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Sökning: WFRF:(Conley Daniel J.)

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1.
  • Smith, Jennifer A, et al. (författare)
  • Genome-wide association study identifies 74 loci associated with educational attainment
  • 2016
  • Ingår i: Nature (London). - : Springer Science and Business Media LLC. - 1476-4687 .- 0028-0836. ; 533:7604, s. 539-542
  • Tidskriftsartikel (refereegranskat)abstract
    • Educational attainment is strongly influenced by social and other environmental factors, but genetic factors are estimated to account for at least 20% of the variation across individuals. Here we report the results of a genome-wide association study (GWAS) for educational attainment that extends our earlier discovery sample of 101,069 individuals to 293,723 individuals, and a replication study in an independent sample of 111,349 individuals from the UK Biobank. We identify 74 genome-wide significant loci associated with the number of years of schooling completed. Single-nucleotide polymorphisms associated with educational attainment are disproportionately found in genomic regions regulating gene expression in the fetal brain. Candidate genes are preferentially expressed in neural tissue, especially during the prenatal period, and enriched for biological pathways involved in neural development. Our findings demonstrate that, even for a behavioural phenotype that is mostly environmentally determined, a well-powered GWAS identifies replicable associated genetic variants that suggest biologically relevant pathways. Because educational attainment is measured in large numbers of individuals, it will continue to be useful as a proxy phenotype in efforts to characterize the genetic influences of related phenotypes, including cognition and neuropsychiatric diseases.
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2.
  • Frazier-Wood, Alexis C., et al. (författare)
  • Genetic variants associated with subjective well-being, depressive symptoms, and neuroticism identified through genome-wide analyses
  • 2016
  • Ingår i: Nature Genetics. - : Nature Research (part of Springer Nature). - 1061-4036 .- 1546-1718. ; 48, s. 624-
  • Tidskriftsartikel (refereegranskat)abstract
    • Very few genetic variants have been associated with depression and neuroticism, likely because of limitations on sample size in previous studies. Subjective well-being, a phenotype that is genetically correlated with both of these traits, has not yet been studied with genome-wide data. We conducted genome-wide association studies of three phenotypes: subjective well-being (n = 298,420), depressive symptoms (n = 161,460), and neuroticism (n = 170,911). We identify 3 variants associated with subjective well-being, 2 variants associated with depressive symptoms, and 11 variants associated with neuroticism, including 2 inversion polymorphisms. The two loci associated with depressive symptoms replicate in an independent depression sample. Joint analyses that exploit the high genetic correlations between the phenotypes (vertical bar(p) over cap vertical bar approximate to 0.8) strengthen the overall credibility of the findings and allow us to identify additional variants. Across our phenotypes, loci regulating expression in central nervous system and adrenal or pancreas tissues are strongly enriched for association.
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3.
  • Clymans, Wim, et al. (författare)
  • Silica uptake and release in live and decaying biomass in a northern hardwood forest
  • 2016
  • Ingår i: Ecology. - : Wiley. - 0012-9658 .- 1939-9170. ; 97:11, s. 3044-3057
  • Tidskriftsartikel (refereegranskat)abstract
    • In terrestrial ecosystems, a large portion (20-80%) of the dissolved Si (DSi) in soil solution has passed through vegetation. While the importance of this terrestrial Si filter is generally accepted, few data exist on the pools and fluxes of Si in forest vegetation and the rate of release of Si from decomposing plant tissues. We quantified the pools and fluxes of Si through vegetation and coarse woody debris (CWD) in a northern hardwood forest ecosystem (Watershed 6, W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire, USA. Previous work suggested that the decomposition of CWD may have significantly contributed to an excess of DSi reported in stream-waters following experimental deforestation of Watershed 2 (W2) at the HBEF. We found that woody biomass (wood+bark) and foliage account for approximately 65% and 31%, respectively, of the total Si in biomass at the HBEF. During the decay of American beech (Fagus grandifolia) boles, Si loss tracked the whole-bole mass loss, while yellow birch (Betula alleghaniensis) and sugar maple (Acer saccharum) decomposition resulted in a preferential Si retention of up to 30% after 16yr. A power-law model for the changes in wood and bark Si concentrations during decomposition, in combination with an exponential model for whole-bole mass loss, successfully reproduced Si dynamics in decaying boles. Our data suggest that a minimum of 50% of the DSi annually produced in the soil of a biogeochemical reference watershed (W6) derives from biogenic Si (BSi) dissolution. The major source is fresh litter, whereas only similar to 2% comes from the decay of CWD. Decay of tree boles could only account for 9% of the excess DSi release observed following the experimental deforestation of W2. Therefore, elevated DSi concentrations after forest disturbance are largely derived from other sources (e.g., dissolution of BSi from forest floor soils and/or mineral weathering).
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4.
  • Leavitt, Peter R., et al. (författare)
  • Paleolimnological evidence of the effects on lakes of energy and mass transfer from climate and humans
  • 2009
  • Ingår i: Limnology and Oceanography. - 0024-3590 .- 1939-5590. ; 54:6, s. 2330-2348
  • Forskningsöversikt (refereegranskat)abstract
    • The premise of this article is that climate effects on lakes can be quantified most effectively by the integration of process-oriented limnological studies with paleolimnological research, particularly when both disciplines operate within a common conceptual framework. To this end, the energy (E)-mass (m) flux framework (Em flux) is developed and applied to selected retrospective studies to demonstrate that climate variability regulates lake structure and function over diverse temporal and spatial scales through four main pathways: rapid direct transfer of E to the lake surface by irradiance, heat, and wind; slow indirect effects of E via changes in terrestrial development and subsequent m subsidies to lakes; direct influx of m as precipitation, particles, and solutes from the atmosphere; and indirect influx of water, suspended particles, and dissolved substances from the catchment. Sedimentary analyses are used to illustrate the unique effects of each pathway on lakes but suggest that interactions among mechanisms are complex and depend on the landscape position of lakes, catchment characteristics, the range of temporal variation of individual pathways, ontogenetic changes in lake basins, and the selective effects of humans on m transfers. In particular, preliminary synthesis suggests that m influx can overwhelm the direct effects of E transfer to lakes, especially when anthropogenic activities alter m subsidies from catchments.
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5.
  • Lee, James J, et al. (författare)
  • Gene discovery and polygenic prediction from a genome-wide association study of educational attainment in 1.1 million individuals.
  • 2018
  • Ingår i: Nature genetics. - : Springer Science and Business Media LLC. - 1546-1718 .- 1061-4036. ; 50:8, s. 1112-1121
  • Tidskriftsartikel (refereegranskat)abstract
    • Here we conducted a large-scale genetic association analysis of educational attainment in a sample of approximately 1.1million individuals and identify 1,271independent genome-wide-significant SNPs. For the SNPs taken together, we found evidence of heterogeneous effects across environments. The SNPs implicate genes involved in brain-development processes and neuron-to-neuron communication. In a separate analysis of the X chromosome, we identify 10independent genome-wide-significant SNPs and estimate a SNP heritability of around 0.3% in both men and women, consistent with partial dosage compensation. A joint (multi-phenotype) analysis of educational attainment and three related cognitive phenotypes generates polygenic scores that explain 11-13% of the variance in educational attainment and 7-10% of the variance in cognitive performance. This prediction accuracy substantially increases the utility of polygenic scores as tools in research.
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6.
  • Norkko, J., et al. (författare)
  • A welcome can of worms? Hypoxia mitigation by an invasive species
  • 2012
  • Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 18:2, s. 422-434
  • Tidskriftsartikel (refereegranskat)abstract
    • Invasive species and bottom-water hypoxia both constitute major global threats to the diversity and integrity of marine ecosystems. These stressors may interact with unexpected consequences, as invasive species that require an initial environmental disturbance to become established can subsequently become important drivers of ecological change. There is recent evidence that improved bottom-water oxygen conditions in coastal areas of the northern Baltic Sea coincide with increased abundances of the invasive polychaetes Marenzelleria spp. Using a reactive-transport model, we demonstrate that the long-term bioirrigation activities of dense Marenzelleria populations have a major impact on sedimentary phosphorus dynamics. This may facilitate the switch from a seasonally hypoxic system back to a normoxic system by reducing the potential for sediment-induced eutrophication in the upper water column. In contrast to short-term laboratory experiments, our simulations, which cover a 10-year period, show that Marenzelleria has the potential to enhance long-term phosphorus retention in muddy sediments. Over time bioirrigation leads to a substantial increase in the iron-bound phosphorus content of sediments while reducing the concentration of labile organic carbon. As surface sediments are maintained oxic, iron oxyhydroxides are able to persist and age into more refractory forms. The model illustrates mechanisms through which Marenzelleria can act as a driver of ecological change, although hypoxic disturbance or natural population declines in native species may be needed for them to initially become established. Invasive species are generally considered to have a negative impact; however, we show here that one of the main recent invaders in the Baltic Sea may provide important ecosystem services. This may be of particular importance in low-diversity systems, where disturbances may dramatically alter ecosystem services due to low functional redundancy. Thus, an environmental problem in one region may be either exacerbated or alleviated by a single species from another region, with potentially ecosystem-wide consequences.
