• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 539
  • 379
  • 336
  • 121
  • 10
  • 7
  • 5
  • 5
  • 1
  • 1
  • 1
  • Tagged with
  • 4115
  • 1303
  • 702
  • 685
  • 660
  • 644
  • 631
  • 388
  • 295
  • 215
  • 175
  • 75
  • 67
  • 66
  • 61
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Wetland hydrology of the Elmley marshes

Gavin, Helen January 2001 (has links)
Despite the importance of the hydrological regime for the functioning of wetland environments, the understanding of hydrological processes, particularly evaporative dynamics and clay soil moisture fluxes, is limited and the original research outlined in this thesis constitutes a real contribution to further the scientific understanding of wetland systems. Two lines of investigation are followed based upon field experiments and monitoring of groundwater and ditch water levels together with soil moisture over time and space. The first investigation assesses the effectiveness of the Environmentally Sensitive Area (ESA) scheme of the North Kent Marshes to achieve its objective of soft moist ground conditions through the manipulation of ditch water levels. Results demonstrate that due to the clay soil texture and low hydraulic conductivity of the marsh substrate, little relationship exists between the position of ditch water levels and groundwater. Flooding the marsh surface with brackish ditch water does promote high surface soil moistures and optimal habitat for waterfowl. However the deleterious effects upon soil structure and floristic diversity by flooding large areas need to be taken in consideration for the sustainable management of the wetland. Results from this investigation prove that precipitation and evaporation are the dominant fluxes of water, and so the second investigation focused upon the loss of water from the wet grassland by evaporation. There are two foci of research. Firstly the relationship between surface resistance and soil moisture content is examined, so that 'correct' surface resistance values could be input to the Penman Monteith method to compute actual evaporation according to the wetness of the marsh soil. Research results show a complex relationship with decreasing rates of actual evaporation below potential rates with soil moisture loss, and a functional relationship has been quantified. Secondly the actual evaporative water loss of the wetland was determined taking into account the small-scale heterogeneity of surface wetness conditions. The Weighted Penman Monteith (WPM) approach was followed referencing results against data by the Bowen Ratio Energy Balance method. Research results show that the WPM method could be used to compute the actual evaporation loss, however the adoption of the Priestly-Taylor approach with suitable values of a is a simpler method and of equal accuracy.

Global trends in eddy kinetic energy from satellite altimetry

O'Donnell, Christopher John January 2015 (has links)
The temporal changes in the oceanic eddy kinetic energy (EKE) including trends and variability are presented and the dynamical mechanisms are investigated. The domain is near-global with a focus on the North Atlantic, North Pacific and Southern Oceans. Altimeter-derived geostrophic surface velocities are used to compute an 18 year time series of EKE on a 1/3� grid. Linear trends are best-fit to the 18-year time series and their statistical significance assessed using bootstrap techniques. Near-global mean EKE trends are non-statistically significant. However, on a regional scale, statistically significant trends are found in all of the major ocean basins. Widespread negative trends occur primarily in the northern and southern subtropical Pacific as well as the central North Atlantic, while positive trends occur primarily in the North Atlantic subtropical gyre, much of the northeast North Atlantic, the southeast Indian Ocean and in several regions in the Southern Ocean. Buoyancy forcing and non-local wind forcing related to the PDO are significant in the North Pacific. In the North Atlantic, changes in wind stress curl as well as changes in local wind speed are implicated, where a di-polar pattern of correlations with the NAO is observed. In the Southern Ocean, changes in local and/or remote winds appear as the dominant mechanism south of 30�S. On a global scale, EKE trends are slightly positive (0.15% of the mean per decade) but non-statistically significant. EKE has decreased in the northern hemisphere and increased in the southern hemisphere despite an increase in hemispheric mean wind speed in both northern and southern hemispheres. Changing wind speeds are influential across all the ocean basins but other mechanisms are significant including shifting wind stress curl fields, buoyancy forcing, indirect (non-local) winds and intrinsic variability. Statistically significant correlations between annual mean EKE and major modes of climate variability are evident in all the ocean basins.

