An expanded weighted-averaging model for inferring past total phosphorus concentrations from diatom assemblages in eutrophic British Columbia (Canada) lakes

Eighteen lakes were added to a published training set of 46 British Columbia (BC) lakes in order to expand the original range of total phosphorus (TP) concentrations. Canonical correspondence analysis (CCA) was used to analyze the relationship between diatom assemblages and environmental variables. Specific conductivity and [TP] each explained significant (P0.05) directions of variance in the distribution of the diatoms. The relationship between diatom assemblages and [TP] was sufficiently strong to warrant the development of a weighted-averaging (WA) regression and calibration model that can be used to infer past trophic status from fossil diatom assemblages.The relationship between observed and inferred [TP] was not improved by the addition of more eutrophic lakes, however the [TP] range and the number of taxa used in the transfer function are now superior to the original model. Diatom species assemblages changed very little in lakes with TP concentrations greater than 85 µg 1^–1, so we document the development of a model containing lakes with TP85 µg 1^–1. The updated model uses 59 training lakes and covers a range of species optima from 6 to 41.9 µg 1^–1 TP, and a total of 150 diatom taxa.The updated inference model provided a more realistic reconstruction of the anthropogenic history of a highly eutrophic BC lake. The model can now be used to infer past nutrient conditions in other BC lakes in order to assess changes in trophic status.     

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 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.     

Development of a diatom-based model for inferring total phosphorus in southeastern Australian water storages

Diatom-based transfer functions for inferring epilimnetic total phosphorus (TP) have been developed from a data set of 33 southeastern Australian water storages. Regular institutional monitoring of these sites has allowed comparison of models developed from TP data covering different time periods. A model based on mean annual TP performs better than models derived from winter maximum TP, spring minimum TP or TP nearest the time of diatom sampling. A mean annual TP model (WA-PLS 2 component) has a jack-knifed diatom-inferred versus measured TP correlation coefficient (r ² _jack) of 0.69 and a root-mean-square-error of prediction (RMSEP) of 0.246 log₁₀g TP l^–1, while alternative models have RMSEP > 0.27. Deletion of two samples with uncharacteristic species composition and environmental conditions improved performance of the mean annual TP model (r ² _jack= 0.74; RMSEP = 0.233 log₁₀g TP l^–1). Comparison with other published diatom-TP calibration models indicates that this model performs relatively well, with possible contributing factors including the extensive characterisation of TP (with an average 15 determinations making up the annual mean) and the dominance of planktonic diatoms in most sites. Downcore application of the model will allow the reconstruction of reservoir nutrient histories since commissioning, and thus provide a basis for understanding and management of reservoirs.     

A diatom-based inference model for autumn epilimnetic total phosphorus concentration and its application to a presently eutrophic boreal lake

A diatom transfer function to infer epilimnetic total phosphorus (TP) concentration was derived using surface sediment diatom data from 68 medium-sized (10–1000 ha) lakes in Southern Finland. Publicly available monitoring records were used in lake selection to avoid gradients caused by pH and humic substances. Constrained and partially constrained ordinations indicated that TP was an important variable influencing diatom assemblages. A long floristic gradient in relation to TP was also apparent and therefore an inference model was developed for TP using unimodal-based regression and calibration methods. The final model included 61 lakes with epilimnetic TP concentrations between 3 and 89 g P l^–1, measured during the autumnal circulation period. It has a jackknifed-estimated root mean squared error of prediction of 0.16 log g P l^–1, a maximum bias of 0.28 log g P l^–1, and an r² _jack of 0.76.The model was tested in the presently eutrophic Lake Valkjärvi (epilimnetic [TP] 60–85 g P l^–1), located in Southern Finland. It successfully predicted the measured autumnal epilimnetic TP concentration for the past twenty years and the changes in inferred [TP] reflected disturbances known to have occurred before that time. The diatom-based inferences show that Lake Valkjärvi was oligo-mesotrophic as late as the 1930's and has become eutrophic because of nutrient inputs from agriculture and, especially, municipalities. However, epilimnetic TP concentration has not increased further.     