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7.
  • Struyf, E., et al. (författare)
  • The Role of Vegetation in the Okavango Delta Silica Sink
  • 2015
  • Ingår i: Wetlands (Wilmington, N.C.). - : Springer Science and Business Media LLC. - 0277-5212 .- 1943-6246. ; 35:1, s. 171-181
  • Tidskriftsartikel (refereegranskat)abstract
    • We assessed the role of vegetation and hydrology in the Si cycle in the Okavango Delta. Our results show a large storage of biogenic Si (BSi) in vegetation and the sediments. The biological storage is among the highest observed so far for any ecosystem worldwide. Floodplain vegetation accumulates similar amounts of BSi in both the temporary floodplains and the permanent floodplains, with most values observed between 20 and 100 g Si m(-2). This vegetation Si, after litterfall, contributes to a large biogenic Si storage in the sediments. In temporary floodplains, sediments contain less BSi (375-1950 g Si m(-2) in the top 5 cm) than in the permanent floodplains (1950-3600 g Si m(-2) in the top 5 cm). BSi concentrations in the floodplain sediments decline exponentially indicating rapid dissolution. In the occasional and seasonal floodplains, unidirectional solute transfer from floodplains to the islands will remove Si from the riverine systems. Our work clearly emphasizes the crucial role of floodplains and wetlands in Si transport through tropical rivers, and the potential interference of hydrology with this role.
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8.
  • Sutton, Jill N., et al. (författare)
  • A review of the stable isotope bio-geochemistry of the global silicon cycle and its associated trace elements
  • 2018
  • Ingår i: Frontiers in Earth Science. - : Frontiers Media SA. - 2296-6463. ; 5
  • Forskningsöversikt (refereegranskat)abstract
    • Silicon (Si) is the second most abundant element in the Earth’s crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, δ13 C, δ15 N, δ18 O, δ30 Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies.
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9.
  • Cox, T. J. S., et al. (författare)
  • A macro-tidal freshwater ecosystem recovering from hypereutrophication: the Schelde case study
  • 2009
  • Ingår i: Biogeosciences. - 1726-4189. ; 6:12, s. 2935-2948
  • Tidskriftsartikel (refereegranskat)abstract
    • We report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. After decades of high inorganic nutrient concentrations and recurring anoxia and hypoxia, we observe a paradoxical increase in chlorophyll-a concentrations with decreasing nutrient inputs. We hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows a high algal biomass, net oxygen production equilibrium with low ammonia inputs, and a low algal biomass, net oxygen consumption equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. Although not intentional, the numerical output of this model corresponds to observations, giving extra support for assumption of algal growth inhibition. Due to potential algal growth inhibition, the recovery of hypereutrophied systems towards a classical eutrophied state, will need reduction of waste loads below certain thresholds and will be accompanied by large fluctuations in oxygen concentrations. We conclude that also flow-through systems, heavily influenced by external forcings which partly mask internal system dynamics, can display multiple stable states.
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10.
  • Frings, Patrick J, 1986-, et al. (författare)
  • Tracing silicon cycling in the Okavango Delta, a sub-tropical flood-pulse wetland using silicon isotopes
  • 2014
  • Ingår i: Geochimica et Cosmochimica Acta. - 0016-7037 .- 1872-9533. ; 142:0, s. 132-148
  • Tidskriftsartikel (refereegranskat)abstract
    • Chemical weathering of silicate minerals releases elements into solution whereas the neoformation of secondary minerals works in the opposite direction, potentially confounding estimates of silicate weathering rates. Silicon isotopes (δ30Si) may be a useful tool to investigate these processes. Here, we present 82 δ30Si measurements from surface waters, pore waters, biogenic silica (BSi), clays, sand and vegetation from the Okavango Delta, Botswana, a freshwater sub-tropical, flood-pulse wetland. Hydrologically, the Okavango is dominated by evapotranspiration water losses to the atmosphere. It receives an annual pulse of water that inundates seasonal floodplains, while river baseflow is sufficient to maintain a permanent floodplain. δ30Si in dissolved silica (DSi) in surface waters along a 300 km transect at near-peak flood show a limited range (0.36–1.19‰), implying the Delta is well buffered by a balance of processes adding and removing DSi from the surface water. A key control on DSi concentrations is the uptake, production of BSi and recycling of Si by aquatic vegetation, although the net isotopic effect is necessarily small since all BSi re-dissolves on short timescales. In the sediments, BSi δ30Si (n = 30) ranges from −1.49‰ to +0.31‰ and during dissolution, residual BSi tends towards higher δ30Si. The data permit a field-based estimate of the fractionation associated with BSi dissolution, ε30BSi-DSi = −0.26‰, though it is unclear if this is an artefact of the process of dissolution. Clay δ30Si ranges from −0.97‰ to +0.10‰, (n = 15, mean = −0.31‰) and include the highest values yet published, which we speculate may be due to an equilibrium isotope effect during diagenetic transformation of BSi. Two key trends in surface water DSi δ30Si merit further examination: declining δ30Si in an area roughly corresponding to the permanent floodplains despite net DSi removal, and increasing δ30Si in the area corresponding to the seasonal floodplains. We infer that evaporative enrichment of surface waters creates two contrasting regimes. Chemical weathering of low δ30Si phases releases low δ30Si DSi in the relatively dilute waters of the permanent floodplains, whereas silicon removal via clay formation or vegetation uptake is the dominant process in the more enriched, seasonal floodplains.
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11.
  • Okbay, Aysu, et al. (författare)
  • Polygenic prediction of educational attainment within and between families from genome-wide association analyses in 3 million individuals.
  • 2022
  • Ingår i: Nature genetics. - : Springer Science and Business Media LLC. - 1546-1718 .- 1061-4036. ; 54:4, s. 437-449
  • Tidskriftsartikel (refereegranskat)abstract
    • We conduct a genome-wide association study (GWAS) of educational attainment (EA) in a sample of ~3 million individuals and identify 3,952 approximately uncorrelated genome-wide-significant single-nucleotide polymorphisms (SNPs). A genome-wide polygenic predictor, or polygenic index (PGI), explains 12-16% of EA variance and contributes to risk prediction for ten diseases. Direct effects (i.e., controlling for parental PGIs) explain roughly half the PGI's magnitude of association with EA and other phenotypes. The correlation between mate-pair PGIs is far too large to be consistent with phenotypic assortment alone, implying additional assortment on PGI-associated factors. In an additional GWAS of dominance deviations from the additive model, we identify no genome-wide-significant SNPs, and a separate X-chromosome additive GWAS identifies 57.
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12.
  • Rabalais, N. N., et al. (författare)
  • Eutrophication-driven deoxygenation in the coastal ocean
  • 2014
  • Ingår i: Oceanography. - : The Oceanography Society. - 1042-8275. ; 27:1, s. 172-183
  • Tidskriftsartikel (refereegranskat)abstract
    • Human activities, especially increased nutrient loads that set in motion a cascading chain of events related to eutrophication, accelerate development of hypoxia (lower oxygen concentration) in many areas of the world's coastal ocean. Climate changes and extreme weather events may modify hypoxia. Organismal and fisheries effects are at the heart of the coastal hypoxia issue, but more subtle regime shifts and trophic interactions are also cause for concern. The chemical milieu associated with declining dissolved oxygen concentrations affects the biogeochemical cycling of oxygen, carbon, nitrogen, phosphorus, silica, trace metals, and sulfide as observed in water column processes, shifts in sediment biogeochemistry, and increases in carbon, nitrogen, and sulfur, as well as shifts in their stable isotopes, in recently accumulated sediments. © 2014 by The Oceanography Society. All rights reserved.
