Mean July Air Temperature Research Articles

Comparison between chironomid-inferred July temperatures and meteorological data AD 1850–2001 from varved Lake Silvaplana, Switzerland

Inferred temperatures from chironomids preserved in the varved sediment of Lake Silvaplana in the Eastern Swiss Alps were compared with instrumental data obtained from a meteorological station in Sils-Maria, on the shore of Lake Silvaplana, for the time interval 1850–2001. At near-annual resolution, the general patterns of chironomid-inferred temperature changes followed the meteorological record over the last ∼150 years (r Pearson = 0.65, P = 0.01) and 87% of the inferences had deviations from the instrumental data below the root-mean-square error of prediction (RMSEP). When the inferences were compared with a 2-year running mean in the meteorological data, 94% of the inferences had differences with the instrumental data below the RMSEP, indicating that more than half of the inaccurate inferences may have been due to errors in varve counting. Larger deviations from the instrumental data were also obtained from samples with low percentages of fossil taxa represented in the training set used for temperature reconstruction and/or assemblages with poor fit to temperature. Changes in total phosphorus (TP, as inferred by diatoms) and/or greater precipitation were possible factors affecting the accuracy of the temperature reconstruction. Although these factors might affect the quantitative estimates, obtaining >80% accurate temperature inferences suggests that chironomid analysis is a reliable tool for reconstructing mean July air temperature quantitatively over the last ∼150 years in Lake Silvaplana.

Palaeoclimatic applications of large databases: developing and testing methods of palaeotemperature reconstruction using nonmarine ostracods

Non-analogue methods for Quaternary palaeoclimatic reconstruction, such as the coleopteran Mutual Climate Range method and the Mutual Ostracod Temperature Range (MOTR) method, use large geographical databases combined with modern climate datasets to establish the modern climate/temperature ranges of species, which can then be applied to the interpretation of fossil assemblages. Such approaches have been criticized for their lack of attention to variation in the distributions of species within their climate ranges. The MOTR method, for example, assumes that a species has an equal probability of occurring anywhere within its temperature range (e.g. mean July air temperature range), but tests using calculated averages of all records (as opposed to just the two end-points that define the range) show this to be false. The MOTR method remains valid, nevertheless, as long as it is only used to estimate the temperature ranges within which a fossil assemblage could have existed. Analyses of the distributions of 65 species within European climate space show that most are not normal, but often skewed and sometimes polymodal. The factors influencing such variations are discussed and consideration is given to ways of taking them into account in palaeotemperature estimations based on ostracod assemblages. A revised calibration table is presented, showing the modern air temperature ranges of 65 species, for use with the MOTR method.

Chironomid response to environmental drivers during the Holocene in a shallow treeline lake in northwestern Fennoscandia

The significance of different environmental drivers for the chironomid compositional changes during the Holocene was assessed using partial redundancy analysis. These drivers included pollen-inferred mean July air temperature, vegetation cover as pollen percentages, diatom-inferred water pH and total organic carbon (TOC), and sediment properties as loss on iginition (LOI). Variance partitioning showed that on the Holocene timescale, chironomid assemblages were significantly responding to changes in Betula percentages. This is explained by the co-occurring response of chironomids to the Holocene climatic development, which has also been responsible for the development of vegetation in the catchment. However, during the very early Holocene period from 10 500 cal. yr BP to 8300 cal. yr BP chironomids were responding more directly to the changes in water quality, ie, pH. During the latter part of the Holocene, no environmental variable appeared to be significant in explaining the variation in chironomid assemblages. This may be due to the underestimation of the actual water TOC by the diatom-based TOC model. However, for the latter part of the Holocene, from 5000 cal. yr BP to the present, the importance of limnological changes attributed to the development of marginal mires on the midges is evident. Our study shows that factors other than climate may have had more direct influence on the chironomid abundances at certain time periods. Although these factors are probably indirectly connected to climate through catchment processes, more understanding of the catchment-mediated climate effects on lake ecology and chironomids in high latitudes is needed, in addition to careful site selection, in order to better understand the environmental reconstructions.

