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1.
Stable oxygen isotope measurements on fossil chironomid head capsules from lake sediments show that these chitinous remains can be used to reconstruct past lake water δ18O and, indirectly, past climate change. We examined the impact of chemical pretreatment procedures on the chemical and stable oxygen isotope composition, and morphology of chironomid cuticles. Use of alkali, acids, and sodium chlorite alters the chemical composition and the morphological structure of chironomid cuticles by selective removal of chitin or proteins. Gas chromatograms of pyrolyzates show that NaClO2 causes deproteination, whereas the combined use of HCl and HF results in partial chitin removal. Head capsules pretreated with KOH contained both chitin- and protein-derived moieties, although the concentration of protein was reduced, especially after KOH treatment at high concentration (28%) and temperature (100°C). Scanning electron microscopy confirmed that a proteinaceous matrix is still present in modern and fossil head capsules after KOH treatment. This matrix, however, is largely absent in head capsules pretreated with NaClO2. A change in the proportion of chitin and proteins in our samples was associated with differences in chironomid δ18O values. Our results suggest that deproteination results in a relative increase of chironomid δ18O, whereas removal of chitin leads to decreased δ18O values. We therefore discourage the use of acids or prolonged (≥1 h) exposure to hot alkali (70°C) prior to chironomid δ18O analysis. Chitin purification by sodium chlorite causes significant weight loss, which may preclude down-core chironomid δ18O measurements. Caution and standardization are required when pretreating samples for chironomid δ18O analysis to ensure reliable, comparable, and reproducible results.  相似文献   

2.
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 δDTOM 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.  相似文献   

3.
We used a series of experiments to determine whether stable carbon isotope analysis of modern and fossil larval head capsules of chironomids allowed identification of their dietary carbon source. Our main focus was to assess whether carbon from naturally 13C-depleted methane-oxidizing bacteria (MOB) can be traced in chironomid cuticles using stable carbon isotope analysis. We first showed that a minimum sample weight of ~20 μg was required for our equipment to determine head capsule δ13C with a standard deviation of 0.5‰. Such a small minimum sample weight allows taxon-specific δ13C analyses at a precision sufficient to differentiate whether head capsules consist mainly of carbon derived from MOB or from other food sources commonly encountered in lake ecosystems. We then tested the effect of different chemical pre-treatments that are commonly used for sediment processing on δ13C measurements on head capsules. Processing with 10% KOH (2 h), 10% HCl (2 h), or 40% HF (18 h) showed no detectable effect on δ13C, whereas a combination of boiling, accelerated solvent extraction and heavy chemical oxidation resulted in a small (0.2‰) but statistically significant decrease in δ13C values. Using culturing experiments with MOB grown on 13C-labelled methane, we demonstrated that methanogenic carbon is transferred not only into the larval tissue, but also into chironomid head capsules. Taxon-specific δ13C of fossil chironomid head capsules from different lake sediments was analyzed. δ13C of head capsules generally ranged from −28 to −25.8‰, but in some instances we observed δ13C values as low as −36.9 to −31.5‰, suggesting that carbon from MOB is traceable in fossil and subfossil chironomid remains. We demonstrate that stable carbon isotope analyses of fossil chironomid head capsules can give insights into dietary links and carbon cycling in benthic food webs in the past and that the method has the potential to reconstruct the importance of MOB in the palaeo-diet of chironomid larvae and, indirectly, to infer past changes in methane flux at the sediment water interface in lakes.  相似文献   

4.
Kerosene, a grade mineral oil, is commonly used to extract beetles from sediment. Here, the use of kerosene to extract chironomid head capsules was tested on 10 samples from sediment of different lakes, with different organic matter content as measured by loss on ignition, and estimated ages. Our results revealed that this flotation tool is very effective in extracting either full or half chironomid head capsules. The mean extraction efficiency was 89.3 ± 8.0% with an estimated relative abundance error ranging from −1% to 1% for 46 of the 57 identified taxa. Larger chironomids (400–500 μm width), which are often full of sediment particles, have the highest relative abundance error, with a maximum of 4.3% for Corynocera oliveri-type. A canonical correspondence analysis showed that, despite this small bias, samples retrieved with the kerosene flotation do not differ from the whole sample assemblages. These results give us confidence in the use of this flotation technique for chironomid sample preparation.  相似文献   

