eutrophication and unusual and/or noxious algal bloom, etc. Except the surface water during summer flooding season, high concentrations of nutrients appeared generally in dry season, whereas low values in spring, possibly because of the occurrence of spring phytoplankton bloom. On the other hand, nutrient flux through the estuary seems to be primarily depending on river discharge, sewage discharge and agricultural activities, especially during the rainy season. Also, nutrients in this coastal waters are considered to be supplied from the sediments of tidal-flats, which developed extensively around the Han-River mouth, especially during fall and winter of dry and low discharge seasons, possibly due to the stirring of tidal flat sediments with highly enriched pore-water nutrients by storm. And also, COD and DIN concentrations in the study area consistently increased during the last 20 years, probably because of agricultural activities and increasing discharge of industrial and domestic wastes.

Physical mixing effects on iron biogeochemical cycling: FeCycle experiment

The effects of physical processes on the distribution, speciation, and sources/sinks for Fe in a high‐nutrient low‐chlorophyll (HNLC) region were assessed during FeCycle, a mesoscale SF6 tracer release during February 2003 (austral summer) to the SE of New Zealand. Physical mixing processes were prevalent during FeCycle with rapid patch growth (strain rate γ = 0.17–0.20 d−1) from a circular shape (50 km2) into a long filament of ∼400 km2 by day 10. Slippage between layers saw the patch‐head overlying noninfused waters while the tail was capped by adjacent surface waters resulting in a SF6 maximum at depth. As the patch developed it entrained adjacent waters containing higher chlorophyll concentrations, but similar dissolved iron (DFe) levels, than the initial infused patch. DFe was low ∼60 pmol L−1 in surface waters during FeCycle and was dominated by organic complexation. Nighttime measurements of Fe(II) ∼20 pmol L−1 suggest the presence of Fe(II) organic complexes in the absence of an identifiable fast Fe(III) reduction process. Combining residence times and phytoplankton uptake fluxes for DFe it is cycled through the biota 140–280 times before leaving the winter mixed layer (WML). This strong Fe demand throughout the euphotic zone coupled with the low Fe:NO3− (11.9 μmol:mol) below the ferricline suggests that vertical diffusion of Fe is insufficient to relieve chronic iron limitation, indicating the importance of atmospheric inputs of Fe to this region.

Short-term and interannual variability of redox-sensitive chemical parameters in hypoxic/anoxic bottom waters of the Chesapeake Bay

A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn 2+ and Fe 2+ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry. Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn 2+ and Fe 2+. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn 2+ and Fe 2+ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.

Mixing and spreading in stratified flow

G. I. Taylor [Proc. R. Soc. London, Ser. A 219, 186 (1953)] quantified enhanced solute mixing in the flow through a tube at asymptotically long times by the constant Taylor dispersion coefficient, which provides a good representation of both the asymptotic dispersion dynamics and evolution of the solute concentration. At preasymptotic times, however, the use of the constant Taylor dispersion coefficient does not facilitate a faithful representation of either the actual mixing or spreading, which are controlling factors for chemical reaction rates. Transport in spatially varying flow fields often displays non-Fickian or anomalous behavior, which is reflected by the fact that effective dispersion evolves in time. Here we study and quantify the mechanisms leading to enhanced solute mixing in spatially nonhomogeneous flow fields using “local” spatial moments, i.e., moments of the transport Green function. On the basis of such a local moment formulation, we define effective dispersion coefficients to characterize effective preasymptotic solute spreading and mixing. We apply these concepts to the characterization of effective transport in general stratified flows, and illustrate them for the particular case of transport in a two-dimensional channel. We study effective mixing and spreading at preasymptotic times in terms of explicit analytical expressions for the effective dispersion coefficients as well as by numerical random walk simulations. We find that the vertically averaged concentration profiles contain little information on the physical mixing and spreading processes occurring at preasymptotic times. This leads us to define an alternative average, effective concentration, whose evolution is characterized by the effective dispersion coefficient and which reflects mainly the average effective solute mixing rather than purely advective spreading of the initial solute distribution.

