1,720,980 research outputs found
The influence of the subtropical oceans on atmospheric carbon dioxide (abstract of paper presented at ASLO/AGU Ocean Sciences Meeting, Honolulu, HI, 11-15 Feb 2002)
No full textModelling the effects of chromatic adaptation on phytoplankton community structure in the oligotrophic ocean
No full textWe explored the role of chromatic adaptation in shaping vertical phytoplankton community structures using a trait-based ecosystem model. The model included 1000 ‘phytoplankton types’ and was applied to the oligotrophic South Atlantic Gyre in a 1-dimensional framework, where ‘phytoplankton types’refers to the model phytoplankton that were stochastically assigned unique physiological characteristics. The model incorporates multi-spectral optics and light absorption properties for the different phytoplankton. The model successfully reproduced observed vertical gradients in the nitrate, bulk phytoplankton properties and community structure. Model phytoplankton types with Synechococcus-like spectral light absorption properties were outcompeted at depth, where eukaryote-like spectral properties were advantageous. In contrast, photoinhibition was important for vertical separation of high-light and low-light Prochlorococcus model analogues. In addition, temperature dependence was important for selection of phytoplanktontypes on the temperature gradient. The fittest, or successful, phytoplankton types were characterised by combinations of simultaneously optimal traits that suited them to a particular depth in the water column, reflecting the view that phytoplankton have co-evolved multiple traits that are advantageous in a particular environmental condition or niche
The wind-driven, subtropical gyres and the solubility pump of CO2
Full text link[1] Using a suite of abiotic, ocean-atmosphere, carbon cycle models we demonstrate that the representation of the ventilated thermocline leads to a significant enhancement of the sensitivity of atmospheric pCO(2) to subtropical surface ocean properties. In particular, we study an idealized sector, ocean circulation and abiotic carbon cycle model with a coupled atmospheric reservoir and examine the solubility pump of CO2 in the subtropical oceans. We compare solutions for atmospheric pCO(2) when driven only by buoyancy forces to those with both buoyancy and wind stress forcing. Introducing the wind stress leads to the formation of a subtropical gyre and the warm lens of the ventilated thermocline. This lens is depleted in carbon relative to the surrounding waters since its properties are inherited from the warm, subtropical surface ocean. It is undersaturated in carbon since subduction quickly follows the strong cooling in the western boundary current before equilibration with the overlying atmosphere can occur. Plausible wind stress patterns increase atmospheric pCO(2), relative to the case without wind forcing, and double the sensitivity of atmospheric pCO(2) to perturbations of the low latitude surface carbon system properties. We suggest that it is the resolution of the ventilated thermocline in global, three-dimensional, ocean circulation models that enhances their sensitivity of atmospheric pCO(2) to warm surface water properties relative to highly idealized box models
Maximal feeding with active prey-switching: A kill-the-winner functional response and its effect on global diversity and biogeography
No full text17 pages, 10 figures, 2 tablesPredators' switching towards the most abundant prey is a mechanism that stabilizes population dynamics and helps overcome competitive exclusion of species in food webs. Current formulations of active prey-switching, however, display non-maximal feeding in which the predators' total ingestion decays exponentially with the number prey species (i.e. the diet breadth) even though the total prey biomass stays constant. We analyse three previously published multi-species functional responses which have either active switching or maximal feeding, but not both. We identify the cause of this apparent incompatibility and describe a kill-the-winner formulation that combines active switching with maximal feeding. Active switching is shown to be a community response in which some predators become prey-selective and the formulations with maximal or non-maximal feeding are implicitly assuming different food web configurations. Global simulations using a marine ecosystem model with 64 phytoplankton species belonging to 4 major functional groups show that the species richness and biogeography of phytoplankton are very sensitive to the choice of the functional response for grazing. The phytoplankton biogeography reflects the balance between the competitive abilities for nutrient uptake and the degree of apparent competition which occurs indirectly between species that share a common predator species. The phytoplankton diversity significantly increases when active switching is combined with maximal feeding through predator-mediated coexistence. © 2013 Elsevier Ltd.This work was supported by a Marie Curie Fellowship (IOF – FP7) to S.M.V. from the European Union (EU) and was performed within the MIT’s Darwin ProjectPeer Reviewe
Microzooplankton regulation of surface ocean POC:PON ratios
Full text linkThe elemental composition of particulate organic matter in the surface ocean significantly affects the efficiency of the ocean's store of carbon. Though the elemental composition of primary producers is an important factor, recent observations from the western North Atlantic Ocean revealed that carbon-to-nitrogen ratios (C:N) of phytoplankton were significantly higher than the relatively homeostatic ratio of the total particulate pool (particulate organic carbon:particulate organic nitrogen; POC:PON). Here we use an idealized ecosystem model to show how interactions between primary and secondary producers maintain the mean composition of surface particulates and the difference between primary producers and bulk material. Idealized physiological models of phytoplankton and microzooplankton, constrained by laboratory data, reveal contrasting autotrophic and heterotrophic responses to nitrogen limitation: under nitrogen limitation, phytoplankton accumulate carbon in carbohydrates and lipids while microzooplankton deplete internal C reserves to fuel respiration. Global ecosystem simulations yield hypothetical global distributions of phytoplankton and microzooplankton C:N ratio predicting elevated phytoplankton C:N ratios in the high-light, low-nutrient regions of the ocean despite a lower, homeostatic POC:PON ratio due to respiration of excess carbon in systems subject to top-down control. The model qualitatively captures and provides a simple interpretation for, a global compilation of surface ocean POC:PON data
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
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