Changes in the importance of lotic and littoral diatoms in a high arctic lake over the last 191 years

Both habitat preferences and community associations for some of the most prominent pennate taxa in our profile were established using samples from the modern lake and its watershed. Habitat preferences were used to establish a simple indicator for the relative contribution of lotic communities in this lake (Lotic Index) and indicator taxa for both littoral (Achnanthes spp. and Cymbella spp.) and lotic (Hannaea arcus and Meridion circulare) communities. When profiled, the Lotic Index showed a clear positive relationship to sedimentation rate as recorded in the varves, while profiles of littoral indicator taxa show the opposite trends. It seems likely that the patterns we observed in the Lotic Index are related to changes in runoff. Apparently, there was a period of declining runoff beginning ca. two centuries ago and ending in the late 1800's. This was followed by increasing runoff lasting until the middle of the 20th century. A brief minimum occurred in ca. 1970 followed by a recovery by ca. 1980.A strong positive relationship was also found between the dates of major turbidites, exceptionally thick varves and the concentration of valves in the sediment. It is possible that many of the thicker varves in the profiles contain littoral material transported to the site of deposition by turbid interflows and underflows. For this reason, the concentration of valves in the sediment in our cores appears to be a proxy for sediment deposition from turbidity currents.This is the sixth in a series of papers published in this issue on the Taconite Inlet Lakes project. These papers were collected by Dr R. S. Bradley.     

Paleoindication of acidification in Kleiner Arbersee (Federal Republic of Germany,Bavarian Forest) by chydorids,chrysophytes, and diatoms

In a sediment core of Lake Kleiner Arbersee, located in the Bavarian Forest on gneiss bedrock and with spodosolic soils in the catchment, acidification was reconstructed using assemblages of chydorids, scaled chrysophytes, and diatoms as paleoindicators. All paleoindicators demonstrated acidification of Kleiner Arbersee. There were marked differences, however, in the extent of the indicated acidification: In lower layers, where scaled chrysophytes and chydorids clearly indicate beginning acidification, pH-decline inferred by diatoms lies within the standard deviation, and is statistically not significant until the 7 cm layer. We conclude that mallomonadaceans reflect the acidification of the water body and strongly respond to the lowest seasonal pH, as their indicated rate of acidification is higher than that of diatoms, which appear to more closely indicate the pH of the sediment/water interface, especially when the pH of the water column has dropped below ca. 5.5. Chydorids may additionally be influenced by proton release of dense Sphagnum-stands, which cover the littoral of Kleiner Arbersee completely.We dedicate this paper to Prof. Dipl.-Ing. L. Strobel, who encouraged new perspectives in limnological water management, on the occasion of his 65^th anniversary.     

On diatom concentrations in lake sediments: results from an inter-laboratory comparison and other tests performed on a uniform sample

The diatom concentration of a uniform lake sediment sample was estimated in 15 laboratories. Whilst 21 of 35 estimates were between 13 and 19 × 10⁷ valves g^-1, values ranged from 8.2 × 10⁶ to 2.0 × 10⁹ valves g^-1. However, counts from replicate sediment digestions, replicate slide preparations from a single digested slurry, and multiple counts from an individual slide all yielded internally consistent results, with acceptably low coefficients of variation (±4.5–6.3%). Without greater efforts towards harmonization, for example by calibrating techniques to a standard of known concentration, reported diatom concentration values, as well as derived data such as diatom accumulation rates, are not directly comparable between most laboratories. This sharply contrasts the strong reproducibility of diatom relative frequency data. A calibration exercise, using the Eucalyptus pollen spike technique to estimate a known concentration of Lycopodium spores, demonstrates that this method performs best when approximately equal proportions of microfossils and introduced markers are counted.     

