Unknown Fraction Research Articles

Aquaporin-4 (AQP4) Associations and Array Dynamics Probed by Photobleaching and Single-molecule Analysis of Green Fluorescent Protein-AQP4 Chimeras

The plasma membrane assembly of aquaporin-4 (AQP4) water channels into orthogonal arrays of particles (OAPs) involves interactions of AQP4 N-terminal domains. To study in live cells the site of OAP assembly, the size and dynamics of plasma membrane OAPs, and the heterotetrameric associations of AQP4, we constructed green fluorescent protein (GFP)-labeled AQP4 "long" (M1) and "short" (M23) isoforms in which GFP was inserted at the cytoplasm-facing N or C terminus or between Val-141 and Val-142 in the second extracellular loop of AQP4. The C-terminal and extracellular loop GFP insertions did not interfere with the rapid unrestricted membrane diffusion of GFP-labeled M1 or the restricted diffusion and OAP assembly of GFP-labeled M23. Photobleaching of brefeldin A-treated cells showed comparable and minimally restricted diffusion of M1 and M23, indicating that OAP assembly occurs post-endoplasmic reticulum. Single-molecule step photobleaching and intensity analysis of GFP-labeled M1 in the absence versus presence of excess unlabeled M1 or M23 with an OAP-disrupting mutation indicated heterotetrameric AQP4 association. Time-lapse total internal reflection fluorescence imaging of M23 in live cells at 37 degrees C indicated that OAPs diffuse slowly (D approximately 10(-12) cm(2)/s) and rearrange over tens of minutes. Our biophysical measurements in live cells thus reveal extensive AQP4 monomer-monomer and tetramer-tetramer interactions.

Two-dimensional tomography for gas concentration and temperature distributions based on tunable diode laser absorption spectroscopy

A tomography system is presented that uses wavelength-scanned direct absorption of two transitions of a target species (NH3 in the demonstration experiment) to determine the distributions of gas concentration and temperature. The absorption measurements are performed simultaneously from four platforms that each rotate a beam from a single laser through an 11° arc, acquiring a data set from all four laser platforms in 100 ms to enable observation of dynamic flow events. The laser is wavelength scanned through two absorption transitions with different internal energy producing two sets of equations with species mole fraction and temperature as independent variables. The mole fraction and temperature distributions are reconstructed using the algebraic reconstruction technique (ART) for this set of incomplete projections. A numerical simulation is used to evaluate the measurement accuracy for measurements of an NH3 mixture escaping from an open pipe. This phantom distribution is then realized in the laboratory and the measurement strategy is demonstrated using a tunable diode laser absorption spectroscopy (TDLAS) measurement using a single laser near 1.5 µm to scan adjacent transitions in NH3. The reconstruction of NH3 concentration and gas temperature is compared with independently determined values to illustrate the fidelity of the tomographically reconstructed distributions for the NH3 mole fraction assuming a fixed temperature and for unknown mole fraction and temperature. Potential extensions of this research in the future include evaluation of other reconstruction algorithms and investigation of the dynamic distribution of various gases for combustion diagnostics.

Geochemical Resilience of a Ferralsol Subjected to Anoxia and Organic Matter Amendment

The spreading of vinasse on soils may favor not only anoxia but also mineral alterations and metal mobilization. Our aims were to record the geochemical changes occurring in a ferralsol subjected to vinasse input and anaerobiosis, and to check for soil resilience after a return to aerobic conditions. Soil slurries were therefore incubated under successive 7 d of aerobic, 0 to 28 d of anaerobic, and 28 d of aerobic conditions, vinasse being supplied at one of three levels when anaerobiosis began. At several dates, the slurry solution was characterized (pH and organic and mineral compounds) and Mn and Fe oxidation states in solids were assessed. Before incubations, about 20% of the Fe in solids was already reduced, whereas almost all Mn was found as Mn(IV). During the first aerobic period, mobilized metals included Pb, Zn, Ni, Cu, and Cr. After vinasse input during the first 14 d of anaerobiosis, the principal biotransformations were fermentation and true acetogenesis; Fe and Mn were mobilized during this period (up to 4.05 and 6.2 mmol L−1, respectively), and most Mn and an unknown but small fraction of Fe in solids were reduced. During the subsequent 14 d of anaerobiosis, only acetoclastic methanogenesis was observed. The return to aerobic conditions led to rapid oxidative immobilization of Fe, desorption of exchangeable Fe(II), and the partial immobilization of other metals (Ca, Mg, K, Na, Mn, Pb, and Ni). Manganese was not oxidized, and there was no return to the initial conditions for Fe and Mn in solids.