Environmental inferences and chironomid-based temperature reconstructions from fragmentary records of the Weichselian Early Glacial and Pleniglacial periods in the Niederlausitz area (eastern Germany)

We inferred past climate conditions from lacustrine sediments intercalated in Weichselian Early Glacial and Early Pleniglacial fluvial and aeolian sediments, exposed in two opencast lignite mines from the Niederlausitz area (eastern Germany). A chronology was established using radiocarbon and luminescence dating methods. Both lithology and chironomid fauna indicate that the former shallow lakes were situated on a floodplain. Palaeotemperature estimates calculated from the fossil chironomid-assemblages of the Early Glacial lacustrine deposit indicate mean July air temperatures of ca. 15 °C, which is in line with results derived in earlier studies from the Niederlausitz area and from northwestern Europe. The Early Pleniglacial lacustrine deposits consist of an organic-rich gyttja, intercalated with sand and silt lenses. The chironomid-assemblages show that a shallow meso- to eutrophic lake was present at the study site, and chironomid-inferred palaeotemperature estimates indicate an abrupt decline in July air temperatures from 15–16 °C to ca. 13 °C. In combination with other proxies from the same record, this suggests a Dansgaard/Oeschger like climate event.

Timing of sexual reproduction in chydorid cladocerans (Anomopoda, Chydoridae) from nine lakes in southern Finland

INTRODUCTION Cladocerans (Crustacea) have two strategies of reproduction. During most of their active period, which occurs in the ice-free season in northern temperate and subarctic climates, they reproduce asexually (parthenogenetically). Sexual reproduction, or gamogenesis, usually begins towards the end of the active period and leads to hatching of males and gamogenetic females, which results in fertilized resting eggs. Resting eggs are very tolerant of extreme environmental conditions, such as freezing and drying, and ensure the survival of cladoceran populations during the winter, from which new asexually reproducing females hatch in spring. Much of the research on cladoceran sexual reproduction has focused on the genus Daphnia (e.g. Stross & Hill, 1965, 1968; Kleiven et al., 1992; Pijanowska & Stolpe, 1996; Slusarczyk, 1995, 2001), while the patterns of sexual reproduction in the family Chydoridae (chydorids) have not been widely examined. It is generally known that sexual reproduction in chydorids also occurs in autumn in central and northern Europe (e.g. Poulsen, 1940; Maemets, 1961; Flossner, 1964; Green, 1966; Frey, 1982; Koksvik, 1995), probably triggered by declining temperatures and shortening photoperiods (Shan, 1974; Frey, 1982). Nevertheless, the mechanisms that induce gamogenetic reproduction in chydorids have not yet been entirely clarified. Frey (1982) generalized the life cycles of chydorids as predominantly monocyclic with autumnal gamogenetic periods, with some species having variable and insignificant earlier periods of gamogenesis. Berg (1929) and Flossner (1964) stated that some chydorid communities on the European mainland also have sexual individuals present during the summer months; i.e. they are dicyclic. Some chydorid species are dicyclic as far north as in Denmark (Raen, 1995) and Estonia (Maemets, 1961). There is also one record of summer gamogenesis from southern Finland. Jarnefelt (1956) recorded gamogenetic females ofAlonella excisa (Fischer) from Lake Tuusulanjarvi in early June during a single year. The aim of the present study was to examine the patterns of sexual reproduction in chydorids from several lakes in southern Finland. Our focus was on the timing of sexual reproduction among chydorid populations, i.e. does summer gamogenesis occur in most species and are induction and incidence of autumnal gamogenesis synchronous among populations? Nine lakes in southern Finland were monitored weekly for living chydorids and their sexual reproduction during the open-water season of 2005. Most of the lakes chosen for the study differed in their physical, chemical, and ecological conditions, but were situated in the same climate region. SITES The nine study lakes are located in southern Finland, near Helsinki (Fig. 1). The limnological variables of the lakes measured during three seasons are presented in Table 1. The mean annual air temperature in the area is approximately 4.5 [degrees]C, the mean July air temperature 17[degrees]C, and the mean January air temperature -6[degrees]C (Helminen, 1987). The open-water season lasts about 7 months, from early May to late November (Kuusisto, 1986), depending on yearly variation in spring and autumn temperatures. Lakes Hauklampi (60[degrees] 18' N, 24[degrees]36' E, 76.7 m above sea level, area 2.7 ha), Iso Majaslampi (60[degrees]19' N, 24[degrees]36' E, 92.7 m a.s.l., area 6.3 ha), and Pieni Majaslampi (60[degrees]19.3' N, 24[degrees]35.7' E, 97.3 m a.s.l., area about 1 ha) are located close to one another. They are acidic and oligotrophic lakes whose catchments are small and characterized by patches of mire, bedrock outcrops, and pine (Pines) forests. The shores are inhabited by sedges and Sphagnum mosses, and the aquatic macrophyte zone consists only of European white waterlily (Nymphaea alba). The lakes were severely acidified during the 1980s as a result of acid deposition, but experienced a recovery process during the 1990s. …

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 r2jack = 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.

