A multiple stable isotope record of Late Quaternary limnological changes and chironomid paleoecology from northeastern Iceland

We present results from multiple stable isotope analyses (δ¹⁸O of chironomid larval head capsules, chironomid adult thoraxes and other insect remains and δD, δ¹³C, δ¹⁵N of total organic matter—TOM) of a lake sediment core (04-SVID-03) taken from Stora Vidarvatn in northeastern Iceland to reconstruct past environmental, limnological and δ¹⁸O of past lake water changes during the Holocene. Core 04-SVID-03 represents a ∼12,000 cal. yrs BP to present record. Large magnitude changes in δ¹⁸O occurred during the Holocene at the site. Downcore shifts in δ¹⁸O of chironomids did not correlate with measurements of the δ¹³C and δ¹⁵N of chironomid head capsules, implying that the δ¹⁸O changes were not primarily driven by changes in chironomid diet during the Holocene. The δD of TOM provided a proxy of relative lake-water δD changes at the site and also showed large magnitude changes during the record. This approach was supported by analyses of a modern training set where δD_TOM analyses were conducted using surface sediments from a suite of freshwater lakes over a large latitudinal gradient. The magnitude of changes in both the δ¹⁸O and δD and the relatively negative δ¹⁸O values throughout much of the core suggest that the proxies represent more paleoenvironmental information than solely temperature. Additional possible influences on lake-water isotopic composition are discussed, including changes in the seasonality of precipitation, in the patterns of air masses supplying precipitation to Iceland and in the dominant mode of the North Atlantic Oscillation.     

Midges (Chironomidae,Ceratopogonidae, Chaoboridae) as a temperature proxy: a training set from Tasmania,Australia

Chironomid and ceratopogonid head capsules, along with Chaoborus mandibles, were used to model mean temperature of the warmest quarter (TWARM) in Tasmania. Our transfer function is based on midge assemblages and 21 environmental variables sampled from 47 lakes. Canonical correspondence analysis (CCA) revealed seven variables that account for a significant (P ≤ 0.05) portion of the explainable variance. In order of explanatory power, these were pH, TWARM, annual radiation, magnesium, annual precipitation, SiO₂, and depth. TWARM was modeled using weighted averaging partial least squares (WA-PLS) and generated a model with and RMSEP = 0.94. Advances in chironomid paleoecology are progressing very quickly in the Southern Hemisphere. Chironomid identification guides and autecological data are available for many regions, highlighting the potential for developing midge-based quantitative models to address hemispheric and interhemispheric climate hypotheses.     

Assessing chironomid head capsule concentrations in sediment using exotic markers

All the chironomid head capsules in a sediment sample are normally extracted to determine the concentration of chironomids. This method may be tedious since a sample may contain hundreds of head capsules. We have tested a method to estimate the concentration of chironomids in sub-samples and assessed the potential taxonomic bias generated when only a part of each sediment sample is sorted. To allow calculation of concentrations, a known quantity of exotic markers (microspheres with diameter 149–350 μm) was added to the sample. The number of head capsules in the total sample was estimated according to the ratio between the retrieved numbers of microspheres to head capsules in the sub-sample. According to our results, the chironomid concentrations estimated from sub-samples were statistically similar to the concentrations obtained by processing the full sample. Also, a sub-sample containing at least 75 head capsules was likely to have similar taxa abundances and taxa richness as the full sediment sample. These results indicate that exotic markers may be added to chironomid samples for representative concentration estimation. The method may potentially be used for other biological groups than chironomids.     

Holocene environmental change at Lake Njulla (999 m a.s.l.), northern Sweden: a comparison with four small nearby lakes alongan altitudinal gradient

We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.     