Quantifying Predation on Galaxiids and Other Native Organisms by Introduced Rainbow Trout in an Ultraoligotrophic Lake in Northern Patagonia, Argentina: A Bioenergetics Modeling Approach

AbstractExotic rainbow trout Oncorhynchus mykiss support an economically valuable recreational fishery in Patagonia but also create concern for impacts on native organisms. These concerns are intensified by the possibility of hatchery release programs in this region. We estimated losses of different prey from predation by rainbow trout in Lake Moreno, Río Negro Province, Argentina, using a bioenergetics model combined with input data from directed sampling on growth, seasonal diet, distribution, and thermal experience. The fish community was sampled seasonally using gill nets, hydroacoustics, and ichthyoplankton nets. Pelagic galaxiid larvae and benthic juvenile and adult small puyen Galaxias maculatus were the most important components of the diet. Bioenergetics simulations showed that over a 6‐year life span in the lake (ages 1–7), rainbow trout attained a body mass of 2.3 kg and consumed 74.7 kg of food, of which 20% consisted of galaxiid larvae and 16% consisted of adult small puyen. Based on an estimated abundance of 29,000 rainbow trout of ages 1–7, this predator exerted significant but sustainable mortality on the native prey populations, consuming 44 metric tons or an estimated 23% of the annual larval galaxiid production and 35 metric tons of adult small puyen, which represented an unknown fraction of the postlarval population. Galaxiids supported the estimated predation demand under current conditions. However, simulations of stocking strategies normally proposed for this region showed that consumption demands on prey would increase to unsustainable levels, reducing native fish populations and likely reducing growth of rainbow trout. It is also probable that the fish community composition would shift further in response to the increased demand for prey by stocked predators. This implies that in some cases, stocking could jeopardize sport fisheries; stocking strategies should be evaluated on a case‐by‐case basis to be consistent with specific objectives for native fish conservation and sustainable food web interactions.

An improved Heaviside approach to partial fraction expansion and its applications†

In this note, we present an improved Heaviside approach to compute the partial fraction expansions of proper rational functions. This method uses synthetic divisions to determine the unknown partial fraction coefficients successively, without the need to use differentiation or to solve a system of linear equations. Examples of its applications in indefinite integration, inverse Laplace transforms and linear ordinary differential equations are included. †This work is an extended version of my paper presented at the International Conference of Applied and Engineering Mathematics (ICAEM) held at London, UK, on 2–4 July 2008.

A discretization of the phase mass balance in fractional step algorithms for the drift-flux model

We address in this paper a parabolic equation used to model the phase mass balance in two-phase flows, which differs from the mass balance for chemical species in compressible multicomponent flows by the addition of a nonlinear term of the form ∇ · ρφ (y)u r , where y is the unknown mass fraction, ρ stands for the density, φ is a regular function such that φ(0) = φ(1) = 0 and u r is a (not necessarily divergence free) velocity field. We propose a finite-volume scheme for the numerical approximation of this equation, with a discretization of the nonlinear term based on monotone flux functions. Under the classical assumption that the discretization of the convection operator must be such that it vanishes for a constant y, we prove the existence and uniqueness of the solution, together with the fact that it remains within its physical bounds, i.e., within the interval [0, 1]. Then this scheme is combined with a pressure correction method to obtain a semi-implicit fractional step scheme for the so-called drift-flux model. To satisfy the above-mentioned assumption a specific time stepping algorithm with particular approximations for the density terms is developed. Numerical tests are performed to assess the convergence and stability properties of this scheme.

