Dissertations / Theses on the topic 'Diatomées fossiles'

Les reconstitutions paleoenvironnementales antérieurement réalisées sur l'holocène des Andes de Bolivie se sont appuyées pour la plus part sur des études concernant les sédiments des lacs glaciaires profonds ou de très grands lacs, tels que le lac Titicaca. Le présent mémoire prend en considération un milieu très diffèrent : les tourbières des fonds de vallées, jusqu'à présent peu étudiées. Ces tourbières sont caractérisées par une forte hétérogénéité à différentes échelles spatiales, métriques, hectométriques et kilométriques. La première partie est consacrée à l'étude des diatomées actuelles contenues dans des échantillons de sédiment superficiel. Ces échantillons ont été prélevés sur une tranche d'altitude comprise entre 4200 et 4950 m le long du profil longitudinal (20 km) d'une vallée glaciaire. Ils proviennent de différents milieux définis d'après les données hydrologiques locales (surfaces d'eau libre, sols humides, chenaux, etc. ), la composition spécifique des végétaux supérieurs et certains paramètres physico-chimiques (ph, conductivité, température). Le traitement statistique montre que les assemblages de diatomées sont corrélés à l'altitude et aux conditions hydrologiques locales. L’enregistrement du gradient altitudinal de la température par les diatomées est masque par un bruit de fond qui est lie à l'hétérogénéité spatiale des tourbières, aux différences observées dans la qualité des eaux entre le haut et le bas de la vallée et à une très forte variabilité journalière de la température. L’enregistrement des conditions hydrologiques se révèle par contre excellent dans les assemblages de diatomées. La deuxième partie propose une reconstitution à haute résolution temporelle de l'évolution des tourbières d'après l'étude de deux carottes respectivement situées à 4300 et 4900 m d'altitude. Cette reconstitution s'appuie sur les résultats obtenus dans l'étude des diatomées actuelles. Un évènement paleohydrologique majeur, date de 2600-2000 ans bip. Correspond à une modification régionale du climat ayant été déjà reconnue par l'étude des carottes du lac Titicaca. Les changements hydrologiques des 300 dernières années sont corrélés aux variations de l'accumulation nette dans les carottes de glace de quelccaya (sud Pérou, près de la frontière avec la Bolivie). Ils mettent en évidence une forte variabilité des précipitations durant la petit Age de la glace. Mots clés : cordillère des Andes, Bolivie, holocène, diatomées, écologie, paléoécologie, statistiques.

Les diatomées forment un groupe important d’algues brunes microscopiques colonisant l’ensemble des milieux aquatiques. Organismes photosynthétiques possédant un exosquelette siliceux, elles sont reconnues depuis plusieurs décennies comme d’excellents bio-indicateurs des caractéristiques physico-chimiques de l’eau. Cette bio-indication repose sur la connaissance de l’écologie des espèces. Peu de travaux ont été conduit dans le Sahel et en particulier au Tchad, alors qu’il possède l’un des plus grands lacs d’eau douce d’Afrique, le lac Tchad. A partir d’une étude exhaustive menée dans tous les lacs tchadiens, Tchad, Iro, Fitri et d’Ounianga (à l’exception du lac Léré inaccessible pour des raisons de sécurité), l’inventaire des diatomées est dressé. La distribution des diatomées en fonction des caractéristiques hydrochimiques de l’eau est explorée et une modélisation est proposée afin de quantifier les paramètres de l’environnement aquatique à partir de l’étude des diatomées fossiles
Diatoms form a large group of microscopic brown algae colonizing all aquatic environments. Photosynthetic organisms with siliceous exoskeletons have been recognized and used for decades as excellent bio-indicators of the water chemistry. This bio-indication is based on knowledge of the self-ecology of the species. Little work has been carried out in the Sahel and especially in Chad, while it has one of the largest freshwater lakes in Africa, Lake Chad. Based on a comprehensive study carried out in all Chadian lakes, Chad, Iro, Fitri and Ounianga (with the exception of Lake Léré due to safety reasons), the diatom inventory is drawn up. Diatomaceous distributions based on water hydrochemical characteristics are explored and modeling is proposed to quantify aquatic environmental parameters from the fossil diatom study