Fossil diatoms and the mid to late holocene paleolimnology of Lake Turkana,Kenya: a reconnaissance study

A 12 m sediment core recovered from the south basin of Lake Turkana, northwestern Kenya, reveals four major diatom assemblages that span approximately 5450 to 1070 years BP based on AMS radiocarbon analyses. The oldest assemblage, Zone D (5450 to 4850 yr BP), is dominated by Melosira nyassensis and Stephanodiscus spp. and is interpreted to reflect higher lake levels, fresher water and more variable seasonal mixing of the water column than the modern lake. Melosira dominates the assemblage in Zone C (4850 to 3900 yr BP) with some Surirella engleri and Stephanodiscus. This assemblage indicates a continuation of relatively high lake levels and seasonal mixing of a stratified lake. The brief peak of Surirella, interpreted as benthic, suggests an episode of slightly lower lake level. Thalassiosira rudolfi and Surirella predominate since the beginning of Zone B (3900 to 1900 yr BP), reflecting a decrease in lake level and increase in water column salinity. Increasing dominance of Surirella in Zone A (1900 to 1070 yr BP) may suggest that the lake continued to decrease in depth. Salinity probably rose to levels comparable with the modern lake. These results are consistent with paleoclimatic interpretations based on carbonate abundance, lamination thickness, oxygen isotope and bulk geochemistry profiles from this core and cores recovered from the north basin. It extends the known paleolimnology beyond 4000 yr BP of the earlier research to 5450 yr BP and into the early to mid Holocene pluvial phase in northern intertropical east Africa.     

Relationships between sedimentary diatom assemblages and lakewater pH values in the Experimental Lakes Area

Relationships between sedimentary diatom assemblages and lakewater pH values from 36 lakes and experimentally acidified Lake 223, in the Experimental Lakes Area, were analyzed. The relationships were used to assign diatoms in the 36 lakes into pH preference groupings. Based on their regional distribution Cyclotella stelligera and Tabellaria flocculosa strain IIIp were categorized as pH indifferent, in comparison to other areas were they have been categorized as acidophilic. Two models were then applied to calculate transfer coefficients which were used to calculate diatom-inferred pH values for Lake 223. Index B and a multiple linear regression of the pH groupings yielded similar correlations (r² 0.82 and 0.84 respectively, p=0.0001). The multiple linear regression inferred a pH of 5.36 for acidified Lake 223 compared to a measured pH of 5.46.     

Distribution of diatoms among intermittent ponds on the Atlantic Coastal Plain: development of a model to preduct drought periodicity from surface-sediment assemblages

Inferences of past climate from the fossil record in lakes rely on the accurate quantification of a relationship of fossilizing organisms to their environment. Whereas the relationship of diatoms to water chemistry parameters has been modeled in many systems, few studies adequately address the relationship of diatoms to physical properties, such as water depth or hydrology, that may be more directly tied to climate. We examined the composition of modern diatoms in surface sediments of 75 isolated ponds (mostly Carolina bays) of the Atlantic Coastal Plain to: (1) assess the influence of physical and chemical variables on the distribution of diatoms among ponds of the region, and (2) develop a model that predicts hydroperiod (a measure of pond permanence) from diatom assemblages. We constructed two hydroperiod calibration models: the first infers hydroperiod from the weighted-average optima and tolerances of taxa along the hydroperiod gradient, the second bases inferences on the hydroperiod estimates of compositionally similar samples. Both approaches incorporate a-priori and post-hoc tests of assumptions often inherent in the construction of transfer functions. Diatom assemblage composition had strong, approximately linear relationships to hydroperiod, water depth, and calcium concentration in non-metric multidimensional ordination space; effects of other variables, including pH, were non-linear or ambiguous. Overall, the assemblages reflected the dilute, acidic chemical characteristics of bays. The assemblages contained differing abundances of euterrestrial, benthic and planktonic taxa, depending on a pond's susceptibility to drying. A weighted-averaging regression model based on taxon-specific hydroperiod optima generated adequate, unbiased hydroperiod inferences from diatom species composition (r² = 0.81). This model may be used to infer past drought episodes from fossil diatom assemblages at appropriate sites on the Atlantic Coastal Plain.     

A method for the extraction of carbonaceous particles from lake sediment

The methods found in the literature for the extraction of carbonaceous particles from lake sediment are discussed. The technique used by Griffin & Goldberg (1975) on Lake Michigan sediments was improved by modifying the procedure to halve the extraction time and reduce the risk of fragmentation. This method was then applied to a sediment core taken from Loch Tinker in Western Scotland, which had previously been analysed using the Renberg & Wik method. Although the basic trends for both methods are the same, the new method is found to be more sensitive to low particle numbers and more accurate, due to a more efficient extraction and a higher magnification for microscope counting.     