Storage of CO2 in saline aquifers–Lessons learned from 10 years of injection into the Utsira Formation in the Sleipner area

The ongoing CO2 injection at Sleipner has demonstrated that 2/3 of the injected CO2 has not reached the top of the Utsira Formation, but has instead migrated laterally below imperfect intra-reservoir seals. The CO2 trapping below the structural spill point in the Utsira Formation is due to local mini traps, capillary flow resistance, and the hydrodynamic drive of the injection. About 40% of the CO2 that has entered the pore systems will remain as residually trapped CO2, whereas an unknown fraction of the remaining CO2 will migrate towards the top of the reservoir.

The Micro‐Arcsecond Scintillation‐Induced Variability (MASIV) Survey. II. The First Four Epochs

We report on the variability of 443 flat-spectrum, compact radio sources monitored using the VLA for 3 days in four epochs at ~4 month intervals at 5 GHz as part of the Micro-Arcsecond Scintillation-Induced Variability (MASIV) survey. Over half of these sources exhibited 2%-10% rms variations on timescales over 2 days. We analyzed the variations by two independent methods and find that the rms variability amplitudes of the sources correlate with the emission measure in the ionized interstellar medium along their respective lines of sight. We thus link the variations with interstellar scintillation of components of these sources, with some (unknown) fraction of the total flux density contained within a compact region of angular diameter in the range 10-50 μas. We also find that the variations decrease for high mean flux density sources and, most importantly, for high-redshift sources. The decrease in variability is probably due either to an increase in the apparent diameter of the source or to a decrease in the flux density of the compact fraction beyond z ~ 2. Here we present a statistical analysis of these results, and a future paper will discuss the cosmological implications in detail.

Inapparent infections and cholera dynamics

In many infectious diseases, an unknown fraction of infections produce symptoms mild enough to go unrecorded, a fact that can seriously compromise the interpretation of epidemiological records. This is true for cholera, a pandemic bacterial disease, where estimates of the ratio of asymptomatic to symptomatic infections have ranged from 3 to 100 (refs 1-5). In the absence of direct evidence, understanding of fundamental aspects of cholera transmission, immunology and control has been based on assumptions about this ratio and about the immunological consequences of inapparent infections. Here we show that a model incorporating high asymptomatic ratio and rapidly waning immunity, with infection both from human and environmental sources, explains 50 yr of mortality data from 26 districts of Bengal, the pathogen's endemic home. We find that the asymptomatic ratio in cholera is far higher than had been previously supposed and that the immunity derived from mild infections wanes much more rapidly than earlier analyses have indicated. We find, too, that the environmental reservoir (free-living pathogen) is directly responsible for relatively few infections but that it may be critical to the disease's endemicity. Our results demonstrate that inapparent infections can hold the key to interpreting the patterns of disease outbreaks. New statistical methods, which allow rigorous maximum likelihood inference based on dynamical models incorporating multiple sources and outcomes of infection, seasonality, process noise, hidden variables and measurement error, make it possible to test more precise hypotheses and obtain unexpected results. Our experience suggests that the confrontation of time-series data with mechanistic models is likely to revise our understanding of the ecology of many infectious diseases.

