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.     

Trace-element and stable-isotope geochemistry of non-marine ostracod shells in Quaternary palaeoenvironmental reconstruction

Ostracods are small bivalved aquatic crustancean. They secrete shells of low-Mg calcite that are often preserved in lake sediments. Recent work has shown that the uptake of trace elements (especially Mg and Sr) into the shell may be a function of the salinity and temperature of the host water. Furthermore, ostracod shells are a source of carbonate for stable-isotope analysis. This paper reviews the application of ostracod shell chemistry to Quaternary palaeolimnology. Although such work has revealed the excellent potential of these techniques to provide quantitative palaeolimnological reconstructions, a number of problems have also emerged. These problems relate to (1) methods used for extraction of ostracod shells from sediment and their subsequent cleaning (2) post-mortem diagenesis and alteration of the shell (3) complications with the calcification mechanism (4) spatial and temporal variability in shell composition (5) the ecological tolerances of individual species and (6) the relationships between shell chemistry and palaeohydrology. To some extent, these problems are an inevitable outcome of the diversity of lacustrine systems: they may be overcome by developing a thorough understanding of the physiology, life-cycle and ecology of the species concerned, together with the modern limnology of the study site. Overall, these techniques have excellent potential in Quaternary palaeolimnology, especially when used with other palaeoenvironmental indicators.     

Late Glacial and Holocene chironomid assemblages in Lac Long Inférieur (southern France, 2090 m): palaeoenvironmental and palaeoclimatic implications

Changes in lake water temperature and trophic states were inferred using chironomid fossil assemblages from Lac Long Inférieur (Southern Alps, France). In the Late Glacial, a colder period, possibly analogous to the Younger Dryas, is characterised by a peak in Micropsectra, a cold stenothermic taxon. The increase in temperatures during the Late Glacial interstadial is indicated by a decrease in the percentages of cold stenothermic taxa (Tanytarsus lugens/Corynocera oliveri grp.) and by an increase in taxa linked to the development of vegetation in the littoral zone. The beginning of the Holocene is marked by the presence of taxa adapted to warmer and more eutrophic waters. During the Holocene, the progressive warming of the climate and increase in lake trophic status were indicated by the increase of eutrophic and warmer water indicators. An increase in tributary inflow into Lac Long Inférieur was also inferred by the increase in rheophilous taxa, reflecting increased snowmelt. During the Subatlantic, the composition of the chironomid spectra suggests a re-cooling of the climate and/or a decrease in lake trophic status.     

A heuristic model for potential geomorphic influences on trophic interactions in streams

Whereas certain linkages between stream channel morphology and stream ecology are fairly well-understood, how geomorphology influences trophic interactions remains largely unknown. As a first step, a simple, heuristic model is developed that couples reach-scale geomorphic morphology with trophic dynamics between vegetation, detritus, herbivores, and predators. Predation is assumed to increase with depth beyond a threshold depth, and herbivory is assumed to decrease with velocity beyond a threshold velocity. Results show that the modeled food chain is sensitive to channel geometry, particularly around the threshold conditions for predators and herbivores. Importantly, geomorphic influences are not isolated to a particular trophic level, but rather are transferred through the food chain via top-down and bottom-up effects. The modeled system is particularly sensitive to changes in the end-members of the food chain: vegetation and predators. Results illustrate that geomorphic disturbances, known to affect a single trophic level (e.g., fish), likely impact multiple trophic levels in the stream ecosystem via trophic interactions. Such impacts at the multiple trophic level are poorly understood. While limited by the lack of empirical long-term data for testing and calibration, this simple model provides a structure for generating hypotheses, collecting targeted data, and assessing the potential impacts of stream disturbance or restoration on entire stream ecosystems. Further, the model illustrates the potential for future coupled stream models to explore spatial and temporal linkages.     

Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of past environmental conditions in the Alps. I. Climate

Diatom, chrysophyte cyst, benthic cladocera, planktonic cladocera, and chironomid assemblages were studied in the surface sediments of 68 small lakes along an altitudinal gradient from 300 to 2350 m in Switzerland. In addition, 43 environmental variables relating to the physical limnology, geography, catchment characteristics, climate, and water chemistry were recorded or measured for each lake. The explanatory power of each of these predictor variables for the different biological data-sets was estimated by a series of canonical correspondence analyses (CCA) and the statistical significance of each model was assessed by Monte Carlo permutation tests. A minimal set of environmental variables was found for each biological data-set by a forward-selection procedure within CCA. The unique, independent explanatory power of each set of environmental variables was estimated by a series of CCAs and partial CCAs. Inference models or transfer functions for mean summer (June, July, August) air temperature were developed for each biological data-set using weighted-averaging partial least squares or partial least squares. The final transfer functions, after data screening, have root mean squared errors of prediction, as assessed by leave-one-out cross-validation, of 1.37 °C (chironomids), 1.60 °C (benthic cladocera), 1.62 °C (diatoms), 1.77 °C (planktonic cladocera), and 2.23 °C (chrysophyte cysts).     

Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of past environmental conditions in the Alps. II. Nutrients

Surface sediments from 68 small lakes in the Alps and 9 well-dated sediment core samples that cover a gradient of total phosphorus (TP) concentrations of 6 to 520 g TP l^-1 were studied for diatom, chrysophyte cyst, cladocera, and chironomid assemblages. Inference models for mean circulation log₁₀ TP were developed for diatoms, chironomids, and benthic cladocera using weighted-averaging partial least squares. After screening for outliers, the final transfer functions have coefficients of determination (r², as assessed by cross-validation, of 0.79 (diatoms), 0.68 (chironomids), and 0.49 (benthic cladocera). Planktonic cladocera and chrysophytes show very weak relationships to TP and no TP inference models were developed for these biota. Diatoms showed the best relationship with TP, whereas the other biota all have large secondary gradients, suggesting that variables other than TP have a strong influence on their composition and abundance. Comparison with other diatom – TP inference models shows that our model has high predictive power and a low root mean squared error of prediction, as assessed by cross-validation.     

The modern distribution of chironomid sub-fossils (Insecta: Diptera) in the Sierra Nevada, California: Potential for paleoclimatic reconstructions

Surface lake sediment was recovered from 57 lakes along an elevation gradient in the central, eastern Sierra Nevada of California. The surface sediment was analysed for subfossil chironomid remains in order to assess the modern distribution of chironomids in the region. The lakes sampled for the calibration dataset were between 2.0 and 40.0 m in depth, spanned an altitudinal gradient of 1360 m and a surface water temperature gradient of approximately 14 °C. Redundancy analysis (RDA) identified that five of the measured environmental variables – surface water temperature, elevation, depth, strontium, particulate organic carbon – accounted for a statistically significant amount of the variance in chironomid community composition. Quantitative transfer functions, based on weighted-averaging (WA), partial least squares (PLS) and weighted-averaging partial least squares (WA-PLS), were developed to estimate surface water temperature from the chironomid assemblages. The best model was a WA model with classical deshrinking, which had a relatively high coefficient of determination (r² = 0.73), low root mean square error of prediction (RMSEP = 1.2 °C) and a low maximum bias (0.90 °C). The results from this study suggest that robust quantitative estimates of past surface water temperature can be derived from the application of these models to fossil chironomid assemblages preserved in late-Quaternary lake sediment in this region.     

A protocol for preparing subfossil chironomid head capsules (Diptera: Chironomidae) for stable isotope analysis in paleoclimate reconstruction and considerations of contamination sources

Several techniques are available to examine the isotopic composition of historic lake waters, providing data that can subsequently be used to examine environmental changes. A recently-developed technique is the stable oxygen isotope analysis of subfossil chironomid (Diptera: Chironomidae) head capsules (mostly chitin) preserved in lake sediments. This technique involves a high Temperature Conversion Elemental Analyzer (TC/EA), which has been a relatively recent addition to the suite of online peripherals for analyzing the stable isotopic composition of organic samples. The highly precise and accurate ¹⁸O/¹⁶O and D/H measurements obtainable using the TC/EA with samples in the microgram range make this instrumentation suitable for studying geochemical and biological processes. Preparation of organic samples for isotopic analysis typically requires first weighing each sample into silver/tin capsules. These capsules can introduce oxygen and hydrogen contamination (a “blank effect”), which is especially problematic for analysis of small organic samples (e.g. less than 100 μg). Here we tested tin and silver capsules from two manufacturers and a range of sizes to assess contamination to small organic samples on the TC/EA. We also assessed how a method for cleaning silver capsules affected our analysis of commercial chitin. In general, capsules made of silver have less detectible oxygen than those made of tin, and capsules from the two manufacturers varied in their detectible oxygen. There was no detectable H contamination from silver capsules. In addition to our empirical findings, we present a model demonstrating the influence that contaminant oxygen can have on the δ¹⁸O of small organic samples. Sample mass becomes an important issue for such analyses. In light of our findings, we recommend a minimum sample mass ≥50 μg (approximately 120 whole chironomid head capsules) on a TC/EA-IRMS (Delta^plus XP system). Finally, we present a detailed protocol for preparing and transferring chironomid head capsules into silver capsules that minimizes the influence of contaminant oxygen. This protocol provides the paleo-community with another potential method for reconstructing paleoenvironments.     