A summary of past and recent plankton of Lake Winnipeg, Canada using algal fossil remains

Although the phytoplankton and protozoan assemblages of Lake Winnipeg are quite variable both, spatial and temporally, knowledge of their current distribution and ecology enables them to be used as indicators providing unique information concerning past and recent ecosystem conditions. Many of the main taxa have been similar throughout history although there have been dramatic changes in their abundance. Lake Winnipeg is currently very responsive to the climatic conditions in its drainage basin and it appears, historically, to have been a diatom-bluegreen algal lake with Tintinnids and thecate amoeba forming significant components of the protozoan community. A wide variety of microfossil remains have been used to infer past conditions in this large prairie lake. Microfossils reported from the long core 103 (8 m) taken from the north basin of LakeWinnipeg and short core Namao 7a taken from the south basin of the lake in August 1994 indicate that the lake has experienced several changes over its history. Diatom assemblages, low throughout the core 103 below 50 cm, except for a peak around 300–400 cm are virtually absent below 690–800 cm (Lake Agassiz). Stephanodiscus and Aulacoseira are the two major pelagic diatom genera represented throughout the lake's history. Species changes occur near the top of the core indicate increased anthropogenic eutrophication. Shallow water littoral taxa were never abundant at either coring sites. The presence of planktic cyanoprokaryote (cyanophyte, cyanobacteria, bluegreen algae) remains (akinetes) from 600 cm depth and the progressive increase in abundance from 400 cm to through the top 100 cm of the core indicates increasing phosphorus levels, warming summer temperatures and increasing summer nitrogen limitation in the lake. Nitrogen fixing bluegreens (especially Anabaena and Aphanizomenon akinetes) and the diatoms (Aulacoseira ambigua, A. granulata, A. islandica, S. binderanus, and S. niagarae) were abundant in the upper sediments and changes after 100 cm can be interpreted as the effects of human impact. These taxa indicative of increase eutrophication, in addition to S. agassizensis, Melosira varians and Cyclostephanos dubius, are representative of present day plankton. The short core Namao 7a was dated by ^Pb210 and the fossil remains also show a rise in the Aulacoseira (particularly A. granulata and A. ambigua) and bluegreen akinetes in the last 40 years as was seen in the upper sediments of the 103 core. There was an interesting peak in chrysophyte cysts during the 1930's which corresponds nicely with climatic conditions during this period. The fires accompaning the hot dry period in the mid 70's can be seen in the dramatic rise in charcoal during this period. Multiple types of micro fossils remains provides several useful tools for interpreting past lacustrine conditions.     

A method for the selective removal of inorganic ash particles from lake sediments

Inorganic ash particles are formed by the fusing of inorganic material present during the high temperature combustion of fossil fuels. As they accumulate in lake sediments, they record the history of atmospheric contamination produced from such sources. A technique has been developed for concentrating these particles from lake sediments involving the stepwise removal of unwanted components of the sediment, including organic material and biogenic silica.When applied to a sediment core taken from Loch Tinker, central Scotland, a particle concentration profile, very similar to that of the carbonaceous particle profile (the other component of fossil-fuel combustion ash) is produced. The concentration of the inorganic ash spheres in the sediment is approximately an order of magnitude higher than the carbonaceous particles and there appears to be a continuous pre-industrial background value. This seems to imply a similar source (i.e. coal combustion rather than oil) for both inorganic ash and carbonaceous particles. This method has also been used with success on peat cores.     

Paleolimnological evidence for the recent acidification of Llyn Hir,Dyfed, Wales

A variety of paleolimnological techniques, coupled with historical data on land-use and fisheries, are used to evaluate the magnitude, timing, and causes of acidification of Llyn Hir, a moorland lake in central Wales. pH reconstruction based on diatom analysis suggests a gradual decline in lakewater pH beginning ca. 1870 and intensifying in the mid-1930's, with a total decline of 1.1 pH units between 1870 and 1984. This pH decline correlates with increased sedimentary concentrations of carbonaceous particles, trace metals, and magnetic minerals, which indicate the local deposition of atmospherically transported products of fossil-fuel combustion. Pollen data and the historical record show no significant alterations in land-use or catchment vegetation, indicating that acidification of Llyn Hir is a result of the increased deposition of atmospheric pollutants, not of land-use and vegetation change.Limnological Research Center Contribution No. 374.     