Medium‐ to large‐scale density variability as observed by CHAMP

Atmospheric density accounts for the largest source of error for predicting low‐Earth orbiting satellite positions. Relevant applications include catalog maintenance, collision avoidance, and mission design. In this paper we quantify density model errors due to density variability at submodel grid spatial scales. Densities in the 370–450 km height region inferred from accelerometer measurements on CHAMP between 2001 and 2006 are utilized. Results are presented in terms of root‐mean‐square (RMS) amplitudes of the high‐pass‐filtered density residuals with respect to the trend. RMS one‐sigma amplitudes are found to be on the order of 9% (13%) for low (high) geomagnetic activity and to possess distinct seasonal‐latitudinal and local time patterns. The RMS density amplitude patterns suggest a high‐latitude (i.e., auroral) wave source for many of the density perturbations, dissipation for most waves poleward of 40° latitude, and a preference for penetration to lower latitudes during nighttime and during higher levels of geomagnetic activity and lower levels of solar activity. An unknown fraction of the density variability may also originate from gravity wave sources in the lower atmosphere. The RMS amplitude distributions provide statistical constraints on gravity wave contributions from all sources and in a modeling or theoretical context also help to constrain mean heating and acceleration rates due to wave dissipation. In the context of density model errors, our results can be used to calculate an orbit prediction and attitude dynamics error budget. Other engineering applications such as thruster throttling requirements for a drag‐compensated satellite system are also discussed.

The Energy Output of the Universe from 0.1 to 1000 μm

The dominant source of electromagnetic energy in the universe today (over ultraviolet, optical, and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proved difficult due to interstellar dust grains that attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies, we find that (integrated over all galaxy types and orientations) only 11% ± 2% of the 0.1 μm photons escape their host galaxies; this value rises linearly (with log λ) to 87% ± 3% at 2.1 μm. We deduce that the energy output from stars in the nearby universe is (1.6 ± 0.2) × 1035 W Mpc−3, of which (0.9 ± 0.1) × 1035 W Mpc−3 escapes directly into the intergalactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided.

Relative importance of mass and volume changes to global sea level rise

We examine the relationship between 50‐year‐long records of global sea level (GSL) calculated from 1023 tide gauge stations and global ocean heat content (GOHC), glacier and ice sheet melting. The lack of consistent correlation between changes in GOHC and GSL during the period 1955–2003 argues against GOHC being the dominant factor in GSL as is often thought. We provide clear evidence of the substantial and increasing role in GSL from the eustatic component (47%) compared with the contribution from increasing heat content (25%), suggesting that the primary role is being played by the melting glaciers and ice sheets. There remains about 1/4 of GSL rise unaccounted for by the best estimates of both eustatic and thermosteric effects. This fraction also exhibits large variability that is not readily associated with known causes of sea level variability. The most likely explanation of this unknown fraction is underestimated melting, climate‐driven changes in terrestrial storage components, and decadal timescale variability in global water cycle. This argues for a concerted effort to quantify changes in these reservoirs.

Reply to the comment by Reid and Hudson on “Rate of species introductions in the Great Lakes via ships’ ballast water and sediments”