Molecular ecology of marine isoprene degradation

Johnston, Antonia January 2014 (has links)
Isoprene is an atmospheric trace gas whose emissions to the atmosphere are roughly equal to that of methane. It is highly reactive and has the potential to affect climate through a variety of interactions in the atmosphere, including the formation of ozone. In the marine environment alone, algae produce up to 11 Tg y-1 of isoprene. To date, little is known about its degradation by microbes in the marine environment. In this project, isoprene-degrading bacteria from a range of marine sites were characterised and Illumina sequencing was used to mine the genomes of isoprene-degrading strains related to Gordonia polyisoprenivorans and Mycobacterium hodleri, isolated from the Colne Estuary, Essex. From these genomes, we retrieved novel sequences encoding isoprene monooxygenase, previously identified in a terrestrial Rhodococcus species. This information allowed the design of specific PCR primer sets for the isoA gene, encoding the alpha subunit of isoprene monooxygenase, to retrieve isoprene-specific genes from environmental samples. The primers amplify isoA from a wide range of marine genera. A database of isoA sequences from extant isoprene degraders and isoA sequences retrieved by PCR from DNA from a variety of different marine environments was created. The data obtained demonstrated that isoprene monooxygenase genes are widespread in the marine environment. Other work focused on the physiology of isoprene-degrading bacteria, particularly the marine isolate Gordonia polyisoprenivorans. SDS-PAGE, oxygen electrode assays and RT-PCR were also used to investigate the regulation of soluble diiron centre monooxygenases in this organism, and showed that two separate, inducible monooxygenase enzyme systems exist in this organism and are responsible for the oxidation of isoprene and propane. DNA-Stable Isotope Probing revealed that members of the genera Rhodococcus, Mycobacterium, Gordonia and Microbacterium are active isoprene degraders in the Colne Estuary.

The effects of elevated CO2 and ocean acidification on the production of marine biogenic trace gases

Webb, Alison January 2015 (has links)
The human-induced increases in atmospheric carbon dioxide since the beginning of the industrial revolution have led to increasing oceanic carbon uptake and changes in seawater carbonate chemistry, resulting in lowering of surface water pH. To date, surface ocean acidity has increased by 30% compared with pre-industrial times. The aim of this study was to investigate the relationship between increasing pCO2, decreasing pH and changes in volatile dimethylsulphide (DMS) and halocarbon concentrations, through 70,000 litre, high pCO2 mesocosm experiments and laboratory culture studies. DMS is a climatically important trace gas produced by marine algae: it transfers sulphur into the atmosphere and is a major influence on biogeochemical climate feedbacks. Halocarbons are also important biogenic trace gases which undergo atmospheric photochemical degradation, releasing halide radicals to participate in atmospheric ozone cycling, and transfer halogens from sea to land. Evidence is presented from a Norwegian coastal study which showed a 60% DMS, 30% DMSP and 40% iodocarbon reduction in high pCO2 mesocosms, and in the Baltic Sea, known for its low-salinity, cyanobacterial dominated community, where DMS concentrations showed an 80% reduction under high pCO2 but halocarbon concentrations were unaffected. No decrease in DMS or DMSP concentrations were identified in high pCO2 laboratory cultures of the DMSproducing species Emiliania huxleyi RCC1229, and halocarbons were undetectable. Changes in trace gas concentrations may arise due to pH effects on the interactions between microbial producers and consumers. Other effects may arise from cell biochemistry due to long-term adaptation to increased ρCO2 and reduced pH on the enzymatic activity production of the compounds. Further studies should determine the nature of the pCO2 and pH effect on bacterial interactions with DMS, DMSP and halocarbon production and breakdown. There should also be attention given to the DMS source in the cyanobacterial-dominated community of the Baltic Sea.

Methylated amine-utilising bacteria and microbial nitrogen cycling in Movile Cave