A review of factors that regulate carotenoid and chlorophyll deposition and fossil pigment abundance

Limnological surveys show that fossil pigment concentration is an accurate predictor of algal production. However, experimental and mass flux studies indicate that >90% of pigment is degraded to colourless compounds before permanent burial. To reconcile these views, this paper reviews current literature on pigment degradation and proposes a hierarchical control model for pigment deposition and fossil abundance. Over the widest range of production, pigment deposition and fossil concentration are proportional to algal standing crop. However, within a narrower range, the actual concentration of pigment in sediments is regulated by photo- and chemical oxidation. Three phases of loss exist: rapid oxidation in the water column (T_1/2=days); slower post-depositional loss in surface sediments (T_1/2=years); and very slow loss of double bonds in deep sediments (T_1/2=centuries). Despite losses during deposition, fossil and algal abundance remain correlated through time, so long as there is no change in basin morphometry, light penetration, stratification or deepwater oxygen content. At the finest scale, food-web processes can increase the preservation of pigments from edible algae by incorporating pigments into feces that sink rapidly and bypass water column losses. As a consequence of selective loss during deposition and initial burial, carotenoid relative abundance is an unreliable measure of phytoplankton community composition. Instead, absolute concentration — scaled to the historical maximum — should be used for fossil interpretations.     

Diatom preservation in the recent sediment record of Spanish saline lakes: implications for palaeoclimate study

In palaeoclimate research, fossil diatoms from saline lakes can be excellent indicators of past salinity, a proxy for climate change, although they are sometimes poorly preserved in sediment cores. Spain has numerous salt lakes but the potential of diatoms for studies of climate change has never been investigated. A comprehensive survey of diatom preservation is described based on modern and fossil diatoms from short cores (<50 cm depth) in a representative data-set of 59 sites, and the main factors affecting preservation are investigated using principal components analysis (PCA). Most lakes do not preserve a diatom record; four sites in southern Spain are identified which both contain diatoms and have suitable limnological characteristics for a climate study. Many lakes are ephemeral and the physical effects of desiccation, coupled with other factors such as turbidity and high salinity, are the main factors enhancing diatom dissolution or their failure to be incorporated into the sediment record.     

Environmental Characteristics of Lake Tecocomulco, northern basin of Mexico, for the last 50,000 years

Paleoenvironmental studies have documented the late Pleistocene to Holocene evolution of the lakes in the central and southern parts of the basin of Mexico (Texcoco and Chalco). No information was available, however, for the lakes in the north-eastern part of this basin. The north-eastern and the central and southern areas represent, at present, different environmental conditions: an important gradient exists between the dry north and the moister south. To investigate the late Pleistocene to Holocene characteristics of the north-eastern lakes in the basin of Mexico two parallel cores (TA and TB) were drilled at the SE shore of Lake Tecocomulco. Stratigraphy, magnetic properties, granulometry, diatom and pollen analyses performed on these sediments indicate that the lake experienced a series of changes between ca. > 42,000 yr BP and present. Chronological control is given by five radiocarbon determinations. The base of the record is represented by a thick, rhyolitic air-fall tephra that could be older than ca. 50,000 yr BP. After this Plininan event, and until ca. 42,000 yr BP, Lake Tecocomulco was a moderately deep, freshwater lake surrounded by extended pine forests that suggest the presence of cooler and moister conditions than present. Between ca. 42,000 and 37,000 yr BP, the lake became shallower but with important fluctuations and pollen suggests slightly warmer conditions. Between ca. 37,000 and 30,000 yr BP the lake experienced two relatively deep phases separated by a dry interval. A second Plinian eruption, represented in the sequence by a dacitic an air-fall tephra layer dated at 31,000 yr BP, occurred in the area by the end of this dry episode. Between ca. 30,000 and 25,7000 yr BP Tecocomulco was a fresh to slightly alkaline lake with a trend towards lower level. After ca. 25,700 yr BP very low lake levels are inferred, and after ca. 16,000 yr BP the data indicate the presence of a very dry environment that was persistent until the middle Holocene. After 3,500 yr BP lacustrine conditions were re-established and the vegetation cover shows a change towards higher percentages of herbaceous taxa.     

