Environmental controls on modern chironomid faunas from NW Iceland and implications for reconstructing climate change

Reconstructing climate change quantitatively over millennial timescales is crucial for understanding the processes that affect the climate system. One of the best methods for producing high resolution, low error, quantitative summer air temperature reconstructions is through chironomid analyses. We analysed over 50 lakes from NW and W Iceland covering a range of environmental gradients in order to test whether the distribution of the Icelandic chironomid fauna was driven by summer temperature, or whether other environmental factors were more dominant. A range of analyses showed the main environmental controls on chironomid communities to be substrate (identified through loss-on-ignition and carbon content) and mean July air temperature, although other factors such as lake depth and lake area were also important. The nature of the Icelandic landscape, with numerous volcanic centres (many of which are covered by ice caps) that produce large quantities of ash, means that relative lake carbon content and summer air temperature do not co-vary, as they often do in other chironomid datasets within the Arctic as well as more temperate environments. As the chironomid–environment relationships are thus different in Iceland compared to other chironomid training sets, we suggest that using an Icelandic model is most appropriate for reconstructing past environmental change from fossil Icelandic datasets. Analogue matching of Icelandic fossil chironomid datasets with the Icelandic training set and another European chironomid training set support this assertion. Analyses of a range of chironomid-inferred temperature transfer functions suggest the best to be a two component WA-PLS model with r ² _jack = 0.66 and RMSEP = 1.095°C. Using this model, chironomid-inferred temperature reconstructions of early Holocene Icelandic sequences show the magnitude of temperature change compared to contemporary temperatures to be similar to other NW European chironomid sequences, suggesting that the predictive power of the model is good.     

Impact of early prehistoric farming on chironomid communities in northwest Ireland

This study explored the utility and performance of chironomid (Diptera: Chironomidae) autecology in the investigation of prehistoric farming impacts on freshwater lake systems. Chironomid subfossils, lake sediment geochemistry (δ¹³C, δ¹⁵N and C:N), pollen and macroscopic charcoal analyses were used in a comparative limnological assessment of three archaeologically rich study sites in northwest Ireland. At all three study sites, pastoral farming and its associated nutrient inputs, as represented by non-arboreal pollen indicative of grassland/pasture (NAPp) and lake sediment geochemistry, are concomitant with increases in eutrophic chironomid taxa. Redundancy analysis (RDA) and partial RDAs established that δ¹⁵N and NAPp were controlling factors of chironomid community compositional change during the Neolithic (4000–2500 BC) and Bronze Age (2500–600 BC). Bronze Age farming had a considerably greater impact on the lake systems than Neolithic farming, as indicated by a higher proportion of eutrophic taxa and increases in δ¹⁵N, C:N and δ¹³C values, consistent with increased erosion and agricultural inputs. Findings emphasise the importance of identifying the natural, pre-impacted state of a lake system to determine the extent of agricultural impact accurately. The timing and magnitude of change show that Neolithic and Bronze Age farming exhibited a strong control over chironomid communities at all three sites.     

Two hundred years of environmental change in Picos de Europa National Park inferred from sediments of Lago Enol, northern Iberia

We present a study of two short sediment cores recovered from Lago Enol, in the Picos de Europa National Park, Cantabrian Mountains, northern Iberia. We inferred past climate conditions and anthropogenic impacts using geochemical and biological (pollen and diatoms) variables in the dated sequences, in conjunction with temperature and precipitation data collected since 1871 at meteorological stations in the region. The record provides evidence of environmental changes during the last 200 years. At the end of the Little Ice Age (~1800?C1875 AD) the region was characterized by an open landscape. Long-term use of the area for mixed livestock grazing in the mountains, and cultivation of rye during the nineteenth century, contributed to the expansion of grassland at the expense of forest. Warmer temperatures since the end of the nineteenth century are inferred from a change in diatom assemblages and development of the local forest. Socioeconomic transformation during the twentieth century, such as livestock changes related to dairy specialization, planting of non-native trees, mining activities, and management of the national park since its creation in 1918, caused profound changes in the catchment and in the lake ecology. The last several decades (~1970?C2007 AD) of the Lago Enol sediment record are strikingly different from previous periods, indicating lower runoff and increasing lake productivity, particularly since AD 2000. Today, the large number of tourists who visit the area cause substantial impacts on this ecosystem.     

