Vignette d'image

Mitchell, Edward

Nom
Mitchell, Edward
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
edward.mitchell@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/357
_
0000-0003-0358-506X

Résultat de la recherche

Filtres

5
5
Réinitialiser les filtres

Paramètres

Voici les éléments 1 - 5 sur 5
  • Publication
    Accès libre
    A multi-proxy, high-resolution record of peatland development and its drivers during the last millennium from the subalpine Swiss Alps
    (2011)
    van der Knaap, W.O.
    ;
    Mariusz Lamentowicz
    ;
    van Leeuwen, J.F.N
    ;
    Hangartner, S.
    ;
    Leuenberger, M.
    ;
    Dmitri Mauquoy
    ;
    Golslar, T.
    ;
    Mitchell, Edward 
    ;
    Kamenikg, C.
    We present a record of peatland development during the last 1000 years from Mauntschas mire in the eastern Swiss Alps (Upper Engadine valley; 1818 m a.s.l.) inferred from testate amoebae (pH and depth to the water table (DWT) reconstructions), stable oxygen isotopes in Sphagnum (δ18O; proxy for water vapour pressure) and carbon isotopes in Sphagnum (δ13C; proxy for mire surface wetness), peat accumulation rates, charcoal (indicating local burning), pollen and spores (proxies for human impact), and plant macrofossils (reflecting local vegetation and trophic state). Past human impact on the local mire conditions was strong but fluctuating during AD 1000–1570 (±50 yr; depth–age model based on 29 14C AMS dates) with local irrigation of nutrient-enriched water and grazing. Human impact was minor AD 1570–1830 (±30 yr) with partial recovery of the local mire vegetation, and it was absent AD 1830 (±30 yr)–present when hummock formation took place. Correlations among DWT, pH, δ13C, and δ18O, carried out both with the raw data and with linear trends removed, suggest that the factors driving peatland development changed over time, since only testate amoeba-based pH and DWT co-varied during all the three aforementioned periods. δ18O correlates with δ13C only in the period AD 1830–present and with DWT only during AD 1570–1830, δ13C correlates with DWT only during AD 1000–1570. Part of this apparent instability among the four time series might be attributed to shifts in the local mire conditions which potentially formed very different (non-analogue) habitats. Lack of analogues, caused, for example, by pre-industrial human impact, might have introduced artefacts in the reconstructions, since those habitats are not well represented in some proxy transfer functions. Human impact was probably the main factor for peatland development, distorting most of the climate signals.
  • Publication
    Accès libre
    Testate amoebae analysis in ecological and paleoecological studies of wetlands: past, present and future
    (2008)
    Mitchell, Edward 
    ;
    Charman, Daniel J.
    ;
    Warner, Barry G.
    Testate amoebae are an abundant and diverse polyphyletic group of shelled protozoa living in aquatic to moist habitats ranging from estuaries to lakes, rivers, wetlands, soils, litter, and moss habitats. Owing to the preservation of shells in sediments, testate amoebae are useful proxy indicators complementary to long-established indicators such as pollen and spores or macrofossils. Their primary use to date has been for inferring past moisture conditions and climate in ombrotrophic peatlands and, to a lesser extent, to infer pH in peatlands and the trophic or nutrient status of lakes. Recent research on these organisms suggests other possible uses in paleoecology and ecology such as sea-level reconstruction in estuarine environments, as indicators of soil or air pollution, and monitoring recovery of peatland. We review the past and present use of testate amoebae, the challenges in current research, and provide some ideas on future research directions.
  • Publication
    Accès libre
    Effect of taxonomic resolution on ecological and palaeoecological inference: a test using testate amoeba water table depth transfer functions
    Mitchell, Edward 
    ;
    Lamentowicz, Mariusz
    ;
    Payne, Richard J
    ;
    Mazei, Yuri
    Sound taxonomy is a major requirement for quantitative environmental reconstruction using biological data. Transfer function performance should theoretically be expected to decrease with reduced taxonomic resolution. However for many groups of organisms taxonomy is imperfect and species level identification not always possible. We conducted numerical experiments on five testate amoeba water table (DWT) transfer function data sets. We sequentially reduced the number of taxonomic groups by successively merging morphologically similar species and removing inconspicuous species. We then assessed how these changes affected model performance and palaeoenvironmental reconstruction using two fossil data sets. Model performance decreased with decreasing taxonomic resolution, but this had only limited effects on patterns of inferred DWT, at least to detect major dry/wet shifts. Higher-resolution taxonomy may however still be useful to detect more subtle changes, or for reconstructed shifts to be significant.
