51 research outputs found
Phytoplankton dynamics in contrasting early stage North Atlantic spring blooms: composition, succession, and potential drivers
The spring bloom is a key annual event in the phenology of pelagic ecosystems, making a major contribution to the oceanic biological carbon pump through the production and export of organic carbon. However, there is little consensus as to the main drivers of spring bloom formation, exacerbated by a lack of in situ observations of the phytoplankton community composition and its evolution during this critical period.
We investigated the dynamics of the phytoplankton community structure at two contrasting sites in the Iceland and Norwegian basins during the early stage (25 Marchâ25 April) of the 2012 North Atlantic spring bloom. The plankton composition and characteristics of the initial stages of the bloom were markedly different between the two basins. The Iceland Basin (ICB) appeared well mixed down to >400 m, yet surface chlorophyll a (0.27â2.2 mg mâ3) and primary production (0.06â0.66 mmol C mâ3 dâ1) were elevated in the upper 100 m. Although the Norwegian Basin (NWB) had a persistently shallower mixed layer (<100 m), chlorophyll a (0.58â0.93 mg mâ3) and primary production (0.08â0.15 mmol C mâ3 dâ1) remained lower than in the ICB, with picoplankton (<2 Îźm) dominating chlorophyll a biomass. The ICB phytoplankton composition appeared primarily driven by the physicochemical environment, with periodic events of increased mixing restricting further increases in biomass. In contrast, the NWB phytoplankton community was potentially limited by physicochemical and/or biological factors such as grazing.
Diatoms dominated the ICB, with the genus Chaetoceros (1â166 cells mLâ1) being succeeded by Pseudo-nitzschia (0.2â210 cells mLâ1). However, large diatoms (>10 Îźm) were virtually absent (<0.5 cells mLâ1) from the NWB, with only small nano-sized (<5 Îźm) diatoms (i.e. Minidiscus spp.) present (101â600 cells mLâ1). We suggest microzooplankton grazing, potentially coupled with the lack of a seed population of bloom-forming diatoms, was restricting diatom growth in the NWB, and that large diatoms may be absent in NWB spring blooms. Despite both phytoplankton communities being in the early stages of bloom formation, different physicochemical and biological factors controlled bloom formation at the two sites. If these differences in phytoplankton composition persist, the subsequent spring blooms are likely to be significantly different in terms of biogeochemistry and trophic interactions throughout the growth season, with important implications for carbon cycling and organic matter export
Generalised quantum weakest preconditions
Generalisation of the quantum weakest precondition result of D'Hondt and
Panangaden is presented. In particular the most general notion of quantum
predicate as positive operator valued measure (POVM) is introduced. The
previously known quantum weakest precondition result has been extended to cover
the case of POVM playing the role of a quantum predicate. Additionally, our
result is valid in infinite dimension case and also holds for a quantum
programs defined as a positive but not necessary completely positive
transformations of a quantum states.Comment: 7 pages, no figures, added references, changed conten
Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of the inter-particle interactions
We study the magnetic properties of spherical Co clusters with diameters
between 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputtering
of Co and Al2O3. The particle size distribution has been determined from the
equilibrium susceptibility and magnetization data and it is compared to
previous structural characterizations. The distribution of activation energies
was independently obtained from a scaling plot of the ac susceptibility.
Combining these two distributions we have accurately determined the effective
anisotropy constant Keff. We find that Keff is enhanced with respect to the
bulk value and that it is dominated by a strong anisotropy induced at the
surface of the clusters. Interactions between the magnetic moments of adjacent
layers are shown to increase the effective activation energy barrier for the
reversal of the magnetic moments. Finally, this reversal is shown to proceed
classically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.
Encoding and retrieval in a CA1 microcircuit model of the hippocampus
Recent years have witnessed a dramatic accumulation of
knowledge about the morphological, physiological and molecular characteristics,
as well as connectivity and synaptic properties of neurons in
the mammalian hippocampus. Despite these advances, very little insight
has been gained into the computational function of the different neuronal
classes; in particular, the role of the various inhibitory interneurons in
encoding and retrieval of information remains elusive. Mathematical and
computational models of microcircuits play an instrumental role in exploring
microcircuit functions and facilitate the dissection of operations
performed by diverse inhibitory interneurons. A model of the CA1 microcircuitry
is presented using biophysical representations of its major cell
types: pyramidal, basket, axo-axonic, bistratified and oriens lacunosummoleculare
cells. Computer simulations explore the biophysical mechanisms
by which encoding and retrieval of spatio-temporal input patterns
are achieved by the CA1 microcircuitry. The model proposes functional
roles for the different classes of inhibitory interneurons in the encoding
and retrieval cycles
Effective interaction between helical bio-molecules
The effective interaction between two parallel strands of helical
bio-molecules, such as deoxyribose nucleic acids (DNA), is calculated using
computer simulations of the "primitive" model of electrolytes. In particular we
study a simple model for B-DNA incorporating explicitly its charge pattern as a
double-helix structure. The effective force and the effective torque exerted
onto the molecules depend on the central distance and on the relative
orientation. The contributions of nonlinear screening by monovalent counterions
to these forces and torques are analyzed and calculated for different salt
concentrations. As a result, we find that the sign of the force depends
sensitively on the relative orientation. For intermolecular distances smaller
than it can be both attractive and repulsive. Furthermore we report a
nonmonotonic behaviour of the effective force for increasing salt
concentration. Both features cannot be described within linear screening
theories. For large distances, on the other hand, the results agree with linear
screening theories provided the charge of the bio-molecules is suitably
renormalized.Comment: 18 pages, 18 figures included in text, 100 bibliog
Comparison and Implementation of a Rigid and a Flexible Multibody Planetary Gearbox Model
We propose algorithms for developing (1) a rigid (constrained) and (2) a flexible planetary gearbox model. The two methods are compared against each other and advantages/disadvantages of each method are discussed. The rigid model (1) has gear tooth reaction forces expressed by Lagrange multipliers. The flexible approach (2) is being compared with the gear tooth forces from the rigid approach, first without damping and second the influence of damping is examined. Variable stiffness as a function of base circle arc length is implemented in the flexible approach such that it handles the realistic switch between one and two gear teeth in mesh. The final results are from modelling the planetary gearbox in a 500 kW wind turbine which we also described in Jørgensen et.al (2013)
Atividade antiproliferativa e mutagĂŞnica dos extratos aquosos de Baccharis trimera (Less.) A. P. de Candolle e Baccharis articulata (Lam.) Pers. (Asteraceae) sobre o sistema teste de Allium cepa
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