Дисертації з теми "Deep and elongated lake"

This doctoral thesis provides the first comprehensive study on the physical processes controlling hydrodynamics and transport in Lake Garda. The investigation is carried out in parallel on three different levels: data collection and analysis, three-dimensional numerical modeling and theoretical study. On the first level, data are collected by building up a network of research institutes and local administrations in the lake area. New data are acquired through traditional field campaigns (CTD, thermistor chains, satellite imagery), while a citizen-science approach, based on local knowledge harvesting, is successfully tested to gather qualitative data on surface circulation. On the second level, a three-dimensional modeling chain is set up, by coupling one-way a mesoscale atmospheric model to a hydrodynamic model. Both models are validated on multiple temporal and spatial scales, allowing to identify the main interactions between the weather forcing and the hydrodynamic response of the lake. Circulations in Lake Garda are found to be very sensitive to the thermal stratification, to the spatial distribution of the wind forcing and to the Earth’s rotation. Surface cyclonic gyre patterns develop in the lake as a residual outcome of alternating wind forcing of local breezes and differential acceleration induced by Earth’s rotation, whereas unidirectional currents flow under a nearly uniform and constant wind. Both model and observations evidences show that, under weak thermal stratification, Ekman transport activates a secondary circulations in the northern part of the lake, driving surface water to the deep layers and possibly preconditioning the lake for subsequent buoyancy-driven deep mixing events. On the third level, the relevance of the Coriolis term in the equations of motion for relatively narrow closed basins is analytically addressed. The classical Ekman problem is solved by including the presence of lateral boundaries and a new analytical solution is formulated. The validity of the new solution is proved by numerical tests of idealized domains of different size, geographical location and turbulent regime, and on Lake Garda as a real test case. The meaningful length scales are discussed, and the significance of Rossby radious as a reference horizontal scale is disproved for steady-state circulations driven by wind and planetary rotation.

This doctoral thesis provides the first comprehensive study on the physical processes controlling hydrodynamics and transport in Lake Garda. The investigation is carried out in parallel on three different levels: data collection and analysis, three-dimensional numerical modeling and theoretical study. On the first level, data are collected by building up a network of research institutes and local administrations in the lake area. New data are acquired through traditional field campaigns (CTD, thermistor chains, satellite imagery), while a citizen-science approach, based on local knowledge harvesting, is successfully tested to gather qualitative data on surface circulation. On the second level, a three-dimensional modeling chain is set up, by coupling one-way a mesoscale atmospheric model to a hydrodynamic model. Both models are validated on multiple temporal and spatial scales, allowing to identify the main interactions between the weather forcing and the hydrodynamic response of the lake. Circulations in Lake Garda are found to be very sensitive to the thermal stratification, to the spatial distribution of the wind forcing and to the Earth’s rotation. Surface cyclonic gyre patterns develop in the lake as a residual outcome of alternating wind forcing of local breezes and differential acceleration induced by Earth’s rotation, whereas unidirectional currents flow under a nearly uniform and constant wind. Both model and observations evidences show that, under weak thermal stratification, Ekman transport activates a secondary circulations in the northern part of the lake, driving surface water to the deep layers and possibly preconditioning the lake for subsequent buoyancy-driven deep mixing events. On the third level, the relevance of the Coriolis term in the equations of motion for relatively narrow closed basins is analytically addressed. The classical Ekman problem is solved by including the presence of lateral boundaries and a new analytical solution is formulated. The validity of the new solution is proved by numerical tests of idealized domains of different size, geographical location and turbulent regime, and on Lake Garda as a real test case. The meaningful length scales are discussed, and the significance of Rossby radious as a reference horizontal scale is disproved for steady-state circulations driven by wind and planetary rotation.

