1,720,988 research outputs found
Macronutrient supply, uptake and recycling in the coastal ocean of the west Antarctic Peninsula
Full text linkNutrient supply, uptake and cycling underpin high primary productivity over the continental shelf of the west Antarctic Peninsula (WAP). Here we use a suite of biogeochemical and isotopic data collected over five years in northern Marguerite Bay to examine these macronutrient dynamics and their controlling biological and physical processes in the WAP coastal ocean.
We show pronounced nutrient drawdown over the summer months by primary production which drives a net seasonal nitrate uptake of 1.83 mol N m-2 yr-1, equivalent to net carbon uptake of 146 g C m-2 yr-1. High primary production fuelled primarily by deep-sourced macronutrients is diatom-dominated, but non-siliceous phytoplankton also play a role. Strong nutrient drawdown in the uppermost surface ocean has the potential to cause transient nitrogen limitation before nutrient resupply and/or regeneration. Interannual variability in nutrient utilisation corresponds to winter sea ice duration and the degree of upper ocean mixing, implying susceptibility to physical climate change.
The nitrogen isotope composition of nitrate (δ15NNO3) shows a utilisation signal during the growing seasons with a community-level net isotope effect of 4.19 ± 0.29‰. We also observe significant deviation of our data from modelled and observed utilisation trends, and argue that this is driven primarily by water column nitrification and meltwater dilution of surface nitrate.
This study is important because it provides a detailed description of the nutrient biogeochemistry underlying high primary productivity at the WAP, and shows that surface ocean nutrient inventories in the Antarctic sea ice zone can be affected by intense recycling in the water column, meltwater dilution and sea ice processes, in addition to utilisation in the upper ocean
Nitrogen cycling in the warming Arctic Ocean
Full text linkThis PhD thesis is investigating fixed nitrogen cycling at a Pan-Arctic scale, in order to better understand the sensitivity of Arctic biogeochemistry to ongoing warming and to identify future implications for long-term carbon fixation, nitrogen mass balance and oxygenation of the Arctic Ocean. Using stable isotopes of nitrate (δ¹⁵N-NO3 and δ¹⁸O-NO3), alongside an extensive suite of biogeochemical and hydrographic data, this project provides spatially and temporally integrated measurements of N supply, uptake and recycling in the Central Arctic Ocean (Nansen, Amundsen and Makarov basins) and the Atlantic inflow regions (Barents Sea, Fram Strait), outlining the processes that control nutrient budgets and fluxes on a Pan-Arctic scale.
This study proves the paramount role of bordering Arctic shelves in the reprocessing and redistribution of nutrients throughout the entire Arctic Ocean. Stable isotope data demonstrates that the entire nutrient pool in the Central Arctic halocline has been regenerated and laterally advected from the surrounding shelves. However, nutrient supply and uptake across the Central Arctic basins exhibits an east-west gradient dictated by location of the Transpolar Drift (TPD).
Regions situated on the TPD path (i.e. Amundsen and Makarov basins) exhibit surface NO3-depletion and a halocline isotopic signal (δ15N-NO3 ~6.5±0.1‰, δ18O-NO3~-1±0.2‰) consistent with lateral advection of partially nitrified and denitrified shelf bottom waters, traced back to the Siberian shelves. The strong salinity stratification in Amundsen and Makarov basins impedes surface nutrient
recharge from the halocline, restricting biological uptake to the NO₃-depleted meltwater layer (top 20-30m). Using an isotopically constrained mass balance model, this study shows that the TPD is not an additional source of (riverine) NO3 to Amundsen/Makarov basins and cannot support a future increase in Central Arctic productivity because any riverine nitrogen inputs concentrated in the TPD are lost to denitrification on the shallow, productive Siberian shelves.