神农架大九湖湿地摇蚊群落组成及其环境指示意义

泥炭地是一种水-陆过渡型的湿地生态系统。水体沼泽化过程中,植被等生物群落也随之改变,进而影响泥炭地碳埋藏和发育过程。作为泥炭地生态系统重要的次级生产者,摇蚊群落结构变迁可以为追溯泥炭地环境演变和发育历史提供关键线索。泥炭地在发育过程中将经历各种不同阶段,然而,现有的泥炭地摇蚊群落调查仅限于泥炭生境或泥炭地中开阔水域,较短的生境梯度不足以提供更为全面的摇蚊群落变化信息。神农架大九湖湿地包含泥炭地、湖泊、临时性水体等多种生境,本研究选择该湿地公园作为研究区域,通过采集不同生境的表层沉积物及水体样品,提取底泥亚化石摇蚊头壳,分析大九湖湿地中不同生境下的摇蚊群落结构差异,并对其与环境因子间的关系进行探讨。结果表明:1)大九湖湿地不同生境中摇蚊优势种迥异,泥炭藓藓丘生境中Limnophyes、Psilometriocnemus、Pseudosmittia等半陆生种类为主要优势种,湖泊生境中则以典型静水种Polypedilum nubeculosum-type为主要优势种,而过渡性水域(泥炭地洼地、沟渠及水洼生境)中,静水种与半陆生种共存;2)烧失量、水位埋深和pH是塑造不同生境类型下摇蚊群落结构的显著环境因子,水文条件主要通过改变碳积累过程等其他环境条件间接影响生物群落组成;3)沿水生-半陆生生境梯度,有机质含量和类型均发生显著变化,而摇蚊群落也由静水种、静水/半陆生共存转变为以半陆生种为主,摇蚊群落对生境变化表现出良好的响应过程。本研究揭示了不同生境条件下摇蚊群落的结构差异及影响摇蚊群落结构的潜在因子,为未来基于摇蚊的长时间尺度上泥炭地发育过程分析提供参考依据。     

Development of a chironomid-based air temperature inference model for the central Canadian Arctic

Subfossil midge remains were identified in surface sediment recovered from 88 lakes in the central Canadian Arctic. These lakes spanned five vegetation zones, with the southern-most lakes located in boreal forest and the northern-most lakes located in mid-Arctic tundra. The lakes in the calibration are characterized by ranges in depth, summer surface-water temperature (SSWT), average July air temperature (AJAT) and pH of 15.5 m, 10.60°C, 8.40°C and 3.69, respectively. Redundancy analysis (RDA) indicated that maximum depth, pH, AJAT, total nitrogen-unfiltered (TN-UF), Cl and Al capture a large and statistically significant fraction of the overall variance in the midge data. Inference models relating midge abundances and AJAT were developed using different approaches including: weighted averaging (WA), weighted averaging-partial least squares (WA-PLS) and partial least squares (PLS). A chironomid-based inference model, based on a two-component WA-PLS approach, provided robust performance statistics with a high coefficient of determination (r ² = 0.77) and low root mean square error of prediction (RMSEP = 1.03°C) and low maximum bias. The use of a high-resolution gridded climate data set facilitated the development of the midge-based inference model for AJAT in a region with a paucity of meteorological stations and where previously only the development of a SSWT inference model was possible. David Porinchu and Nicolas Rolland contributed equally to the work.     

Intraregional variability in chironomid-inferred temperature estimates and the influence of river inundations on lacustrine chironomid assemblages

Floodplain lakes are rarely analysed for fossil chironomids and usually not incorporated in modern chironomid-climate calibration datasets because of the potential complex hydrological processes that could result from flooding of the lakes. In order to investigate this potential influence of river inundations on fossil chironomid assemblages, 13 regularly inundated lakes and 20 lakes isolated from riverine influence were sampled and their surface sediments analysed for subfossil chironomid assemblages. The physical and chemical settings of all lakes were similar, although the variation in the environmental variables was higher in the lakes isolated from riverine influence. Chironomid concentration and taxon richness show significant differences between the two classes of lakes, and the variation in these variables is best explained by loss-on-ignition of the sediments (LOI). Relative chironomid abundances show some differences between the two groups of lakes, with several chironomid taxa occurring preferentially in one of the two lake-types. The variability in chironomid assemblages is also best explained by LOI. Application of a chironomid-temperature inference model shows that both types of lakes reconstruct July air temperatures that are equal to, or slightly underestimating, the measured temperature of the region. We conclude that, although there are some differences between the chironomid assemblages of floodplain lakes and of isolated lakes, these differences do not have a major effect on chironomid-based temperature reconstruction.     

