1,720,970 research outputs found
The response of isoprene emission rate and photosynthetic rate to photon flux and nitrogen supply in aspen and white oak trees
No full textThe relationship between isoprene emission rate and dark respiration rate in white poplar (Populus alba L.) leaves
No full textEcology of Marine Phytoplankton
No full textMarine phytoplankton account for about 45 % of global net primary production (NPP). In addition, they perform other important biogeochemical functions including nitrogen fixation, calcium carbonate precipitation, and the production of climatically active gases such as dimethyl sulfide. Oceanographers employ a wide variety of platforms for studying marine phytoplankton ecology, including sampling from ships, sampling from autonomous remotely operated vehicles, and collecting observations from Earth-orbiting satellites. Marine phytoplankton range in size from <1 ?m in diameter to about 1 mm in length and include representatives from at least five eukaryotic phyla together with the cyanobacteria. This wide size range and phylogenetic diversity presents challenges for quantifying and characterizing phytoplankton communities. Functional traits that quantify responses of growth rate, photosynthesis and nutrient uptake to temperature, irradiance, and nutrient availability provide a useful basis for understanding phytoplankton ecology. A variety of complementary approaches are used to measure gross and net primary production. These include measuring production of O2 and organic matter in bottle experiments and measuring diel and seasonal changes of O2 in open waters. Information obtained from satellite remote sensing of ocean color is used to calculate NPP on regional and global scales. The physical and chemical variables that drive NPP include temperature, nutrient availability, and solar radiation. These vary in time and space, and our understanding of this variability is largely encapsulated in the concepts of the seasonal production cycle and marine biogeochemical provinces. Nutrient limitation sets an upper limit to NPP over most of the ocean surface, with either inorganic iron or nitrogen being the proximate limiting element in different regions. The upper water column is stably stratified over much of the ocean, and pronounced vertical gradients of light and nutrients lead to depth separation of ecotypes with differing adaptations to nutrient availability and the light environment. Although the growth of individual phytoplankton cells is often limited by temperature and the availability of nutrients and light, biotic interactions including predation and disease often control the growth of phytoplankton populations and the species composition of phytoplankton communities. Anthropogenic impacts on the ocean, including nutrient loading to coastal waters, climatic forcing associated with global warming, and ocean uptake of anthropogenic CO2, are influencing the chemistry and physics of the upper ocean, with multiple potential impacts on the phytoplankto
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Within-plant isoprene oxidation confirmed by direct emissions of oxidation products methyl vinyl ketone and methacrolein
No full textIsoprene is emitted from many terrestrial plants at high rates, accounting for an estimated 1/3 of annual global volatile organic compound emissions from all anthropogenic and biogenic sources combined. Through rapid photooxidation reactions in the atmosphere, isoprene is converted to a variety of oxidized hydrocarbons, providing higher order reactants for the production of organic nitrates and tropospheric ozone, reducing the availability of oxidants for the breakdown of radiatively active trace gases such as methane, and potentially producing hygroscopic particles that act as effective cloud condensation nuclei. However, the functional basis for plant production of isoprene remains elusive. It has been hypothesized that in the cell isoprene mitigates oxidative damage during the stress-induced accumulation of reactive oxygen species (ROS), but the products of isoprene-ROS reactions in plants have not been detected. Using pyruvate-2-13C leaf and branch feeding and individual branch and whole mesocosm flux studies, we present evidence that isoprene (i) is oxidized to methyl vinyl ketone and methacrolein (iox) in leaves and that iox/i emission ratios increase with temperature, possibly due to an increase in ROS production under high temperature and light stress. In a primary rainforest in Amazonia, we inferred significant in plant isoprene oxidation (despite the strong masking effect of simultaneous atmospheric oxidation), from its influence on the vertical distribution of iox uptake fluxes, which were shifted to low isoprene emitting regions of the canopy. These observations suggest that carbon investment in isoprene production is larger than that inferred from emissions alone and that models of tropospheric chemistry and biotachemistryclimate interactions should incorporate isoprene oxidation within both the biosphere and the atmosphere with potential implications for better understanding both the oxidizing power of the troposphere and forest response to climate change
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