5.
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.  相似文献   

6.
We conducted a rearing experiment with the chironomid species Chironomus riparius to assess the relationship between the δ13C values of chironomid larvae and the δ13C values of their exuvial head capsules. Our experiment was also designed to study the extent of the trophic fractionation factor (Δ13C) under different dietary conditions. Three food sources were used (Tetramin, oats and corn), covering a range in δ13C values of 14.55 ‰. For each of the four successive instars, carbon isotope ratios were measured in larval tissues and head capsules. This approach highlighted the variability in δ13C for both larvae and their head capsules during larval development. Once the larvae reached the 3rd instar, their δ13C values were stabilised and did not significantly differ from their food δ13C (Δ13C = 0 ‰). It is probable that the variability in the δ13C offset during larval development reflected a difference in the carbon turnover for the chironomid cuticle compared with the whole body. At the 4th instar, the δ13C offset did not significantly differ between the three food sources and was ?0.9 ± 0.2 ‰. The proposed Δ13C and δ13C offset values can be considered as a first step for the reconstructions of the chironomid larvae paleo-diets with the aim of deciphering the different organic carbon sources supporting chironomid larvae productions. However, the influence of the environment (e.g. temperature, oxygen), other food sources (e.g. different nutritive values) as well as taxonomy (i.e. other chironomid species) should be assessed to strengthen the robustness of these results.  相似文献   

7.
Short sediment cores from two gulfs, Monche Bay and Kunchast Bay, of Lake Imandra (Kola Peninsula, northern Russia) were analysed for sediment chemistry and chironomid head capsule remains. Monche Bay has been receiving metals from the Severonikel copper-nickel smelter since the late 1930's. Kunchast Bay was selected in the remotest lake basin as an internal reference site. There were no pronounced changes in the chironomid assemblages with the beginning of slight metal contamination of Kunchast Bay. Based on the reconstructed environmental variables and chironomid assemblages, three developmental stages were distinguished from the chironomid fauna history of Monche Bay: (1) A natural development stage; (2) the early warning stage; and (3) the developing crises stage. During the first period, the changes in the chironomid fauna reflect an anthropogenically undisturbed assemblage, with Micropsectra insignilobus dominating (17–23%). The changes during the second period reflect the initial phase of anthropogenic succession associated with the beginning of metal pollution. The main species showed opposite distributional patterns in this period: the abundance of M. insignilobus decreased, whereas the abundance of Chironomus, Procladius and Sergentia coracina increased. At the same time, maximal numbers were attained for species richness (45) and Shannon-Weaver diversity (4.85) of chironomid assemblages, and the highest head capsule concentration (75 head capsules · g–1 of dry sediment). The third period was characterized by a major shift in the faunal assemblages, from M. insignilobus to other dominant species, including Chironomus (22–44%), Procladius (10–30 %) and S. coracina (15–18%). Besides fauna changes, assemblages of the third period are distinguished by the occurrence of mouthpart deformities in Chironomus head capsules.  相似文献   

8.
Cotton blue was added to sediment samples at least 2 h before chironomid head capsules were picked under a binocular microscope and mounted on slides for identification. The use of stain greatly increased the visibility of chironomid head capsules during picking and enhanced the contrast of various parts of the head capsules (pores, ventromental plates, striations on ventromental plates), which could aid identification. In the seven samples studied, there was no significant difference between the percentages of taxa found in stained and unstained samples. The number of taxa were also similar in stained and unstained samples. This method allowed samples to be picked faster.  相似文献   