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13.
  • Schoelynck, Jonas, et al. (författare)
  • The trapping of organic matter within plant patches in the channels of the Okavango Delta : a matter of quality
  • 2017
  • Ingår i: Aquatic Sciences. - : Springer Science and Business Media LLC. - 1015-1621 .- 1420-9055. ; 79:3, s. 661-674
  • Tidskriftsartikel (refereegranskat)abstract
    • The role of in-stream aquatic vegetation as ecosystem engineers in the distribution of organic matter was investigated in the Okavango Delta, one of the world’s largest oligotrophic wetlands. The Okavango channel beds are covered up to 50% with submerged macrophyte patches. By accumulating and concentrating organic matter in the sediments below the patches, macrophytes are likely able to locally forestall a deficiency of nutrients. Up to 21 times more N, 18 times more C, 13 times more P and 6 times more Si can be found in vegetated sediments compared to non-vegetated sediments. Nutrient specific accumulation relates to its relative scarcity in the overlaying water. There is a depletion of dissolved N relative to P, whereas Si is relatively abundant. The Okavango Delta water can generally be characterised as oligotrophic based on plant species composition (e.g. presence of carnivorous plants and absence of floating plants), low plant N:P ratios, and low nutrient- and element-concentrations. Local mineralization and intensified nutrient cycling in the sediments is hypothesized to be crucial for the macrophytes’ survival because it provides a key source of the essential nutrients which plants otherwise cannot obtain in sufficient quantities from the nutrient poor water. By engineering the ecosystem as such, channel vegetation also retards the loss of elements and nutrients to island groundwater flow, contributing to one of the key processes driving the high productivity of the Okavango Delta, making it unique among its kind.
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14.
  • Alvarez, Belinda, et al. (författare)
  • Assessing the potential of sponges (Porifera) as indicators of ocean dissolved Si concentrations
  • 2017
  • Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4
  • Tidskriftsartikel (refereegranskat)abstract
    • We explore the distribution of sponges along dissolved silica (dSi) concentration gradients to test whether sponge assemblages are related to dSi and to assess the validity of fossil sponges as a palaeoecological tool for inferring dSi concentrations of the past oceans. We extracted sponge records from the publically available Global Biodiversity Information Facility (GBIF) database and linked these records with ocean physiochemical data to evaluate if there is any correspondence between dSi concentrations of the waters sponges inhabit and their distribution. Over 320,000 records of Porifera were available, of which 62,360 met strict quality control criteria. Our analyses was limited to the taxonomic levels of family, order and class. Because dSi concentration is correlated with depth in the modern ocean, we also explored sponge taxa distributions as a function of depth. We observe that while some sponge taxa appear to have dSi preferences (e.g., class Hexactinellida occurs mostly at high dSi), the overall distribution of sponge orders and families along dSi gradients is not sufficiently differentiated to unambiguously relate dSi concentrations to sponge taxa assemblages. We also observe that sponge taxa tend to be similarly distributed along a depth gradient. In other words, both dSi and/or another variable that depth is a surrogate for, may play a role in controlling sponge spatial distribution and the challenge is to distinguish between the two. We conclude that inferences about palaeo-dSi concentrations drawn from the abundance of sponges in the stratigraphic records must be treated cautiously as these animals are adapted to a great range of dSi conditions and likely other underlying variables that are related to depth. Our analysis provides a quantification of the dSi ranges of common sponge taxa, expands on previous knowledge related to their bathymetry preferences and suggest that sponge taxa assemblages are not related to particular dSi conditions.
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15.
  • Ashuiev, Anton, et al. (författare)
  • Geometry and electronic structure of Yb(iii)[CH(SiMe3)2]3 from EPR and solid-state NMR augmented by computations
  • 2024
  • Ingår i: Physical Chemistry, Chemical Physics - PCCP. - 1463-9076 .- 1463-9084. ; 26:11, s. 8734-8747
  • Tidskriftsartikel (refereegranskat)abstract
    • Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and – still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre–two-electron Yb-γ-Me-β-Si secondary metal–ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb–carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.
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16.
  • Bianchi, Thomas S., et al. (författare)
  • Redox Effects on Organic Matter Storage in Coastal Sediments during the Holocene : A BiomarkerProxy Perspective
  • 2016
  • Ingår i: Annual Review of Earth and Planetary Sciences. - : Annual Reviews. - 0084-6597 .- 1545-4495. ; 44, s. 295-319
  • Forskningsöversikt (refereegranskat)abstract
    • Coastal margins play a significant role in the burial of organic matter (OM) on Earth. These margins vary considerably with respect to their efficiency in OM burial and to the amounts and periodicity of their OM delivery, depending in large part on whether they are passive or active margins. In the context of global warming, these coastal regions are expected to experience higher water temperatures, changes in riverine inputs of OM, and sea level rise. Low-oxygen conditions continue to expand around the globe in estuarine regions (i.e., hypoxic zones) and shelf regions (i.e., oxygen minimum zones), which will impact the amounts and sources of OM stored in these regions. In this review, we explore how these changes are impacting the storage of OM and the preservation of sedimentary biomarkers, used as proxies to reconstruct environmental change, in coastal margins.
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17.
  • Brown, Sabrina R., et al. (författare)
  • Multi-proxy record of Holocene paleoenvironmental conditions from Yellowstone Lake, Wyoming, USA
  • 2021
  • Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 274
  • Tidskriftsartikel (refereegranskat)abstract
    • A composite 11.82 m-long (9876–67 cal yr BP) sediment record from Yellowstone Lake, Wyoming was analyzed using a robust set of biological and geochemical proxies to investigate the paleoenvironmental evolution of the lake and its catchment in response to long-term climate forcing. Oxygen isotopes from diatom frustules were analyzed to reconstruct Holocene climate changes, and pollen, charcoal, diatom assemblages, and biogenic silica provided information on terrestrial and limnological responses. The long-term trends recorded in the terrestrial and limnic ecosystems over the last 9800 years reflect the influence of changes in the amplification of the seasonal cycle of insolation on regional climate. The early Holocene (9880–6700 cal yr BP) summer insolation maximum and strengthening of the northeastern Pacific subtropical high-pressure system created warm dry conditions and decreasing summer insolation in the middle (6700–3000 cal yr BP) and late (3000–67 cal yr BP) Holocene resulted in progressively cooler, wetter conditions. Submillenial climate variation is also apparent, with a wetter/cooler interval between 7000 and 6800 cal yr BP and warmer and/or drier conditions from 4500 to 3000 cal yr BP and at ca. 1100 cal yr BP. These data show that the Yellowstone Lake basin had a climate history typical of a summer-dry region, which helps to better define the spatial variability of Holocene climate in the Greater Yellowstone Ecosystem.
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18.
  • Brylka, Karolina, et al. (författare)
  • Looking for the oldest diatoms
  • 2024
  • Ingår i: Marine Micropaleontology. - 0377-8398. ; :190
  • Tidskriftsartikel (refereegranskat)abstract
    • Paleontological observations of ancient flora and fauna provide powerful insights into past diversity and relationshipdynamics between organisms and their environments. Diatoms are globally distributed protists thatinfluence major biogeochemical cycles and sustain oceanic food webs. The fossil diatom record extends 120million years back to the Early Cretaceous where rare deposits were discovered worldwide and are occasionallyrepresented by diverse communities. However scarce, the taxonomic richness and geographical spread of thesediatom communities suggest prior evolutionary events and therefore earlier deposits. To complement the existingfossil information and to discover diatom deposits predating 120 Ma, we examined 33 study sites from cores andoutcrops across oceans and continents. These efforts did not generate new fossil discoveries, however. Ourassessment suggests biogenic silica that comprises the cell wall of diatoms was likely dissolved from Mesozoicsediments through diagenetic processes. Altogether, the search for the oldest diatoms must continue but shouldtarget sediments that experienced shallow burial and concretions.
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19.
