Dan J. Charman

Testate amoebae analysis in ecological and paleoecological studies of wetlands: past, present and future

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.

Quantitative estimates of water tables and soil moisture in Holocene peatlands from testate amoebae

Changes in surface wetness on Holocene ombrotrophic mires have principally been estimated from plant macrofossils and humification. Testate amoebae (Protozoa: Rhizopoda) provide an additional technique and have the potential to provide improved quantitative estimates of water-table depths and soil moisture. The relationship between hydrology and testate amoebae assemblages from 163 samples on nine British mires is explored using canonical correspondence analysis (CCA). Mean annual water-table depth and percentage soil moisture are two of the most important environmental variables related to the distribution of testate amoebae within peat. Transfer functions for these variables are developed using four underlying models; weighted aver aging (WA), tolerance downweighted weighted averaging (WA-Tol), weighted averaging partial least squares (WA-PLS) and partial least squares (PLS). In ‘jack-knifed’ validation, WA produced the lowest prediction errors for water table, but was outperformed by WA-Tol for percentage moisture. WA and WA-Tol based transfer functions are then applied to a fossil data set from Bolton Fell Moss, Cumbria. This methodology offers a new technique for reconstructing surface wetness changes on British ombrotrophic and oligotrophic mires and provides data in terms of a meaningful environmental parameter. The cosmopolitan distribution of testate amoebae species suggests that the technique has a much wider geographical potential.

The preparation of testate amoebae (Protozoa: Rhizopoda) samples from peat

The analysis of testate amoebae (Protozoa: Rhizopoda) in peat is proving to be a useful new quantitat ive technique for assessing hydrological change on peatlands. Preparation experiments were carried out on peat extracted from Coom Rigg Moss, Northumberland, England, in order to assess the impact of different procedures on the concentration and species composition of the faunas. Five replicates of each of six samples were treated by different techniques and the testate amoebae counted together with 200 Lycopodium spores. All preparation procedures suitable for pollen extraction reduce the number of tests recorded by up to 80% and the number of taxa recorded by 60%. KOH digestion did not destroy tests but damaged them, making identification difficult, although it enabled extraction of a higher number of tests. A simple water-based prep aration, with sieving to remove coarse and fine detritus, appears to give the best results and is an improvement on earlier techniques in removing more extraneous material from the samples. Testate amoebae counts from samples prepared for pollen analysis must be viewed with extreme caution and are unrepresentative of the faunal content of the peat in both absolute and relative terms.

Biostratigraphic and palaeoenvironmental applications of testate amoebae

Abstract Testate amoebae (also referred to as rhizopods, thecamoebians and arcellaceans) are single-celled organisms in which the cytoplasm is enclosed within an external shell (the test). They live in a wide range of terrestrial and aquatic habitats, including wet soils, lakes and saltmarshes, and fossil tests have been recovered from sediments from all these environments. This paper reviews existing and recently developed applications of testate amoebae analysis to biostratigraphic description of Quaternary sediments and palaeoenvironmental reconstruction. Advances in testate amoebae analysis are discussed for three key areas of Quaternary research and the potential for the further exploration and use of the technique is emphasised. These areas are: (1) Peatlands and palaeoclimates : Testate amoebae have been used as a new method of reconstructing palaeoclimates from ombrotrophic peats; (2) lake sediments : Testate amoebae respond to acidity levels in lakes but they may also be useful as indicators of pollution and temperature; (3) saltmarshes and sea-level change : Testate amoebae can now be added to diatoms and foraminifera as potential indicators of sea-level change. The paper concludes with an examination of future developments in testate amoebae research and highlights the need for further work to explore their full range of occurrence in Quaternary sediments.

A Database and Synthesis of Northern Peatland Soil Properties and Holocene Carbon and Nitrogen Accumulation

Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non-Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm3, organic matter bulk density averaged 0.11 ± 0.05 g/cm3, and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non-Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m2/yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m2/yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu.

Summer water deficit variability controls on peatland water-table changes: implications for Holocene palaeoclimate reconstructions:

Interpretation of proxy-climate records depends on a thorough understanding of the proxy-climate relationship. Peatland surface wetness records have been interpreted as reflecting changes between cool and/or wet conditions and warm and/or dry conditions. This paper analyses a high-resolution record of reconstructed water-table changes based on testate amoebae analysis in relation to instrumental weather records since AD 1775. Replicate peat records are reconciled by multiple chronological techniques and tuning, and demonstrate that the reconstructions preserve many replicable high-frequency changes. Water-table variability is highly correlated with the total seasonal moisture deficit (precipitation—evapotranspiration, P-E) expressed as the sum of all months with negative P-E. The reconstructed water-table record reflects antecedent periods of 5 or 10 years (maximum r2 = 52.4%) and proxy bog surface wetness records can therefore be interpreted as reflecting the length and intensity of the summer water deficit period. Response surfaces of the summer deficit in relation to temperature and precipitation variability support the hypothesis that the summer deficit is determined by summer precipitation in midlatitude oceanic peatlands and that summer temperature plays a greater but still subsidiary role in higher latitude, continental settings. These relationships apply for all plausible past Holocene climate changes and future twenty-first century climate scenarios. Non-linear responses to longer-term climate states prevent the direct application of a calibration of the reconstructed water-table records to infer quantitative estimates of climate variables. Models that combine peat accumulation, mire growth and hydrological processes are required to undertake this task.

