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Geochemical evidence of denitrification in the Benguela upwelling system
No full textThis paper presents analysis of nitrate, phosphate and silicate data from the Benguela upwelling system. Evidence is presented that suggests denitrification occurring close to shore, and also nutrient trapping. Denitrification leaves an imprint on the water properties in terms of a nitrate deficit, that is to say nitrate concentrations that are significantly less than predicted by multiplying the phosphate concentrations by the Redfield ratio. It is probable that denitrification also causes a decoupling of nitrate and carbon compared to Redfield processes, and large-scale losses of nitrate in the Benguela which are not accompanied by losses of carbon. Nitrate-driven CO2 drawdown following upwelling will be less than it might otherwise be, because of denitrification.Nutrient trapping (secondary remineralisation) is apparent as enhanced phosphate concentrations, some of which are several [mu]mol higher than in the offshore source waters for upwelling. Waters also become enriched in silicate and to a lesser extent nitrate as they advect across the shelf. By implication the same process should also "supercharge" waters in dissolved inorganic carbon, leading to stronger outgassing of CO2 immediately after upwelling. The effect is again to increase the size of the estimated Benguela upwelling system CO2 sourc
Physical controls on biogeochemical zonation in the Southern Ocean
No full textThe primary control on the N–S zonation of the Southern Ocean is the wind-induced transport of the Antarctic Circumpolar Current (ACC). The ACC divides the Southern Ocean into three major zones: the Subantarctic Zone (SAZ) north of the ACC; the ACC transport zone; and the zone south of the ACC (SACCZ). The zone of ACC transport is most often subdivided into two zones, the Polar Frontal Zone (PFZ) and the Antarctic Zone (AAZ), but it may be appropriate to define more subzones or indeed only one at some longitudes. To maintain geostrophic balance, isopycnals must slope upwards to the south across the ACC, thus raising nutrient-rich deep water closer to the surface as one goes polewards. In addition, silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate-rich bottom water. Surface silicate concentrations therefore decrease northwards from high levels in the SACCZ to low levels in the SAZ. Within the SAZ and PFZ and even in the northern part of the AAZ, silicate levels may drop to limiting levels for siliceous phytoplankton production during summer. Nitrate concentrations also decrease northwards, but only become limiting in the Subtropical Zone north of the SAZ. The second circumpolar control is the changing balance of stratification, with temperature dominating near-surface stratification in the SAZ and salinity dominating further south because of fresh water input to the surface from melting ice. This results in circumpolar features such as the subsurface 2°C temperature minimum and the subduction of the salinity minimum of Antarctic Intermediate Water, which are often but not always associated with frontal jets and large transports. The transport of the ACC is dynamically constrained into narrow bands, the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude. Thus it is not the fronts that are circumpolar, but the total ACC transport and scalar properties of the salinity and temperature fields. Evidence of summer silicate and nitrate uptake in all zones (SAZ, PFZ and AAZ) shows that there is productivity despite their high-nutrient low-chlorophyll status. Blooms covering large areas (say 400 km across) in the PFZ and AAZ are found in the vicinity of submarine plateaux, which suggest benthic iron fertilization
Variability of chlorophyll profiles on the west coast of Southern Africa in June/July 1999
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Basin-scale variability of phytoplankton bio-optical characteristics in relation to bloom state and community structure in the Northeast Atlantic
No full textPhytoplankton physiological data collected throughout the Iceland Basin and Rockall Trough during the North Atlantic spring bloom from May to June 2001 are presented. Physiological parameters including the maximum photochemical quantum efficiency (Fv/Fm) and the functional absorption cross section of photosystem II (?PSII) were measured using fast repetition rate fluorometry. Information on the taxonomic and pigment characteristics of the phytoplankton populations was also collected, with pigment data being used to reconstruct absorption spectra. Significant changes in the physiological properties of PSII were found to be associated with the progression of the spring bloom from diatom to flagellate domination. Changes in both community composition and physiology were in turn correlated with environmental parameters. Lower Fv/Fm, higher ?PSII and corresponding decreases in cell size were associated with the observed decrease of nutrients that accompanied increasing stratification. Differences in ?PSII were primarily associated with the changing pigment composition of the phytoplankton populations, with the largest changes appearing to be governed by the amount of absorption by photosynthetic carotenoids. The physiological state of PSII was thus found to be an indicator of bloom status and community structure in this productive temperate region principally as a result of taxon specific variability
Real-time forecasting of ecosystem dynamics during the CROZEX experiment and the roles of light, iron, silicate, and circulation
No full textReal-time coupled physical and biological forecasting was conducted prior to and during the CROZet natural bloom and EXport experiment (CROZEX) programme in the Southern Ocean (SO) between November 2004 and January 2005. The programme was aimed at investigating, through both measurements and modelling, the origin and fate of an intense and long-lasting phytoplankton bloom that is unusual for the otherwise high-nutrient low-chlorophyll conditions that characterise the SO. It was hypothesised that dissolved iron supplied to the system from shallow sediments accumulates downstream of the Crozet Islands during winter and creates conditions that favour phytoplankton production.Regular 14-day forecasts and reanalysis of the biogeochemical fields were issued on a weekly basis using a 3D primitive equation model coupled with an 11-component ecosystem model, along with assimilation of remotely sensed and in situ data. Real-time modelling assisted not only in optimisation of the cruise strategy, but also provided an ongoing synthesis of observations and test of the working hypotheses proposed to explain bloom dynamics in the study area.The model results indicated that a complex interplay between light, iron, and silicate limitation on primary production and grazing control determined the spatial extent and dynamics of the phytoplankton bloom. The model suggested that natural fertilisation of the area by dissolved iron was insufficient on its own to initiate a phytoplankton bloom. The persistence of deep mixing in the southern part of the iron-enriched waters was delaying, or in some areas preventing, the development of the bloom
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
Phytoplankton, nutrients and hydrography in the frontal zone between the Southwest Indian Subtropical gyre and the Southern Ocean
No full textA survey was made of the Southwest Indian Ocean frontal region between 30 and 50°E containing the Agulhas Return, Subtropical and Subantarctic Fronts. From CTD, SeaSoar and extracted samples the distribution of nitrate, silicate and chlorophyll a is shown to be strongly linked to the front and water mass structure, varying zonally and meridionally. Surface chlorophyll a concentrations were low to the north and south leaving a band of elevated chlorophyll between the Subtropical and Subantarctic Fronts. The low concentration of chlorophyll a to the north, in Subtropical Water, was clearly due to nitrate limitation. Between the Subtropical and Subantarctic Fronts, where the chlorophyll a concentrations were highest, the surface layer showed silicate depletion limiting diatom growth. South of the Subantarctic Front there were deep extending, low concentrations of chlorophyll a, but despite plentiful supplies of macro-nutrients and a well-stratified surface layer, high concentrations of chlorophyll a were absent. Changes from west to east were associated with the meandering of the Southern Ocean Fronts, especially the Subtropical Front, and their strength and proximity to each other. Concentrations of chlorophyll a peaked where the Agulhas Return, Subtropical and Subantarctic Fronts were in close proximity. Combined frontal structures appear to have particularly pronounced vertical stability and are associated with enhanced upwelling of nutrients and leakage of nutrients across the front. Light levels are high within the shallow stable layer. Such conditions are clearly favourable for biological growth and support the development of larger-celled phytoplankton communities
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
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