Diatom and chrysophyte functions and inferences of post-industrial acidification and recent recovery trends in Killarney lakes (Ontario, Canada)

The surface sediment diatom and chrysophyte assemblages from 33 Sudbury lakes were added to our published 72 lake data set to expand and refine the diatom and chrysophyte-based inference models that we had earlier developed for this region. Our calibration data set now includes 105 lakes, representing gradients for multiple environmental variables (e.g., lakewater pH, metals, and transparency). The revised models are based on the weighted averaging calibration and regression approach and include bootstrap error estimates. The pH model was the strongest (r² _boot = 0.75, RMSE _boot = 0.50). The chrysophyte-inferred pH model (r² _boot = 0.79, RMSE _boot = 0.48) that we developed was as robust as the diatom pH model. Diatom and chrysophyte inferred pH models were then applied to top (surface sediments representing current conditions) and bottom (generally from > 30 cm deep representing pre-industrial conditions) sediment diatom and chrysophyte assemblages of 19 Killarney area lakes near Sudbury. The top and bottom inferred pH results were compared to early-1970s measured pH data. These data suggest that, although many of the poorly buffered Killarney lakes had experienced acidification, marked pH recovery has occurred in many lakes within the last 25 years. Despite the stunning pH recovery, the present-day diatom and chrysophyte assemblages are significantly different from assemblages present during pre-industrial times. Our results suggest that biological recovery may require more time than chemical recovery. It is also likely that these lakes may never recover biologically because other anthropogenic stressors (e.g., climate warming and increased exposure to UV-B radiation) may now have greater influence on biological communities in Killarney/Sudbury area lakes than acidification.     

The detection of Rhizosolenia (Bacillariophyceae) in sediment of Ontario lakes and implications for paleoecology

The detection of the freshwater genus Rhizosolenia (Bacillariophyceae) remains in sediments is affected by the oxidants used in the digestion procedures. The Rhizosolenia counts decrease from 30% H₂O₂>50% HNO₃>50% chromic acid>100% HNO₃>100% H₂SO₄. Rhizosolenia is mostly found in Ontario lakes with summer average pH ranging from 5.6 to 8.3, Gran alkalinity from 0.2 to 120 mg L^–1 as CaCO₃ and water colour from 4 to 105 Hazen units. Large Rhizosolenia populations are generally found in clear lakes, neutral pH and low alkalinity.     

Diatoms and pollen as indicators of water quality and land-use change: a case study from the Oak Ridges Moraine,Southern Ontario,Canada

Swan Lake is a small kettle lake located on the Oak Ridges Moraine; a moraine that is recognized as an important source of ground water for the nearby and rapidly expanding Greater Toronto Area. A paleolimnological reconstruction using pollen and diatoms from the lake sediments showed significant changes in biological community composition through the last ∼400 years. Alterations in the diatom and pollen assemblages were most dramatic ca. A.D. 1850, correlating with the highest sediment flux in the lake between the period ca. A.D. 1850 and A.D. 1870. These changes were directly linked to regional deforestation and agricultural activities associated with European settlement. The pollen record from ca. A.D. 1850 to present day indicated that tree species (e.g. Pinus spp., Tsuga canadensis) were declining, while grass (Poaceae) and invasive species (e.g. Ambrosia) were increasing. Around A.D. 1850, the diatom flora changed from an assemblage dominated by large, benthic species (e.g. Sellaphora pupula, Pinnularia cf. maior, and Stauroneis phoenicenteron) to an assemblage characterized by smaller, tychoplanktonic (e.g. Fragilaria tenera, Staurosirella pinnata) and epiphytic (e.g. Achnanthidium minutissimum, Rossithidium linearis) taxa. This diatom community change supports the intermediate disturbance hypothesis which predicts a high level of diversity and richness following an intermediate to intense disturbance of short duration. Phosphorus concentrations in Swan Lake were inferred using a diatom-based regional calibration model, and the results indicated marked changes in lake water chemistry through time (from below detection limits before land clearance and settlement to 19.3 μg l⁻¹ in the current sediments), which were concurrent with episodes of regional deforestation and land-use change. Although the sediment and biological records indicate that the lake ecology has stabilized over the last 30–50 years, paleolimnological records show that the water quality and biology of Swan Lake has changed dramatically and not returned to pre-settlement conditions. Swan Lake presents a detailed record of the impact created by deforestation and urban development with a population of <50 individuals per km². Detailed paleolimnological studies like Swan Lake, in tandem with global human footprint studies, can create realistic estimates of land-use impacts at the global scale.     