Uranium distribution in the coastal waters and pore waters of Tampa Bay, Florida

The geochemical reactivity of uranium ( 238U) and dissolved organic carbon (DOC), Fe, Mn, Ba, and V was investigated in the water column, pore waters, and across a river/estuarine mixing zone in Tampa Bay, Florida. This large estuary is impacted both by diverse anthropogenic activity and by extensive U-rich phosphatic deposits. Thus, the estuarine behavior of uranium may be examined relative to such known U enrichments and anthropogenic perturbations. Dissolved (< 0.45 m) uranium exhibited both removal and enrichment processes across the Alafia River/estuarine mixing zone relative to conservative mixing. Such non-conservative U behavior may be attributed to: i) physical mixing processes within the river; ii) U carrier phase reactivity; and/or iii) fluid exchange processes across sediment/water interface. In the bay proper, U concentrations were ∼ 2 to 3 times greater than those reported for other estuarine systems and are likely a result of erosional inputs from the extensive, underlying U-rich phosphatic deposits. Whereas dissolved U concentrations generally did not approach seawater values (13.6 nM) along the Alafia River salinity transect, water column U concentrations exceeded 16 nM in select regions of the bay. Within the hydrogeological framework of the bay, such enriched U may also be derived from advective fluid transport processes across the sediment/water interface, such as submarine groundwater discharge (SGD) or hyporheic exchange within coastal rivers. Pore water profiles of U in Tampa Bay show both a flux into and out of bottom sediments, and average, diffusive U pore water fluxes ( J diff) ranged from − 82.0 to 116.6 mol d − 1 . It is likely that negative U fluxes imply seawater entrainment or infiltration (i.e., submarine groundwater recharge), which may contribute to the removal of water column uranium. For comparison, a bay-wide, Ra-derived submarine groundwater discharge estimate for Tampa Bay (8 L m − 2 d − 1 ) yielded an average, advective ( J SGD) U flux of 112.9 mol d − 1 . In Tampa Bay, the estuarine distribution of U indicates a strong natural, geologic control that may also be influenced by enhanced fluid transport processes across the sediment/water interface.

Composites Derived from Zirconia Nanopowders Stabilized with Yttria and Containing Alumina Particles Incorporated Physically or Chemically

Two methods of incorporating Al2O3 inclusions into a polycrystalline matrix of tetragonal zirconia stabilized with yttria (3Y-TZP) were used, and their influence on the microstructure and properties of TZP/Al2O3 composites has been studied. The first method consisted in the physical mixing of the component powders by means of intensive milling. The second method utilized the possibilities of the coprecipitation method of chemically homogeneous deposits followed by their calcination. An aqueous solution of zirconium, yttrium, and aluminum salts was used. Green compacts were shaped by cold isostatic pressing under a pressure of 300 MPa, and then they were sintered under no pressure for 2 h at 1500–1650°C in air. The powders were also consolidated using hot pressing under 25 MPa in argon and under the same heating cycle as in the case of pressureless sintering. The composites studied contained from 0 to 20 vol % of alumina inclusions, whose sizes depended on the powder preparation method. The morphology and phase composition of the powders and the microstructure of the green compacts and sintered materials were characterized. Bending strength, fracture toughness, and wear were measured. The coprecipitation method enabled us to produce composites that contained nanosize alumina inclusions. The inclusions derived from this method were much smaller than those derived from the physical mixing method (d 50 = 0.21 ± 0.01 and 0.32 ± 0.2 µm, respectively). The reduction in the alumina inclusion size nearly to the nanometer level did not increase the fracture strength and fracture toughness of the composites. The density of the composites and the size of microstructural flaws were the critical factors controlling the fracture strength. The highest strength value, namely, 1.7 ± 0.2 GPa, was measured for the TZP containing 5 vol % of alumina particles incorporated by means of the physical mixing process. The 20 vol % content of alumina particles increased the wear resistance of the 3Y-TZP materials by 51 and 41% for the physical and chemical methods of inclusion incorporation, respectively.

Relative influences of bioturbation and physical mixing on degradation of bloom-derived particulate organic matter: Clue from microcosm experiments