  • Publication
    Accès libre
    Eight species in the Nebela collaris complex: Nebela gimlii (Arcellinida,Hyalospheniidae), a new species described from a Swiss raised bog
    Singer, David 
    ;
    Kosakyan, Anush 
    ;
    Pillonel, Amandine 
    ;
    Mitchell, Edward 
    ;
    Lara, Enrique 
    We describe here a new species of sphagnicolous testate amoeba found abundantly in the forested part of the Le Cachot peatland (Jura Mountains, Neuchâtel, Switzerland) based on microscopical observations (LM, SEM). The new species, called Nebela gimlii was placed in a phylogenetic tree based on mitochondrial cytochrome oxidase sequences (COI), and branched robustly within the N. collaris complex next to the morphologically similar N. guttata and N. tincta. It is however genetically clearly distinct from these two species, and differs morphologically from them by its smaller size and stouter shape of the shell. This new species completes the phylogeny of the Nebela collaris species complex, with now eight species described, mostly from peatlands and acidic forest litter, and further demonstrates the existence of an unknown diversity within testate amoebae. Improving the taxonomy of testate amoebae in peatlands and clarifying the ecology of newly discovered species should make these organisms even more valuable as bioindicator and for palaeoecological reconstruction.
  • Publication
    Accès libre
    Towards a Holarctic synthesis of peatland testate amoeba ecology: Development of a new continental-scale palaeohydrological transfer function for North America and comparison to European data
    Amesbury, Matthew J
    ;
    Booth, Robert K
    ;
    Roland, Thomas P
    ;
    Bunbury, Joan
    ;
    Clifford, Michael J
    ;
    Charman, Dan J
    ;
    Elliot, Suzanne
    ;
    Finkelstein, Sarah
    ;
    Garneau, Michelle
    ;
    Hughes, Paul D.M
    ;
    Lamarre, Alexandre
    ;
    Loisel, Julie
    ;
    Mackay, Helen
    ;
    Magnan, Gabriel
    ;
    Markel, Erin R
    ;
    Mitchell, Edward 
    ;
    Payne, Richard J
    ;
    Pelletier, Nicolas
    ;
    Roe, Helen
    ;
    Sullivan, Maura E
    ;
    Swindles, Graeme T
    ;
    Talbot, Julie
    ;
    van Bellen, Simon
    ;
    Warner, Barry G
    Fossil testate amoeba assemblages have been used to reconstruct peatland palaeohydrology for more than two decades. While transfer function training sets are typically of local-to regional-scale in extent, combining those data to cover broad ecohydrological gradients, from the regional-to continental- and hemispheric-scales, is useful to assess if ecological optima of species vary geographically and therefore may have also varied over time. Continental-scale transfer functions can also maximise modern analogue quality without losing reconstructive skill, providing the opportunity to contextualise understanding of purely statistical outputs with greater insight into the biogeography of organisms. Here, we compiled, at moderate taxonomic resolution, a dataset of nearly 2000 modern surface peatland testate amoeba samples from 137 peatlands throughout North America. We developed transfer functions using four model types, tested them statistically and applied them to independent palaeoenvironmental data. By subdividing the dataset into eco-regions, we examined biogeographical patterns of hydrological optima and species distribution across North America. We combined our new dataset with data from Europe to create a combined transfer function. The performance of our North-American transfer function was equivalent to published models and reconstructions were comparable to those developed using regional training sets. The new model can therefore be used as an effective tool to reconstruct peatland palaeohydrology throughout the North American continent. Some eco-regions exhibited lower taxonomic diversity and some key indicator taxa had restricted ranges. However, these patterns occurred against a background of general cosmopolitanism, at the moderate taxonomic resolution used. Likely biogeographical patterns at higher taxonomic resolution therefore do not affect transfer function performance. Output from the combined North American and European model suggested that any geographical limit of scale beyond which further compilation of peatland testate amoeba data would not be valid has not yet been reached, therefore advocating the potential for a Holarctic synthesis of peatland testate amoeba data. Extending data synthesis to the tropics and the Southern Hemisphere would be more challenging due to higher regional endemism in those areas.