7,082 research outputs found
Recommended from our members
Growth dynamics of Phaeocystis antarctica-dominated plankton assemblages from the Ross Sea
Large-volume experiments were conducted using natural seawater assemblages collected in the southern Ross Sea during austral spring 1994 and summer 1995 to assess the carbon and nitrogen exchanges among phytoplankton, bacteria and dissolved organic carbon pools, and to compare the elemental partitioning in these experimental enclosures with those observed in situ. Large concentrations of particulate matter were produced in these enclosures, which were at all times dominated by the colonial haptophyte Phaeocystis antarctica. Particulate organic carbon concentrations exceeded 200 mu mol l(-1) at the end of the experiment. Bacterial carbon comprised only a small (%) fraction of the particulate carbon, but bacteria grew at 0.15 to 0.3 d(-1) and were not limited by bacteriovores. Nutrient levels decreased concomitantly with POC increases, and nitrate was reduced to undetectable levels. Dissolved organic carbon (DOC) levels remained low (less than 50 mu M) while nutrients were present, but increased dramatically (to more than 200 mu M) after nitrate was depleted. Growth rates calculated from changes in particulate matter concentrations were slightly below the predicted maximum based on temperature. Field studies, however, showed no depletion of nitrate, similar levels of particulate organic carbon to those found during exponential growth, low levels of DOG, and relatively low levels of bacterial biomass. It appears that P. antarctica in the Ross Sea does not produce large amounts of DOC during nutrient-replete growth; furthermore, because macronutrients are rarely, if ever, depleted where P. antarctica is dominant in the Ross Sea, it is likely that much of the carbon generated during its growth remains in the particulate pool
Measuring Global Similarity between Texts
We propose a new similarity measure between texts which, contrary to the
current state-of-the-art approaches, takes a global view of the texts to be
compared. We have implemented a tool to compute our textual distance and
conducted experiments on several corpuses of texts. The experiments show that
our methods can reliably identify different global types of texts.Comment: Submitted to SLSP 201
Outstanding Issues in Solar Dynamo Theory
The magnetic activity of the Sun, as manifested in the sunspot cycle,
originates deep within its convection zone through a dynamo mechanism which
involves non-trivial interactions between the plasma and magnetic field in the
solar interior. Recent advances in magnetohydrodynamic dynamo theory have led
us closer towards a better understanding of the physics of the solar magnetic
cycle. In conjunction, helioseismic observations of large-scale flows in the
solar interior has now made it possible to constrain some of the parameters
used in models of the solar cycle. In the first part of this review, I briefly
describe this current state of understanding of the solar cycle. In the second
part, I highlight some of the outstanding issues in solar dynamo theory related
to the the nature of the dynamo -effect, magnetic buoyancy and the
origin of Maunder-like minima in activity. I also discuss how poor constraints
on key physical processes such as turbulent diffusion, meridional circulation
and turbulent flux pumping confuse the relative roles of these vis-a-vis
magnetic flux transport. I argue that unless some of these issues are
addressed, no model of the solar cycle can claim to be ``the standard model'',
nor can any predictions from such models be trusted; in other words, we are
still not there yet.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Speciesistic Veganism: An Anthropocentric Argument
The paper proposes an anthropocentric argument for veganism based on a speciesistic premise that most carnists likely affirm: human flourishing should be promoted. I highlight four areas of human suffering promoted by a carnistic diet: (1) health dangers to workers (both physical and psychological), (2) economic dangers to workers, (3) physical dangers to communities around slaughterhouses, and (4) environmental dangers to communities-at-large. Consequently, one could ignore the well-being of non-human animals and nevertheless recognize significant moral failings in the current standard system of meat production
Toward Understanding Massive Star Formation
Although fundamental for astrophysics, the processes that produce massive
stars are not well understood. Large distances, high extinction, and short
timescales of critical evolutionary phases make observations of these processes
challenging. Lacking good observational guidance, theoretical models have
remained controversial. This review offers a basic description of the collapse
of a massive molecular core and a critical discussion of the three competing
concepts of massive star formation:
- monolithic collapse in isolated cores
- competitive accretion in a protocluster environment
- stellar collisions and mergers in very dense systems
We also review the observed outflows, multiplicity, and clustering properties
of massive stars, the upper initial mass function and the upper mass limit. We
conclude that high-mass star formation is not merely a scaled-up version of
low-mass star formation with higher accretion rates, but partly a mechanism of
its own, primarily owing to the role of stellar mass and radiation pressure in
controlling the dynamics.Comment: 139 pages, 18 figures, 5 tables, glossar
Cocaine-associated Chest Pain How Common Is Myocardial Infarction?
Objective: Prior studies addressing the incidence of acute myocardial infarction (AMI) in patients with cocaine-associated chest pain have found divergent results. Previous prospective studies, which found approximately a 6% incidence of AMI, have been criticized for selection bias. This study sought to determine the rate of AMI in patients with cocaine-associated chest pain. Methods: All patients seen in an urban university-affiliated hospital between July 1996 and February 1998 were identified by ICD-9 medical records search for cocaine use and chest pain/acute coronary syndromes. In this system, all faculty admit all patients with cocaine-associated chest pain for at least 23-hour observation periods. Data collected included demographics, medical and cocaine use history, presenting characteristics, hospital course, cardiovascular complications, and diagnostic tests using a 119-item closed-question data instrument with high interrater reliability. The main outcome measure was AMI according to World Health Organization (WHO) criteria. Results: There were 250 patients identified with a mean age of 33.5 ± 8.5 years; 77% were male; 84% were African American. Of 196 patients tested, 185 had cocaine or cocaine metabolites in the urine (94%). The incidence of cardiac risk factors were: hypercholesterolemia, 8%; diabetes, 6%; family history, 34%; hypertension, 26%; tobacco use, 77%; prior MI, 6%; and prior chest pain, 40%. Seventy-seven percent admitted to cocaine use in the preceding 24 hours: crack, 85%; IV, 2%; nasal, 6%. Twenty-five patients (10%) had electrocardiographic evidence of ischemia. A total of 15 patients experienced an AMI (6%; 95% CI = 4.1% to 8.9%) using WHO criteria. Complications were infrequent: bradydysrrhythmias, 0.4%; congestive heart failure, 0.4%; supraventricular tachycardia, 1.2%; sustained ventricular tachycardia, 0.8%. Conclusion: The incidence of AMI was 6% in patients with cocaine-associated chest pain. This result is identical to that found in prior prospective studies.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71896/1/j.1553-2712.2000.tb02064.x.pd
Bone marrow mesenchymal stem cells do not enhance intra-synovial tendon healing despite engraftment and homing to niches within the synovium
Intra-synovial tendon injuries display poor healing, which often results in reduced functionality and pain. A lack of effective therapeutic options has led to experimental approaches to augment natural tendon repair with autologous mesenchymal stem cells (MSCs) although the effects of the intra-synovial environment on the distribution, engraftment and functionality of implanted MSCs is not known. This study utilised a novel sheep model which, although in an anatomically different location, more accurately mimics the mechanical and synovial environment of the human rotator cuff, to determine the effects of intra-synovial implantation of MSCs
- …