  • Cartier, Rosine, et al. (författare)
  • Hydrological changes in Yellowstone Lake (USA) during the Holocene based on the analysis of oxygen isotopes in diatoms
  • 2019
  • Ingår i: Hydrological changes in Yellowstone Lake (USA) during the Holocene based on the analysis of oxygen isotopes in diatoms.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Northern Yellowstone Lake is on the southeast edge of the 631-ka Yellowstone caldera and is an area with high heat flow, high seismicity, and an abundance of active hydrothermal features and structures. Several large hydrothermal explosions since the last glacial recession formed craters of more than 100 m in diameter. These large craters raise the question on how climate and hydrological changes have affected the hydrothermal system and the lake ecosystem at millennial timescales.This study focuses on an 11.6-m-long core collected in 2016 in the Lake Hotel graben covering the last 9,900 cal years according to radiocarbon ages. Past hydrological changes were inferred from oxygen isotopes values of biogenic silica that comprises the cell wall of the diatoms. d 18O values reflect silica-lake water fractionation during diatom growth. The d 18O values vary according to changes in sources of precipitation, supply of runoff by tributaries, lake water temperature, and evaporation. Currently, precipitation occurs mainly as winter snow from weather systems originating in the Pacific.Periods of high d 18O in diatoms (enrichment in the heavy isotope) occur from the base of the record 9900 to ca. 7500 cal years BP, from 4500 to 3000 cal years BP and ca. 1000 cal years BP. These isotopic enrichments have been interpreted as to be mostly the result of increased water evaporation and/or reduced snowmelt flowing into the lake from the Yellowstone River and other tributaries. This inference is supported by d 18O measurements from water samples showing that lake water is progressively more evaporated with increased distance from the Yellowstone River inlet . The base of the record also is characterized by lower abundance of Pinus pollen suggesting a more open Pinus contorta forest until 5800 cal years BP, with more-frequent fire than today. Additionally, a long-term decrease in d 18Odiatomin the record and a progressive increase in the duration of spring water mixing shown by diatom assemblages (i.e. higher A. subarctica/S. minutulusratio) are associated with decreased summer insolation during the Holocene. These results compare well with other paleoclimatic records from the Yellowstone region that show a transition to cool, wet conditions in the late Holocene.
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20.
  • Churakova, Yelena, et al. (författare)
  • Biogenic silica accumulation in picoeukaryotes : Novel players in the marine silica cycle
  • 2023
  • Ingår i: Environmental Microbiology Reports. - : John Wiley & Sons. - 1758-2229. ; 15:4, s. 282-290
  • Tidskriftsartikel (refereegranskat)abstract
    • It is well known that the biological control of oceanic silica cycling is dominated by diatoms, with sponges and radiolarians playing additional roles. Recent studies have revealed that some smaller marine organisms (e.g. the picocyanobacterium Synechococcus) also take up silicic acid (dissolved silica, dSi) and accumulate silica, despite not exhibiting silicon dependent cellular structures. Here, we show biogenic silica (bSi) accumulation in five strains of picoeukaryotes (<2-3 mu m), including three novel isolates from the Baltic Sea, and two marine species (Ostreococcus tauri and Micromonas commoda), in cultures grown with added dSi (100 mu M). Average bSi accumulation in these novel biosilicifiers was between 30 and 92 amol Si cell(-1). Growth rate and cell size of the picoeukaryotes were not affected by dSi addition. Still, the purpose of bSi accumulation in these smaller eukaryotic organisms lacking silicon dependent structures remains unclear. In line with the increasing recognition of picoeukaryotes in biogeochemical cycling, our findings suggest that they can also play a significant role in silica cycling.
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21.
  • Conley, Daniel J., et al. (författare)
  • Biosilicification drives a decline of dissolved si in the oceans through geologic time
  • 2017
  • Ingår i: Frontiers in Marine Science. - : Frontiers Media SA. - 2296-7745. ; 4:DEC
  • Forskningsöversikt (refereegranskat)abstract
    • Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis.
  •  
22.
  • Fontorbe, Guillaume, et al. (författare)
  • A silicon depleted North Atlantic since the Palaeogene : Evidence from sponge and radiolarian silicon isotopes
  • 2016
  • Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X. ; 453, s. 67-77
  • Tidskriftsartikel (refereegranskat)abstract
    • Despite being one of Earth's major geochemical cycles, the evolution of the silicon cycle has received little attention and changes in oceanic dissolved silica (DSi) concentration through geologic time remain poorly constrained. Silicon isotope ratios (expressed as δ30Si) in marine microfossils are becoming increasingly recognised for their ability to provide insight into silicon cycling. In particular, the δ30Si of siliceous sponge spicules has been demonstrated to be a useful proxy for past DSi concentrations. We analysed δ30Si in radiolarian tests and sponge spicules from the Blake Nose Palaeoceanographic Transect (ODP Leg 171B) spanning the Palaeocene–Eocene (ca. 60–30 Ma). Our δ30Si results range from +0.32 to +1.67‰ and −0.48 to +0.63‰ for the radiolarian and sponge records, respectively. Using an established relationship between ambient dissolved Si (DSi) concentrations and the magnitude of silicon isotope fractionation in siliceous sponges, we demonstrate that the Western North Atlantic was DSi deplete during the Palaeocene–Eocene throughout the water column, a conclusion that is robust to a range of assumptions and uncertainties. These data can constitute constraints on reconstructions of past-ocean circulation. Previous work has suggested ocean DSi concentrations were higher than modern ocean concentrations prior to the Cenozoic and has posited a drawdown during the Early Palaeogene due to the evolutionary expansion of diatoms. Our results challenge such an interpretation. We suggest here that if such a global decrease in oceanic DSi concentrations occurred, it must predate 60 Ma.
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23.
  • Fontorbe, Guillaume, et al. (författare)
  • Constraints on Earth System Functioning at the Paleocene-Eocene Thermal Maximum From the Marine Silicon Cycle
  • 2020
  • Ingår i: Paleoceanography and Paleoclimatology. - 2572-4517. ; 35:5
  • Tidskriftsartikel (refereegranskat)abstract
    • The Paleocene-Eocene Thermal Maximum (PETM, ca. 56 Ma) is marked by a negative carbon isotope excursion (CIE) and increased global temperatures. The CIE is thought to result from the release of 13C-depleted carbon, although the source(s) of carbon and triggers for its release, its rate of release, and the mechanisms by which the Earth system recovered are all debated. Many of the proposed mechanisms for the onset and recovery phases of the PETM make testable predictions about the marine silica cycle, making silicon isotope records a promising tool to address open questions about the PETM. We analyzed silicon isotope ratios (δ30Si) in radiolarian tests and sponge spicules from the Western North Atlantic (ODP Site 1051) across the PETM. Radiolarian δ30Si decreases by 0.6‰ from a background of 1‰ coeval with the CIE, while sponge δ30Si remains consistent at 0.2‰. Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long-term carbon sink. We find no resolvable evidence for a volcanic trigger for carbon release, or for a change in regional oceanography. Better understanding of radiolarian Si isotope fractionation and more Si isotope records spanning the PETM are needed to confirm the global validity of these conclusions, but they highlight how the coupling between the silica and carbon cycles can be exploited to yield insight into the functioning of the Earth system.
  •  
24.
  • Fontorbe, Guillaume, et al. (författare)
  • Enrichment of dissolved silica in the deep equatorial Pacific during the Eocene-Oligocene
  • 2017
  • Ingår i: Paleoceanography. - 0883-8305 .- 1944-9186. ; 32, s. 848-863
  • Tidskriftsartikel (refereegranskat)abstract
    • Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23 Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from 3.16 to +0.18‰ and from 0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37 Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past. 
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25.