Les études antérieures faites dans le Sud-Congo et l' Ouest-Cameroun ont montré qu'une période d'extension des savanes au détriment de la forêt a débuté vers 3 000/2 500 ans B. P. Cette période, qui a été en outre caractérisée par d'intenses perturbations au sein des écosystèmes forestiers humides, s'est prolongée jusqu'à une date qui ne remonte pas à plus de quelques siècles dans la plupart des régions étudiées. L'objectif du présent mémoire est de fournir une reconstruction à haute résolution temporelle ( 70 ans) de l'évolution du lac Ossa (3°3 N, 9°36 E) afin d'en extraire une estimation des changements paléohydrologiques et paléoclimatiques. Cette estimation est nécessaire pour mieux comprendre la réponse de la végétation aux variations du climat. Les reconstructions paléohydrologiques s'appuient sur une étude des assemblages de diatomées (environ 200 espèces) fossilisés dans les sédiments holocènes. Pour mieux interpréter ces assemblages, il a été nécessaire de procéder à une analyse détaillée de la distribution des diatomées actuelles en fonction des caractéristiques physico-chimiques des eaux du lac Ossa. La bathymétrie et le pH sont les principaux facteurs de cette distribution. L'estimation des modifications passées de la bathymétrie et du pH a permis d'identifier les grandes étapes de l'évolution du lac Ossa durant les 5 000 dernières années. Un changement brusque, date d'environ 2 700 ans B. P. , correspond à un renforcement de la saisonnalité. Celui-ci a entraîné les fortes modifications qui ont été mises en évidence par la palynologie dans la végétation. Un autre évènement important a été enregistré par un abaissement du niveau lacustre à 600/200 ans B. P. L 'instauration des conditions climatiques humides actuelles est marquée par une remontée du niveau lacustre durant les derniers siècles. Elle correspond à une reconquête forestière sur les bordures du massif forestier et à une atténuation des perturbations au sein de la forêt humide. D' autres changements ont eu lieu durant l'holocène mais ils n'ont pas entraîné de modifications assez fortes dans la végétation pour que celles-ci soient enregistrées dans les spectres polliniques.

Dans la présente recherche, l’influence humaine et climatique des 1500 dernières années sur quatre lacs de Nouvelle-Angleterre (États-Unis) a été reconstituée par l’analyse des diatomées fossiles provenant des sédiments. Différentes méthodes ont été testées pour inférer certaines variables environnementales à l’aide des diatomées. Les méthodes des moyennes pondérées et de la probabilité maximale se sont avérées les plus appropriées pour la reconstitution des variables du gradient principal, tandis que les réseaux neuronaux artificiels et la probabilité maximale sont préférables pour les variables du gradient secondaire. Différentes transformations des données sont appropriées pour chacune des méthodes. Les diatomées fossiles des lacs ont enregistré un patron régional propre à la déforestation suite à la colonisation par les Européens. Malgré le rétablissement des forêts autour des sites depuis 1900 AD, trois lacs montrent à l’état actuel des conditions qui ne correspondent pas à celles de la période avant-perturbation. Ceci est dû aux caractéristiques locales des sites, tels que l’évolution naturelle des lacs, les perturbations naturelles (ouragans), ainsi que le développement des marais. Pour rétablir l’état « naturel » théorique envisagé dans la restauration, il est donc important de tenir compte de l’évolution naturelle des lacs. Au lac Walden Pond (Massachusetts), un enrichissement accéléré en nutriments a été inféré suite à l’usage récréatif intensif depuis env. 1950 AD. Depuis 1975 AD, le taux d’eutrophisation a diminué grâce aux mesures de gestion. L’état naturel du lac reconstitué par notre étude représente un bon guide pour les futurs mesures de gestion. Au lac Levi Pond (Vermont), une tendance accrue à l’humidité pendant les dernières 2000 années, qui correspond au refroidissement global du Néoglaciaire, a été inférée par les diatomées. Les diatomées sont donc d’une utilité potentielle pour les études paléohydrologiques dans les régions tempérées. L’analyse mensuelle de la flore diatomifère dans Bates Pond (Connecticut), a indiqué que les assemblages sont liés à la stratification, ce qui a permis d’identifier une période de mélange prolongée dans le passé à partir des assemblages fossiles. Le maximum de productivité diatomifère en automne indique que c’est la saison privilégiée pour l’échantillonnage d’eau en vue du développement des modèles d’inférence. Cette thèse apporte des nouvelles connaissances sur l’évolution et l'écologie des lacs en Nouvelle-Angleterre, et sur deux aspects méthodologiques pertinents pour la discipline de la paléolimnologie.