Climate of the Little Ice Age and the past 2000 years in northeast Iceland inferred from chironomids and other lake sediment proxies

A sedimentary record from lake Stora Viearvatn in northeast Iceland records environmental changes over the past 2000 years. Downcore data include chironomid (Diptera: Chironomidae) assemblage data and total organic carbon, nitrogen, and biogenic silica content. Sample scores from detrended correspondence analysis (DCA) of chironomid assemblage data are well correlated with measured temperatures at Stykkishólmur over the 170 year instrumental record, indicating that chironomid assemblages at Stora Viearvatn have responded sensitively to past temperature changes. DCA scores appear to be useful for quantitatively inferring past temperatures at this site. In contrast, a quantitative chironomid-temperature transfer function developed for northwestern Iceland does a relatively poor job of reconstructing temperature shifts, possibly due to the lake’s large size and depth relative to the calibration sites or to the limited resolution of the subfossil taxonomy. The pre-instrumental climate history inferred from chironomids and other paleolimnological proxies is supported by prior inferences from historical documents, glacier reconstructions, and paleoceanographic studies. Much of the first millennium AD was relatively warm, with temperatures comparable to warm decades of the twentieth century. Temperatures during parts of the tenth and eleventh centuries AD may have been comparably warm. Biogenic silica concentrations declined, carbon:nitrogen ratios increased, and some chironomid taxa disappeared from the lake between the thirteenth and nineteenth centuries, recording the decline of temperatures into the Little Ice Age, increasing soil erosion, and declining lake productivity. All the proxy reconstructions indicate that the most severe Little Ice Age conditions occurred during the eighteenth and nineteenth centuries, a period historically associated with maximum sea-ice and glacier extent around Iceland.

Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland

Multi-proxy data, both lithostratigraphic and biostratigraphic, are presented from Efstadalsvatn, a lake in NW Iceland. The sequence covers the period 10,000 to 3500 ¹⁴C yr B.P. The biostratgraphic data include the first Icelandic chironomid-based reconstruction of Holocene mean July air temperatures, using a Norwegian training set in the absence of modern Icelandic data. The results show that deglaciation and ecosystem development probably began before 10,000 ¹⁴C yr B.P. and that July temperatures were around 4°C at ca. 9500 ¹⁴C yr B.P. Temperatures then rose to ca. 8°C at the time of the deposition of the Saksunarvatn tephra (9100 ¹⁴C yr B.P.), reaching ca. 10°C by 8500 ¹⁴C yr B.P., high enough for the growth of tree birch, although successful birch colonisation did not take place until 6750 ¹⁴C yr B.P. There is some evidence for cooling immediately preceding 9100 ¹⁴C yr B.P. There is little firm biostratigraphic evidence for the 8200 cal. B.P. event, although this may be due to a relatively low resolution pollen sampling interval, but there are changes at this time in the total carbon (TC) and mass susceptibility (MS) data. Optimal temperatures and relative vegetation stability may have occurred between 8000–6100 ¹⁴C yr B.P. but the chironomid assemblages indicate higher temperatures after 5000 ¹⁴C yr B.P. This latter interpretation may, however, reflect delayed colonisation of thermophilous taxa and requires further investigation. There is evidence in the lithostratigraphy for greater local terrestrial instability after 6100 ¹⁴C yr B.P. but it seems unlikely that this led to the redevelopment of ice in the catchment. The biostratigraphic records appear to show a degree of resistence to climate forcing throughout the early and middle Holocene. The new chironomid-based temperature reconstruction needs to be refined by further studies in Iceland, particularly the development of an Icelandic training set, but has already demonstrated the problems of paleoclimatic interpretations based on pollen and/or macrofossil evidence alone.     

