Mitchell, Edward

2013, Qin, Yangmin, Mitchell, Edward, Lamentowicz, Mariusz, Payne, Richard J., Lara, Enrique, Gu, Yansheng, Huang, Xianyu, Wang, Hongmei

Testate amoebae are a diverse and abundant group of protozoa that constitute a large proportion of biomass in many ecosystems and probably fill important roles in ecosystem function. These microorganisms have attracted the interest of paleoecologists because the preserved shells of testate amoebae and their known hydrological preferences enable reconstruction of past hydrological change. In ombrotrophic peatlands, surface wetness reflects hydroclimate, so testate amoebae can play an important role in reconstruction of Holocene climate change. Previous studies, however, have been geographically restricted, mostly to North America and Europe. We studied the ecology of testate amoebae in peatlands from central China in relation to hydrology, pH and metal concentrations. We found that testate amoeba community structure was correlated with depth to water table (DWT) and that the hydrological preferences of species generally matched those of previous studies. We developed a weighted average DWT transfer function that enables prediction of water table depth with a cross-validated mean error of < 5 cm. Our results demonstrate the potential for using testate amoebae to reconstruct paleohydrology in China. Such studies could contribute to our understanding of Holocene climate changes in China, particularly regarding past Asian monsoon activity.

2012, Payne, Richard J., Lamentowicz, Mariusz, van der Knaap, W. O., Van Leeuwen, Jacqueline F. N., Mitchell, Edward, Mazei, Yuri

Among the non-pollen micro-fossils commonly encountered in Quaternary sediment samples prepared for pollen analysis are many shells of testate amoebae. Testate amoebae are eukaryotic micro-organisms which are increasingly used in ecological and palaeoecological studies, particularly as indicators of hydrological change in Sphagnum-dominated peatlands. In this study we address the extent to which testate amoebae are used in palynological research, the key challenges to more widespread use, and the extent to which ecological information is retained in the testate amoeba assemblages of standard palynological slides. To achieve this we review the literature on the use of testate amoebae in palynology, compare testate amoeba records produced by palynological and water-based preparation methods and carry out simulations using previously-derived datasets. Our results show that testate amoebae are widely encountered in Quaternary palynological studies, primarily in peatlands, but the information which they can provide is undermined by limited taxonomic knowledge. Many taxa are destroyed in pollen preparations, but for taxa that are retained patterns of abundance parallel those determined using water-based preparation methods. Although the loss of sensitive taxa limits the ecological information contained in testate amoeba assemblages the information preserved is likely to be useful in a multiproxy approach to palaeoenvironmental reconstruction. To help improve taxonomic awareness and encourage the use of testate amoebae in palynology we present a basic introduction to testate amoeba taxonomy and a guide to the taxonomic literature. (C) 2011 Elsevier B.V. All rights reserved.

2009, Payne, Richard J., Mitchell, Edward

Testate amoebae are increasingly used in ecological and palaeoecological studies of wetlands. To characterise the amoeba community a certain number of individuals need to be counted under the microscope. To date, most studies have aimed for 150 individuals, but that sample size is not based on adequate evidence. When testate amoeba concentrations are low, it can be difficult or impossible to reach this total. The impacts of lower count totals have never been seriously scrutinised. We investigated the impact of count size on number of taxa identified, quantitative inferences of environmental variables and the strength of the links between amoebae and environmental data in the context of predicting depth to water table. Low counts were simulated by random selection of individuals from four existing datasets. Results show progressively diminishing returns by all criteria as count size increases from low numbers to counts of 150. A higher count is required to identify all taxa than to adequately characterise the community for transfer function inference. We suggest that in most cases, it will be a more efficient use of time to count a greater number of samples to a lower count. While a count of 50 individuals may be sufficient for some samples from some sites we recommend that counts of 100 individuals should be sufficient for most samples. Counts need only be increased to 150 or more where the aim is to identify relatively minor, but still potentially ecologically relevant community changes. This approach will help reduce lack of replication and low resolution, which are common limitations in testate amoeba-based palaeoecological and ecological studies.

2007, Payne, Richard J., Mitchell, Edward, Payne, Richard J., Mitchell, Edward

Testate amoebae are useful environmental indicators in ecological and palaeoecological studies from peatlands. Previous quantitative studies have focused on the peatlands of Northern and Central Europe, North America, and New Zealand and have considered a relatively restricted variety of peatland types, mostly ombrotrophic or Sphagnum-dominated while more minerotrophic fens have been less studied. Here we present the first quantitative ecological study of testate amoebae from four small mesotrophic fens (pH 5.5–8.1) in the Elatia Forest, northern Macedonia province, Greece. Relationships with the environmental data were investigated using redundancy analysis and mantel tests. Transfer function models were derived using a variety of techniques. Results demonstrate that as for Sphagnum-dominated mires hydrology is the most important control on amoebae community composition. Transfer function models should enable water tables to be predicted within 2.5 cm, when data selection is used this is reduced to less than 2 cm. pH is also an important environmental control on testate amoebae communities, a transfer function model enables pH prediction within 0.4 pH units. The hydrological transfer function is the best performing such model yet produced in terms of prediction error. This study provides new data on the ecology of testate amoebae in fens, and the transfer function models should allow quantitative palaeohydrological reconstruction.