Wischer, Daniela January 2014 (has links)
Movile Cave is an unusual, isolated ecosystem which harbours a complex population of microorganisms, fungi and endemic invertebrates. In the absence of light and with no fixed carbon entering the cave, life is sustained by non-phototrophic microorganisms such as sulfur and methane oxidisers. Also present are methylotrophs that use one-carbon compounds such as methanol and methylated amines as their sole source of carbon and energy. Produced during putrefaction, methylated amines are likely to be major degradation products in Movile Cave. Further to being methylotrophic substrates, they are also a nitrogen source for many non-methylotrophic bacteria. The role of methylated amines as carbon and nitrogen sources for Movile Cave bacteria was investigated using a combination of DNA stable isotope probing and cultivation studies. Both, well-characterised and novel methylotrophs were identified: Methylotenera mobilis dominated 13C-monomethylamine SIP enrichments, while members of Catellibacterium, Cupriavidus and Altererythrobacter were also active. Cultivation studies consolidated SIP results in obtaining the first methylotrophic isolates from the genera Catellibacterium and Mesorhizobium. Pathways for monomethylamine (MMA) metabolism were investigated using new PCR primers designed to target gmaS, the gene for gammaglutamylmethylamide synthetase, a key enzyme of the recently characterised indirect MMA oxidation pathway. This pathway is also present in bacteria that use MMA only as a nitrogen source, while the well-characterised, direct MMA oxidation pathway involving methylamine dehydrogenase (mauA) is found only in methylotrophs. gmaS was present in all MMAutilising isolates, while mauA was found only in some methylotrophs, suggesting the indirect pathway is the major mode of MMA oxidation both in methylotrophs and non-methylotrophs from Movile Cave. Preliminary gmaS surveys revealed a high diversity of gmaS-containing bacteria. The roles of N2 fixers and nitrifiers were also investigated. Both bacterial and archaeal ammonia oxidisers were found to be active; however, sulfur oxidisers appeared to be the dominant autotrophs in Movile Cave.

Modelling biological invasions : population cycles, waves and time delays

Jankovic, Masha January 2015 (has links)
Biological invasions are rapidly gaining importance due to the ever-increasing number of introduced species. Alongside the plenitude of empirical data on invasive species there exists an equally broad range of mathematical models that might be of use in understanding biological invasions. This thesis aims to address several issues related to modelling invasive species and provide insight into their dynamics. Part I (Chapter 2) documents a case study of the gypsy moth, Lymantria dispar, invasion in the US. We propose an alternative hypothesis to explain the patchiness of gypsy moth spread entailing the interplay between dispersal, predation or a viral infection and the Allee effect. Using a reaction-diffusion framework we test the two models (prey-predator and susceptible-infected) and predict qualitatively similar patterns as are observed in natural populations. As high density gypsy moth populations cause the most damage, estimating the spread rate would be of help in any suppression strategy. Correspondingly, using a diffusive SI model we are able to obtain estimates of the rate of spread comparable to historical data. Part II (Chapters 3, 4 and 5) is more methodological in nature, and in a single species context we examine the effect of an ubiquitous phenomenon influencing population dynamics time delay. In Chapter 3 we show that contrary to the general opinion, time delays are not always destabilising, using a delay differential equation with discrete time delay. The concept of distributed delay is introduced in Chapter 4 and studied through an integrodifferential model. Both Chapters 3 and 4 focus on temporal dynamics of populations, so we further this consideration to include spatial effects in Chapter 5. Using two different representations of movement, we show that the onset of spatiotemporal chaos in the wake of population fronts is possible in a single species model.

An investigation of the relationship between the construing of the environment and its physical form

Honikman, Basil Clive January 1972 (has links)
This is a study of the relationships between the physical characteristics of an environment and the way people evaluate it. George Kelly's theory of personal constructs forms the philosophical basis and the experimental work uses a combination of repertory grid, construct laddering, resistance to change, and implication grid techniques, in order to describe the way each informant construed colour photographs of living rooms. A set of 15 colour photographs was used for construct eliciting and 10 of these formed the elements for the repertory grids. These were scored by each informant in terms of constructs which had been personally and separately elicited from him. Principal components analysis, laddering, resistance to change and implication grids identified how the constructs were organised in hierarchical networks. The laddering process established how 'chains' of subordinate constructs relating to one of the photographs which had been preselected, were derived from initially elicited constructs. The principal components analysis enabled relationships to be identified between superordinate constructs and the main parts or factors of each informant's system or evaluation. The implication grids established implicative relationships between super and subordinate constructs whether or not they belonged to the same factor of evaluation or chain of constructs. The physical characteristics of the selected photograph were frequently nominated as subordinate constructs and their relationship with other constructs in the informant's whole network of construing was established by five different kinds of links. In this way an indication may be gained as to the contribution of the physical characteristics of environments to the general way that people evaluate them.

Ecological studies on Sargassum muticum (Yendo) Fensholt

Critchley, A. T. January 1981 (has links)
No description available.

Ecological complexity : a conceptual analysis of dynamics and organisation in contrasting environments

Dyer, Jacqueline Ruth January 2004 (has links)
No description available.

A study of the ecology of leptophyes punctatissima

Deura, K. January 1980 (has links)
No description available.

Page generated in 0.05 seconds