Chloropigment profiles observed in coastal marine sediments imply that the fate of bloom-derived organic matter is simultaneously affected by physical and biological mixing processes, but these influences are poorly differentiated by field measurements. To test relative influences of biological versus physical mixing processes on organic matter degradation, we conducted a series of microcosm experiments in three simulated mixing regimes: bioturbated, episodically physically mixed, and unmixed. Algal materials (uniformly 13C and 15N-labeled) were added on the surface of homogenized (pre-sieved) sediment cores as a simulation of natural deposition of bloom-derived organic matter (with an increase of 30–40 nmol chlorophyll- a in 1 g of dry sediment). Biological mixing was initiated by adding a group of macrofauna or individual species into the sediment cores while physical mixing was manipulated by mechanically stirring the surface sediment at variable frequency. We followed the time-dependent and depth-dependent variations of algal organic matter ( 13C-POC and 15N-PON) and chloropigments in simulated mixing regimes over 1-month incubations. The analytical results showed that algal organic matter and chlorophyll- a (Chl- a) degraded coincidentally but the degradation rates varied in different mixing regimes, which was likely related to variable redox conditions created by different mixing processes. In general, physical mixing immediately transported the fresh particulate organic matter from the oxic surface to the deep anoxic sediments, where 13C-POC, 15N-PON and Chl- a degraded at a 3–5× slower rate than those in the unmixed and bioturbated sediments. No matter how frequently the sediments were stirred, similar amounts of 13C-POC, 15N-PON, and Chl- a remained in all physically mixed cores after 1 month. On the other hand, biological mixing created an oscillating oxic/anoxic environment for degradation of algal organic matter through irrigation to deepen dissolved oxygen penetration and via reworking to mix particles between oxic and anoxic sediments. However, the influences of biological mixing on organic matter degradation varied with macrofaunal species and their behaviors. In the unmixed regime, 13C-POC, 15N-PON, and Chl- a from the added algal materials degraded under continuous oxic conditions on the sediment–water interface at similar rates to those in the bioturbated sediments.

Detailed Modeling of Gas Flow in Liquid Steel: Bubble Size Distribution and Voidage Calculation

Although the role of gas purging in liquid steel systems is well recognized, it has yet to be adequately analyzed. One key aspect of this process is the prediction of gas voidage in the bath, which has been studied in great detail beginning with water modeling in the early days and using advanced multiphase models more recently. Still, there are significant unresolved issues with gas purging systems. When gas is introduced through a nozzle at high flow rate, a jet may form which is undesirable. The break-up of this jet into bubbles is a separate topic of research. The more common practice in the steel industry is to use porous plugs for gas injection. Gas entry through a porous plug can be characterized by the stretched bubble regime, and the laws of coalescence and fragmentation used to analyze bubble column reactors are generally applicable. Calculation of the bubble size distribution is important for two reasons. First, the voidage distribution in the bath is significantly modified by the injection system and flow rates used, primarily due to changes in flow regime and bubble dynamics (collision, break-up, coalescence). Second, the voidage distribution directly determines the buoyancy, that influences the physical mixing process, and the specific-area-density, that influences surface reactions (for example, decarburization, desulfurization and nitrogen pick-up). In this paper, a numerical study is presented that combines a bubble dynamics model with an Eulerian multiphase model. The results of the simulation are compared with the experimental data from Anagbo and Brimacombe (1990). Relevant discussion and reviews will be presented to distinguish the differences of this detailed bubble dynamics model with the uniform bubble diameter approximations reported in various recent studies.

CHANGES IN TURBULENT MIXING SHIFT COMPETITION FOR LIGHT BETWEEN PHYTOPLANKTON SPECIES

The intriguing impact of physical mixing processes on species interactions has always fascinated ecologists. Here, we exploit recent advances in plankton models to develop competition theory that predicts how changes in turbulent mixing affect competition for light between buoyant and sinking phytoplankton species. We compared the model predictions with a lake experiment, in which the turbulence structure of the entire lake was manipulated using artificial mixing. Vertical eddy diffusivities were calculated from the measured temperature microstructure in the lake. Changes in turbulent mixing of the lake caused a dramatic shift in phytoplankton species composition, consistent with the predictions of the competition model. The buoyant and potentially toxic cyanobacterium Microcystis dominated at low turbulent diffusivity, whereas sinking diatoms and green algae dominated at high turbulent diffusivity. These findings warn that changes in the turbulence structure of natural waters, for instance driven by climate change, may induce major shifts in the species composition of phytoplankton communities.

Transport of plutonium ( 239/240Pu) and caesium ( 137Cs) in the Irish Sea: comparison between observations and results from sediment and contaminant transport modelling