The human and climatic impact on four New England lakes was reconstructed by analysis of fossil diatoms preserved in lake-bottom sediments. Different diatom-based methods for the reconstruction of environmental variables (pH, total phosphorus, alkalinity) were tested. Weighted averaging and Gaussian logit regression were the most appropriate methods for the reconstruction of variables representing the principal gradient, whereas artificial neural networks and Gaussian logit regression performed better for secondary gradient variables. It was shown that different data transformations may be useful for different reconstructions. Fossil diatom assemblages of the sites recorded a regional pattern of deforestation and agricultural activity following European settlement. Despite the re-establishment of forests in the lake watersheds since 1900 AD, three lakes did not return to pre-disturbance conditions. This is due to local site characteristics, such as natural lake evolution, local natural disturbance patterns (hurricanes), and peatland development. It is therefore important to consider the natural dynamics of lakes when establishing the theoretical « natural » state for restoration purposes. At Walden Pond, Massachusetts, an accelerated nutrient enrichment was inferred from 1950 AD onwards, related to intensive recreational use of the lake. Since 1975 AD, the rate of eutrophication has diminished after management measures were implemented. The natural state of the lake reconstructed by this study represents a useful benchmark for future management decisions. In Levi Pond, a trend of increasing humidity during the past ca. 2000 years has been inferred by diatoms, corresponding to the period of Neoglacial cooling. This result indicates that diatoms may be a useful tool for future paleohydrological studies in temperate regions. The seasonal study of diatoms in Bates Pond, Connecticut, indicated that diatom assemblages are strongly influenced by stratification, which helped to identify a period of prolonged full-circulation in the past based on the fossil assemblages of the same lake. The maximum diatom productivity in autumn indicated that this is the most appropriate season for taking water samples for diatom inference model development. This thesis has provided new knowledge of the evolution of New England lakes before and after European settlement and on two pertinent methodological aspects of paleolimnologic study.

The Berg River is a pivotal source of fresh water for domestic, industrial and agricultural use as well as for in stream ecology, therefore knowledge of what impacts this rivers water quality and assessing whether ecological resilience has been surpassed are of the utmost importance. Since diatom assemblages are inextricably linked to the chemical, physical and biological characteristics of their environment, they were chosen for this study to investigate long-term changes in water quality and availability and suggest potential drivers of such changes at an agricultural site (Rhenostervlei Farm) adjacent the Berg River in the Cape lowlands through fossil diatom analysis of a sediment core (RV3). Diatoms were extracted along the length of the RV3 core, chemically and physically treated to remove unwanted material and then were mounted on a slide to be counted. Twenty abundant diatoms were chosen as environmental indicators for the analysis. Their abundances were plotted against depth and age and changes in water quality and availability were inferred based on their autecological characteristics. The most prominent shift in the indicator diatoms at Rhenostervlei Farm as shown by the stratigraphic diagrams, CONISS analysis and the PCA occurred at the onset of the 20th century CE and was characterised by a shift from a saline, dry and nutrient-poor environment (ca. 1790-1890 CE) to a more turbid, nutrient-rich environment with increased freshwater influence that was prone to periodic flooding (ca. 1890-2011 CE). Through the chronological analysis and the interpretation of the historical record (climate and land-use) as well as other environmental proxies (sediment accumulation rate and macro-charcoal), the causes of the detected change in water availability and quality at the floodplain site were likely related to land-use change in the form of agricultural intensification at Rhenostervlei Farm and potentially in the Berg River catchment as a whole. This could have involved burning, clearance of natural vegetation, soil disturbance and fertilizer use - all of which contributed to increased surface runoff, erosion and nutrient and sediment loading into the site. Furthermore, water extraction and diversion in the 1950s could explain the decreased flooding signal (i.e. lower abundance of Aulacoseira granulata). Although no evidence of a catastrophic regime shift was identified, if land-use practices continue to intensify (e.g. increased fertilizer use) and future climate change interacts and influences the agricultural alterations to hydrological systems, we may expect increased vulnerability to global change and unexpected ecological outcomes such as regime shifts. In order to improve the interpretation of fossil diatom records in terms of changing water quality and availability, a study with multiple proxies should be undertaken to help infer environmental conditions in a complex environment that has many potential drivers, such as the Berg River.