9.
The mass transport of nutrients by migratory animals can markedly alter the biogeochemistry and ecology of recipient ecosystems, particularly in nutrient-poor regions such as the Arctic. However, the role of biovectors in the global cycling of nutrients is often overlooked. Here we investigate nitrogen dynamics in two seabird-affected ponds in the Canadian High Arctic. The ponds lie at the base of a large seabird colony and have been greatly enriched in nutrients due to the input of guano and other wastes. Using sediment cores that span the last ~200 years, we measured stable isotopes of nitrogen (δ15N) in bulk sediments as well from the subfossil remains of chironomid (Diptera) head capsules and Daphnia ephippia. The bulk-sediment samples from our seabird-affected ponds had elevated δ15N values relative to seabird-free sites elsewhere in the Arctic. In general, the chironomid δ15N profiles roughly paralleled those of bulk sediments in both study ponds, while the Daphnia profile remained relatively stable in contrast to the considerable variation recorded in the bulk sediments and chironomids. Interestingly, no apparent pattern emerged among δ15N values recorded in the bulk sediments, chironomids, and Daphnia between the two study ponds. The stability recorded in the δ15N profiles from bulk sediments relative to the more variable invertebrate profiles point towards the complexity of nitrogen uptake by chironomids and Daphnia at these sites. These data suggest that the bulk sediments are integrating the different fractions of the overall δ15N pool and thus may be most appropriate for reconstructing overall trends in lake trophic status.  相似文献   

10.
Hypolimnetic oxygen depletion has been accelerated in many lakes due to cultural eutrophication. However, the extent and magnitude of environmental change is difficult to ascertain due to the lack of historical records. Larval Chironomidae (Diptera) are useful proxy indicators of oxygen, as they show a wide range of tolerances to oxygen conditions and their chitinous head capsules preserve well in lake sediments. Using paleolimnological techniques, chironomid assemblages from the surface sediments of 42 southeastern Ontario lakes were related to environmental conditions. Hypolimnetic oxygen conditions, measured as the average endofsummer hypolimnetic dissolved oxygen (AvgDO(Summ)), explained the most variation in the chironomid assemblages, whereas dissolved inorganic carbon, the Anoxic Factor, max. depth and total phosphorus concentrations were also correlated with assemblage composition. Based on the relative abundances of 45 chironomid taxa, a robust, partial least squares (PLS) regression transfer function for AvgDO(Summ) was constructed (r2 = 0.74, r2 (jack) = 0.58, n = 40). This new transfer function should allow paleolimnologists to directly track past trends in hypolimnetic oxygen levels.  相似文献   

11.
Initial investigations of Holocene carbonate sediment from Hawes Water, Northwest England, yielded lower numbers of chironomid head capsules than anticipated. Standard techniques used to prepare sediment for chironomid analysis were ineffective in breaking up the coarse crystalline sediment structure sufficiently. This led to large amounts of sediment being retained and increased sample processing times. The low yield of head capsules also meant that more sediment was needed to produce adequate numbers of head capsules for analysis. The use of ultra-sound as part of the sediment processing was investigated. This technique reduced the amount of sediment left for sorting and yielded significantly more head capsules which were of equivalent structural condition and cleaner than those produced by conventional methods. The technique was extended to clay samples where similar results were obtained although shorter treatment times are recommended. The proportion of Tanytarsini and Tanypodinae heads increased significantly in carbonate and clay samples, respectively; both sediment types showed a significant decline in the proportion of Chironomini. The results indicate that ultra-sonic preparation of samples will yield a more accurate representation of chironomid assemblages in sediments leading to greater sensitivity and reliability in analysing past environmental conditions.  相似文献   

12.
Sieving samples for chironomid analysis with a 150 μm mesh was shown to greatly reduce sample preparation time, and use of only larger specimens did not affect chironomid-inferred salinities in African lakes. Here, we tested if this method is suitable for temperature reconstruction in colder lakes at higher latitudes. Removal of specimens <150 μm in two training sets, one from Canada and one from Sweden, had little impact on the performance statistics of the calibration models. Chironomid abundance, however, decreased greatly because more than half of the head capsules in assemblages were <150 μm. This had major impacts on the temperature reconstructions. Inferences were on average 2°C warmer with the modified models (all specimens >150 μm) than those obtained with the full model (all specimens >100 μm). General patterns of temperature change were also altered. For Lake 7 on Southampton Island, Canada, a cooling trend was reconstructed with the full Canadian model while the modified Canadian model yielded a warming trend. When only specimens >150 μm were used, two to three times more wet sediment was needed to obtain a sufficient number of head capsules. These results indicate that, in cold lakes (mean July/August air temperature ≤11°C), large proportions of head capsules are <150 μm, and sieving the samples in a 150 μm mesh leads to altered temperature reconstructions.  相似文献   