Midge-inferred Holocene summer temperatures in Southeastern British Columbia, Canada

Using fossil midge stratigraphies, we inferred Holocene summer temperatures at three subalpine lakes in eastern British Columbia. The late-glacial sediment indicated cool conditions, with an abundance of Microspectra atrofasciata/ radialis type fossils at Thunder Lake and Redmountain Lake, and Sergentia at Windy Lake. Sergentia and Tanytarsus lugens/ Corynocera oliveri type were dominant in the early Holocene, together with Chironomus at Redmountain Lake. At Thunder and Windy lakes, the early Holocene was dominated by warm-adapted taxa such as Microtendipes. Quantitative midge-temperature inference models reconstruct a 4 to 8 °C rise in mean July air temperature for Windy and Thunder lakes at the Pleistocene/Holocene transition. Early-Holocene temperatures averaged 3 to 4 °C warmer than those extant today. In contrast, no long-term temperature trend was evident at Redmountain Lake. This site may not reflect actual trends in air temperature due to runoff from a persistent snow pack in the watershed. Comparison of midge and pollen data suggests an inverse relationship between summer temperature and precipitation through the middle to late Holocene.

Chironomid-based palaeotemperature estimates for northeast Finland during Oxygen Isotope Stage 3

Quantitative palaeotemperature estimates for the earlier part of Oxygen Isotope Stage (OIS-) 3 are inferred from subfossil chironomid remains. The high-latitudinal study site of Sokli, northeast Finland, provides for a unique lacustrine deposit covering the earlier part of OIS-3, and the chironomid remains found in the sediments show that a shallow lake with a diverse fauna was present at the study site throughout the record. Using a Norwegian calibration data set as a modern analogue, mean July air temperatures are reconstructed. The chironomid-inferred July air temperatures are surprisingly high, reaching values similar to the current temperature at the study site. Other proxies that were applied to the sediments included the analysis of botanical and zoological macro-remains, and our results concur with temperature estimates derived from climate indicator taxa. Summer temperatures for interstadial conditions, reconstructed with climate models, are as high as our proxy-based palaeotemperatures.

Influence of the Arctic Circumpolar Vortex on the Mass Balance of Canadian High Arctic Glaciers

AbstractVariability in July mean surface air temperatures from 1963 to 2003 accounted for 62% of the variance in the regional annual glacier mass balance signal for the Canadian High Arctic. A regime shift to more negative regional glacier mass balance occurred between 1986 and 1987, and is linked to a coincident shift from lower to higher mean July air temperatures. Both the interannual changes and the regime shifts in regional glacier mass balance and July air temperatures are related to variations in the position and strength of the July circumpolar vortex. In years when the July vortex is “strong” and its center is located in the Western Hemisphere, positive mass balance anomalies prevail. In contrast, highly negative mass balance anomalies occur when the July circumpolar vortex is either weak or strong without elongation over the Canadian High Arctic, and its center is located in the Eastern Hemisphere. The occurrence of westerly positioned July vortices has decreased by 40% since 1987. The associated shift to a dominantly easterly positioned July vortex was associated with an increased frequency of tropospheric ridging over the Canadian High Arctic, higher surface air temperatures, and more negative regional glacier mass balance.

Modeling mass and nitrogen remaining in litterbags for Canadian forest and climate conditions

A new Forest Litter Decomposition Model (FLDM) is presented to simulate mass, N and carbon/nitrogen ratios (C/N) according to the 1992–1998 leaf litterbag data of the Canadian Intersite Decomposition Experiment (CIDET). This experiment involved 10 litter types, with litterbags placed on the ground of 18 upland and 3 wetland sites across Canada. The calibrated model based on first-order reaction kinetics calculates total mass, N concentration and C/N for each litter type and location using: three compartments (fast, slow, and very slow), four parameters for compartment initialization; three for compartment-based decay; three to assess the climate influence on decay; and one each to determine the rate o f N-mineralization and the final C/N ratio. With FLDM, the initial fast fraction is determined from the initial water-extractable and acid-hydrolyzable or acid-unhydrolyzable portions of the litter; the initial ash content determines the ratio between the slow and very slow fractions. Mean July and January air temperatures, and annual precipitation, determine the climate effect on litter decay. Initial N concentration and the upland-wetland difference determine the N-mineralization coefficient. Model performance was fairly consistent by litter type, location, and year; r2 ranged from 0.83 to 0.97, from 0.64 to 0.92 and from 0.54 to 0.93 for mass remaining, N concentration and C/N, respectively. Key words: Forest litterbags, organic matter decomposition, N concentrations, climate, litter type, CIDET