Ecology of Testate Amoebae (Protozoa: Rhizopoda) on Peatlands in Western Russia with Special Attention to Niche Separation in Closely Related Taxa

Testate amoebae (Protozoa: Rhizopoda) are frequently used as indicators of past environmental changes, and the interpretation of fossil assemblages depends upon our knowledge of ecological affinities of taxa in modern environments. A variety of taxonomic approaches have been used in fossil studies, mostly involving grouping of closely related taxa. This paper presents data from peatlands in western Russia relating surface wetness parameters to species occurrence. Relationships between species abundance, water table depth and soil moisture are modelled using weighted averaging, and species niches are calculated as optima and tolerance for these parameters. Niche separation of closely related taxa is examined in detail and it is shown that there is often a gradient of hydrological preference within each group of taxa. Wet to dry gradients include those found in the Trigonopyxis arcula group (T. arcula var. major > T. arcula > T. minuta), the Assulina-Valkanovia group (A. seminulum > A. muscorum > V. elegans), and the Trinema lineare group (T. lineare var. truncatum/ T. lineare > T. lineare var. terricola), all of which are associated with a large to small size gradient. In addition, spined forms within the Euglypha and Placocista genera are shown to consistently occur in wetter habitats than glabrous forms or those with shorter spines. It is concluded that palaeoecological studies should attempt the lowest taxonomic divisions possible within these groups, to maximise the ecological indicator value of the assemblages recorded.

Multiproxy surface wetness records from replicate cores on an ombrotrophic mire: implications for Holocene palaeoclimate records

Ombrotrophic mires can provide records of palaeoclimate over the mid- to late- Holocene in several areas of the world. Their potential is currently partly limited by difficulties with scaling indices based on plant macrofossils and humification, and the need to account for the internal variability of the mire system. This paper explores the use of testate amoebae analysis as a third technique and assesses the minimum within-site variability by comparing results from two closely spaced cores. Reconstruction of surface wetness changes was carried out on cores from the centre of an intermediate raised-blanket mire, Coom Rigg Moss, Northumberland, by analysis of testate amoebae, plant macrofossils and humification. Surface wetness changes were expressed as mean annual water table changes inferred from testate amoebae assemblages, two separate indices based on plant macrofossils and percentage transmission of humification extracts. Comparisons between the proxies suggest good agreement of general trends in Sphagnum peats but some differences in monocot and ericaceous peats. The magnitude of surface wetness changes also differs within Sphagnum peats. The records from the separate cores converge over time and replicability between cores is best in the last 1000 yr. Changes over this period are similar to those shown by estimates based on documentary sources. Good agreement is obtained between a normalised plant macrofossil index and normalised reconstructed water-table values and it is suggested that this approach could form the basis for improved composite, multiproxy records from peatlands. Copyright

Palaeohydrological records derived from testate amoebae analysis from peatlands in northern England: within-site variability, between-site comparability and palaeoclimatic implications:

Testate amoebae analysis was undertaken on eight cores from three mires within a restricted geographical area of northern England. This was used to assess, first, the amount of autogenically produced variability in palaeohydrological records from ombrotrophic mires, and, second, to determine whether a palaeoclimatic signal can be derived from testate amoebae analysis from peatlands. Past mean annual water tables were reconstructed by calibrating the testate amoebae record with an existing transfer function. There is a good degree of replicability between the water table reconstructions for the upper peats within each site (especially since cal. AD 600) and the magnitude and timing of most changes are similar. The results show that autogenic factors have a relatively minor control on palaeohydrological records from ombrotrophic peatlands, even for marginal locations where these effects should be greatest. Records from the centres of peatlands are compared to assess the replicability of hydrological changes between the sites to determine which of these changes are attributable to climate. The major fluctuations are well replicated in all central cores, especially for the last 2000 years, suggesting that these shifts are climatically forced and that there is an increasing climatic influence through time, even at the valley mire site. Comparisons with other proxy climatic records suggest that mire surface wetness changes occurred in concert across a broad region of northern England and southern Scotland during at least the last 2000 years.

Carbon dynamics in a forested peatland in north-eastern Ontario, Canada

The long-term carbon accumulation rate in a forested peatland in north-east Ontario was examined in relation to gas production and factors which control carbon transport. Plots of cumulative total mass and cumulative carbon mass against calibrated radiocarbon age estimates when applied to an existing model of peat accumulation, suggest that only very slow decay is occurring within the catotelm. Gas samples collected from depth show that both carbon dioxide and methane are present. Accelerator mass spectrometry (AMS) radiocarbon analysis yields age estimates of both gases which are between 500 and 2000 years younger than conventional age estimates on adjacent peat (...)