Assessing potential environmental influences on killer whale (Orcinus orca) distribution patterns in the Bremer Canyon,south-west Australia

ABSTRACT

Submarine canyons play an important role in the regional distribution, abundance and dispersal of marine biota and are increasingly being recognised as geomorphic features of high conservation significance along Australia’s continental margin. Certain canyons have been described as foraging ‘hotspots’ attributable to the high abundance of apex cetacean species aggregating in these areas. Anecdotal evidence of large seasonal aggregations of killer whales in the Bremer Canyon, south-west Australia, has attracted significant research attention in the last decade. To identify important environmental drivers influencing aggregation patterns, a predictive spatial habitat model using the Maxent model was developed based on presence-only whale sighting data. In addition, remotely sensed sea surface temperature and chlorophyll-a concentrations were assessed to investigate the spatio-temporal variation in sea surface conditions. Habitat preference was predicted in areas between canyon heads, with the most influential predictor variables being depth and distance from the continental shelf break. Analysis of remote-sensing data highlighted low localised variability in surface waters and illustrated the seasonal trends of the Leeuwin Current. This study demonstrates the influence of bathymetry and submarine geomorphology on enhanced cetacean abundance and highlights the need for recognition of this potential foraging area in marine reserve planning.     

美国西部近8000年来树木年轮重建降雨量序列中的气候突变

1 IntroductionThe U niversity of A rizona Laboratory of Tree-Ring Research collected tree-ring sam ples from anum ber of long-lived trees, dating back thousands of years, from eight states in the w esternU nited States. H ughes and G raum lich (1996) pres…     

Light-absorbing impurities on Keqikaer Glacier in western Tien Shan: concentrations and potential impact on albedo reduction

Light-absorbing impurities on glaciers are important factors that influence glacial surface albedo and accelerate glacier melt. In this study, the quantity of light-absorbing impurities on Keqikaer Glacier in western Tien Shan, Central Asia, was measured. We found that the average concentrations of black carbon was 2,180 ng/g, with a range from 250 ng/g to more than 10,000 ng/g. The average concentrations of organic carbon and mineral dust were 1,738 ng/g and 194 μg/g, respectively. Based on simulations performed with the Snow Ice Aerosol Radiative model simulations, black carbon and dust are responsible for approximately 64% and 9%, respectively, of the albedo reduction, and are associated with instantaneous radiative forcing of 323.18 W/m2(ranging from 142.16 to 619.25 W/m2) and 24.05 W/m2(ranging from 0.15 to69.77 W/m2), respectively. For different scenarios, the albedo and radiative forcing effect of black carbon is considerably greater than that of dust. The estimated radiative forcing at Keqikaer Glacier is higher than most similar values estimated by previous studies on the Tibetan Plateau, perhaps as a result of black carbon enrichment by melt scavenging. Light-absorbing impurities deposited on Keqikaer Glacier appear to mainly originate from central Asia, Siberia, western China(including the Taklimakan Desert) and parts of South Asia in summer, and from the Middle East and Central Asia in winter.A footprint analysis indicates that a large fraction(60%) of the black carbon contributions on Keqikaer Glacier comes from anthropogenic sources. These results provide a scientific basis for regional mitigation efforts to reduce black carbon.     

Reconstructing temperature at Egelsee,Switzerland, using North American and Swedish chironomid transfer functions: potential and pitfalls

The temperature reconstruction obtained from chironomids preserved in the sediment of Egelsee, Switzerland, was partially flawed by the low percentages of fossil taxa represented in the Swiss calibration set (Larocque-Tobler et al. 2009a). Transfer functions (TFs) from other regions, which allow a good representation of the fossil taxa (>80%), could be applied to the fossil assemblages of Egelsee. First, the validity of using two (a Swedish and a North American (NA)) TFs was tested by comparing the chironomid-inferred temperatures with instrumental data. Since good relationships (r _Pearson = 0.71 and 0.61, p = 0.001 for the NA and Swedish TFs, respectively) were obtained, these two models were used to reconstruct the Late Glacial and early Holocene periods at Egelsee. Reconstructions using both models showed clear cold periods during the Younger Dryas and the so-called 8,200 calibrated years BP event. However, the amplitude of changes during these periods was higher when the NA transfer function was used, probably due to the fact that 37% of the taxa in the core had temperature optima colder in the NA than in the Swedish and Swiss models. The results indicate that TFs from other regions can be applied when they are based on samples with good modern analogues, however, caution should be taken when the amplitude of temperature changes is considered.     