13.
Subfossil chironomids in the surface sediments of five small and shallow Norwegian lakes were studied to determine the within-lake variability of fossil assemblages, changes in chironomid assemblages with respect to water depth, and the representativeness of single samples for the entire chironomid fauna of a lake. In each of the lakes studied, six short sediment cores in the deepest part of the lake basin and two littoral to deep-water transects of seven cores each were obtained using a gravity corer, and chironomid assemblages in the uppermost centimetre of sediment were analysed. In three of the five lakes, chironomid concentrations were highest in the deepest parts of the lake basins. In the remaining two lakes, concentrations were either very variable or, in a lake with clear indications of anoxia in the bottom waters, highest at intermediate water depth. Chironomid assemblages tended to be dominated by the same taxa within a lake basin. However, in each of the lakes studied there was a clear and statistically significant shift in chironomid assemblages with respect to water depth. The organic content of the sediments was statistically significant in explaining the variance in the chironomid assemblages only in lakes where organic matter content was closely related to water depth. Only a few chironomid taxa were restricted to the shallowest parts of the lake basins, whereas a number of chironomids were found exclusively in deep-water sediments. Chironomid head capsules of running water taxa and simuliid remains were generally found in sediments close to lake tributaries and in the deepest parts of the lake basins. Although any individual sample contained only a part of the total subfossil chironomid fauna (21–63% of the total taxa per lake), chironomids dominant in any section of the study lakes were found in most of the transect and mid-lake samples.  相似文献   

14.
Surface sediment samples from Lake Moaralmsee in the Austrian Alps were examined for fossil remains of aquatic insects and mites. This study investigated the influence of water depth on the fauna, to explore the possibility of using such fossil remains in sediment cores to reconstruct past water level changes. In addition, instar-specific patterns of chironomid (Diptera: Chironomidae) head capsule accumulation were examined to evaluate whether the smaller, lighter-weight early instars are more easily transported within the lake basin, creating a potential source of error for paleolimnological inferences. Results showed that intra-lake distribution of these zoological remains is closely related to water depth and suggested that the fossils accumulate near each species’ habitat. In addition, the ratio between exoskeletons of oribatid mites (Acari: Oribatida) and chironomid head capsules was strongly related to water depth. Examination of instar-specific accumulation patterns of all chironomid remains showed no significant relationship between specific instars and water depth, though littoral samples consisted only of the 3rd and 4th instars. A taxon-specific examination revealed that the early instars of Paracladius are significantly focused to the deeper parts of the basin. Because most taxa displayed significant relations with water depth, a transfer function was developed, relating fossil chironomids to water depth. This model has a high coefficient of determination and a low estimate of prediction error. In this study, Paracladius was found to prefer shallow and intermediate water depths, hence enhanced offshore transport of early instar head capsules may weaken model performance statistics. Results indicate that intra-lake calibration sets of invertebrate remains have great potential in paleolimnological research, though there is a possible risk of spatial autocorrelation. Such datasets also contribute to the understanding of the modern ecology of the fauna because fossil assemblages in surface deposits provide habitat-specific autecological information. More effort should be directed at evaluating how remains of different instars are transported within other lake basins, because selective offshore transport of head capsules of different larval stages can potentially cause bias in environmental reconstructions.  相似文献   

15.
Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability of surface-sediment chironomid assemblages in the fairly large (∼100–170 km2 since 1983) and shallow (Z max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island Crater, 1.9 km2, 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects, and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes, but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages can be exploited for within-lake calibration of environmental gradients.  相似文献   