Drake and David M. Lodge 555 With Reid and Hudson (2008), we lament the weak representation of taxonomic expertise in modern biology. In fact, we have advocated increased funding for research in systematics and programs for training taxonomists (Lodge 2002). We are cognizant of our own lack of expertise in taxonomy and how the lack of taxonomic information and expertise hinders progress in research and management of biological invasions. Nevertheless, taxonomic expertise does not explain the differences between the conclusions of our papers (Drake and Lodge 2007a, 2007b) and the conclusions Reid and Hudson think we ought to have drawn. Rather, their disagreement is with our conceptual framework for thinking about invasive species and the uncertainties concerning species that might establish in regions outside their native ranges. Specifically, they assess invasion potential differently than we did. That is, to categorize species that have not invaded the Great Lakes, and for which a paucity of distributional and ecological information exists, we adopted different conceptual criteria than Reid and Hudson to designate these species as indigenous or nonindigenous. In addition, for species that have not invaded the Great Lakes, it is impossible to say if they might become established if they were to be introduced, leaving room for informed judgments and improvement of those judgments as new information becomes available. At issue are four species of freshwater copepods that we collected from the ballast water of ships entering the Great Lakes: Microcyclops rubellus, Microcyclops varicans, Paracyclops chiltoni, and Maraenobiotus insignipes. Reid and Hudson believe the first three should be considered indigenous to the Great Lakes because they occur elsewhere in North America and are reported from the Great Lakes in Hudson and Lesko (2003). Unfortunately, Hudson and Lesko (2003), an Internet source, was not available when the taxonomic component of this work was being performed, does not appear in a peer-reviewed journal, and does not refer to previously published reports of these species. Furthermore, Hudson and Lesko (2003) indicate that these species were not collected in the Great Lakes prior to 1984. Thus, we were and are justified in considering these species to be nonindigenous to the Great Lakes. Proper analysis and publication of previously collected specimens may yet reveal that these species were established prior to our collection of them from ballast water, in which case we would consider them as established, nonindigenous species. Concerning Maraenobiotus insignipes, Reid and Hudson concede that it has not been reported from the Great Lakes, but think we should have excluded it on the basis of its habitat, which Reid and Hudson report is primarily “streams, seeps, and damp moss”. Ideally, for all species we collected, we would have attempted to indicate their ability to establish in various Great Lakes habitats. Unfortunately, the Great Lakes ecosystem is so heterogeneous and the habitat tolerances of zooplankton are so poorly documented that we concluded that the only designation we could reliably apply to all the species we collected was whether or not they were known to tolerate fresh waters. Of course, the species we collected are just a sample of a much larger species pool, for which our primary goal was to estimate its size. An unknown fraction of this pool will not be able to inhabit the Great Lakes for various reasons. That this fraction is unknown is nonetheless a poor reason for rejecting our estimate of the total pool. Reid and Hudson also object to our categorization of Oithona similis as “freshwater” (Drake and Lodge 2007b). As can be understood from the context and the more complete listing in Annex 1 of Drake and Lodge (2007b), our designation indicates we were unable to rule out freshwater tolerance. This is different than asserting it to prefer fresh-

Chemical analysis of atmospheric aerosols

An aerosol is an assembly of liquid or solid particles which are suspended in a gaseous medium long enough to enable observation and measurement. The atmospheric aerosol thus describes the system of particles suspended in our atmosphere. Generally, the sizes of aerosol particles are in the range 0.001–100 μm. Sampled aerosol particles are also called particulate matter (PM). According to the cutoff used during sampling, a suffix is often used, thus, PM10 denotes all aerosol particles with an aerodynamic particle diameter d≤10 μm. Most legal standards are related to PM10. Atmospheric aerosols are of interest mainly because of their effects on health and climate. Concerning health, many epidemiological studies have shown a link between increased mortality/morbidity and increased PM10 or PM2.5 (see [1] and references therein). Concerning climate, aerosol particles scatter and absorb light (known as the direct effect on climate), and modify cloud properties (with a variety of effects known as indirect effects) [2]. These effects are influenced by the chemical and physical properties of the aerosol particles, which makes these properties important to be measured. In addition, the chemical composition reveals information about the sources of the atmospheric aerosols. Thus, for optimal mitigation processes the chemical composition needs to be known. Figure 1 presents the average chemical composition of PM10 from Zurich (data from [3]). Elemental carbon (EC), trace elements and mineral dust are so-called primary aerosol constituents, i.e., they are directly emitted into the atmosphere in particulate form. On the other hand, nitrate and sulfate are only formed in the atmosphere through the oxidation of the gaseous species NOx (i.e., sum of NO and NO2) and SO2 to nitric and sulfuric acid and subsequent neutralization of ammonia. These components are thus called secondary. Organic matter (OM) is composed of organic carbon (OC), which includes a variety of heteroatoms such as oxygen, hydrogen, or nitrogen. A conversion factor is therefore used to calculate OM from the measured OC concentration. OM can be of primary or secondary origin. The sum of OM and EC is also called carbonaceous aerosol. The unknown fraction consists of a considerable amount of water [3]. While the sources of the inorganic constituents are quite well known, very little is still known about the chemical composition of OM. This paper therefore focuses on the chemical analysis of OM, and includes a discussion of primary versus secondary and anthropogenic versus biogenic carbon.