2013, Mitchell, Edward, Payne, Richard J., van der Knaap, Willem O., Lamentowicz, Lukasz, Gabka, Maciej, Lamentowicz, Mariusz

Peatlands are widely exploited archives of paleoenvironmental change. We developed and compared multiple transfer functions to infer peatland depth to the water table (DWT) and pH based on testate amoeba (percentages, or presence/absence), bryophyte presence/absence, and vascular plant presence/absence data from sub-alpine peatlands in the SE Swiss Alps in order to 1) compare the performance of single-proxy vs. multi-proxy models and 2) assess the performance of presence/absence models. Bootstrapping cross-validation showing the best performing single-proxy transfer functions for both DWT and pH were those based on bryophytes. The best performing transfer functions overall for DWF were those based on combined testate amoebae percentages, bryophytes and vascular plants; and, for pH, those based on testate amoebae and bryophytes. The comparison of DWT and pH inferred from testate amoeba percentages and presence/absence data showed similar general patterns but differences in the magnitude and timing of some shifts. These results show new directions for paleoenvironmental research, 1) suggesting that it is possible to build good-performing transfer functions using presence/absence data, although with some loss of accuracy, and 2) supporting the idea that multi-proxy inference models may improve paleoecological reconstruction. The performance of multi-proxy and single-proxy transfer functions should be further compared in paleoecological data. (C) 2012 University of Washington. Published by Elsevier Inc. All rights reserved.

2012, Payne, Richard J., Telford, Richard J., Blackford, Jeffrey J., Blundell, Antony, Booth, Robert K., Charman, Dan J., Lamentowicz, Lukasz, Lamentowicz, Mariusz, Mitchell, Edward, Potts, Genevieve, Swindles, Graeme T., Warner, Barry G., Woodland, Wendy

Transfer functions are widely used in palaeoecology to infer past environmental conditions from fossil remains of many groups of organisms. In contrast to traditional training-set design with one observation per site, some training-sets, including those for peatland testate amoeba-hydrology transfer functions, have a clustered structure with many observations from each site. Here we show that this clustered design causes standard performance statistics to be overly optimistic. Model performance when applied to independent data sets is considerably weaker than suggested by statistical cross-validation. We discuss the reasons for these problems and describe leave-one-site-out cross-validation and the cluster bootstrap as appropriate methods for clustered training-sets. Using these methods we show that the performance of most testate amoeba-hydrology transfer functions is worse than previously assumed and reconstructions are more uncertain.

2008, Mitchell, Edward, Payne, Richard J., Lamentowicz, Mariucz

Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what, if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance. However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous experimental work did not lead to significant difference in inferred water table. Our results show that differential shell preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant, and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic peatlands.

2012, Payne, Richard J., Mitchell, Edward, Hung, Nguyen-Viet, Gilbert, Daniel

Peatland testate amoebae are widely used to reconstruct paleohydrological/climatic changes, but many species are also known to respond to pollutants. Peatlands around the world have been exposed to anthropogenic and intermittent natural pollution through the late Holocene. This raises the question: can pollution lead to changes in the testate amoeba paleoecological record that could be erroneously interpreted as a climatic change? To address this issue we applied testate amoeba transfer functions to the results of experiments adding pollutants (N, P, S, Pb, O-3) to peatlands and similar ecosystems. We found a significant effect in only one case, an experiment in which N and P were added, suggesting that pollution-induced biases are limited. However, we caution researchers to be aware of this possibility when interpreting paleoecological records. Studies characterising the paleoecological response to pollution allow pollution impacts to be tracked and distinguished from climate change. (c) 2012 University of Washington. Published by Elsevier Inc. All rights reserved.

2011, Payne, Richard J., Lamentowicz, Mariucz, Mitchell, Edward

A fundamental requirement of quantitative palaeoecology is consistent taxonomy between a modern training set and palaeoecological data. In this study we assess the possible consequences of violation of this requirement by simulating taxonomic errors in testate amoeba data. Combinations of easily confused taxa were selected, and data manipulated to reflect confusion of these taxa; transfer functions based on unmodified data were then applied to these modified data sets. Initially these experiments were carried out one error at a time using four modern training sets; subsequently, multiple errors were separately simulated both in four modern training sets and in four palaeoecological data sets. Some plausible taxonomic confusions caused major biases in reconstructed values. In the case of two palaeoecological data sets, a single consistent taxonomic error was capable of changing the pattern of environmental reconstruction beyond all recognition, totally removing any real palaeoenvironmental signal. The issue of taxonomic consistency is one that many researchers would rather ignore; our results show that the consequences of this may ultimately be severe.

2007, Payne, Richard J., Mitchell, Edward

Testate amoebae are useful environmental indicators in ecological and palaeoecological studies from peatlands. Previous quantitative studies have focused on the peatlands of Northern and Central Europe, North America, and New Zealand and have considered a relatively restricted variety of peatland types, mostly ombrotrophic or Sphagnum-dominated while more minerotrophic fens have been less studied. Here we present the first quantitative ecological study of testate amoebae from four small mesotrophic fens (pH 5.5-8.1) in the Elatia Forest, northern Macedonia province, Greece. Relationships with the environmental data were investigated using redundancy analysis and mantel tests. Transfer function models were derived using a variety of techniques. Results demonstrate that as for Sphagnum-dominated mires hydrology is the most important control on amoebae community composition. Transfer function models should enable water tables to be predicted within 2.5cm, when data selection is used this is reduced to less than 2cm. pH is also an important environmental control on testate amoebae communities, a transfer function model enables pH prediction within 0.4 pH units. The hydrological transfer function is the best performing such model yet produced in terms of prediction error. This study provides new data on the ecology of testate amoebae in fens, and the transfer function models should allow quantitative palaeohydrological reconstruction. (C) 2006 Elsevier GmbH. All rights reserved.