Regional assessment of long-term hypolimnetic oxygen changes in Ontario (Canada) shield lakes using subfossil chironomids

Subfossil chironomid assemblages were used to infer long-term water quality changes in south-central Ontario shield lakes, which are currently being impacted by anthropogenic eutrophication, acid rain, and recent climate change. Using a transfer function developed to infer average end-of-summer volume-weighted hypolimnetic oxygen (avg VWHO), a top-bottom paleolimnological approach was used to reconstruct pre-industrial (pre-1850) deepwater oxygen conditions. Comparison with present-day (top surface sediments) chironomid-based inferences of avgVWHO results suggest that hypolimnetic oxygen levels are presently similar to natural, pre-industrial (bottom sediments) levels in most lakes. Approximately half of the study lakes recorded an increase in hypolimnetic oxygen since the 19th century. Inferred changes in avgVWHO correlate well with our results from another chironomid-based oxygen model which reconstructs the Anoxic Factor (AF). When study lakes are separated according to their hydrological status (i.e., natural versus managed), lakes with a dam at their outlet and seasonally managed lake levels had significantly different changes in avgVWHO compared to lakes with natural hydrology. Lakes with a dam at their outlet generally recorded increases in avgVWHO, while natural hydrology lakes mostly recorded declines. There was no relationship between inferred changes in avgVWHO and the density of cottage and resort development along the shorelines of lakes. Changes in dissolved organic carbon (DOC) possibly related to recent climate changes may also be affecting deepwater oxygen, however patterns of change are very subtle.     

Variability of diatom-inferred phosphorus profiles in a small lake basin and its implications for histories of lake eutrophication

Diatom analyses were undertaken of sediment cores covering a range of water depths in a small eutrophic lake (Lough Augher, Co. Tyrone, N. Ireland). The significance of between-core variability in diatom relative frequency stratigraphy was assessed by Canonical Correspondence Analysis (CCA) where the ordination axes were constrained to external environmental variables (sediment depth, core location coordinates, water depth, effective fetch, distance-from-shore and distance-from-inflow). After the removal of the effect of sediment age by partialling it out, the resultant first two axes from the partial-CCA were significantly correlated with water depth and distance-from-shore, indicating non-uniform diatom stratigraphies across the lake. Despite this variability, all cores show the same succession of species and, therefore, record the eutrophication of the lake. Diatom-inferred total phosphorus (DI-TP) was inferred for six cores using weighted averaging regression and calibration. Apart from considerable differences of DI-TP in surficial sediment samples, there was good between-core repeatability of DI-TP profiles. These data support the use of DI-TP for establishing background nutrient concentrations for lakes, and associated implications for lake restoration schemes using single cores. Comparisons of DI-TP profiles and total diatom accumulation rate data for the individual cores indicate that diatom production peaked prior to the maximum TP concentrations in the lake.     

Acidification and metal contamination in Whitepine Lake (Sudbury,Canada): a paleolimnological perspective

Diatom and chrysophyte assemblages from a sediment core from Whitepine Lake were examined to infer changes in lakewater pH, nickel and aluminum concentrations since pre-industrial times, and to help determine the cause of the virtual extirpation of the lake trout population from the lake during the 1960s and 1970s. Our study indicates that acidification started in the 1920s, and that the maximum inferred pH decline (from 6.2 to 5.8) occurred between 1960 and 1970, coincident with the peak in metal mining and smelting activity in the Sudbury basin. Lakewater [Al] and [Ni], as inferred from our diatom transfer functions, increased. It appears that in addition to the pH decline, elevated [Al] may have played an important role in the decline of lake trout from Whitepine Lake in the 1960s and 1970s. Diatom-inferred lakewater pH and [Ni] have recovered slightly in the recent sediments, which coincides with reductions in emissions that have occurred since the mid-1970s.     