  • Frings, Patrick J, 1986-, et al. (författare)
  • Silicate weathering in the Ganges alluvial plain
  • 2015
  • Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 1385-013X .- 0012-821X. ; 427, s. 136-148
  • Tidskriftsartikel (refereegranskat)abstract
    • The Ganges is one of the world's largest rivers and lies at the heart of a body of literature that investigates the interaction between mountain orogeny, weathering and global climate change. Three regions can be recognised in the Ganges basin, with the Himalayan orogeny to the north and the plateaus of peninsular India to the south together delimiting the Ganges alluvial plain. Despite constituting approximately 80% of the basin, weathering processes in the peninsula and alluvial plain have received little attention. Here we present an analysis of 51 water samples along a transect of the alluvial plain, including all major tributaries. We focus on the geochemistry of silicon and its isotopes. Area normalised dissolved Si yields are approximately twice as high in rivers of Himalaya origin than the plain and peninsular tributaries (82, 51 and 32 kmol SiO2 km(-2) yr(-1), respectively). Such dissolved Si fluxes are not widely used as weathering rate indicators because a large but variable fraction of the DSi mobilised during the initial weathering process is retained in secondary clay minerals. However, the silicon isotopic composition of dissolved Si (expressed as delta Si-30) varies from +0.8 parts per thousand in the Ganges mainstem at the Himalaya front to +3.0 parts per thousand in alluvial plain streams and appears to be controlled by weathering congruency, i.e. by the degree of incorporation of Si into secondary phases. The higher delta Si-30 values therefore reflect decreasing weathering congruency in the lowland river catchments. This is exploited to quantify the degree of removal using a Rayleigh isotope mass balance model, and consequently derive initial silica mobilisation rates of 200, 150 and 107 kmol SiO2 km(-2) yr(-1), for the Himalaya, peninsular India and the alluvial plain, respectively. Because the non-Himalayan regions dominate the catchment area, the majority of initial silica mobilisation from primary minerals occurs in the alluvial plain and peninsular catchment (41% and 34%, respectively). (C) 2015 The Authors. Published by Elsevier B.V.
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26.
  • Frings, Patrick J., et al. (författare)
  • The continental Si cycle and its impact on the ocean Si isotope budget
  • 2016
  • Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541. ; 425, s. 12-36
  • Tidskriftsartikel (refereegranskat)abstract
    • The silicon isotope composition of biogenic silica (δ30SiBSi) in the ocean is a function of the δ30Si of the available dissolved Si (DSi; H2SiO4), the degree of utilisation of the available DSi, and, for some organisms, the concentration of DSi. This makes δ30SiBSi in sediment archives a promising proxy for past DSi concentrations and utilisation. At steady-state, mean δ30SiBSi must equal a weighted average of the inputs, the majority of which are of continental origin. Variation in the functioning of the continental Si cycle on timescales similar to the residence time of DSi in the ocean (~10 ka) may therefore contribute to downcore variability in δ30SiBSi on millennial or longer timescales. The direction and magnitude of change in published δ30SiBSi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers. They document glacial values that are typically 0.5 to 1.0‰ lower than interglacial values and together hint at coherent and predictable glacial-interglacial variability in whole-ocean δ30Si driven by a change in mean δ30Si of the inputs. In this contribution, we review the modern inputs of DSi to the ocean and the controls on their isotopic composition, and assess the evidence for their variability on millennial-plus timescales.Today, 9.55 × 1012 mol yr-1 DSi enters the ocean, of which roughly 64% and 25% are direct riverine inputs of DSi, and DSi from dissolution of aeolian and riverborne sediment, respectively. The remainder derives from alteration or weathering of the ocean crust. Each input has a characteristic δ30Si, with our current best estimate for a weighted mean being 0.74‰, although much work remains to be done to characterise the individual fluxes. Many aspects of the continental Si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean δ30Si of DSi entering the ocean. These changes relate to i) a cooler, drier glacial climate, ii) lowered sea level and the exposure of continental shelves, iii) the presence of large continental ice-sheets, and iv) altered vegetation zonation. Using a simple box-model with a Monte-Carlo approach to parameterisation, we find that a transition from a hypothesised glacial continental Si cycle to the modern Si cycle can drive an increase in whole ocean δ30Si of comparable rate and magnitude to that recorded in δ30SiBSi. This implies that we may need to revisit our understanding of aspects of the Si cycle in the glacial ocean. Although we focus on the transition from the last glacial, our synthesis suggests that the continental Si cycle should be seen as a potential contributory factor to any variability observed in ocean δ30SiBSi on millennial or longer timescales.
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27.
  • Greaves, Deborah, et al. (författare)
  • Environmental Impact Assessment : Gathering experiences from wave energy test centres in Europe
  • 2016
  • Ingår i: International Journal of Marine Energy. - : Elsevier BV. - 2214-1669. ; 14, s. 68-79
  • Tidskriftsartikel (refereegranskat)abstract
    • The wave energy industry is an emerging sector and a new user of maritime space that has potential to contribute significantly to the EU renewable energy goals. International and national regulatory frameworks necessitate Environmental Impact Assessments (EIA) that provide important data to inform development consent decisions. Here we have evaluated experience related to the assessment programmes at EU wave energy test centres combined with knowledge gained from EIA produced for other similar renewable energy developments. From this we have identified key receptors of concern, as well as the type and magnitude of impacts which may be expected. The key environmental receptors of concern for wave energy EIA include the physical environment (e.g. morphology, waves and current) and flora and fauna(1) as represented by marine mammals, seabirds, benthos, fish and shellfish. From a review of the EIAs performed at wave energy test centres, we identified several lessons regarding the wave energy EIA process. There is clear evidence that the receptors of primary interest are dependent on factors such as the local environmental characteristics, the presence/absence of protected species and the regulatory authority under which the EIA is performed. Furthermore, it is recommended that concerns relating to cumulative impacts, from an expanding level of wave energy development taking place in a background of growing utilisation of the marine environment, which are largely unknown at this early stage of the industry may be comprehensively addressed at the national level as part of a Strategic Environmental Assessment (EIA) and/or in Maritime Spatial Planning (MSP) and that it should be regularly reassessed.
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28.
  • Kemp, W. M., et al. (författare)
  • Temporal responses of coastal hypoxia to nutrient loading and physical controls
  • 2009
  • Ingår i: Biogeosciences. - 1726-4189. ; 6:12, s. 2985-3008
  • Forskningsöversikt (refereegranskat)abstract
    • The incidence and intensity of hypoxic waters in coastal aquatic ecosystems has been expanding in recent decades coincident with eutrophication of the coastal zone. Worldwide, there is strong interest in reducing the size and duration of hypoxia in coastal waters, because hypoxia causes negative effects for many organisms and ecosystem processes. Although strategies to reduce hypoxia by decreasing nutrient loading are predicated on the assumption that this action would reverse eutrophication, recent analyses of historical data from European and North American coastal systems suggest little evidence for simple linear response trajectories. We review published parallel time-series data on hypoxia and loading rates for inorganic nutrients and labile organic matter to analyze trajectories of oxygen (O-2) response to nutrient loading. We also assess existing knowledge of physical and ecological factors regulating O-2 in coastal marine waters to facilitate analysis of hypoxia responses to reductions in nutrient (and/or organic matter) inputs. Of the 24 systems identified where concurrent time series of loading and O-2 were available, half displayed relatively clear and direct recoveries following remediation. We explored in detail 5 well-studied systems that have exhibited complex, non-linear responses to variations in loading, including apparent 'regime shifts'. A summary of these analyses suggests that O-2 conditions improved rapidly and linearly in systems where remediation focused on organic inputs from sewage treatment plants, which were the primary drivers of hypoxia. In larger more open systems where diffuse nutrient loads are more important in fueling O-2 depletion and where climatic influences are pronounced, responses to remediation tended to follow non-linear trends that may include hysteresis and time-lags. Improved understanding of hypoxia remediation requires that future studies use comparative approaches and consider multiple regulating factors. These analyses should consider: (1) the dominant temporal scales of the hypoxia, (2) the relative contributions of inorganic and organic nutrients, (3) the influence of shifts in climatic and oceanographic processes, and (4) the roles of feedback interactions whereby O-2-sensitive biogeochemistry, trophic interactions, and habitat conditions influence the nutrient and algal dynamics that regulate O-2 levels.
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29.
  • Kroepelin, S, et al. (författare)
  • Climate-driven ecosystem succession in the Sahara: The past 6000 years
  • 2008
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 1095-9203 .- 0036-8075. ; 320:5877, s. 765-768
  • Tidskriftsartikel (refereegranskat)abstract
    • Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a "green Sahara" to the present hyperarid desert. Our continuous 6000- year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site- specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large- scale dust mobilization from 4300 calendar years before the present ( cal yr B. P.). Today's desert ecosystem and regional wind regime were established around 2700 cal yr B. P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate.
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30.
  • Laruelle, G. G., et al. (författare)
  • Anthropogenic perturbations of the silicon cycle at the global scale: Key role of the land-ocean transition
  • 2009
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 23
  • Forskningsöversikt (refereegranskat)abstract
    • Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century.
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31.