Lake trout (Salvelinus namaycush) were extirpated from Lake Champlain around 1900 and from the lower four Great Lakes by 1960. Their ecological, commercial and recreational importance has prompted extensive restoration efforts. Despite widespread evidence of natural reproduction by stocked lake trout, there is minimal evidence of survival of wild progeny beyond age-0. Various abiotic and biotic impediments may be preventing self-sustaining lake trout populations from becoming established. Unsuccessful restoration in shallow areas has recently prompted a shift to restoration efforts to offshore, deep reefs in the Great Lakes. The first objective of this study was to develop, test, and implement methods for evaluating lake trout reproduction in deep water, where previously established techniques were ineffective. The second objective addressed the recruitment bottleneck between the emergent fry and juvenile life stages in Lake Champlain, by assessing the severity of predation on lake trout fry by epi-benthic fish. In order to quantify egg density on deep-water habitats (>18 m), we paired a deep-water egg trap with egg bags to establish a relationship between the two types of gear in Lake Champlain. There was no significant difference between densities in the egg bags and deep-water traps, but there was a positive correlation of their ranks (correlation coefficient = 0.514, p<0.0001). The deep-water traps were then used in Lake Michigan to successfully acquire the first egg density data from two sites on the deep Mid Lake Reef Complex. A drop electroshocker was developed to detect fry presence and tested in Lake Champlain in conjunction with emergent fry traps. Both types of gear exhibited similar patterns of fry relative abundance. To assess fry predation in Lake Champlain, two-hour gillnet sets during the period of fry emergence to identify fry predators and to describe how predation patterns changed diurnally and temporally. Seven species of epi-benthic fry predators were identified, including five species that had not been previously identified as fry predators. Yellow perch and rock bass dominated the predator community at two study sites (83% of total catch, N=1179, 77% of all fry predators, N=57). Predator presence and fry consumption was almost entirely nocturnal. There was a linear aggregational response in predator CPUE (fish/hr) to increasing fry relative abundance (p<0.033) but confirmed predators did not exhibit a functional response. There was evidence of a threshold of fry relative abundance at 1 fry/trap/day for the onset and conclusion of fry predation. Temperature was a driving factor in the timing of fry emergence and predator abundance, allowing us to predict the relative impact of predators based on temperature scenarios. Only 5% of the potential predators consumed fry. We used empirical probabilities of consumption to model loss of fry due to predation. This consumption model revealed that predator abundances would have to be extremely high for predation to significantly reduce the population of fry. However, given the relatively high species richness of predators observed at the shallow water study sites, lake trout fry survival is likely to be higher at deep, offshore reefs. These results support the recent shift in restoration efforts to focus on deep reefs.

Thesis (M.A.)--West Virginia University, 2007.
Title from document title page. Document formatted into pages; contains v, 42 p. : ill. (some col.), col. maps. Includes abstract. Includes bibliographical references (p. 36-37).

Lake Baikal (Southern Siberia) is the world's oldest, deepest and largest freshwater body by volume. In spite of its enormous depth, episodically (i.e. almost twice a year) large volumes of surface, cold and oxygenated water sink towards the bottom of the lake. This phenomenon is known as deep ventilation and determines the periodical, partial renewal of deep water, playing a key role in the ecology of the whole lacustrine system. Despite deep ventilation has been widely observed, still significant uncertainties exist about the detailed characterization of deep downwellings. In order to tackle this issue, a simplified, one-dimensional numerical model has been developed, which allows for a suitable simulation of deep ventilation in profound lakes. Three main algorithms are at the basis of the model: a reaction-diffusion equation for temperature and other tracers (e.g. dissolved oxygen), and two Lagrangian algorithms, the first to handle buoyancy-driven convection due to density instability (including thermobaric effects) and the other to reproduce the deep downwelling mechanism. Thanks to its simple structure, such a model ensures a considerable computational speed that makes it suitable to perform long-term simulations (i.e. decades, centuries). At the same time, it has been shown to be appropriate for quantitatively and qualitatively simulating deep ventilation, well capturing the relative contribution of the different processes involved. The model has been applied to investigate deep ventilation in the South Basin of Lake Baikal. The numerical results have been shown to be in good agreement with observed data (concerning temperature, CFC-12 