Chrysophyte cyst relationships to water chemistry in Pyrenean lakes (NE Spain) and their potential for environmental reconstruction

Chrysophyte cysts were identified from the surface sediment of 105 mountain lakes in the Pyrenees (NE Spain), and their statistical relationship to water chemistry was examined using canonical correspondence analysis (CCA). The chemical parameters that explained significant and independent amounts of variability were alkalinity, pH, potassium, nitrate and magnesium. In a CCA using these parameters, the first canonical axis was related to a gradient of alkalinity and pH, which reflected the varying nature of the watershed bedrock in the Pyrenees, while the second axis was correlated with potassium (negatively) and nitrate (positively). The potential for environmental reconstructions of the five chemical parameters was further studied by: (i) analyzing the distribution of optima and tolerances calculated by weighted-averaging (WA); (ii) carrying out detrended canonical correspondence analysis (DCCA) with a single environmental variable; and (iii) examining the performance of WA-PLS transfer functions. Acceptable transfer functions were obtained for alkalinity, pH and nitrate. However, for potassium and magnesium the tolerance of cysts was too broad and the distribution of optima too skewed, respectively. The possibility of reconstructing nitrogen-related issues using chrysophyte cysts is particularly interesting because of the lack of direct chemical records of nitrogen compounds in sediments. Nitrate reconstructions using transfer functions may be complemented by a holistic reconstruction using partial CCA, where, after subtracting the effects of other chemicals, samples are ordered on a plain defined by potassium and nitrate. This ordination could show down-core trends in lake productivity and renewal time.     

Arcellaceans (thecamoebians) as indicators of land-use change: settlement history of the Swan Lake area, Ontario as a case study

Arcellaceans have been used as indicators of a variety of paleolimnological conditions including pH, eutrophication, oxygen level, and heavy metal contamination, but there has only been limited application of the group to climate and land-use change research. The limnological evolution of Swan Lake in southwestern Ontario was documented using arcellaceans as proxies, and compared to the results of a palynological analysis, with which it closely correlated. The palynological record documents the rapid disappearance of forest by around 1850 as the area was cleared for agriculture and settlements. The change was characterized by a shift in the palynological record from the Woodland to High Diversity assemblages. Similarly the arcellacean fauna changed from the Pre-European Settlement Assemblage to the distinctive low diversity, stressed environment European Deforestation Assemblage. The introduction of high-yield chemical fertilizers in the post World War II era, and the resultant eutrophication of the lake, was clearly recognizable in the palynological record as indicated by the dramatic increase in the algae Pediastrum (High Nutrient Assemblage). This change in farming practice was also identifiable with arcellacean proxies, as indicated by the appearance of the algalphilic Eutrophication Assemblage. The arcellacean Ecologically Destabilized Assemblage dominated the lake for short intervals during the transition between the Pre-European Settlement and European Deforestation assemblages and again at the transition between the European Deforestation and Eutrophication Assemblages, indicative of periods of ecological destabilization as the lake adjusted to new trophic inputs. A stressed environment arcellacean Ecologically Destabilized Assemblage found in a portion of the core estimated to have been deposited between AD 1350 and AD 1700 may provide evidence of an earlier phase of deforestation associated with Huron Indian agricultural practices. The close correlation between the palynological and arcellacean proxy data clearly demonstrates the potential of arcellaceans as land-use change proxies, and indicates that changes in land-use had an almost immediate impact on the Swan Lake ecosystem. This observation raises concerns about the impact that rapid urbanization is having on the environmentally sensitive Oak Ridges Moraine watershed, of which Swan Lake is a part.     