  • Mosimane, Keotshephile, et al. (författare)
  • Variability in chemistry of surface and soil waters of an evapotranspiration-dominated flood-pulsed wetland : Solute processing in the okavango delta, Botswana
  • 2017
  • Ingår i: Water S.A.. - : Academy of Science of South Africa. - 1816-7950 .- 0378-4738. ; 43:1, s. 104-115
  • Tidskriftsartikel (refereegranskat)abstract
    • Water chemistry is important for the maintenance of wetland structure and function. Interpreting ecological patterns in a wetland system therefore requires an in-depth understanding of the water chemistry of that system. We investigated the spatial distribution of chemical solutes both in soil pore water and surface water, along island-floodplain-channel hydrological gradients in seasonally and permanently inundated habitats between major regions in the Okavango Delta, Botswana. Our results show that major cations (Ca, Na, Mg, and K), dissolved silica (DSi), dissolved boron (B), dissolved organic matter (DOC) and electrical conductivity increased significantly, at p ≤ 0.05, from the inlet of the Delta (the Panhandle) to the distal downstream reaches, suggesting the influence of evapoconcentration. Concentrations of dissolved Fe, Al, Zn, Cu, and Mn significantly decreased, at p ≤ 0.05, from the inflow of the Delta to the distal reaches. Only Na, Mn, Fe, Al, and DOC showed significant differences, at p ≤ 0.05, along the local floodplain-channel hydrological gradients, with higher solute concentrations in the floodplains than the channels. Solute concentrations in soil water exhibited similar distribution patterns to those in surface water, but concentrations were higher in soil water. Based on the results, we hypothesise that floodplain emergent vegetation and the channel-fringing vegetation in the Panhandle (a fault-bounded entry trough to the Delta) and the permanently inundated eco-region together influence the cycling of solutes that enter the Delta through uptake.
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32.
  • Nantke, Carla K.M., et al. (författare)
  • Human influence on the continental Si budget during the last 4300 years : δ30Sidiatom in varved lake sediments (Tiefer See, NE Germany)
  • 2021
  • Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 258
  • Tidskriftsartikel (refereegranskat)abstract
    • The continental silicon (Si) cycle, including terrestrial and freshwater ecosystems (lakes, rivers, estuaries), acts as a filter and modulates the amount of Si transported to the oceans. In order to link the variation in the terrestrial Si cycle to aquatic ecosystems, knowledge on changes in vegetation cover, soil disturbance and the impact of human activity are required. This study on varved lake sediments from Tiefer See near Klocksin (TSK) in northeastern Germany investigates Si isotope variations in diatom frustules (δ30Sidiatom) over the last ∼4300 years. δ30Sidiatom values vary between 0.37 and 1.63‰. The isotopic signal measured in centric (mostly planktonic) and pennate (mostly benthic) diatoms shows the same trend through most of the record. A decrease in δ30Sidiatom coinciding with early deforestation between 3900 and 750 a BP in the catchment area, points to an enhanced export of isotopically light dissolved silica (DSi) from adjacent soils to the lake. The burial flux of biogenic silica (BSi) observed in the lake sediments increases with cultivation due to enhanced nutrient supply (N, P and Si) from the watershed and nutrient redistribution caused by wind-driven increased water circulation. When the cultivation intensifies, we observe a shift to higher δ30Sidiatom values that we interpret to reflect a diminished Si soil pool and the preferential removal of the lighter 28Si by crop harvesting. Human activity influences the DSi supply from the catchment and appears to be the primary driver controlling the Si budget in TSK. Our data shows how land use triggers variations in continental Si cycling on centennial timescales and provides important information on the underlying processes.
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33.
  • Nantke, Carla K.M., et al. (författare)
  • Si cycling in transition zones : a study of Si isotopes and biogenic silica accumulation in the Chesapeake Bay through the Holocene
  • 2019
  • Ingår i: Biogeochemistry. - : Springer Science and Business Media LLC. - 0168-2563 .- 1573-515X. ; 146:2, s. 145-170
  • Tidskriftsartikel (refereegranskat)abstract
    • Si fluxes from the continents to the ocean are a key element of the global Si cycle. Due to the ability of coastal ecosystems to process and retain Si, the ‘coastal filter’ has the potential to alter Si fluxes at a global scale. Coastal zones are diverse systems, sensitive to local environmental changes, where Si cycling is currently poorly understood. Here, we present the first palaeoenvironmental study of estuarine biogenic silica (BSi) fluxes and silicon isotope ratios in diatoms (δ30Sidiatom) using hand-picked diatom frustules in two sediment cores (CBdist and CBprox) from the Chesapeake Bay covering the last 12000 and 8000 years, respectively. Constrained by the well-understood Holocene evolution of the Chesapeake Bay, we interpret variations in Si cycling in the context of local climate, vegetation and land use changes. δ30Sidiatom varies between + 0.8 and + 1.7‰ in both sediment cores. A Si mass balance for the Chesapeake Bay suggests much higher rates of Si retention (~ 90%) within the system than seen in other coastal systems. BSi fluxes for both sediment cores co-vary with periods of sea level rise (between 9500 and 7500 a BP) and enhanced erosion due to deforestation (between 250 and 50 a BP). However, differences in δ30Sidiatom and BSi flux between the sites emphasize the importance of the seawater/freshwater mixing ratios and locally variable Si inputs from the catchment. Further, we interpret variations in δ30Sidiatom and the increase in BSi fluxes observed since European settlement (~ 250 a BP) to reflect a growing human influence on the Si cycle in the Chesapeake Bay. Thereby, land use change, especially deforestation, in the catchment is likely the major mechanism.
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34.
  • Rabouille, C., et al. (författare)
  • Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhone rivers
  • 2008
  • Ingår i: Continental Shelf Research. - : Elsevier BV. - 0278-4343. ; 28:12, s. 1527-1537
  • Tidskriftsartikel (refereegranskat)abstract
    • We examined the occurrence of seasonal hypoxia (O-2 < 2 mg l(-1)) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhone Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhone and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1-10g C m(-2) d(-1)), and lower off the Rhone River (< 1 g C m(-2) d(-1)), which could be one of the reasons of the absence of hypoxia on the Rhone shelf The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhone partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhone show little or no hypoxia. (c) 2008 Elsevier Ltd. All rights reserved.
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35.
  • Schmidtbauer, Kerry, et al. (författare)
  • Linking silicon isotopic signatures with diatom communities
  • 2022
  • Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 323, s. 102-122
  • Tidskriftsartikel (refereegranskat)abstract
    • The use of silicon isotope ratios (expressed as δ30Si) as a paleolimnological proxy in lacustrine systems requires a better understanding of the role of lake processes in setting the δ30Si values of dissolved Si (dSi) in water and in diatom biogenic silica (bSi). We determined the δ30Si of modern dSi (δ30SidSi) and bSi (δ30SibSi) in three lakes in Lassen Volcanic National Park (LAVO), California (USA), and produced diatom assemblage compositional data from the modern system and from sediment core samples. In the modern systems, we observe the largest magnitude diatom Si isotope fractionations yet reported, at −3.4 and −3.9‰ for Fragilaria dominated samples. Using statistical approaches designed to condense multivariate ecological data, we can deconvolve assemblage-specific Si isotope fractionations from the combined diatom assemblage-δ30Si data. For example, samples dominated by generally deeper water euplanktic species have low δ30SibSi values (<−1.10‰). Conversely, samples dominated by shallow water planktic or benthic periphyton have higher δ30SibSi values (>−0.14‰). These data suggest that δ30Si records from LAVO lakes reflect species specific Si isotope fractionations and thus act as paleolimnological proxy for the aquatic-habitat of bSi production. Silicon isotope analysis should be coupled with diatom community composition data and other geochemical proxies for the most robust paleolimnological interpretations. We also construct a Si mass-balance for Manzanita Lake based on elemental fluxes. Despite a short residence time of ∼4 months, it is an efficient Si sink: about 30% of inflowing Si is retained in the lake sediments. An entirely independent Si isotope-based estimate agrees remarkably well. Burial fluxes of bSi derived from radiometrically dated sediment cores yield retention rates of about a factor of three higher, which might suggest groundwater is an important term in the lake Si budget.
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36.
  •  
37.