Observations on the distribution of paraglacial reworking of glacigenic drift in western Norway

This paper presents new data on the extent of and controls on paraglacial reworking of glacigenic drift by debris flows and snow avalanches at eight localities in western Norway. At sites around Jostedalsbreen, gully density is in the order of c. 10-100 gullies per km. Drift slopes at the sites in the Jotunheim massif are generally much less extensively modified by paraglacial processes (gully density nowhere exceeds 8 km^-1 in either site). Factors controlling paraglacial drift modification include gradient, sediment availability and water supply. Gully density generally exceeds 20 gullies per km where drift is steeper than c. 30° and thicker than c. 10 m, and where the void ratio of unreworked sediment exceeds c. 0.35. Widespread gullying is also favoured at sites of high and focused water input, in particular where melting snow and ice are involved. A significant implication of these findings concerns the preservation of glacigenic sediments and landforms in different paraglaciated landscapes. Understanding the detailed constraints on paraglacial processes is essential for realistically assessing the geomorphological significance of paraglaciation in a range of environments.     

Laminated lake sediments in northeast Poland: distribution, preconditions for formation and potential for paleoenvironmental investigation

Glacial landscapes of the Land of Great Masurian Lakes and Suwa?ki Lakelands in northeast Poland are characterized by very high abundance of lakes. These two areas were surveyed for lakes containing laminated sediments. Using bathymetry as a criterion, 60 small, deep lakes, representing preferred conditions for formation and preservation of lacustrine non-glacial varves, were selected for gravity coring. We found laminated sediments in 24 of the lakes, 15 in the Land of Great Masurian Lakes and 9 in the Suwa?ki Lakeland. Seven of these 24 sediment records were laminated in the topmost part only. Analysis of lake morphometric variables showed that the relation between surface area and maximum water depth can be used to identify lakes with laminated sediments. Most of the newly discovered lakes with laminated deposits have surface areas ≤0.3 km² and maximum depths of 15–35 m. Multivariate statistical analysis (Linear Discriminant Analysis) of the lake dataset identified the morphological features of lake basins and their catchments that largely control preservation of laminated sediments. Microscopic and geochemical analyses revealed a biogenic (carbonaceous) type of lamination typical for lakes in northeast Poland. Such lakes are characterized by a spring-summer lamina that is rich in calcium carbonate and an autumn-winter lamina composed of organic and minerogenic detritus. This pattern may be modified by multiple periods of calcite deposition during a single year or substantial contribution of clastic material. Laminations and high sedimentation rates offer the possibility of high-resolution investigation of past climate and environmental changes through application of myriad biological, isotopic and geochemical proxies.     

Carbon and nitrogen stable isotope ratios in surface sediments from lakes of western Ireland: implications for inferring past lake productivity and nitrogen loading

We used statistical analyses to determine which subset of 36 environmental variables best explained variations in surface sediment δ¹³C and δ¹⁵N from 50 lakes in western Ireland that span a human-impact gradient. The factors controlling lake sediment δ¹³C and δ¹⁵N depended on whether organics in the lake sediment were mostly derived from the lake catchment (allochthonous) or from productivity within the lake (autochthonous). Lake sediments with a dominantly allochthonous organic source (high C:N ratio sediments) produced δ¹³C and δ¹⁵N measurements similar to values from catchment vegetation. δ¹³C and δ¹⁵N measurements from lake sediments with a dominantly autochthonous organic source (low C:N ratio sediments) were influenced by fractionation in the lake and catchment leading up to assimilation of carbon and nitrogen by lacustrine biota. δ¹³C values from lake sediment samples in agricultural catchments were more negative than δ¹³C values from lake sediment samples in non-impacted, bogland catchments. Hypolimnetic oxygen concentrations and methane production had a greater influence on δ¹³C values than fractionation due to algal productivity. δ¹⁵N from lake sediment samples in agricultural catchments were more positive than δ¹⁵N in non-impacted bogland catchments. Lower δ¹⁵N values from non-impacted lake catchments reflected δ¹⁵N values of catchment vegetation, while higher δ¹⁵N values in agricultural catchments reflected the high δ¹⁵N values of cattle manure and inorganic fertilisers. The influence of changing nitrogen sources and lake/catchment fractionation processes were more important than early diagenesis for lake sediment δ¹⁵N values in this dataset. The results from this study suggest a possible influence of bound inorganic nitrogen on the bulk sediment δ¹⁵N values. We recommend using a suitable method to control for bound inorganic nitrogen in lake sediments, especially when working with clay-rich sediments. This study confirms the usefulness of δ¹³C and δ¹⁵N from bulk lake sediments, as long as we are mindful of the multiple factors that can influence these values. This study also highlights how stable isotope datasets from lake surface sediments can complement site-specific isotope source/process studies and help identify key processes controlling lake sediment δ¹³C and δ¹⁵N in a study area.     