and dissolved oxygen profiles), indicating a proper performance of the core algorithms. The analysis of results allowed for a detailed description of the major mixing and thermal dynamics of the lake, and for an in-depth characterization of deep water renewal (e.g. typical downwelling temperatures and volumes, vertical distribution of sinking water, energy balance). Numerical simulations have been performed under current conditions and climate change scenarios, thus permitting to assess the future behavior of the lake and the possible impact on deep ventilation, in response to the expected evolution of climate. In addition to the main results discussed above, this study provided some additional outcomes: a simplified lumped model to convert air temperature into surface water temperature of lakes, and a novel downscaling procedure to transform meteorological data (i.e. wind speed and air temperature) from the global scale to the lake scale. In the light of the proven performance of the deep ventilation model, further improvements of the model could bring to the development of a suitable module to simulate biogeochemical processes in the lake, thus providing valuable information to assess the role of deep ventilation in affecting the lake ecosystem.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Precisely dated terrestrial paleoclimate records are indispensable for understanding how different regions respond to global-scale climate variability. Here, we focus on developing precisely dated records from the Bonneville Basin, a large basin in the eastern U.S. Great Basin. This region is known to have been highly sensitive to hydroclimate changes during the past glacial period; closed-basin lakes in this region have been studied for over a century. We present lake and speleothem records from the Bonneville Basin that are anchored by high-precision U-Th ages. Deep-lake carbonates are deposits that form beneath the surface of lakes in crevices and protected spaces. We explore these deposits as an archive of past hydrological change in the Bonneville Basin. Deep-lake carbonates deposits were collected from varying elevations in the basin and microsampled for U-Th dating. The results of this work improve constraints on existing lake level information for Lake Bonneville. Additionally, we present [delta]234U data from these carbonates that records changes in weathering, and by extension, hydrology, in the Bonneville Basin. 87Sr/86Sr ratios of deep-lake carbonates reflect relative changes in fluvial input between the northern and southern sub-basins of the Bonneville Basin, allowing for the reconstruction of past precipitation patterns. This record shows that there were distinctly different patterns of atmospheric circulation over the Bonneville Basin during Heinrich Event 2 and the Last Glacial Maximum. Finally, we present records Mg/Ca ratios and [delta]13C values of speleothems from Lehman Caves, NV, located on the western edge of the Bonneville Basin. These records suggest that the onset of mid- Holocene drying in the Great Basin was at 8.2 ka, coincident with the collapse of the Laurentide Ice Sheet.
by Elena Anne Steponaitis.
Ph. D.

Il cambiamento climatico è considerato una delle minacce più gravi per la terra e gli ecosistemi acquatici e cresce la preoccupazione per gli effetti che il riscaldamento globale può avere sulle comunità biologiche. Negli ultimi anni, diversi studi hanno evidenziato la sensibilità dei corpi idrici alle fluttuazioni climatiche. Gli organismi zooplanctonici, caratterizzati da piccole dimensioni e rapido ciclo riproduttivo, sono sensibili ai cambiamenti ambientali. Inoltre, rappresentano dei componenti chiave delle catene trofiche acquatiche, collegando i produttori primari ai grandi consumatori secondari come pesci. Lo scopo della presente ricerca di dottorato è di ampliare le conoscenze sulle comunità zooplanctoniche dei grandi laghi subalpini, rilevando tendenze e cambiamenti nella struttura delle reti trofiche pelagiche e le loro potenziali cause. L’analisi di dati limnologici a lungo termine combinata a uno studio paleolimnologico del lago d’Iseo ha permesso di distinguere gli effetti dovuti al cambiamento climatico a quelli dovuti a impatti più locali, come l’eutrofizzazione. I risultati evidenziano che le rete trofiche dei laghi profondi subalpini sono fortemente influenzate dal clima. In particolare, la variabilità climatica invernale provoca un effetto a cascata che coinvolge l'intero ecosistema lacustre, modificando le caratteristiche chimiche, fisiche e biologiche del lago. Gli effetti del riscaldamento delle temperature invernali possono ripercuotersi sulle dinamiche di popolazione dello zooplancton fino all'estate, influenzando il funzionamento dell’intera catena trofica. L'indagine paleolimnologica, attraverso lo studio combinato di fattori biotici e abiotici, ha permesso di considerare gli effetti dei più importanti fattori di cambiamento negli ecosistemi lacustri, suggerendo che i fattori climatici dovrebbero essere considerati di fondamentale importanza nel determinare le dinamiche di sviluppo delle comunità planctoniche e la struttura e il funzionamento delle reti trofiche pelagiche.