Regions outside the TPD influence (i.e. Nansen basin) exhibit Si depletion and a well-mixed Atlantic halocline formed by lateral advection of shelf waters from the Barents Sea. Based on the coupled isotopic signature of preformed nitrate in Barents Sea (δ15N-NO3 ~5±0.1‰, δ18O-NO3~2.8±0.2‰) the origin of nutrients within the Atlantic inflow is traced to the north Atlantic subpolar gyre. Biological uptake peaks in the weakly stratified Nansen basin, extending throughout the entire halocline (100m), eventually limited by Si availability and transient Fe availability. In the eventuality that Si limitation is resolved with ongoing warming, this study estimates that new productivity in Nansen Basin can double from the current 15g C/m2 to ~33g C/m2. However, the probability that Nansen basin productivity will increase with warming is low given the Atlantification of Atlantic inflow regions, which generates further Si-limitation and unstable light conditions through deep mixing, creating an environment restrictive to biological uptake. Thus, although the systematics behind nutrient limitation are very different between Nansen versus Amundsen and Makarov basins, leading to the formation of an east-west biogeochemical gradient which will only strengthen with ongoing warming, this study shows that there is limited potential for primary productivity to increase anywhere in the Central Arctic Ocean.
Through complementary use of respiration stoichiometry and δ¹⁸O-NO3 signatures, this study also assesses the role of in situ export production versus lateral advection of shelf organic matter in shaping water column remineralisation and O2 consumption in the Central Arctic. Lighter δ¹⁸O-NO3 (1.4-1.5‰) in Central Arctic deep waters compared to their Atlantic source (~2‰) indicate that ~50% of the deep nutrient pool in the Central Arctic has been recycled, exhibiting a strong correlation with deep O2 concentrations and ventilation age. It is revealed that ~90% of the carbon remineralisation and O2 consumption measured in Central Arctic deep waters are associated with the lateral supply of shelf organic matter. These findings prove that the Arctic shelf carbon pump drives long-term carbon sequestration in the deep Central Arctic, turning the study area into a carbon sink, with potential mitigating effects on global warming trends.
The heavier δ¹⁵N-NO₃ signature (5.2‰-5.5‰) in Central Arctic deep waters compared to their Atlantic source (~4.9‰) reflects regional differences in nitrogen supply, reinforcing the strong sensitivity of deep Arctic biogeochemistry to far-field shelf nutrient cycling. There is a westward increase in deep δ¹⁵N-NO₃, reaching 5.5‰ in Makarov basin explained by remineralisation of isotopically heavy organic matter laterally advected from the Siberian shelves. This study further identified heavier δ¹⁸O-NO3 signatures and lower N* in bottom waters of Makarov basin indicative of benthic denitrification impacting the bottom water nitrate pool. It is argued that this denitrification signal is a recent feature of Makarov basin developing in response to warming-induced increases in Siberian shelf productivity and changes in the control mechanisms of shelf exports. Benthic denitrification is expected to amplify and expand across the Central Arctic bottom waters with ongoing warming, as the concentration-based diffusion of O2 into the deep sediment layer decreases.
Under current climate change, the Arctic Ocean is becoming a greater sink for fixed nitrogen. As a result, the stoichiometry of Arctic exports to the North Atlantic is changing. Excess Si and PO4 exports through Fram Strait fuel 10% of nitrogen fixation in North Atlantic. These fluxes are expected to amplify with ongoing warming which can lead to autotrophic shifts and biogeochemical changes that may propagate at a global scale through the meridional overturning circulation
Diurnal variability in alkaline phosphatase activity and the potential role of zooplankton
Full text linkDaily light–dark cycles drive the circadian rhythm of many ocean processes including photosynthesis, gene expression, and zooplankton diel vertical migration (DVM). In phosphate deplete surface ocean regions, microbes produce metalloenzymes, such as alkaline phosphatases (AP), to access dissolved organic phosphorus. Here, we provide novel evidence of diurnal variation in AP activity (APA) in the subtropical North Atlantic using two independent datasets, with APA being two- to three-fold higher at night. We demonstrate that zooplankton are a source of AP and postulate that zooplankton DVM is a source of enhanced AP in the surface waters at night, with reduction or degradation of AP during the day. Our results challenge the current assumption that APA is linear over a 24-h period. While future ocean scenarios predict intensification and expansion of oceanic phosphate limitation, our findings indicate a role for zooplankton in regenerating phosphate that is currently missing in conceptual and numerical models
Nitrogen and carbon cycling in the South Atlantic Ocean: a stable isotope study along a 40°S transect (UK GEOTRACES)
No full textFixed N (nitrate, nitrite, and ammonium) is a limiting nutrient for photosynthesis in
the surface ocean. The rates and relative importance of N cycling processes,
however, are temporally and spatially complex, which hamper their direct
measurement and quantification. The South Atlantic subtropical front separates the
Atlantic Ocean and the subantarctic, an area which can elucidate information about
water masses both entering and leaving the basin. Through the GEOTRACES
programme, an oceanographic section across 40°S in the South Atlantic is used to
investigate biogeochemical cycling of nitrogen and carbon in this region.