Holocene environmental history in northwest Finnish Lapland reflected in the multi-proxy record of a small subarctic lake

A 2.5-m-long sediment core was retrieved from Lake Somaslampi, a small lake located in a kame field on the north slope of the Scandes Mountains in Finnish Lapland. Holocene environmental changes were inferred from the lithological, geochemical, pollen, diatom and Cladocera records stored in the lake sediment. The chronology was based on six radiocarbon AMS dates supported by a palynological control chronology. The sediment profile consists of a glacial sedimentary sequence truncated by a lacustrine one. A hiatus, tentatively correlated with climate cooling and advances of glaciers during the 8.2 ka yrs BP “Finse cooling Event”, occurs between these sequences. The glacial sequence was composed of fluvioglacial clastics, smoothly changing into glacio-lacustrine diatomaceous ooze deposited in a meromictic proglacial lake that covered the kame field. The meromixis was probably caused by the greater depth of the lake, the extended ice-cover, and the microbial mats covering large areas of the lake bottom. A distinct change in the biota of the glacio-lacustrine sediments indicates higher trophic conditions than during deposition of the fluvioglacial clastics. The late-Pleistocene vegetation was characterised by subarctic birch tundra vegetation (Betula–Salix–Ericaceae) with low biodiversity gradually changing to Betula–Pinus dominance in the early Holocene. The lake was deep and had a diatom inferred pH ~ 7 indicated also by the dominance of planktonic Cladocera. The base of the lacustrine sediment sequence (6,650–6,300 cal. BP) consisted of loess-rich sediment indicating an increase in eolian activity. This is also supported by the pollen record, which is dominated by more long-distant taxa such as Alnus and Pinus, and by the increased C/N ratio of the sediment. After the initial meromictic phase of the lake, an abrupt lowering of the water level occurred. Lake Somaslampi was isolated from the larger Pre-Lake Somas basin and became holomictic, shallow, much warmer and more productive, until the deterioration of climate around 3,000 yr BP and the increased input of clastics from the tundra soils. The vegetation followed the general climatic trend by gradually changing from the dominance of Betula and Pinus to the dominance of more tundra-related vegetation like Poaceae and Cyperaceae. However, the higher frequencies of planktonic Cladocera and centric diatoms in the most recent sediments indicates higher trophic conditions, increased turbulence and a prolonged ice-free period, which can possibly be linked to the recent climate warming especially in areas of higher altitude and latitude.     

Reconstruction of the recent history of a large deep prealpine lake (Lake Bourget,France) using subfossil chironomids,diatoms, and organic matter analysis: towards the definition of a lake-specific reference state

This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period, a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state. Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions, a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological state of the lake.     

The Dynamics of Chironomidae (Insecta: Diptera) Assemblages in Response to Environmental Change during the past 700 years on Svalbard

The impact of recent natural and human-induced environmental change on chironomid faunas on Svalbard has been investigated. The modern chironomid fauna was studied from surface-sediment samples collected from 23 lakes in western Svalbard. A total of 18 taxa was found, of which three had not been recorded previously from Svalbard. The influence of water chemistry and physical variables on the distribution and abundance of the modern chironomid assemblages was investigated using correspondence analysis and multiple regression. The chironomid assemblages fall into four groups, which are primarily influenced by pH, nutrient concentrations, water temperature, and water depth. Sediment cores were taken from three lakes to investigate changes in chironomid assemblages over the last 700 years. At two of the sites there is evidence for a response to regional climatic change occurring about 200 years ago and may have been associated with the ‘Little Ice Age’. At the third site there is a response to local catchment changes, probably brought about, initially, by the establishment of a human settlement close to the lake 70 years ago, and subsequently, as a result of the abandonment of this settlement in 1988.     

Jökulhlaups Circa Landnám: Mid- to Late First Millennium AD Floods in South Iceland and their Implications for Landscapes of Settlement

This paper presents geomorphological and sedimentological evidence for three large‐scale floods to the west of the ice‐capped volcano Katla around the time of Norse settlement or Landnám (AD 870–930). These glacial outburst floods (jökulhlaups), the most recent prehistoric events in a series of Holocene floods in the Markarfljót valley, are securely dated by tephrochronology and radiocarbon dating to between c. AD 500 and c. AD 900. The environmental impact of these events would have been extensive, affecting both the highlands and about 40–50 km² of the coastal lowlands where about 15 of the 400 or so landnám farms in Iceland were sited. An awareness of environmental conditions and landscape stability around the time of the Norse colonisation of Iceland is important to understand the earliest settlement patterns because of the different constraints and opportunities that they represent.     