Short-code paleolimnological investigation of Lake Pihlajavesi in the Saimaa Lake complex, eastern Finalnd: assessment of habitat quality of an endemic and endangered seal population

A short-core paleolimnological investigation was carried out to acquire knowledge of the recent history of Lake Pihlajavesi, Saimaa Lake complex, and define its natural state before cultural disturbances. In the natural state, with negligible human interference, the basin was oligotrophic and oxygen-rich. The bioproductivity in the littoral zone was low and the profundal benthic quality was good according to Wiederholm's Benthic Quality Index.Based on diatom analyses, no significant changes have taken place in the phytoplankton communities during the past decades. Sedimentary chironomid communities reveal, however, slight changes on profundal life in the Pihlajavedenselkä basin, near the main pollution sources, whereas there were no significant changes in the outer basin. Three stages could be distinguished in the pollution history of Pihlajavedenselkä: (1) 'The natural state' up until the 1960s, (2) 'the period of increasing loading' during the 1960s and 1970s, and (3) 'the recovery of the basin' during the past two decades.Untreated municipal waste water from the town of Savonlinna was partly responsible for the commencement of eutrophication in the Pihlajavedenselkä basin in the 1960s. The paper and pulp industry in Varkaus, some 40 kilometres upstream from Lake Pihlajanvesi, has also increased eutrophication, especially during the worst period of water quality in the 1960s and 1970s. More effective waste water purification has markedly reduced effluent loading and led to a general recovery of the area. Neverthless the effects of slight nutrient loading can still be seen, especially in littoral bioproductivity.Our paleolimnological data indicate that the present ecosystem will offer living conditions for the endemic Saimaa ringed seal population that are similar to those that existed prior to human disturbances.     

A new method to compensate for bias in magnetotellurics

Error estimates from statistical regression analysis are often obviously too small, leading to doubts about the given equations, the statistical method itself and finally, with resignation, to the conclusion that mathematical equations and reality never agree. However, for magnetotelluric data we have found an almost perfect fit between observed scattering and predicted confidence limits of regression coefficients after accounting for a systematic error—the bias.
Different methods to compensate for bias in magnetotelluric impedance estimation have been described using additional data from a reference station. However, sufficiently accurate reference data are often not available. A new method has been developed that enables bias compensation without additional data. For the new method we derive a linear relationship between the effect of bias and an expression depending on the data fit. From this we extrapolate the solution for the unbiased impedance. The new method assumes a special model of uncorrelated noise as well as an approximation for the structure of the impedance tensor. From each pair of components of the unrotated impedance tensor corresponding to the same output channel, one of the pair can be compensated if its magnitude is large compared to that of the other.
The method has been successfully applied in many cases. We claim that the solution is closer to the true impedance than any solution based on the selection of events. It gives a measure of the partitioning of noise between the electric and magnetic channels.
We applied the method to measurements from the North Anatolian Fault Zone (Turkey) and from the Merapi volcano (Central Java) in the period range 10–2500 s. Different instrumentation was used for the two sets of measurements, but in both cases we used fluxgate magnetometers to measure the magnetic variations.     

Diatom-salinity relationships in 111 lakes from the Interior Plateau of British Columbia,Canada: the development of diatom-based models for paleosalinity reconstructions