Climate change is considered to be one of the most severe threats to earth and aquatic ecosystems and the concern about the effects that global warming can have on biological communities is growing. Even if the number of studies concerning climate change is worldwide increasing the effects of climate change are challenging to monitor and understand because of the multitude of responses within an ecosystem. In the last years, analyses of long-term data sets provide increasing evidence on the sensitivity of water bodies to climatic fluctuation. Small size and fast reproducing zooplankton organisms are sensitive to environmental change. Moreover, they represent key components of aquatic food webs, connecting primary producers to big secondary consumers as fishes. The purpose of the present PhD research is to expand the knowledge on the zooplankton communities of the large and deep lakes south of the Alps, detecting trends and significant change or shift in the pelagic food webs structure due to global or local impacts. The analysis of long-term series of limnological observations and a paleolimnological study on lake sediment have been combined in order to disentangle the effects of climate change from that due to more local and anthropic stressor as eutrophication. The obtained results suggested that in deep subalpine lakes the effect of climate change on pelagic food webs components is complex, including several direct and indirect effects, and it is mediated by nutrient availability. In particular, in deep lakes winter climatic variability trigger a cascading effect that involves the entire lake ecosystems, modifying lake chemical, physical and biological characteristic. The effects of warmer winter temperature can reverberate on zooplankton seasonal patterns until summer, affecting the food webs functioning. The paleolimnological investigation, through the combined study of biotic and abiotic factor, allowed clarifying the synergic effects of the most important drivers of change in lake ecosystems, suggesting that climatic factors should be considered with nutrient availability as a determinant element in controlling the temporal development of plankton communities and pelagic food web structure.

Understanding the interactions of predators and their prey and their responses to environmental changes is one of the striking features of ecological research. In this thesis, spring dynamics of phytoplankton and its consumers, zooplankton, were considered in dependence on the environmental conditions in a deep lake (Lake Constance) and a shallow marine water (mesocosms from Kiel Bight), using descriptive statistics, multiple regression models, and process-oriented dynamic simulation models. The development of the spring phytoplankton bloom, representing a dominant feature in the plankton dynamics in temperate and cold oceans and lakes, may depend on temperature, light, and mixing intensity, and the success of over-wintering phyto- and zooplankton. These factors are often correlated in the field. Unexpectedly, irradiance often dominated algal net growth rather than vertical mixing even in deep Lake Constance. Algal net losses from the euphotic layer to larger depth were induced by vertical mixing, but were compensated by the input from larger depth when algae were uniformly distributed over the water column. Dynamics of small, fast-growing algae were well predicted by abiotic variables, such as surface irradiance, vertical mixing intensity, and temperature. A simulation model additionally revealed that even in late winter, grazing may represent an important loss factor of phytoplankton during calm periods when losses due to mixing are small. The importance of losses by mixing and grazing changed rapidly as it depended on the variable mixing intensity. Higher temperature, lower global irradiance and enhanced mixing generated lower algal biomass and primary production in the dynamic simulation model. This suggests that potential consequences of climate change may partly counteract each other. The negative effect of higher temperatures on phytoplankton biomass was due to enhanced temperature-sensitive grazing losses. Comparing the results from deep Lake Constance to those of the shallow mesocosm experiments and simulations, confirmed the strong direct effect of light in contrast to temperature, and the importance of grazing already in