Property-Preserving Data Reconstruction

We initiate a new line of investigation into online property-preserving data reconstruction. Consider a dataset which is assumed to satisfy various (known) structural properties; e.g., it may consist of sorted numbers, or points on a manifold, or vectors in a polyhedral cone, or codewords from an error-correcting code. Because of noise and errors, however, an (unknown) fraction of the data is deemed unsound, i.e., in violation with the expected structural properties. Can one still query into the dataset in an online fashion and be provided data that is always sound? In other words, can one design a filter which, when given a query to any item I in the dataset, returns a sound item J that, although not necessarily in the dataset, differs from I as infrequently as possible. No preprocessing should be allowed and queries should be answered online. We consider the case of a monotone function. Specifically, the dataset encodes a function f:{1,…,n} ↦ R that is at (unknown) distance ε from monotone, meaning that f can—and must—be modified at ε n places to become monotone. Our main result is a randomized filter that can answer any query in O(log 2 nlog log n) time while modifying the function f at only O(ε n) places. The amortized time over n function evaluations is O(log n). The filter works as stated with probability arbitrarily close to 1. We provide an alternative filter with O(log n) worst case query time and O(ε nlog n) function modifications. For reconstructing d-dimensional monotone functions of the form f:{1,…,n}d ↦ R, we present a filter that takes (2O(d)(log n)4d−2log log n) time per query and modifies at most O(ε n d) function values (for constant d).

TREE GROWTH INFERENCE AND PREDICTION FROM DIAMETER CENSUSES AND RING WIDTHS

Estimation of tree growth is based on sparse observations of tree diameter, ring widths, or increments read from a dendrometer. From annual measurements on a few trees (e.g., increment cores) or sporadic measurements from many trees (e.g., diameter censuses on mapped plots), relationships with resources, tree size, and climate are extrapolated to whole stands. There has been no way to formally integrate different types of data and problems of estimation that result from (1) multiple sources of observation error, which frequently result in impossible estimates of negative growth, (2) the fact that data are typically sparse (a few trees or a few years), whereas inference is needed broadly (many trees over many years), (3) the fact that some unknown fraction of the variance is shared across the population, and (4) the fact that growth rates of trees within competing stands are not independent. We develop a hierarchical Bayes state space model for tree growth that addresses all of these challenges, allowing for formal inference that is consistent with the available data and the assumption that growth is nonnegative. Prediction follows directly, incorporating the full uncertainty from inference with scenarios for "filling the gaps" for past growth rates and for future conditions affecting growth. An example involving multiple species and multiple stands with tree-ring data and up to 14 years of tree census data illustrates how different levels of information at the tree and stand level contribute to inference and prediction.

An Embarrassment of Switches

What makes a human different from a chimpanzee or a mouse? Of course, we know the answer in broad outline. Mutations in the genome, sifted by natural selection, cause changes in appearance, physiology, and behavior—what geneticists call the phenotype. But we have only a vague picture of a more detailed answer. Precisely which mutations generate phenotypic evolution? It's not that we can't find the mutations. Today's DNA sequencing technology readily identifies all differences between two genomes. There are simply too many differences—tens of millions between human and chimp, for example (1). An unknown fraction of these mutations alter the phenotype. Nonetheless, the molecular effects of mutations provide a rough guide to their phenotypic effects. Some mutations change the amino acid sequence of proteins, thereby altering their functions, and some change so-called cis-regulatory regions, altering when and where proteins are produced. We know a lot about the first class, but much less about the second. Several recent papers, including one by Borneman et al. on page 815 of this issue (2), demonstrate a surprising abundance of cis-regulatory changes between closely related species.