Lateglacial summer temperatures in the Northwest European lowlands: a chironomid record from Hijkermeer, the Netherlands

Lateglacial environments at Hijkermeer, northwest Netherlands, were reconstructed by means of chironomid, diatom and pollen analyses. Diatom assemblages indicate that Hijkermeer was a shallow, oligo- to mesotrophic lake during this period. Pollen assemblages reflect the typical northwest European Lateglacial vegetation development and provide an age assessment for the record from the beginning of the Older Dryas (ca 14 000 calibrated 14C yr BP) into the early Holocene (to ca 10 700 calibrated 14C yr BP). The chironomid record is characterized by several abrupt shifts between assemblages typically found in mid-latitude subalpine to alpine lakes and assemblages typical for lowland environments. Based on the chironomid record, July air temperatures were reconstructed using a chironomid-temperature transfer-function from central Europe. Mean July air temperatures of ca 14.0–16.0 °C are inferred before the Older Dryas, of ca 16.0–16.5 °C during most of the Allerød, of ca 13.5–14.0 °C during the Younger Dryas, and of ca 15.5–16.0 °C during the early Holocene. Two centennial-scale decreases in July air temperature were reconstructed during the Lateglacial interstadial which are correlated with Greenland Interstadial events (GI)-1d and -1b. The results suggest that vegetation changes in the Netherlands may have been promoted by the cooler climate during GI-1d, immediately preceding the Older Dryas biozone, and GI-1b. The Hijkermeer chironomid-inferred temperature record shows a similar temperature development as the Greenland ice core oxygen isotope records for most of the Lateglacial and a good agreement with other temperature reconstructions available from the Netherlands. This suggests that chironomid-based temperature reconstruction can be successfully implemented in the Northwest European lowlands and that chironomids may provide a useful alternative to oxygen isotopes for correlating European lake sediment records during the Lateglacial.

Subfossil Chironomids As Indicators Of Recent Climate Change In Sierra Nevada, California, Lakes

ABSTRACTHigh-resolution chironomid (Insecta: Diptera) stratigraphies were developed for three subalpine lakes in the Sierra Nevada of California to assess whether these lakes have been impacted by recent climate change evident in regional instrumental records for the 19th and 20th centuries. Detrended correspondence analysis (DCA) of the chironomid fauna indicates that the lakes have experienced similar unidirectional change in community composition over the 20th century, with two of the lakes showing particularly sharp gradients of change since the 1970s. Application of a chironomid-based surface water temperature inference model (r2 jack = 0.73, RMSEPjack = 1.1°C, and a maximum bias of 1.24°C) to the subfossil chironomid assemblages preserved in the lake sediment provided quantitative estimates of surface water temperature changes and revealed the existence of similar water temperature trends between the late 19th century and the present. Above average water temperatures characterized the late 20th century and below average surface water temperatures occurred between a.d. 1910 and a.d. 1980. Fluctuations in the surface water temperature of these lakes closely track changes in mean July air temperature as measured in Fresno, California, over the period a.d. 1895–2001. It appears that 20th century climate change has had an overriding influence on the composition of the chironomid communities within these three lakes. This study demonstrates that subfossil chironomid analysis can provide detailed records of community response to local and regional climatic changes at subdecadal time scales. It also suggests that chironomid communities in subalpine lakes in the Sierra Nevada are already recording the impact of recent climate warming.

Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK†

Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18δOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ∼50 and ∼150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ∼1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18δOp) by ∼1.3‰; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.

A northwest North American training set: distribution of freshwater midges in relation to air temperature and lake depth

Freshwater midges, consisting of Chiro- nomidae, Chaoboridae and Ceratopogonidae, were assessed as a biological proxy for palaeoclimate in eastern Beringia. The northwest North American training set consists of midge assemblages and data for 17 environmental variables collected from 145 lakes in Alaska, British Columbia, Yukon, Northwest Territories, and the Canadian Arctic Islands. Canon- ical correspondence analyses (CCA) revealed that mean July air temperature, lake depth, arctic tundra vegetation, alpine tundra vegetation, pH, dissolved organic carbon, lichen woodland vegetation and sur- face area contributed significantly to explaining midge distribution. Weighted averaging partial least squares (WA-PLS) was used to develop midge inference models for mean July air temperature (rboot = 0.818, RMSEP = 1.46� C), and transformed depth (ln (x+1); rboot = 0.38, and RMSEP = 0.58).