A model for radionuclide transport in the marine environment is described and applied, in the Irish Sea, to the movement of caesium ( 137Cs) and plutonium ( 239/240Pu) discharged from Sellafield. The model resolves processes operating on tidal time scales, whilst being simple enough to run over decadal time periods. Modelled processes include transport in the water column, exchange of contaminants between dissolved and particulate phases, wave–current sediment resuspension, sediment transport and mixing of material within the seabed. Transport in the water column is by a combination of tide, wind and density-driven flows. Sediment transport is based on the erosion, advection and deposition of three sediment classes representing sand, flocculated mud/silt and fine background components. Transfer of radionuclides between the dissolved and sediment phases is implemented using rate equations. A layered seabed is incorporated with transfers between layers representing biological and physical mixing processes. We demonstrate that the model is able to reproduce observed concentrations of 137Cs and 239/240Pu in the water column, the build-up of 239/240Pu in the seabed over a 30-year time span, including recent changes in seabed inventory distributions. The model is used to identify and quantify the mechanisms responsible for this recent redistribution of 239/240Pu in the region. The results suggest that whilst sediment transport plays a role in redistributing contaminated sediment in the eastern Irish Sea, desorption, followed by transport in the dissolved phase and reabsorption onto particulate material, is the primary mechanism for redistribution of 239/240Pu.

Degradation of algal lipids in microcosm sediments with different mixing regimes

A series of microcosm experiments was conducted to examine the influence of different mixing processes on algal lipid degradation in surficial sediments. The experiments simulated 3 different mixing regimes: bioturbated, episodically physically mixed, and no mixing. Uniformly 13C- and 15N-labeled algal cells were deposited on the sediment-water interface to simulate natural deposition of bloom-produced organic matter. Degradation rate constants of individual algal lipids, effects of macrofuana species and physical mixing frequency on these rates, and the response of the microbial community were examined during one-month incubations. Our results showed that different mixing processes established different biogeochemical degradation regimes, characterized by variable redox conditions. Algal lipids degraded differently in different mixing regimes. Slower degradation was observed under episodic physical mixing because substrates were mixed deep into the sediment where anaerobic conditions prevailed. The degradation rates of algal lipids subjected to oscillating redox conditions during bioturbation were similar to those in unmixed cores where aerobic conditions dominate at the surface. Our laboratory results support the OET (oxygen exposure time) model based on field observations: longer residence of organic matter in the oxic zone leads to low preservation. Biological and physical mixing processes in surface sediments affect degradation and preservation of organic matter through different controls on redox conditions.

Seasonal dynamics of coastal ecosystems and export production at high latitudes: A modeling study

Export of organic matter from the surface to deeper waters often shows much smaller seasonal variations than primary production or nitrate‐based new production in mid‐ to high‐latitude marine systems. The mechanisms underlying this pattern remain poorly understood, but seasonal shifts in food web structure and dynamics have been implicated. We report here on an ecosystem modeling analysis of a high‐resolution (biweekly) time series of biomass, production, and export flux (sediment trap) measurements conducted in 1991 in Bonne Bay (Newfoundland). This time series shows the classical pattern of a spring bloom followed by a summer low biomass period, yet export is bimodal, with maxima during spring and late summer. The ecosystem model was forced by diagnostic vertical mixing calculations based on temperature and salinity records taken every 3 d and hourly wind data. The physical analysis indicated that the nitrate flux into the euphotic zone during summer was equivalent to that during the spring and fall seasons and accounted for half of the summer export. Statistical adjustments of the parameters of the ecosystem model indicated that strong production of dissolved organic carbon during the spring bloom, high temperature dependence of microbial activity, and physico‐chemical particle aggregation played key roles in explaining the remainder of the summer export. Seasonal changes in trophic pathways between spring and summer, such as a shift from a herbivorous to a microbial food web, played a comparatively smaller role. Our modeling analysis suggests that physical mixing processes and physico‐chemical aggregation processes are at least as important as shifts in food web trophic pathways in explaining the postbloom export flux in mid‐ to high‐latitude marine systems.

Appolonian Packing and Fractal Shape of Grains Improving Geomechanical Properties in Engineering Geology

—Fractal packing and highly irregular shaped particles increase the mechanical properties of rocks and building materials. This suggests that fractal methods are good tools for modeling particle mixes with efficient properties like maximum strength and maximum surface area or minimum porosity and minimum permeability. However gradings and packings are calculated by “Euclidean” disk models and sphere models. Surprisingly even the simplest models are far more complex than they appear. The fractal “Appolonian packing model” is proposed as the most universal two-dimensional packing model. However the inhomogeneity of gradings and the irregularity of natural grain shapes and surfaces are not reflected by these models. Consequently calculations are often far from empirical observations and experimental results. A thorough quantification of packings and gradings is important for many reasons and still a matter of intense investigation and controversial discussion. This study concentrates on fractal models for densely packed non-cohesive rocks, crushed mineral assemblages, concrete and asphalt mixtures. A summary of fractal grain size distributions with linear cumulative curves on log-log plots is presented for these mixtures. It is shown that fractal two-dimensional and three-dimensional models for dense packings reflect different physical processes of material mixing or geological deposition. The results from shear-box experiments on materials with distinct grain size distributions show a remarkable increase of the mechanical strength from non-fractal to fractal mixtures. It is suggested that fractal techniques need more systematical application and correlation with results from material testing experiments in engineering geology. The purpose of future work should lead towards the computability of dense packings of angular particles in three dimensions.