Deaths and Injuries Attributed to Infant Crib Bumper Pads

To document deaths attributed to bumper pads and injuries from their use that are potentially preventable. The US Consumer Product Safety Commission maintains files on cases voluntarily reported to them of deaths and injury related to commercial products. These cases represent an unknown fraction of total occurrences. We searched this database for deaths related to crib bumpers for the years 1985 to 2005. We also searched other Consumer Product Safety Commission databases for crib-related injuries that potentially might have been prevented by bumpers. Additionally, we examined 22 retail crib bumpers and described features that could be hazardous. Twenty-seven accidental deaths reported by medical examiners or coroners were attributed to bumper pads. The mechanism of death included suffocation and strangulation by bumper ties. Twenty-five nonfatal injuries were identified, and most consisted of minor contusions. All retail bumpers had hazardous properties. These findings suggest that crib and bassinet bumpers are dangerous. Their use prevents only minor injuries. Because bumpers can cause death, we conclude that they should not be used.

In vitro gas production profile and the formation of end products from non‐ washable, insoluble washable and soluble washable fractions in some concentrate ingredients

AbstractA procedure that mimics washing in the in situ incubation technique, combined with an in vitro gas and volatile fatty acids (VFAs) production technique, was used to verify the assumption that rumen degradation behaviour of material washed out of nylon bags is instantaneous and complete. In a 6 × 4 factorial arrangement of treatments with three replicates, fractions of maize, barley, milo, yellow peas, lupins (a mixture of white and spotted lupins) and round‐seeded brown faba beans were subjected to an in vitro incubation technique. Fractions were whole (WHO), non‐washable (NWF), insoluble washable (ISWF) and soluble washable (SWF). In a manually operated in vitro fermentation system, another 24 samples of the same substrates were fermented for VFA and ammonia analysis. Except in lupins, ISWF in the concentrate ingredients was very rich in starch. SWF was relatively rich in ash, crude protein, soluble sugars, and a residual unknown fraction but contained only a negligible quantity of starch. Thus, the fermentation characteristics of ISWF were more like WHO and NWF than SWF. Total gas production of SWF was considerably lower than the other fractions. A very rapidly degradable fraction was seen in the first phase of degradation of SWF. The pattern of fermentation end‐product formation for SWF differed from that of the other fractions. Copyright © 2007 Society of Chemical Industry

An Observational Estimate of Volume and Freshwater Flux Leaving the Arctic Ocean through Nares Strait

Abstract The Arctic Ocean is an important link in the global hydrological cycle, storing freshwater and releasing it to the North Atlantic Ocean in a variable fashion as pack ice and freshened seawater. An unknown fraction of this return flow passes through Nares Strait between northern Canada and Greenland. Surveys of ocean current and salinity in Nares Strait were completed in the summer of 2003. High-resolution data acquired by ship-based acoustic Doppler current profiler and via hydrographic casts revealed subtidal volume and freshwater fluxes of 0.8 ± 0.3 Sv and –25 ± 12 mSv (Sv = 103 mSv = 106 m3 s−1), respectively. The observations resolved the dominant spatial scale of variability, the internal Rossby radius of deformation (LD ∼9 km), and revealed a complex, yet coherent along-channel flow with a Rossby number of about 0.13, close to geostrophic balance. Approximately one-third of the total volume flux was associated with across-channel slope of the sea surface and two-thirds (68%) with across-channel slope of isopycnal surfaces. During the period of observation, sustained wind from the southwest weakened the average down-channel flow at the surface. The speed of tidal currents exceeded subtidal components by a factor of 2. Tidal signals were resolved and removed from the observations here using two independent methods resolving horizontal and vertical variability of tidal properties, respectively. Tidal current predictions from a barotropic model agreed well with depth-averaged observations in both amplitude and phase. However, because estimates of freshwater flux require accurate surface currents (and salinity), a least squares fitting procedure using velocity data was judged more reliable, since it permits quantification of vertical tidal current variations.