early spring as soon as moderate algal biomasses developed. In Lake Constance, ciliates dominated the herbivorous zooplankton in spring. The start of ciliate net growth in spring was closely linked to that of edible algae, chlorophyll a and the vertical mixing intensity but independent of water temperature. The duration of ciliate dominance in spring was largely controlled by the highly variable onset of the phytoplankton bloom, and little by the less variable termination of the ciliate bloom by grazing of meta-zooplankton. During years with an extended spring bloom of algae and ciliates, they coexisted at relatively high biomasses over 15-30 generations, and internally forced species shifts were observed in both communities. Interception feeders alternated with filter feeders, and cryptomonads with non-cryptomonads in their relative importance. These dynamics were not captured by classical 1-predator-1-prey models which consistently predict pronounced predator-prey cycles or equilibria with either the predator or the prey dominating or suppressed. A multi-species predator-prey model with predator species differing in their food selectivity, and prey species in their edibility reproduced the observed patterns. Food-selectivity and edibility were related to the feeding and growth characteristics of the species, which represented ecological trade-offs. For example, the prey species with the highest edibility also had the highest maximum growth rate. Data and model revealed endogenous driven ongoing species alternations, which yielded a higher variability in species-specific biomasses than in total predator and prey biomass. This holds for a broad parameter space as long as the species differ functionally. A more sophisticated model approach enabled the simulation of a continuum of different functional types and adaptability of predator and prey communities to altered environmental conditions, and the maintenance of a rather low model complexity, i.e., low number of equations and free parameters. The community compositions were described by mean functional traits --- prey edibility and predator food-selectivity --- and their variances. The latter represent the functional diversity of the communities and thus, the potential for adaptation. Oscillations in the mean community trait values indicated species shifts. The community traits were related to growth and grazing characteristics representing similar trade-offs as in the multi-species model. The model reproduced the observed patterns, when nonlinear relationships between edibility and capacity, and edibility and food availability for the predator were chosen. A constant minimum amount of variance represented ongoing species invasions and thus, preserved a diversity which allows adaptation on a realistic time-span.
Eine der großen Herausforderungen der heutigen ökologischen Forschung ist es, Veränderungen von Ökosys­temen vorher­zusagen, die mit dem Klimawandel einhergehen. Dafür sind ein umfassendes Verständnis der ver­schiedenen Steuerungsfaktoren des entsprechenden Systems und Kenntnisse zur Anpassungs­fähigkeit des Systems nötig. Auf der Grundlage dieses Wissens, können mit mathemati­schen Modellen Klima­szenarien gerechnet und Vorhersagen erstellt werden. Die vorliegende Arbeit untersuchte die Regulation des Phytoplanktons (kleine freischwebende einzellige Algen) und seiner Konsumenten (Zooplankton, tierische Kleinstlebewesen) sowie deren Wechselspiel während des Frühjahrs mit Bezug auf den Klimawandel. Als Basis dienten langjährige Daten von einem großen tiefen See (Bodensee) sowie Daten von Versuchen mit Organis­men aus einem flachen marinen Ge­wässer (Kieler Förde, Ostsee). Diese Daten wurden mit statistischen Verfahren und mathematischen Modellen ausge­wertet. In Gewässern sind Algen als Primärproduzenten die Nahrungsgrundlage für tieri­sche Organismen bis hin zu Fischen und Meeresfrüchten, und bestimmen die Wasserqualität der Ge­wässer. Daher ist es wichtig zu verstehen, welche Mechanismen die Dynamik der Algen steuern. Der Grundstein für die saisonale Entwicklung von Phyto- und Zooplankton in Gewässern un­serer Breiten wird mit dem Be­ginn des Wachstums im Frühjahr gelegt. Diese Arbeit zeigt, dass es bereits im zeitigen, noch kalten Frühjahr ein Wechselspiel physikalischer und biologischer Steuerungsmechanismen für die Algenent­wicklung gibt. Physikalische Faktoren