O presente trabalho descreve as assembléias de diatomáceas, presentes em um perfil sedimentar na Lagoa do Peixe, Litoral Médio do Rio Grande do Sul, com o objetivo de efetuar a reconstrução paleoambiental e identificar os períodos de maior influência marinha e hidrodinâmica, como conseqüências das variações do nível do mar ocorridas durante o Holoceno. O testemunho de sondagem T09 foi selecionado para estudo, devido à boa recuperação e à presença de um espesso pacote de lama próximo à base. Os resultados da análise de diatomáceas revelaram a presença de 114 táxons, constituintes das assembléias fósseis, dominadas pelo complexo de Paralia (Paralia fenestrata, P.sulcata e Paralia sp.), associados à táxons marinhos, entre eles: Auliscus caelatus e Triceratium favus; e os táxons marinhos salobros: Cyclotella striata, Dimeregramma marinum, D. minor var. minor e Psamococconeis disculoides, entre outros. Os resultados da distribuição, tafonomia e paleoecologia do complexo de Paralia e das assembléias presentes ao longo do perfil, forneceram maior detalhamento evolutivo do sistema lagunar associadas à ocorrência de três oscilações de alta freqüência do nível do mar na PCRS, interligadas à períodos de maior influência hidrodinâmica, comprovando que a morfologia lagunar sofreu intensas modificações ao longo do tempo. Entre as idades de 7420-7020 e 5370-5340 anos AP, ocorreram duas oscilações de alta freqüência, que atingiram o sistema lagunar, afetando diretamente as assembléias presentes. Entre as idades de 5370-5340- 2340-2060 anos AP, ocorreu uma terceira oscilação de alta freqüência, concordando com o máximo transgressivo registrado para a região sul do Brasil. A distinta composição observada entre as assembléias fósseis e atuais (preservadas em sedimentos superficiais) é um indicativo de que durante o Holoceno, a Lagoa do Peixe comportou-se como um sistema lagunar, apresentando características morfosedimentares, geomorfológicas, hidrodinâmicas e ecológicas, totalmente diferente das condições atuais, onde assume configurações de lagoa costeira semi-fechada, de ligação restrita com o oceano por meio de um canal de ligação.
This paper describes the diatom assemblages present in a sediment profile in the Lagoa do Peixe, Middle Littoral of Rio Grande do Sul, in order to make the palaeoenvironmental reconstruction and identify periods of increased marine influence and hydrodynamics, as a consequence of changes sea level occurred during the Holocene. The drill core T09 was selected for study due to good recovery and the presence of a thick pack of mud near the base. The results of diatom analysis revealed the presence of 114 taxa, constituents of fossil assemblages, dominated by complex Paralia (Paralia fenestrata, P.sulcata and Paralia sp.), associated with marine taxa: Auliscus caelatus and Triceratium favus, brackish and marine taxa: Cyclotella striata, Dimeregramma marinum, D. minor var.minor and Psamococconeis disculoides, among others. The results of the distribution, taphonomy and paleoecology of Paralia complex assemblages, present along the profile, provided a more detailed evolution of the lagoon system associated with the occurrence of three high-frequency oscillations in sea level in the PCRS, linked to periods of greater influence hydrodynamics, proving that the lagoon morphology underwent enormous changes over time. Between the ages of 7420- 7020 and 5370-5340 years BP, there were two high frequency oscillations, which reached the lagoon system, directly affecting the assemblages present. Between the ages of 5370-5340- 2340-2060 years BP, there was a third high frequency oscillation, in agreement with the transgressive maximum recorded for the southern region of Brazil. The difference in composition observed between the modern and fossil assemblages (preserved in surface sediments), is an indication that during the Holocene, the Lagoa do Peixe, behaved like a lagoon system, whose morphosedimentar, geomorphological, hydrodynamic and ecological characteristics were totally dissimilar from actual conditions representing semi-enclosed coastal lagoon which had restricted connection with the oceano through an inlet.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009.