16.
Cleaning of lake sediment samples for diatom oxygen isotope analysis   总被引：1，自引：0，他引：1  
Detrital grain contamination in a diatom sample can considerably influence the δ18Odiatom signal. In order to obtain a meaningful signal, pure samples must be used. This can be achieved via a series of cleaning stages including organic and carbonate material removal, sieving, differential settling and heavy liquid separation. The method described here works best for sediments with >20% diatom content. Based on testing various clean-up methods, we propose a sequence of four clean-up stages to produce pure diatom samples from a range of lake sediments types starting with a few grams of sediment. The diatom content and the oxygen isotope composition of the samples at each stage were measured in order to assess the effect of differential amounts of contamination. Results show that a four stage clean-up is necessary to produce clean diatom samples and that contamination by silt and clay causes lower δ18O values.  相似文献   

17.
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.  相似文献   

18.
Random distributions for a wide range (1–100,000) of chironomid head capsules (HC) were simulated on a 1-m2 surface. The number of HC found in circular surfaces equivalent to standard core diameters (90 and 63 mm) was estimated 1000 times, over the range of tested densities. For each number of HC found in the samples, the range of simulated densities was estimated using a threshold probability (p > 0.95). This enabled us to develop equations to infer HC density from sample counts. Because of the threshold probability for comparable sample counts, the equations yield higher estimated densities under a random distribution than for a regular distribution. The probability of sampling at least one HC was >0.95 for densities of 900 HC m?2 for the 90-mm core and 1400 HC m?2 for the 63-mm core. For a specific sample count, the range of actual densities was ~10 times higher for the 63-mm core than the 90-mm core. Comparison with field larval densities revealed that most densities were too low to be suitable for annually resolved reconstruction of a quantitative signal, using current corer sizes, although a large number of populations can support sub-decadal analyses. Nonetheless, some lakes exhibit population sizes large enough to reconstruct robust quantitative estimates of past chironomid abundances. This work provides guidance to reconstruct species dynamics and fine-scale time series analyses in paleoecology.  相似文献   

19.
A suite of surface sediment samples from three Icelandic lakes was analysed for subfossil chironomid head capsules, and a quantitative July air temperature inference model was applied to the data to investigate whether there was significant variability among samples taken from a lake. Ordination and simple regression methods were used to analyse the relationships between environmental and sedimentological variables and the chironomid assemblages and inferred temperature data. Substrate was the most important influence on the chironomid assemblages and inferred temperatures, while water depth at the sampling location had no relationship with the chironomid-inferred temperatures. Within-lake variability of the chironomid assemblages and their inferred temperatures, however, were not significant statistically, suggesting that in lakes of western and northwest Iceland within-lake sampling location has no effect on the data obtained, and therefore on training set samples.  相似文献   

20.
Atmospheric contributions of methane from Arctic wetlands during the Holocene are dynamic and linked to climate oscillations. However, long-term records linking climate variability to methane availability in Arctic wetlands are lacking. We present a multi-proxy ~12,000?year paleoecological reconstruction of intermittent methane availability from a radiocarbon-dated sediment core (LQ-West) taken from a shallow tundra lake (Qalluuraq Lake) in Arctic Alaska. Specifically, stable carbon isotopic values of photosynthetic biomarkers and methane are utilized to estimate the proportional contribution of methane-derived carbon to lake-sediment-preserved benthic (chironomids) and pelagic (cladocerans) components over the last ~12,000?years. These results were compared to temperature, hydrologic, and habitat reconstructions from the same site using chironomid assemblage data, oxygen isotopes of chironomid head capsules, and radiocarbon ages of plant macrofossils. Cladoceran ephippia from ~4,000?cal?year BP sediments have ??13C values that range from ~?39 to ?31??, suggesting peak methane carbon assimilation at that time. These low ??13C values coincide with an apparent decrease in effective moisture and development of a wetland that included Sphagnum subsecundum. Incorporation of methane-derived carbon by chironomids and cladocerans decreased from ~2,500 to 1,500?cal?year BP, coinciding with a temperature decrease. Live-collected chironomids with a radiocarbon age of 1,640?cal?year BP, and fossil chironomids from 1,500?cal?year BP in the core illustrate that ??old?? carbon has also contributed to the development of the aquatic ecosystem since ~1,500?cal?year BP. The relatively low ??13C values of aquatic invertebrates (as low as ?40.5??) provide evidence of methane incorporation by lake invertebrates, and suggest intermittent climate-linked methane release from the lake throughout the Holocene.  相似文献   

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