Surface-sediment and epilithic diatom pH calibration sets for remote European mountain lakes (AL:PE Project) and their comparison with the Surface Waters Acidification Programme (SWAP) calibration set

A modern diatom-pH calibration data-set consisting of surface-sediment diatom assemblages from 118 lakes and 530 taxa is presented. The AL:PE data-set is from high-altitude or high-latitude lakes in the Alps, Norway, Svalbard, Kola Peninsula, UK, Slovenia, Slovakia, Poland, Portugal, and Spain (pH range = 4.5-8.0; DOC range = 0.2-3.2 mg l^-1). In addition, 92 epilithon samples from 22 high-altitude or high-latitude lakes comprise an AL:PE epilithon diatom-pH data-set. Weighted averaging partial least squares regression is used to develop pH-inference models. The AL:PE data-set has a root-mean-square-error of prediction (RMSEP) of 0.33 and a maximum bias of 0.36 pH units and r² of 0.82, as assessed by leave-one-out cross-validation. The epilithon data-set has, after data-screening and the deletion of one very obvious outlier, a RMSEP of 0.23 and a maximum bias of 0.18 pH units and r² of 0.88. The 167 sample SWAP diatom-pH data-set from lowland or upland lakes in the UK, Norway, and Sweden has a RMSEP of 0.29 and a maximum bias of 0.23 pH units and r² of 0.86.The pH optima, as estimated by weighted averaging and Gaussian regression, are compared for the three data-sets (AL:PE, SWAP, AL:PE epilithon). There is a good correspondence between the AL:PE and the AL:PE epilithon optima, but a consistent bias between the AL:PE and SWAP optima, with the SWAP optima being lower than the AL:PE estimates.The predictive performances of the AL:PE and SWAP calibration data-sets are compared using independent test samples and six core sequences, all from high-altitude lakes, one in south-east Siberia and five in eastern Scotland. The results show the importance of using the AL:PE data-set for inferring lake-water pH from diatom assemblages in high-altitude or high latitude lakes with low DOC concentrations.     

Geological and geophysical evidence for pre-Nipissing (>5,000 years BP) transgression infilled valleys in the Lake Huron basin,Ontario

Lake Algonquin, the largest glacial lake of the Great Lakes area, ended prior to 10,000 years BP by drainage to the Ottawa Valley as the North Bay outlet was deglaciated. At that time, the outlet area was isostatically downwarped more than 100 m; resulting low water, river-linked lakes Chippewa, Stanley, and Hough, lowstands in the basins of lakes Michigan, Huron, and Georgian Bay respectively, were much below present lake level. While water levels were low, about half of the present lake area was dry land. The land above the lowstands was dissected by streams and became forested. Uplift of the North Bay outlet between 10,000 and 5,000 years BP raised lake level to above the present (the Nipissing transgression), submerging the forest and valley system. Submerged stumps from those forests have often been encountered on the present lake floor; some stumps have been dated. Four sites in Ontario (Parkhill, Owen Sound, St. Joseph Island, Meaford) provide on-land evidence of pre-Nipissing drainage and valley formation. Radiocarbon ages of valley fill organic materials range from 7,310 to 5,410 years BP. At three sites, present drainage is known to be displaced from the pre-Nipissing drainage. Geophysical methods (EM, GPR, resistivity) have been used to refine valley location and morphology at Parkhill and Meaford. There is the potential of tracing the valleys down slope to the low-water shorelines with shipboard geophysics, with implications for archaeology, hydrology and hydrogeology, paleogeography, and Great Lakes history. This is the eighth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M. Lewis were guest editors of this special issue.     

Diatom assemblages and water depth in Lake 239 (Experimental Lakes Area,Ontario): implications for paleoclimatic studies