Title from title screen (site viewed July 21, 2009). PDF text: xi, 160 p. : ill. (some col.), col. maps ; 5.84 Mb. UMI publication number: AAT 3350458. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009.
Title from title screen (site viewed September 08, 2009). PDF text: xii, 168 p. : ill. (some col.) ; 9 Mb. UMI publication number: AAT 3352765. Includes bibliographical references. Also available in microfilm and microfiche formats.

Sediment records of two Italian subalpine lakes (Lake Garda and Lake Ledro) were analyzed in order to reconstruct their ecological evolution over the past several hundred years. A multi-proxy and multi-site approach was applied in order to disentangle the effects of local anthropogenic forcings, such as nutrients, and climate impacts on the two lakes and their catchments. Biological indicators (sub-fossil pigments, diatoms and Cladocera) were used to reconstruct changes in the aquatic food web and to define the lake reference conditions, while geochemical methods, i.e. wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF), were used to provide quantitative information on the different physical or chemical processes affecting both lake and catchment systems. Sub-fossil pigments and diatoms, together with their respective inferred TP values, suggested very stable oligotrophic conditions in both lakes until the 1960s. The period following was affected by nutrient enrichment, which led to a drastic shift in the phytoplanktonic community. The response of sub-fossil pigments and diatoms to major climatic anomalies such as the Medieval Climatic Anomaly (MCA) and the Little Ice Age (LIA) were not pronounced, and the taxonomic composition remained relatively stable. On the contrary, these proxies showed an indirect response to climate variability since the beginning of the nutrient enrichment phase in the 1960s. In Lake Garda, the winter temperature regulates the water column mixing, which in its turn controls the degree of nutrient fertilization of the entire water column, and the related phytoplankton growth. In Lake Ledro a rapid reorganization of planktonic diatoms was observed only during the temperature recovery after the LIA, while recent temperature effects are masked by the prevailing nutrient effects. In Lake Garda, Cladocera remains responded in quantitative and qualitative terms to climatic changes, whereas in Lake Ledro they appeared to be mainly affected by variations in hydrological regimes, i.e. flood events. Cladocera remains corroborated the nutrient enrichment after the 1960s in both lakes as inferred by diatoms and pigments. In Lake Garda, the geochemical data showed a pronounced shift in elemental composition since the mid-1900s, when major elements and lithogenic tracers started to decrease, while some elements related to redox conditions and other (contaminant) trace elements increased. The general trends since the mid-1900s agree with the biological records. However, some differences recorded in the two different basins of Lake Garda reflected the effects of local conditions, both related to hydrology and sedimentation patterns. Lake Ledro showed higher short-term variability for most elements, even though some features were comparable to Lake Garda. The geochemical record of Lake Ledro revealed a major influence of human-induced lake-level fluctuations and catchment properties. This paleolimnological study allows us to place temporally restricted limnological surveys into a longer-term secular perspective, which is highly valuable for the definition of lake reference conditions. Because the restoration targets are usually based on the lake reference conditions, this study highlighted also the necessity to pay particular attention to the lake-specific sensitivity patterns. The multi-proxy and multi-site approach showed that the lake conditions of large and deep lakes in northern Italy, such as Lake Garda, are mainly driven by nutrient enrichment and/or climate change. In contrast, smaller lakes with larger catchment areas, such as Lake Ledro, are seemingly more impacted by conditions and processes occurring in the drainage basin.