Climate and causation in the Swedish Iron Age: learning from the present to understand the past

The paper reassesses the role of climate as a factor shaping changes in settlement and landscape in the Swedish Iron Age (500 BC–AD 1050). Two reasons motivate this re-evaluation. First, high-resolution data based on climate proxies from the natural sciences are now increasingly available. Second, the climate-related social sciences have yielded conceptual and theoretical developments regarding vulnerability and adaptability in the present and recent past, creating new ways to analyse the effects of climatic vs. societal factors on societies in the more distant past. Recent research in this field is evaluated and the explicitly climate deterministic standpoint of many recent natural science texts is criticized. Learning from recent approaches to climate change in the social sciences is crucial for understanding society–climate relationships in the past. The paper concludes that we are not yet in a position to fully evaluate the role of the new evidence of abrupt climate change in 850 BC, at the beginning of the Iron Age. Regarding the crisis in the mid-first millennium AD, however, new climate data indicate that a dust veil in AD 536–537 might have aggravated the economic and societal crisis known from previous research.     

Natural and anthropogenic forcing of Holocene lake ecosystem development at Lake Uddelermeer (The Netherlands)

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication.     

Proximity to ice fields and lake depth as modulators of paleoclimate records: a regional study from southwest Yukon,Canada

Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted.     

Sub-fossil chironomids are significant indicators of turbidity in shallow lakes of northeastern USA

Small, shallow, temperate lakes are predominant landscape features in North America, however, little is known about their long-term ecosystem dynamics, and few data exist on the chironomid fauna they harbor. Using multivariate analyses, we defined relationships between sub-fossil chironomid assemblage composition and environmental variables in 26 shallow lakes of northeastern USA and quantified how differences in taxonomic resolution affect transfer function model performance. Using redundancy analysis, we found that chironomid assemblages are best explained by turbidity, dissolved inorganic carbon and drainage basin/lake area ratio. Turbidity explained the greatest proportion of variance found in the chironomid assemblage (10.4%), followed by total nitrogen. Through ordination analyses and an analysis of similarity, we found that macrophyte density was also a significant predictor of chironomid assemblages. We used partial least squares analysis to develop a robust model for quantitative reconstruction of turbidity, with r _jack² = 0.62. When using a more coarsely resolved taxonomic dataset, we found that model performance statistics were weaker, suggesting the need for fine-resolution taxonomy. Overall, our findings highlight the importance of variables related to lake trophic state in structuring chironomid assemblages in shallow, temperate lakes and provide tools for inferring past ecological changes in these ecosystems.     

A record of vegetation dynamics and lake level changes from Lake Emakat,northern Tanzania,during the last c. 1200 years

Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (¹⁴C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS ¹⁴C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.     

An overview of late Holocene climate and environmental change inferred from Arctic lake sediment

The 14 papers in this Special Issue of the Journal of Paleolimnology report new records of Holocene climate and environmental change from Arctic lakes, with emphasis on the last 2000 years. The study sites span the high latitudes of North America and extend into northwestern Europe. The studies rely on multiple proxy indicators to reconstruct past climate, including: varve thicknesses, chironomid, diatom, and pollen assemblages, biogenic-silica and organic-matter content, oxygen-isotope ratios in diatoms, and the frequency of lake-ice-rafted aggregates. These proxies primarily document changes in past summer temperatures, the main control on physical and biological processes in lakes at high latitudes. The records will be integrated into a larger network of paleoclimate sites to investigate the spatial and temporal variability of climate change and to compare the paleoclimate inferences with the output of general circulation models. This is the Introduction to a series of fourteen papers published as a special issue dedicated to reconstructing late Holocene climate change from Arctic lake sediments. The special issue is a contribution to the International Polar Year and was edited by Darrell Kaufman.     