Quantitative paleotemperature estimates from δ18O of chironomid head capsules preserved in arctic lake sediments

A paleoenvironmental perspective of temperature change is paramount to understanding the significance of recent warming in the Arctic. Late Quaternary sediments from many arctic lakes provide environmental archives with decadal resolution, but reconstructions are hampered by the relative insensitivity of many traditional proxies to temperature. Here, we show that the δ¹⁸O of head capsules of chironomid larvae are equilibrated with the δ¹⁸O of lakewaters in which they live. In suitable lakes, lakewater δ¹⁸O is controlled by the δ¹⁸O of local precipitation, which is strongly correlated to mean annual air temperature (MAT). From this correlation, chironomid δ¹⁸O can be used to examine past changes in MAT. We illustrate the potential of this novel approach to paleothermometry with examples from two arctic lakes that reveal strong regional paleoclimatic gradients in the early Holocene.     

New Zealand chironomids as proxies for human-induced and natural environmental change: Transfer functions for temperature and lake production (chlorophyll a)

The analysis of chironomid taxa and environmental datasets from 46 New Zealand lakes identified temperature (February mean air temperature) and lake production (chlorophyll a (Chl a)) as the main drivers of chironomid distribution. Temperature was the strongest driver of chironomid distribution and consequently produced the most robust inference models. We present two possible temperature transfer functions from this dataset. The most robust model (weighted averaging-partial least squares (WA-PLS), n = 36) was based on a dataset with the most productive (Chl a > 10 μg l⁻¹) lakes removed. This model produced a coefficient of determination () of 0.77, and a root mean squared error of prediction (RMSEP_jack) of 1.31°C. The Chl a transfer function (partial least squares (PLS), n = 37) was far less reliable, with an of 0.49 and an RMSEP_jack of 0.46 Log₁₀μg l⁻¹. Both of these transfer functions could be improved by a revision of the taxonomy for the New Zealand chironomid taxa, particularly the genus Chironomus. The Chironomus morphotype was common in high altitude, cool, oligotrophic lakes and lowland, warm, eutrophic lakes. This could reflect the widespread distribution of one eurythermic species, or the collective distribution of a number of different Chironomus species with more limited tolerances. The Chl a transfer function could also be improved by inputting mean Chl a values into the inference model rather than the spot measurements that were available for this study.     

A late Quaternary paleotemperature record from Hanging Lake, northern Yukon Territory, eastern Beringia

The late Quaternary paleoclimate of eastern Beringia has primarily been studied by drawing qualitative inferences from vegetation shifts. To quantitatively reconstruct summer temperatures, we analyzed lake sediments for fossil chironomids, and additionally we analyzed the sediments for fossil pollen and organic carbon content. A comparison with the δ¹⁸O record from Greenland indicates that the general climatic development of the region throughout the last glaciation–Holocene transition differed from that of the North Atlantic region. Between  17 and 15 ka, mean July air temperature was on average 5°C colder than modern, albeit a period of near-modern temperature at  16.5 ka. Total pollen accumulation rates ranged between  180 and 1200 grains cm^− 2 yr^− 1. At  15 ka, approximately coeval with the Bølling interstadial, temperatures again reached modern values. At  14 ka, nearly 1000 yr after warming began, Betula pollen percentages increased substantially and mark the transition to shrub-dominated pollen contributors. Chironomid-based inferences suggest no evidence of the Younger Dryas stade and only subtle evidence of an early Holocene thermal maximum, as temperatures from  15 ka to the late Holocene were relatively stable. The most recognizable climatic oscillation of the Holocene occurred from  4.5 to 2 ka.