Interglacial and Holocene temperature reconstructions based on midge remains in sediments of two lakes from Baffin Island, Nunavut, Arctic Canada

Midge remains (Insecta: Diptera: Chironomidae and Chaoboridae) from two upland lake basins on the Cumberland Peninsula of Baffin Island were analyzed and the results used in a transfer function to reconstruct paleotemperatures. This upland region remained ice-free throughout the last glacial cycle and the two lake basins retain sediments deposited during the previous interglacial period as well as during the Holocene period, but they did not accumulate sediment during most of the glacial period. Midge remains are abundant and well preserved at both sites. Midge-inferred summer surface water temperatures and mean July air temperatures were estimated using an inference model built on modern samples spanning a geographic range extending from Devon Island, Canada to Maine, USA. Data from twenty-nine new surface samples from Baffin Island were added to an existing inference model. The new weighted averaging (WA) model for summer surface water temperature yields r 2 jack = 0.88 and RMSEP = 2.22 °C for summer water temperatures, and r 2 jack = 0.88 and RMSEP = 1.53 °C for mean July air temperatures. Reconstructions at both sites indicate that summer temperatures during the last interglacial were higher than at any time in the Holocene, and 5 to 10 °C higher than present. Peak Holocene temperatures occurred in the first half of the period, and have decreased since about the mid-Holocene.

Holocene climatic and environmental changes inferred from midge records (Diptera: Chironomidae, Chaoboridae, Ceratopogonidae) at Lake Berkut, southern Kola Peninsula, Russia

A radiocarbon-dated sediment sequence from Lake Berkut in the southern part of the Kola Peninsula, northwest Russia, was investigated by means of midge analysis in order to reconstruct the Holocene climatic and environmental history of the region. Past mean July air temperatures at the study site and hypolimnetic oxygen contents of the lake water were inferred from chironomid-based transfer functions. The early Holocene ( c. 10 100-8400 cal. yr BP) is characterized by summer temperatures and hypolimnetic oxygen concentrations broadly similar to present-day conditions. The midge records give evidence of a lake-level fall at c. 7000 cal. yr BP, resulting in weakened thermal stratification of the lake water, and improvement of the hypolimnetic oxygen conditions. After c. 4000 cal. yr BP midge assemblages suggest a lake-level rise related to increased effective moisture. A secondary mid-Holocene climatic optimum with inferred mean July air temperatures of c. 12.1° C was recorded between 6000 and 4400 cal. yr BP, followed by minimum values (c. 11.3°C) between 3500 and 1500 cal. yr BP, accompanied by the onset of natural acidification of the lake. The late-Holocene midge assemblages suggest weakly acidic conditions, low hypolimnetic oxygen contents and a general warming trend during the last c. 1500 years, although paludification and natural acidification may have influenced the temperature reconstruction at this stage. However, the modern chironomid-inferred mean July air temperature (12.5°C) is consistent with meteorological data from a nearby station. In general, the midge records reflect relatively warm and moist conditions at Lake Berkut before c. 7000 cal. yr BP, a relatively warm and dry climate at c. 7000-4000 cal. yr BP, and a shift to cooler and wetter climatic conditions in the region after c. 4000 cal. yr BP.

Holocene environmental history and climate of R�t�sj�en, a low-alpine lake in south-central Norway