Hydrographic data, in combination with the isotopic composition of nitrate (NO3-),
particulate organic carbon and particulate nitrogen (δ15NNO3, δ18ONO3, δ13CPOC,
δ15NPN), is used to provide integrative measurements for temporally and spatially
variable processes of the marine N-cycle and C-cycle.
A thorough examination of the stable isotope cycling of particulate and dissolved N
across the subtropical front is used to quantify the supply of fixed N to the mixed
layer. The relative importance of nitrate from the subsurface, N2 fixation, terrestrial
input and atmospheric deposition in supplying production is determined. Typically,
30-50% of the export flux in the subtropical water masses is sourced from N2 fixers
and up to 75% within the Brazil Current. This finding suggests that diazotrophs may
be abundant in the South West Atlantic providing a source of new N to this region.
To assess the basin scale N-cycling processes, the deep water masses were analysed
to reveal the origin and history of NO3-. Intermediate waters formed in the
subantarctic are enriched in δ15NNO3 and δ18ONO3 from partial utilisation by algae.
This enrichment in δ15NNO3 is not present in the subtropical North Atlantic or the
return flow of the North Atlantic Deep Water (NADW), which decreases from
~5.9‰ in the newly formed intermediate waters to ~4.8‰ in the NADW at 40°S.
The modification of isotopic signatures through the subtropical Atlantic can be
calculated as an incorporation of 26-36 Tg N yr-1 of newly fixed N from an isotopic
source of -1‰ (N2 fixation). The extent of N addition is higher than estimated rates
of N loss within the Atlantic and surpasses the amount of N deficit supplied to the
basin.
Fixed N inputs and losses through the global ocean are investigated by the
assessment of remineralised nitrate added to the ocean interior. A lower δ15N is
observed in Atlantic remineralised nitrate in comparison to the Pacific. The relative
importance of N2 fixation and pelagic denitrification within each ocean basin is
quantified and through this approach, N2 fixation rates are estimated at 92-116 Tg N
yr-1 in the Pacific and 24-32 Tg N yr-1 in the Indian Ocean. Combining Atlantic N2
fixation of ~32 Tg N yr-1 with Indo-Pacific, global N2 fixation rates can be estimated
at 142-184 Tg N yr-1. The high inputs in the Pacific suggest that excess P is the
dominant control on the success of N2 fixers. However, estimates of new N addition
to the Atlantic indicate other mechanisms such as the recycling efficiency of P and
supply of Fe to the surface ocean increase N2 fixation rates above this threshold.
The organic matter supplied to sediments is principally derived from phytoplankton
across the subtropical front. High organic content is associated with the productive
Brazil-Malvinas Confluence region where a diverse supply of nutrients sustains
elevated biomass. The Rio Plata outflow is characterised with high δ15NNO3 and
δ15NPN, suggesting denitrification processes occur in the estuary. A low δ13C source
associated with high Al concentrations is identified on the western slope, indicating a
supply of terrestrial derived C to the deep ocean. The fractionation of C uptake by
phytoplankton is assessed in subtropical and subantarctic waters. In the subantarctic,
CO2[aq] and growth rates determine the extent of C isotope fractionation. In this
region, low species diversity and a small range in cell size enable the fractionation
from CO2[aq] and growth rate to be expressed in phytoplankton. In subtropical water
masses a larger range of cell size is the principal determinant of C fractionation.
Increased surface area to volume is the main mechanism for increasing C uptake,
arguing against the use of δ13CPOC as a palaeoproxy. The low δ13CPOC and δ15NPN
observed in the subtropics (from C fractionation and N2 fixation) contrast the heavier
signatures in the subantarctic. These observations are propagated to the sediments,
wherein organic matter shifts are determined by changes in the subtropical front over
time. The results of this study have greatly improved knowledge of N and C cycling
within the South Atlantic, providing new insight into the cycling of these two
important elements in the surface and deep ocean, on a regional and global scale
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
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