sind die Wassertemperatur, die Globalstrahlung und die Durchmischung des Gewässers, die durch die Stärke des Windes beeinflusst wird. All diese Steue­rungsmechanismen sind eng miteinander verwoben und werden unterschiedlich stark vom Klimawan­del beeinflusst. Mit mathematischen Modellen gelang es den Einfluss einzelner Faktoren voneinander zu trennen und zu zeigen, dass Effekte durch den Klimawandel sich gegenseitig aufheben oder aber auch verstärken können. Schon geringe Änderungen an der Basis der Nahrungsnetze können weitrei­chende Auswirkungen auf höhere Ebenen habe. Wie stark diese Auswirkungen im Einzelnen sind, hängt entscheidend von der Anpassungsfähigkeit gesamter Ökosysteme und ihrer Artengemeinschaf­ten sowie einzelner Individuen ab. Beispielsweise reagiert die Algengemeinschaft auf einen starken Fraßdruck ihrer Räuber mit einer Verschiebung zu weniger gut fressbaren Algenarten. Diese weniger gut fressbaren Arten unterscheiden sich jedoch auch in anderen Eigenschaften, wie zum Beispiel der Ressourcenausnutzung, von besser fressbaren Algen. In dieser Arbeit wurden Modellansätze entwi­ckelt, die diese Fähigkeit zur Anpassung berücksichtigen. Auf dieser Grundlage und mit Einbeziehung der physikalischen Steuerungsfaktoren können Klimaszenarien gerechnet werden und Vorhersagen für den Einfluss des Klimawandels auf unsere Gewässer gemacht werden, die letztlich auch Perspektiven für Handlungsmöglichkeiten aufzeigen.

Xi, Hongyan.
"December 2010."
Thesis (Ph.D.)--Chinese University of Hong Kong, 2011.
Includes bibliographical references (leaves 163-176).
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Abstract also in Chinese.

This thesis explores ideas about the deep past in Australia in the context of contemporary notions of geological heritage, cultural property, cultural identity and antiquity. Moving between disciplines, localities, stories and timescales it examines the complexities of changing intellectual agenda. But it does not pretend to present a complete history of the earth sciences in Australia. Rather it brings together an array of related themes, places, and stories, that knit into a narrative about the construction and interpretation of signs of age in Australian landscapes. Taking as its starting point the discovery by European settlers in 1830 of the Wellington Caves megafaunal fossils, which first suggested a long chronology for Australian vertebrate fauna, this work considers 'ordinary time' and 'deep time', geological heritage, the appropriation and celebration of deep time by settler Australians, and the naturalisation of narrative and sequence in geological writing. The body of the thesis involves discussion of three landscapes which have been celebrated for the deep pasts revealed in their sediments, landforms and material remains: Hallett Cove and Lake Callabonna in South Australia and the Willandra Lakes in New South Wales. Each of these is regarded as more or less canonical in the respective histories of Australian geology, vertebrate palaeontology and archaeology, but each is also a living historical and geological site where people have lived, interacted with and interpreted the shape of the country for upwards of forty thousand years.
In 2010, material in this thesis was reworked and published as Pictures of time beneath: science, heritage and the uses of the deep past (CSIRO Publishing, Collingwood, Victoria): http://www.publish.csiro.au/pid/6342.htm.

The faunal remains from four archaeological sites on the northwest shore of Mingo Lake, in the interior of Southern Baffin Island, are examined in this thesis. All four sites are radiocarbon dated to Pre-Dorset times (4500 – 2700 BP). The faunal assemblage is dominated by caribou remains. As such, this study is the first, large-scale faunal analysis of an interior Pre-Dorset site with caribou as a main subsistence resource. In total 18,710 faunal bones were examined. Elemental frequencies, fracture patterns, bone burning, and butchering patterns will provide important insights into the lifeway of the Pre-Dorset. The results of the thesis indicate that the Pre-Dorset were utilizing the Mingo Lake area during the late summer into early fall. The main activity at all four sites was caribou hunting with a focus on marrow extraction. The sites served dual purposes as habitation and butchering sites and were occupied for varying lengths of time.