High-resolution paleoecological analysis of recent disturbance in a southern Chilean Nothofagus forest

A 1600 year paleoecological record of environmental change is developed from a small lake in the Nothofagus forest of southern Chile (45.5°S, 72°W). High resolution fossil pollen, charcoal, sedimentological, and chrysophycean stomatocyst analyses are used to investigate the impacts of natural and anthropogenic disturbances on terrestrial and lacustrine environments. Chronological control is based on a combination of ²¹⁰Pb and ¹⁴C dating. Temporal resolution during the past 150–200 yr is ca. 8 yr/sample. The macroscopic charcoal record correlates very closely with historical and dendroecological records of 20th century anthropogenic burning in this region. The chrysophyte stratigraphy indicates that this burning had immediate impacts on the lake itself, while the pollen record provides evidence for a succession of vegetation changes lagging slightly behind the disturbance. These palynological changes are very similar to the pollen signal of European disturbance in northeastern North America. Pre-European shifts in chrysophyte assemblages may be due to the influence of earthquake activity on the lake, though there is no corroborative evidence in the pollen or charcoal records. This study demonstrates that high resolution paleoecological methods can be used to help bridge the temporal gap between traditional ecological and paleoecological studies of environmental change in the temperate forests of southern South America.     

Response of littoral chironomid communities and organic matter to late glacial lake—level,vegetation and climate changes at Lago dell’Accesa (Tuscany,Italy)

This study focuses on the response of lacustrine littoral chironomid communities to late glacial changes in limnological, environmental and climate conditions in the Mediterranean context. Late glacial chironomid (Diptera: Chironomidae) assemblages, organic petrography and geochemistry were analysed in a sediment core from the littoral zone of Lago dell’Accesa (Tuscany, Italy), where the lake-level fluctuations and the vegetation history have been previously reconstructed. Comparison of the chironomid stratigraphy to other proxies (pollen assemblages, organic petrography and geochemistry, lake-level) and regional climate reconstruction suggested the predominant influence of lake-level changes on the littoral chironomid fauna. The main lowering events that occurred during the Oldest and the Younger Dryas were followed by higher proportions of taxa typical of littoral habitats. A complementary study of organic matter suggested the indirect impact of lake-level on the chironomids through changes in humic status and habitat characteristics, such as the type of substrate and aquatic macrophyte development. Several chironomid taxa, such as Glyptotendipes, Microtendipes and Cricotopus type patens, were identified as possible indicators of low lake-level in the late glacial records. Nevertheless, this study suggested that parallel analyses of organic matter and chironomid assemblages may be needed to circumvent misinterpretation of littoral chironomid assemblage stratigraphy. There was a weak response of the chironomid assemblages to small lake-level lowerings that corresponded to the Older Dryas and Preboreal oscillations. A higher level of determination, e.g. to the species group level, may be necessary to increase the sensibility of the indicators to lake-level changes.     

9000 years of chironomid assemblage dynamics in an Alpine lake: long-term trends, sensitivity to disturbance, and resilience of the fauna

Subfossil chironomid analysis was applied to a sediment core from Sägistalsee, a small lake at present-day tree-line elevation in the Swiss Alps. During the whole 9000-year stratigraphy the chironomid fauna was dominated by taxa typical of alpine lakes. Major faunistic trends were caused by changes in accumulation rates of three taxa, namely Procladius, Stictochironomus, and Tanytarsus lugens-type. In the early Holocene Procladius was the dominant taxon. In younger samples, Stictochironomus tended to have as high or higher abundances and both taxa showed an increase in accumulation rates. A possible cause of this succession is the decrease of lake-water depth due to infilling of the lake basin and changes in associated limnological parameters. The immigration of Picea (spruce) at ca. 6500 cal. ¹⁴C yrs BP and the resulting denser woodlands in the lake's catchment may have promoted this trend. During three phases, from ca. 70–1450, 1900–2350, and 3500–3950 cal. BP, remains of Procladius, Stictochironomus, and Tanytarsus lugens-type are absent from the lake sediment, whereas other typical lake taxa and stream chironomids show no change in accumulation rate. Together with sediment chemistry data, this suggests that increased oxygen deficits in the lake's bottom water during these intervals caused the elimination of chironomids living in the deepest part of the lake. All three periods coincide with increased human activity in the catchment, as deduced from palaeobotanical evidence. Therefore, enhanced nutrient loading of the lake due to the presence of humans and their livestock in the catchment is the most likely cause of the increased anoxia. The chironomid fauna reacted the same way to intensive pasturing during the last ca. 1500 years as to Bronze Age clear-cutting and more moderate pasturing during the Bronze, Iron, and Roman Ages, suggesting that alpine lake ecosystems can be extremely sensitive to human activity in the catchment. On the other hand, the chironomid assemblages show a considerable amount of resilience to human disturbance, as the chironomid fauna reverted to the pre-impact stage after the first two periods of human activity. In recent years, even though pasturing decreased again, the chironomid fauna has only partly recovered. This is possibly due to other human-induced changes in the lake ecosystem, e.g., the stocking of the lake with fish. The chironomid stratigraphy is difficult to interpret climatologically as the strongest changes in chironomid-inferred temperatures coincide with periods of intensive human activity in the catchment.     