Diatom assemblages in surface sediments were sampled along three transects in Lake 239, from the Experimental Lakes Area (NW Ontario), and analyzed in order to explore the relationship between modern species distributions and water depth. Approximately 170 diatom species were identified in surficial sediments at lake depths from 2 to 30 m. The species composition varied with sample depth but remained highly similar across all three transects. The main patterns of variation in the diatom assemblages across transects, derived from a detrended correspondence analysis (DCA), showed that assemblages were highly correlated (r = 0.97 to 0.98). At depths > 8 m the pattern of predominantly benthic composition changed to a planktonic assemblage dominated by Cyclotella stelligera. This depth currently corresponds to the depth of 1% light penetration as assessed from extinction coefficient measurements. Diatom species diversity increases with the switch to the near-shore benthic taxa in all three transects. Additionally, there is a large decrease in the ratio of chrysophyte scales to diatoms at depths < 8 m. Light transmission data from wet and dry periods over the last 35 years suggests that during dry periods the extent of the littoral zone should change by over 2 m. We suggest that cores along a transect from 8 to 14 m should provide a highly sensitive location for detailed paleoclimatic study.     

Changes in diatom assemblages in Lake C2 (Ellesmere Island, Arctic Canada): response to basin isolation from the sea and to other environmental changes

Diatoms preserved in the sediments of Lake C2 (82°50 N, 76°00 W), a high arctic meromictic lake, track changes in the lake's salinity which have occurred as the basin was isolated from the sea. An assemblage dominated by marine taxa, such as Chaetoceros species, Nitzschia cylindrus and Diploneis spp., was replaced by a Cyclotella kuetzingiana var. planetophora dominated freshwater flora. A brief brackish period separates the two assemblages. Relatively little floristic change occurred within either the marine or freshwater periods, indicating rather stable environmental conditions, except that rheophilous diatoms fluctuated in relative abundance during the lacustrine phase, perhaps tracking past changes in discharge from inflowing streams. These may reflect periods of warmer, wetter environmental conditions.This is the eighth in a series of papers published in this issue on the Taconite Lakes Project. These papers were collected by Dr R. S. Bradley.     

Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake

We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr ⁻¹ and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr ⁻¹ is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.     

Surface sediment Chironomidae (Insecta: Diptera) distributions along an ecotonal transect in subarctic Fennoscandia: developing a tool for palaeotemperature reconstructions

We used multivariate statistical techniques to analyse the distributions of surface sediment chironomid assemblages with respect to surface-water temperature, and an additional set of 27 environmental variables, in 30 freshwater lakes of northern Fennoscandia. Our study transect spans boreal coniferous forest to subarctic tundra and includes a steep temperature gradient. Canonical correspondence analysis (CCA) with forward selection and associated Monte Carlo permutation tests revealed that there were statistically significant (P<0.05) relationships between chironomid distributions and two environmental variables, namely lakewater temperature and maximum lake depth. A constrained CCA with temperature as the only predictor variable suggested that the relationship between lakewater temperature and chironomid composition was sufficiently robust for developing a weighted-averaging (WA) based quantitative inference model that will allow palaeotemperature reconstructions using subfossil chironomid remains preserved in lake sediments.     

The environmental history of a mountain lake (Lago Paione Superiore, Central Alps, Italy) for the last c. 100 years: a multidisciplinary, palaeolimnological study

A palaeoecological study of an oligotrophic alpine lake, Paione Superiore (Italy), provided a record of historical changes in water quality. Historical trends in lake acidification were reconstructed by means of calibration and regression equations from diatoms, chrysophycean scales and pigment ratios. The historical pH was inferred by using two different diatom calibration data sets, one specific to the alpine region. These pH trends, together with the record of sedimentary carbonaceous particles and chironomid remains, indicate a recent acidification of this low alkalinity lake.Concentration of total organic matter, organic carbon, nitrogen, biogenic silica (BSiO₂), chlorophyll derivatives (CD), fucoxanthin, diatom cell concentration and number of chironomid head capsules increased during the last 2–3 decades. When expressed as accumulation rates, most of these parameters tended to decrease from the past century to c. 1950, then all except P increased to the present day. A marked increase in sedimentary nitrogen may be related to atmospheric pollution and to the general increases in output of N in Europe. High C/N ratios indicate a prevailing allochthonous source of organic matter.Finally, the increase in measured air temperature from the mid-1800's appeared to be related to lake water pH before industrialization: cold periods generally led to lower pH and vice-versa. The more recent phenomenon of anthropogenic acidification has apparently decoupled this climatic-water chemistry relationship.