Diatoms were identified and enumerated from the surface sediments of 111 lakes, 45 from the Kamloops region and 66 from the Cariboo/Chilcotin region, located on the southern Interior Plateau of British Columbia, Canada. This paper is an extension of another study which investigated the relationship of diatoms to salinity and ionic composition in 65 lakes from the Cariboo/Chilcotin region. The 111 lakes spanned a large gradient in salinity, ranging from fresh through hypersaline (late-summer salinity values ranged from 0.04 to 369 g l^–1), and included both carbonate- and sulphate-dominated lakes with sodium and magnesium as the dominant cations. The Kamloops region had more sulphate-dominated, hypersaline lakes and fewer carbonate-rich lakes than the Cariboo/Chilcotin region. Most lakes had higher salinities in the late-summer compared to the spring.Both salinity and brine-type were important variables that could explain the different diatom assemblages present in the lakes. The majority of diatom taxa had salinity optima in the freshwater to subsaline range (<3 g l^–1), and the taxa displayed a range of both narrow and broad tolerances along the salinity gradient. Weighted-averaging regression and calibration, and maximum likelihood techniques were used to develop salinity inference models from the diatom assemblages based on their relationship to the spring, late-summer and average lakewater salinity measurements. Simple weighted-averaging (WA) models generally produced the same or lower bootstrapped RMSEs of prediction than weighted-averaging with tolerance downweighting (WA(tol)) in the two regional and the combined datasets. Weighted averaging partial least squares (WA-PLS) showed little or no improvement in the predictive abilities of the datasets, as judged by the jackknifed RMSE of prediction. In all cases, the combined dataset of 102 lakes performed better than either of the smaller regional datasets, with relatively little difference between spring, average and late-summer salinity models. The maximum likelihood models gave lower apparent RMSEs of prediction in comparison to other methods; however, independent validation of this technique using methods such as bootstrapping were not undertaken because of the computer intensive nature of such analyses. These diatom-based salinity models are now available for reconstructing salinity and climatic trends from appropriately chosen closed-basin lakes in the Interior region of British Columbia.This is the second in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Development of paleolimnological inference models for pH, total nitrogen and specific conductivity based on planktonic diatoms

Canonical correspondence analysis (CCA was used to explore and identify statistically significant relationships between the distributions of planktonic diatoms and the physical and chemical properties of 50 Connecticut lakes. Six variables (pH, total nitrogen, calcium, sulfate, potassium and chlorophyll- a concentrations) were found to be significantly correlated with either or both of the first two extracted axes. The pH and calcium concentration, and to a lesser extent total nitrogen concentrations, were the most important variables controlling the distributions of planktonic diatoms in this suite of lakes. Paleolimnological inference models were developed for pH, total nitrogen (TN) and specific conductivity. Weighted averaging with (WAtol) and without (WA) tolerance downweighting, with and without bootstrap resampling techniques, and using either classical or inverse deshrinking methods were used to develop inference models for each variable. The pH and TN yielded sufficiently high 1/2 ratios and a highly significant first (constrained) axis when entered as single variables in both constrained and partially constrained CCA analyses, supporting the idea that reliable inference models could be developed for these variables. The r2 and RMSE of prediction values ranged from 0.73 to 0.86 and 0.37 to 0.6, respectively for pH, and from 0.4 to 0.64 and 59 g/l to 95 g/l, respectively for TN. Inference models for specific conductivity also yielded significant goodness-of-fit statistics. However, because specific conductivity was removed from the CCA analysis due to its high variance inflation factor and did not yield a significant relationship when entered as the sole variable in a partial constrained CCA, inference models for this variable will probably not yield any additional environmental information. The use of only planktonic diatoms in construction of inference models is discussed.     

Development of a diatom-based specific conductivity model for the glacio-isostatic lakes of Truelove Lowland: implications for paleoconductivity and paleoenvironmental reconstructions in Devon Island lakes, N.W.T., Canada

Diatoms are identified and enumerated from the surface sediments of 100 lakes of Truelove Lowland, Devon Island, N.W.T., Canada. These lakes range from large oligotrophic lakes, to small tundra ponds, to coastal marine lagoons which are diverse in terms of ionic concentration and composition. The relationship between diatoms and 15 limnological variables is examined using Canonical Correspondence Analysis (CCA). Specific conductivity is identified as the most important variable influencing the distribution of diatoms in the Truelove lakes. A Weighted Averaging (WA) calibration model is developed to predict diatom-inferred specific conductivity. The reliability of the model is tested by evaluating the correlation between observed and diatom-inferred values and determining the error of prediction by bootstrapping. The applicability of the predictive conductivity equation is demonstrated by reconstructing the paleoconductivity history of Fish Lake.