Population trends in the Slavonian grebe <Emphasis Type="Italic">Podiceps auritus</Emphasis> (L.) and Chironomidae (Diptera) at a Scottish loch

Loch Ruthven holds the largest British population of the rare water-bird Podiceps auritus, the Slavonian or horned grebe. The breeding success of this bird has fluctuated annually since records began in 1970. To investigate whether these trends are linked to the abundance of chironomid midges, which are an important food-source for the grebe chicks, we analysed a sediment core from the lake, which was sliced at 2.5-mm intervals and provided near-annual sampling resolution. We also analysed diatoms and algal pigments in the lake sediments and inferred changes in total phosphorus from the diatom assemblage to determine whether changes in lake productivity have influenced the abundance of chironomids. Trends in grebe productivity, chironomid abundance and algal assemblages were compared against climate data to determine whether climate, specifically, the North Atlantic Oscillation, was the ultimate driver of the trends we recorded. Our results show that grebe breeding success is positively correlated with chironomid abundance and chironomid abundance is positively correlated with diatom-inferred total phosphorus. Lake productivity and chironomid abundance began to rise early in the twentieth century and continued to rise on a steeper trajectory from the mid-twentieth century to the present. Since the mid-1960s, chironomid abundance began to fluctuate erratically and since 1970 was in phase with grebe productivity, with the grebe trends most plausibly lagging by 1 year. These trends appear to correlate with inter-annual fluctuations in diatom-inferred total phosphorus. No correlation was found between grebe productivity or chironomid abundance and climate variables, suggesting that the size of the chironomid population and breeding success of Podiceps auritus at Loch Ruthven is resource-linked.     

Climate changes in East Siberia (Russia) in the Holocene based on diatom,chironomid and pollen records from the sediments of Lake Kotokel

We analysed a 620-cm-long sediment record from Lake Kotokel located in East Siberia (Russia) for subfossil diatoms, chironomids and pollen to provide a reconstruction of the climate history of the area for the last 12.2 kyr. The subfossil records show differing time lags in their responses to climate change; diatoms and chironomids were more sensitive to climate change than the pollen record. Changes in the biogenic proxies seem related with changes in insolation, the temperature of the North Atlantic and solar activity. The chironomids Chironomus plumosus-type and Einfeldia carbonaria-type and the diatom Aulacoseira granulata were interpreted as markers of warm climate condition. The proxy records were divided into four periods (A, B, C and D) suggesting differing climate in East Siberia during the Holocene. Period D (12.2–9.5 kyr BP) at the beginning of the Holocene, according to chironomid and diatom records, was characterized by warm climate with summer temperatures close to modern. However, forest vegetation had not become fully established yet. During Period C (9.5–5.8 kyr BP), the climate seemed to gradually become colder and wetter from the beginning of Period C to 7 kyr BP. From 7 to 5.8 kyr BP, the climate seemed to remain cold, but aridity increased. Period B (5.8–1.7 kyr BP) was characterised by frequent and sharp alternations between warm and cold conditions. Unstable conditions during this time are also registered in records from Lakes Baikal, Khubsugul and various other shallow lakes of the region. Optimal warm and wet conditions seemed to occur ca. 4 kyr BP. During Period A (the last 1.5 kyr) the diatom and chironomid records show evidence of cold conditions at 1.5–1 kyr BP, but the forest vegetation did not change significantly.