Phytoplankton species structure in the central North Pacific 1973-1996: variability and persistence

Acquisition of recent phytoplankton samples from JGOFS' station ALOHA and the Climax area prompted updating and re-evaluation of 23 years of floristic data from the summer central North Pacific environment. Recurrent group analysis clearly shows the dichotomy between shallow and deep associations. A new finding is that the deep flora consists of two related groups of species whose abundance centers are vertically displaced from one another. At the Climax area, the species structure in both shallow and deep strata persisted throughout the study period. Within each stratum, differences in rank orders of species' abundances between station ALOHA and the Climax area are no greater than expected from physical mixing processes. However, abundances of species in the deeper recurrent groups were significantly lower at ALOHA.

Equivariant adaptive selective transmission

In this paper, we consider the problem of selective transmission-the dual of the blind source separation task-in which a set of independent source signals are adaptively premixed prior to a nondispersive physical mixing process so that each source can be independently monitored in the far field. Following similar procedures for information-theoretic blind source separation, we derive a stochastic gradient algorithm for iteratively estimating the premixing matrix in the selective transmission problem, and through a simple modification, we obtain a second algorithm whose performance is equivariant with respect to the channel's mixing characteristics. The local stability conditions for the algorithms about any selective transmission solution are shown to be the same as those for similar source separation algorithms. Practical implementation issues are discussed, including the estimation of the combined system matrix and the reordering and scaling of the received signals within the algorithm. Mean square error-based selective transmission algorithms are also derived for performance comparison purposes. Simulations indicate the useful behavior of the premixing algorithms for selective transmission.

Mingling in the magmatic system of ischia (Italy) in the past 5 ka

In the past 5 ka volcanism has been very intense at Ischia, a densely populated and still active volcanic island in the Gulf of Napoli. Mineralogical, geochemical and isotopic investigations have been carried out on the pyroclastic deposits and lava flows of the historical eruptions of Fiaiano (530–960 A. D.) and Arso (1302 A. D.). These products are characterized by strong petro-chemical disequilibria. Mineralogical disequilibria are shown by the occurrence of resorbed and reversely zoned phenocrysts, and by the coexistence of phenocrysts and microlites with variable composition. Chemical inhomogeneity characterizes the glass of the juvenile fragments. Isotopic disequilibria have been detected among all separated mineral phases and whole-host rock. The detected disequilibria are interpreted as the result of a physical mixing process between variable fractionated and isotopically distinct batches of magmas. Petrochemical data suggest that mingling occurred between a highly crystallized alkali-trachytic and an isotopically distinct latitic to trachytic magmas, for both the eruptions. The pre-eruptive scenario proposed is the uprising of a latitic-to-trachytic magma (characterized by a Sr-isotope ratio of about 0.7068) from a deep reservoir and successive mingling with batches of more viscous, highly crystallized alkali-trachytic magma (with a Sr-isotope composition of about 0.7061) at shallow depth.

Surface seawater distributions of inorganic carbon and nutrients around the Galapagos Islands: results from the PlumEx experiment using automated chemical mapping

During the second leg (PlumEx) of the 1993 IRONEX cruise, the partial pressure of CO 2 and the concentrations of nitrate and silicate in the surface waters around the Galapágos Islands were continuously measured using automated underway systems. Based on salinity-versus-constituent mixing diagrams, physical mixing processes dominate the pCO 2 and nutrient distributions upstream of the Galápagos Islands. Downstream of the islands, slight removal of nitrate and CO 2 can be discerned because of the high resolution of the underway measurements. The high spatial resolution of the underway measurements allowed evaluation of fine features such as sharp fluorescence peaks on the “warm” side of frontal boundaries. In the waters immediately adjacent to Fernandina and Isabela islands (Bolivar channel), dramatic drawdown of pCO 2 and nutrients was measured, coincident with the highest measured levels of iron (3 nM) and chlorophyll (>13 μg l -1) (Martin et al., 1994). The nearly constant alkalinity of the waters was combined with the measured pCO 2 to calculate total carbon dioxide in the waters. Based on mixing diagrams, the ratio of ΔTCO 2 to ΔNO - 3 was found to be highly variable, ranging from approximately 6.4 to >10 in the waters near Isabela Island. The ratio of ΔTCO 2 to ΔNO - 3 is approximately 8.5 in the waters west of the Galápagos where slight removal of nitrate and TCO 2 occurs. In these waters, the physical process of mixing and CO 2 degassing due to warming of the water becomes significant relative to the biological uptake and the ratio is driven higher.