  • Tenn, William J., III, et al. (författare)
  • Oxy-Functionalization of Nucleophilic Rhenium(I) Metal Carbon Bonds Catalyzed by Selenium(IV)
  • 2009
  • Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:7, s. 2466-2468
  • Tidskriftsartikel (refereegranskat)abstract
    • We report that SeO(2) catalyzes the facile oxy-functionalization of (CO)(5)Re(I)-Me(delta-) with IO(4)(-) to generate methanol. Mechanistic studies and DFT calculations reveal that catalysis involves methyl group transfer from Re to the electrophilic Se center followed by oxidation and subsequent reductive functionalization of the resulting CH(3)Se(VI) species. Furthermore, (CO)(3)Re(I)(Bpy)-R (R = ethyl, n-propyl, and aryl) complexes show analogous transfer to SeO(2) to generate the primary alcohols. This represents a new strategy for the oxy-functionalization of M-R(delta-) polarized bonds.
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38.
  • Zahajská, Petra, et al. (författare)
  • The Holocene silicon biogeochemistry of Yellowstone Lake, USA
  • 2023
  • Ingår i: Quaternary Science Reviews. - 0277-3791. ; 322
  • Tidskriftsartikel (refereegranskat)abstract
    • Silicon (Si) is an essential macronutrient for diatoms, an important component of lacustrine primary productivity that represents a link between the carbon and silicon cycles. Reconstructions of lake silicon cycling thus provide an underexploited window onto lake and catchment biogeochemistry. Silicon isotope geochemistry has potential to provide these reconstructions, given the competing source and process controls can be deconvolved. The silica-rich volcanic and hydrothermal systems in Yellowstone National Park are a great source of dissolved silicon into Yellowstone Lake, a system with high silicon, and thus carbon, export rates and the formation of diatom–rich sediment. Yellowstone Lake sediments should be an archive of past silicon biogeochemistry, although the effect of sublacustrine hydrothermal activity or hydrothermal explosion events is unclear. Here, we analysed lake water, tributaries, and hydrothermal vent fluids from Yellowstone Lake for their dissolved Si concentrations, isotope composition (δ30Si) and Ge/Si ratios to evaluate the sources of variability in the lake's Si cycle. Bulk elemental composition and biogenic SiO2 (bSiO2) content, together with δ30Si and Ge/Si ratios from a single diatom species, Stephanodiscus yellowstonensis, were analysed in two sediment cores spanning the last 9880 cal. yr BP. We investigate these datasets to identify long term Holocene changes in hydrothermal activity and effects of large and short-term events i.e., hydrothermal and a volcanic eruption. Combinations of bSiO2, δ30Si and Ge/Si with XRF and lithology data revealed that Yellowstone Lake has a resilient biogeochemical system: hydrothermal explosions are visible in the lithology but have no identifiable impact on bSiO2 accumulation or on the δ30Si signature. Both cores show similarities that suggest a stable and homogeneous dSi source across the entire lake. A narrow range of δ30Si and Ge/Si values suggests that the productive layer of the lake was well mixed and biogeochemically stable, with consistently high hydrothermal inputs of Si throughout the Holocene to buffer against the disturbance events. Variation in bSiO2 concentration through time is weakly correlated with an increase towards younger sediment in the δ30Si fossil diatom record in both cores. This increase mirrors that seen in ocean records, and follows changes known in summer insolation, summer temperatures and lake water-column mixing since the deglaciation. This suggests that climate forcing, and soil formation ultimately govern the silicon isotope record, which we suggest is via a combination of changes in weathering stoichiometry, diatom production, and relative proportion of dSi sources.
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39.
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40.
  • Asmala, Eero, et al. (författare)
  • A reply to the comment by Karlsson et al.
  • 2019
  • Ingår i: Limnology and Oceanography. - : Wiley. - 1939-5590 .- 0024-3590. ; 64:4, s. 1832-1833
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
  •  
41.
  • Asmala, Eero, et al. (författare)
  • Efficiency of the coastal filter : Nitrogen and phosphorus removal in the Baltic Sea
  • 2017
  • Ingår i: Limnology and Oceanography. - : Wiley. - 1939-5590 .- 0024-3590. ; 62, s. 222-238
  • Tidskriftsartikel (refereegranskat)abstract
    • An important function of coastal ecosystems is the reduction of the nutrient flux from land to the open sea, the coastal filter. In this study, we focused on the two most important coastal biogeochemical processes that remove nitrogen and phosphorus permanently: denitrification and phosphorus burial. We compiled removal rates from coastal systems around the Baltic Sea and analyzed their spatial variation and regulating environmental factors. These analyses were used to scale up denitrification and phosphorus burial rates for the entire Baltic Sea coastal zone. Denitrification rates ranged from non-detectable to 12 mmol N m−2 d−1, and correlated positively with both bottom water nitrate concentration and sediment organic carbon content. The rates exhibited a strong decreasing gradient from land to the open coast, which was likely driven by the availability of nitrate and labile organic carbon, but a high proportion of non-cohesive sediments in the coastal zone decreased the denitrification efficiency relative to the open sea. Phosphorus burial rates varied from 0.21 g P m−2 yr−1 in open coastal systems to 2.28 g P m−2 yr−1 in estuaries. Our analysis suggests that archipelagos are important phosphorus traps and account for 45% of the coastal P removal, while covering only 17% of the coastal areas. High burial rates could partly be sustained by phosphorus import from the open Baltic Sea. We estimate that the coastal filter in the Baltic Sea removes 16% of nitrogen and 53% of phosphorus inputs from land.
  •  
42.
  • Barão, Lúcia, et al. (författare)
  • Alkaline-extractable silicon from land to ocean: A challenge for biogenic silicon determination
  • 2015
  • Ingår i: Limnology and Oceanography. - : Wiley. - 1541-5856. ; , s. n/a-n/a
  • Tidskriftsartikel (refereegranskat)abstract
    • The biogeochemical cycling of silicon (Si) along the land-to-ocean continuum is studied by a variety of research fields and for a variety of scientific reasons. However, there is an increasing need to refine the methodology and the underlying assumptions used to determine biogenic silica (BSi) concentrations. Recent evidence suggests that contributions of nonbiogenic sources of Si dissolving during alkaline extractions, not corrected by standard silicate mineral dissolution correction protocols, can be substantial. The ratio between dissolved Si and aluminum (Al) monitored continuously during the alkaline extraction can be used to infer the origin of the Si fractions present. In this study, we applied both a continuous analysis method (0.5 M NaOH) and a traditional 0.1 M Na2CO3 extraction to a wide array of samples: (1) terrestrial vegetation, (2) soils from forest, cropland and pasture, (3) lake sediments, (4) suspended particulate matter and sediments from rivers, (5) sediments from estuaries and salt marshes and (6) ocean sediments. Our results indicate that the 0.1 M Na2CO3 extraction protocol can overestimate the BSi content, by simultaneously dissolving Si fractions of nonbiogenic origin that may represent up to 100% of the Si traditionally considered as biogenic, hampering interpretation especially in some deeper soil horizons, rivers and coastal oceanic sediments. Moreover, although the term amorphous Si was coined to reflect a growing awareness of nonbiogenic phases we show it is actually inappropriate in samples where silicate minerals may account for a large part of the extracted Si even after linear mineral correction.
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43.
  • Barker, Philip A., et al. (författare)
  • Carbon cycling within an East African lake revealed by the carbon isotope composition of diatom silica: a 25-ka record from Lake Challa, Mt. Kilimanjaro
  • 2013
  • Ingår i: Quaternary Science Reviews. - : Elsevier BV. - 0277-3791. ; 66, s. 55-63
  • Tidskriftsartikel (refereegranskat)abstract
    • The carbon cycle of a lake is a balance between supply from the atmosphere and catchment, and the net demand exerted by primary producers, minus losses back to the atmosphere and to sediment storage. Evaluating the sum of these processes and reconstructing them from sediment records of lake history requires a range of methods and a multi-proxy approach. One promising technique is to explore the carbon-isotope composition (delta C-13(diatom)) of organic matter incorporated within the silica frustules of diatom algae. Here we present a 25,000-year record of delta C-13(diatom) from the sediments of crater Lake Challa on the eastern flank of Mt. Kilimanjaro, and along with other proxy data we make inferences about the three major phases in the history of the lake's carbon cycle. From 25 ka to 15.8 ka years BP, delta C-13(diatom) is positively correlated with the delta C-13 of bulk sediment organic matter (delta C-13(bulk)), indicating that high diatom productivity, as recorded by high % biogenic silica at this time, was preferentially removing C-12 and enriching the delta C-13 of lake-water dissolved inorganic carbon. From 15.8 to 5.5 ka the correlation between delta C-13(diatom) and delta C-13(bulk) breaks down, suggesting carbon supply to the lake satisfied or exceeded the demand from productivity. From 5.5 ka BP the positive correlation resumes, indicating an increase in the internal demand for carbon relative to external supply. Diatom frustule-bound carbon isotopes offer an original tool in examining long-term fluctuations in a lake's carbon budget and how the balance between supply and demand has changed through time. (C) 2012 Elsevier Ltd. All rights reserved.