吉林省西部土地沙化动态变化

吉林省西部沙地位于中国温带半湿润与半干旱的过渡地带 ,是生态环境变化的敏感地区。采用多时相、多波段TM影像 ,结合实地考察 ,建立沙区生态环境地理信息系统 ;选取植被退化状态、风蚀强度和土层厚度为沙化程度评价的综合指标 ,在GIS平台支持下对沙区土地沙漠化进行动态评价。结果表明 ,近 2 0年来土地沙漠化基本得到控制 ,局部有所发展。沙漠化形成的主要自然因素是干燥的气候条件和地表丰富的沙源 ;人类不合理的经济活动加速了沙漠化进程。     

西部欠发达城市人口空间分布与演变

探索西部欠发达城市人口空间格局及其演变特征,与东部发达城市人口分布形成对照,有利于认识城市人口分布规律,引导城市人口有序疏解。以西部欠发达城市兰州为例,对其乡镇街道尺度的常住人口数据进行空间化处理,在此基础上借鉴标准差椭圆分析思路,提出变形椭圆圈层法,并结合GIS空间分析和人口密度模型拟合等方法,多角度揭示2000-2010年兰州市人口空间分布格局及其演变特征。研究发现：兰州市人口分布呈现“带状集聚”特征和“圈层-分异”结构,形成“中心区和近郊区人口缓慢增长,外围区人口快速增长,远郊区人口明显减少”的格局;兰州市人口重心在10年间向东偏北方向移动,人口空间呈“西北-东南”分布格局,总体上表现出集聚态势,且10年间集聚态势有所强化;Smeed模型是刻画兰州市人口分布的最佳单中心模型,10年间人口分布的单中心趋势增强,人口多中心结构尚不成熟。     

Functional attributes of epilithic diatoms for palaeoenvironmental interpretations in South-West Greenland lakes

Benthic diatoms are commonly used for palaeoenvironmental reconstruction in Arctic regions, but interpretation of their ecology remains challenging. We studied epilithic diatom assemblages from the shallow margins of 19 lakes from three areas (coast-inland-ice sheet margin) along a climate gradient in Kangerlussuaq, West Greenland during two periods; shortly after ice-off (spring) and in the middle of the growth season (summer). We aimed to understand the distribution of Arctic epilithic diatoms in relation to water chemistry gradients during the two seasons, to investigate their incorporation into lake sediments and to assess their applicability as palaeoenvironmental indicators. Diatoms were correlated with nutrients in the spring and alkalinity/major ions in the summer, when nutrients were depleted; approximately half of the variance explained was independent of spatial factors. When categorised by functional attributes, diatom seasonal succession differed among regions with the most obvious changes in inland lakes where summer temperatures are warmer, organic nutrient processing is prevalent and silicate is limiting. These conditions led to small, motile and adnate diatoms being abundant in inland lakes during the summer (Nitzschia spp., Encyonopsis microcephala), as these functional attributes are suited to living within complex mats of non-siliceous microbial biofilms. Seasonal succession in silica-rich lakes at the coast was less pronounced and assemblages included Tabellaria flocculosa (indicating more acidic conditions) and Hannaea arcus (indicating input from inflowing rivers). The nitrogen-fixing diatom Epithemia sorex increased from the coast to the ice sheet, negatively correlating with a gradient of reactive nitrogen. The presence of this diatom in Holocene sediment records alongside cyanobacterial carotenoids during arid periods of low nitrogen delivery, suggests that it is a useful indicator of nitrogen limitation. Nitzschia species appear to be associated with high concentrations of organic carbon and heterotrophy, but their poor representation in West Greenland lake sediments due to taphonomic processes limits their palaeoenvironmental application in this region. Proportions of epilithic taxa in lake sediment records of coastal lakes increased during some wetter periods of the Holocene, suggesting that snowpack-derived nutrient delivery may offer diatom taxa living at lake margins a competitive advantage over planktonic diatoms during the “moating” ice melt period. Thus, further research investigating linkages between epilithic diatoms, snowpack and nutrient delivery in seasonally frozen lakes is recommended as these taxa live on the ‘front-line’ during the spring and may be especially sensitive to changes in snowmelt conditions.