Original Articles: Sources of Dissolved Organic Carbon Supporting Planktonic Bacterial Production in the Tidal Freshwater Hudson River

Planktonic bacterial production in the tidal freshwater Hudson River is a major component of secondary productivity and is uncoupled from planktonic primary productivity. There are several major sources of allochthonous dissolved organic carbon (DOC) whose potential contribution to heterotrophic bacterial growth was examined with bioassays. Supply of DOC from the upper Hudson drainage basin and a large tributary in the mid-Hudson together comprise 70 kT DOC/year, which is the bulk of the DOC load to the tidal freshwater Hudson River. Two contrasting tidal wetlands contribute DOC to the main-stem river but were only a few percent of the tributary load even during summer low-flow conditions. The quantity of DOC released from fine sediments was intermediate to the other two loadings considered. Bacterial growth in bioassays receiving water from the sources varied, but differences in thymidine incorporation between reference and DOC sources were small, usually less than 2 nmol/L/h. Similarity in thymidine incorporation suggests that all sources of DOC were capable of supporting bacterial growth at approximately equal rates. Seasonal shifts in carbon availability were clear in several cases, for example, greater growth on wetland-derived DOC at times of peak plant productivity. Seasonal differences in tributary DOC bioavailability were not large despite the well-known seasonality of tributary inputs. Activities of a suite of extracellular enzymes were used as a biologically based characterization of DOC from the various sources. Shifts in allocation among enzymes were apparent, indicating that there are biologically relevant differences in composition among the sources. Fluorescence characteristics and absorbance per unit carbon also varied among sources, providing an independent confirmation of compositional differences among sources. The absence of large differences in bacterial productivity among sources suggests that growth is supported by a wide range of DOC, and the relative importance of the sources is probably related to the quantitative differences in inputs. Efforts to classify carbon supplies to ecosystems must recognize that organism plasticity in carbon use and physical mixing processes will both act to homogenize what might initially appear to be quite distinctive carbon inputs.

The Roles of Hydrographic and Biogeochemical Processes in the Distribution of Dissolved Inorganic Nutrients in a Scottish Sea-loch: Consequences for the Spring Phytoplankton Bloom

Observations taken in Loch Linnhe, a fjord on the west coast of Scotland in 1992, indicate that nutrient distributions are controlled by physical mixing processes (conservative behaviour), temporally varying end-member concentrations (apparently non-conservative behaviour) and biogeochemical processes (real non-conservative behaviour). One-dimensional steady-state mixing diagrams conventionally employed for estuarine nutrient studies cannot be used for Loch Linnhe because of the complex hydrography observed. A one-dimensional box model has been developed which accurately reproduces the hydrography of the system and has been used to isolate and quantify the different types of nutrient behaviour in the system: non-conservative behaviour is identified in the isolated bottom-waters of the basin and apparently non-conservative behaviour is identified at depths greater than 60m. Chlorophyll data show the onset of a phytoplankton bloom coinciding with a deep water renewal event. A similar sequence of events was also noted in 1990 and 1991 and it is suggested that these events are linked.

Pulsed-Accelerated-Flow Spectrometer with Position-Resolved Observation

The title instrument (PAF-PRO) permits the progress of rapid reactions to be monitored at 128 positions along a 2 cm observation cell as reactants flow down the cell. A decelerated push is used to give velocity changes from 21 to 2 m s(-1) during a time interval of 384 ms. Variation of flow velocity allows reaction rate constants to be resolved from the physical mixing process. The flow system brings the reacting solutions together in a 10-jet radial mixer 0.32 cm before the mixture enters the observation tube (0.203 cm width, 1.945 cm length). A masked charge-coupled device (CCD) is the array detector used to obtain transmittance data as a function of position and velocity. The masked portion of the CCD serves as a dynamic memory buffer for fast data acquisition. Pseudo-first-order rate constants are measured from 200 to 12 000 s(-1). The instrument also is calibrated for second-order reactions (equal concentrations) with initial half-lives of 0.3-1.5 ms. Applications of the PAF-PRO system for the study of fast multistep reactions are presented.