The Holocene environmental history and climate are reconstructed for Ratasjoen, a low-alpine lake in south-central Norway. The reconstructions are based on chironomids, diatoms, pollen, plant macrofossils, and sediment characteristics. From plant macrofossil evidence, birch trees (Betula pubescens) immigrated ca. 10,000 cal BP. The chironomid-inferred mean July air temperature was high, but may be unreliable during the early stages of the lake’s history due to the high abundance of Chironomus anthracinus type, a taxon that may include several species. From ca. 9000 cal BP the inferred mean July temperature was lower (ca. 9 °C). Temperatures increased towards 8000 cal BP and pine (Pinus sylvestris) reached its upper limit near the lake. July temperature may have become a significant factor controlling long-term pH in the lake, starting shortly after 8000 cal BP. High pH values were associated with periods of warm summers and lower pH values occurred during periods of colder summers. Alkalinity processes within the lake and/or the catchment are possible factors controlling this relationship. A temperature decline at ca. 5400 cal BP separated two 10.6 °C temperature maxima around 6400 and 4500 cal BP. The 1.5 °C decline in July air temperatures from ca. 4400 cal BP was paralleled by a decrease of pH from 7.2 to 6.8. Following the temperature drop, first pine and then birch trees declined and disappeared from the catchment and organic accumulation in the lake increased. The increased organic accumulation rate had a positive effect on diatom production. At ca. 2700 cal BP the temperature reached a minimum (ca. 9.2 °C) and correspondingly a second pH minimum was reached. Temperature decreased again slightly at ca. 400 cal BP during the ‘Little Ice Age’, before increasing by about 0.5 °C towards the present. Percentage organic carbon as estimated by loss-on-ignition appears to be better correlated with chironomid-inferred July temperatures than organic accumulation rates, at least for the last 9000 years. Accumulation rates of organic sediments are more coupled with catchment-related processes, such as erosion and major changes in vegetation, than is percentage organic carbon.

Distribution of Midge Remains (Diptera: Chironomidae) in Surficial Lake Sediments in New England

Abstract A survey of larval midge remains from surficial sediments in 37 New England lakes was undertaken in order to relate midge distributions to environmental factors. The lakes are located along a transect from northern New Hampshire to southern Connecticut. The midges proved to be a very diverse group of insects in these freshwater habitats. A total of 65 chironomid taxa were recovered. Canonical correspondence analyses indicated that the environmental variables which best explain the distribution of chironomid taxa were mean July air temperature, percent sediment organic matter, pH, and lake depth. This knowledge about the relationship between midge distribution and mean July air temperature can be applied to midge assemblages preserved in older lacustrine sequences to improve our understanding of past environmental conditions in the region.

Palaeolimnological and sedimentary responses to Holocene forest retreat in the Scandes Mountains, west-central Sweden

A suite of analyses was performed on sediments accumulated during the last 10 700 years in Lake Spaime, a small, hydrologically open water body in the modem alpine tundra zone of the Scandes Mountains, west-central Sweden. The study aimed to evaluate (1) the nature of climate changes that forced the late-Holocene lowering of altitudinal tree limit in the region, the timing of which is known from prior studies based on radiocarbon dating of subfossil wood, and (2) the impact of these vegetational changes on an aquatic ecosystem. Arboreal pollen and plant macrofossil data confirm the persistence of trees in the lake catchment at least from c. 9700 cal. BP until c. 3700 cal. BP. Although growing-season temperature is commonly believed to be the dominant factor driving boreal forest tree-limit variations in the region, a chironomid-based reconstruction of mean July air temperature suggests that local deforestation during the late Holocene was not accompanied by a significant cooling. The tree-limit retreat was more likely caused by increasing effective moisture and declining length of the growing season. The ecohydrological response of Lake Spaime to this combination of climate and vegetational changes included a decline in primary productivity, as indicated by an abrupt decrease in sediment organic matter content, while associated increases in organic 613C, 615N and C/N point to diminished fluxes and altered balance of catchment derived nutrients following deforestation. The decline in aquatic productivity is also marked by a distinct change in the mineral magnetic properties, from a high magnetic concentration assemblage dominated by fine-grained magnetite of biogenic origin to one dominated by background levels of coarse-grained detrital magnetite.

Holocene temperature estimates and chironomid community composition in the Abisko Valley, northern Sweden

Multi-proxy paleoenvironmental reconstructions are useful to determine the various factors affecting the biological communities of a lake, but to assess if changes in community composition of one indicator organism accurately reconstructs climatic changes through time, it may be more useful to compare temperature reconstructions using the same indicator in several lakes. Here, we compare reconstructions of mean July air temperature using chironomid-based transfer functions from Holocene records at three nearby lakes in the Abisko Valley of northern Sweden to assess if chironomids can be used as indicators of regional temperature changes. The three study lakes experience the same regional climatic conditions, but are located along gradients of elevation (348–999 m a.s.l), temperature (8.1–12°C) and terrestrial vegetation (coniferous to alpine). Chironomid-temperature reconstructions from the three sites indicate a general pattern of temperature decrease (1.5–2.4°C) during the Holocene, consistent with decreases observed from analyses of other proxies in this area, and from other alpine regions in Europe and North America. Similarities between these reconstructions suggest that chironomids can adequately record general patterns of temperature changes through the Holocene, although effects of site-specific factors such as variations in lake water pH can cause deviations in inferred temperature among sites during some periods.