  •  
44.
  • Breitburg, Denise, et al. (författare)
  • Declining oxygen in the global ocean and coastal waters
  • 2018
  • Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 359:6371
  • Forskningsöversikt (refereegranskat)abstract
    • Oxygen is fundamental to life. Not only is it essential for the survival of individual animals, but it regulates global cycles of major nutrients and carbon. The oxygen content of the open ocean and coastal waters has been declining for at least the past half-century, largely because of human activities that have increased global temperatures and nutrients discharged to coastal waters. These changes have accelerated consumption of oxygen by microbial respiration, reduced solubility of oxygen in water, and reduced the rate of oxygen resupply from the atmosphere to the ocean interior, with a wide range of biological and ecological consequences. Further research is needed to understand and predict long-term, global-and regional-scale oxygen changes and their effects on marine and estuarine fisheries and ecosystems.
  •  
45.
  • Caffrey, Jane M., et al. (författare)
  • Short exposure to oxygen and sulfide alter nitrification, denitrification, and DNRA activity in seasonally hypoxic estuarine sediments
  • 2019
  • Ingår i: FEMS Microbiology Letters. - : Oxford University Press (OUP). - 1574-6968. ; 366:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Increased organic loading to sediments from eutrophication often results in hypoxia, reduced nitrification and increased production of hydrogen sulfide, altering the balance between nitrogen removal and retention. We examined the effect of short-term exposure to various oxygen and sulfide concentrations on sediment nitrification, denitrification and DNRA from a chronically hypoxic basin in Roskilde Fjord, Denmark. Surprisingly, nitrification rates were highest in the hypoxic and anoxic treatments (about 5 μmol cm-3 d-1) and the high sulfide treatment was not significantly different than the oxic treatment. Denitrification in the hypoxic treatment was highest at 1.4 μmol cm-3 d-1 and significantly higher than the high sulfide treatment. For DNRA, the rate in high sulfide treatment was 2 μmol cm-3 d-1. This was significantly higher than all oxygen treatments that were near zero. In this system, nitrifiers rapidly recovered from conditions typically considered inhibiting, while denitrifiers had a more muted response. DNRA bacteria appear to depend on sulfide for nitrate reduction. Anammox was insignificant. Thus, in estuaries and coastal systems that experience short-term variations in oxygen and sulfide, capabilities of microbial communities are more diverse and tolerant of suboptimal conditions than some paradigms suggest.
  •  
46.
  • Carstensen, Jacob, et al. (författare)
  • Baltic Sea Hypoxia Takes Many Shapes and Sizes
  • 2019
  • Ingår i: Limnology and Oceanography Bulletin. - : Wiley. - 1539-607X .- 1539-6088. ; 28:4, s. 125-129
  • Tidskriftsartikel (refereegranskat)abstract
    • The Baltic Sea is naturally prone to hypoxia, but the frequency and extent have increased multifold over the last century. Hypoxia manifests itself as perennial in the open central part, seasonal at the entrance area, and episodic at many coastal sites, and the expression of hypoxia is largely driven by differences in bottom water residence times and stratification patterns. Enhanced nutrient inputs from land and atmosphere are the main drivers of expanding hypoxia in the Baltic Sea although deoxygenation has also been exacerbated by increasing temperature over the past 3–4 decades. Hypoxia severely influences ecosystem functions such as fish production through reduced trophic efficiency and harmful cyanobacteria blooms sustained by phosphorus release from sediments. Nutrient inputs from land have created the largest man-made hypoxic area in the world and the only viable long-term solution to mitigation is to continue efforts to reduce nutrient loading.
  •  
47.
  • Carstensen, Jacob, et al. (författare)
  • Factors regulating the coastal nutrient filter in the Baltic Sea
  • 2020
  • Ingår i: Ambio. - : Springer Science and Business Media LLC. - 0044-7447 .- 1654-7209. ; 49:6, s. 1194-1210
  • Tidskriftsartikel (refereegranskat)abstract
    • The coastal zone of the Baltic Sea is diverse with strong regional differences in the physico-chemical setting. This diversity is also reflected in the importance of different biogeochemical processes altering nutrient and organic matter fluxes on the passage from land to sea. This review investigates the most important processes for removal of nutrients and organic matter, and the factors that regulate the efficiency of the coastal filter. Nitrogen removal through denitrification is high in lagoons receiving large inputs of nitrate and organic matter. Phosphorus burial is high in archipelagos with substantial sedimentation, but the stability of different burial forms varies across the Baltic Sea. Organic matter processes are tightly linked to the nitrogen and phosphorus cycles. Moreover, these processes are strongly modulated depending on composition of vegetation and fauna. Managing coastal ecosystems to improve the effectiveness of the coastal filter can reduce eutrophication in the open Baltic Sea.
  •  
48.
  • Clymans, Wim, et al. (författare)
  • Si precipitation during weathering in different Icelandic Andosols
  • 2014
  • Ingår i: Geochemistry of the Earth's Surface GES-10. - : Elsevier BV. - 1878-5220. ; 10, s. 260-265
  • Konferensbidrag (refereegranskat)abstract
    • Basaltic weathering from volcanic islands plays a critical role in the climate feedback loop. Geochemical and climate models require information on the rate of secondary mineral formation. We provide direct evidence for precipitation of amorphous Si in organic rich and acidic Histic Andosols compared to preferential allophane formation in organic poor and less acidic Haplic Andosols. Similar results have been obtained from the pioneering work by Opfergelt et al(1) using Si isotope composition. Additionally, enhanced allophane precipitation in Haplic Andosols, independent of long-term soil property changes, highlight the potential role of land use and management on secondary mineral formation. (C) 2014 The Authors. Published by Elsevier B.V.
  •  
49.
  •  
50.
  • Conley, Daniel, et al. (författare)
  • Long-term changes and impacts of hypoxia in Danish coastal waters
  • 2007
  • Ingår i: Ecological Applications. - : Wiley. - 1051-0761. ; 17:5, s. 165-184
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • A 38-year record of bottom water dissolved oxygen concentrations in coastal marine ecosystems around Denmark (1965-2003) and a longer partially reconstructed record of total nitrogen (TN) inputs (1900-2003) were assembled to describe long-term patterns in hypoxia and anoxia. Interannual variations in bottom water oxygen concentrations were analyzed in relation to various explanatory variables (bottom temperature, wind speed, advective transport, TN loading). Reconstructed TN loads peaked in the 1980s with a gradual decline to the present, commensurate with a legislated nutrient reduction strategy. Mean bottom water oxygen concentrations during summer have significantly declined in coastal marine ecosystems, decreasing substantially during the 1980s and were extremely variable thereafter. Despite decreasing TN loads, the worst hypoxic event ever recorded in open waters occurred in 2002. For estuaries and coastal areas, bottom water oxygen concentrations were best described by TN input from land and wind speed in July-September, explaining 52% of the interannual variation in concentrations. For open sea areas, bottom water oxygen concentrations were also modulated by TN input from land, however, additional significant variables included advective transport of water and Skagerrak surface water temperature and explained 49% of interannual variations in concentrations. Reductions in benthic species number and alpha diversity were significantly related to the duration of the 2002 hypoxic event. Gradual decreases in diversity measures (species number and alpha diversity) over the first 2-4 weeks show that the benthic community undergoes significant changes before the duration of hypoxia is severe enough to cause the community to collapse. Enhanced sediment-water fluxes of NH4+ and PO43- occur with hypoxia, increasing nutrient concentrations in the water column, and stimulating additional phytoplankton production. Repeated hypoxic events have changed the character of benthic communities and how organic matter is processed in sediments. Our data suggest that repeated hypoxic events lead to an increase in susceptibility of Danish waters to eutrophication and further hypoxia.
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