Gaute Velle

Midges as quantitative temperature indicator species: lessons for palaeoecology.

Calibration data sets give a unique opportunity to establish patterns of biological existence and their statistical associations with environmental variables. By use of calibration data sets, environmental variables can be inferred quantitatively. The resulting long time-series may assist in distinguishing natural environmental variability from human-induced variability, both in terms of climate change and biotic turnover. However, the validity of the palaeoenvironmental reconstructions depends on their accuracy, precision and sensibility. Before performing palaeoenvironmental inferences, key mechanisms controlling contemporary species’ distribution, abundances and dynamics should be identified and understood. An inference model is developed to produce reconstructions. A major challenge lies in validating and interpreting the reconstructions. Calibration data sets involving midges (Diptera: Chironomidae) suggest that climate has a broad-scale, regional control over midge existence and abundance, often over-riding the influence of local within-lake variables. In recent years, the use of midges as quantitative indicators of past temperatures has greatly expanded. As the number of reconstructions increase, especially in Fennoscandia and North America, it seems the among-site variability is so large that it is unlikely to be due only to local differences in climate. Hence, we question whether the long climate gradients in calibration data sets can accurately be used to calibrate local variables, when most local gradients in time and space are short. Ten Holocene chironomid-inferred temperature curves from Fennoscandia are compared. We illustrate some general principles in palaeoecology by identifying factors that may cause bias. Especially, we consider how calibration data sets simplify the complexity of the real world by maximizing single ecological gradients and by not taking into account co-varying variables. We give some recommendations and criteria that chironomid analysis should meet in order to improve the reliability of the temperature inferences. Finally, we discuss how the complex interactions between species and environment may have implications when we aim at predicting future biodiversity.

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.

Effects of within-lake variability of fossil assemblages on quantitative chironomid-inferred temperature reconstruction

Abstract We studied the effects of within-lake variation of subfossil chironomid assemblages on chironomid-inferred temperatures in five lakes in southern Norway. In each lake six replicate surface-sediment samples in the deepest part of the lake basin and two transects of seven samples each towards the littoral were analysed for subfossil chironomid assemblages. A quantitative chironomid–July air temperature inference model applied to the subfossil assemblages inferred temperatures with a standard deviation of 0.35–0.54°C for the six mid-lake samples and of 0.38–0.59°C for all 20 sediment samples. This variation seems high when compared with the cross-validated root mean square error of prediction (RMSEP) of the model of 1.04°C. However, we show that the variability of the replicate mid-lake samples can only account for a maximum of 15% of the model prediction error if the RMSEP is decomposed into a component due to uncertainties associated with sampling the chironomid assemblage of a lake and into a residual error component. Thus, according to our results analysing multiple samples in the model lakes to obtain better estimates of their subfossil assemblage composition may only slightly reduce the model prediction error. It was difficult to detect a clear pattern of chironomid-inferred temperatures with respect to water depth in any single lake due to the high variability of inferred values. However, a comparison of all five study lakes revealed that, relative to the mid-lake samples, cooler temperatures were inferred at intermediate depths and warmer temperatures in the shallowest parts of the lakes, although two shallow-water samples with very low inferred temperatures were exceptions in this respect. This depth-related bias of inferred temperatures in our relatively shallow study lakes indicates that chironomid–temperature models calibrated on mid-lake samples should only be applied with caution to near-shore sediments and that a systematic offset in the inferred temperatures may occur.

Minimizing the edge-effect in environmental reconstructions by trimming the calibration set: Chironomid-inferred temperatures from Spitsbergen:

A core from Lake Skardtjørna on western Spitsbergen was analysed for subfossil chironomids with the aim of inferring past temperatures. The core spans the last 1760 years and has a low concentration and low diversity of chironomids. Three taxa dominate the record: Oliveridia tricornis, Micropsectra radialistype, and M. insignilobus-type, the latter not previously recorded on Svalbard. Compared with the full Norwegian modern climate—chironomid calibration data set, Skardtjørna is at the coldest end of the modern temperature gradient. In an attempt to decrease potential bias caused by the numerical edge-effect and to improve the taxon response functions, the training set was trimmed by excluding lakes dissimilar to the study site. Two trimming approaches are tested. In the first, lakes at the warm end of the gradient are excluded. In the second, the calibration data-set lakes are partitioned by TWINSPAN on the basis of their modern chironomid assemblages, and lakes most dissimilar to Skardtjørna are excluded. According to the training-set performance diagnostics, a WA-PLS inference model excluding lakes >9°C has the most promising results (component 1, R 2 = 0.85, maximum bias = 0.58°C, RMSEP= 0.73°C). The performance diagnostics based on the trimmed inference model out-performs the performance diagnostics from the full model, and the reconstructed temperatures indicate different trends. According to the <9°C inference model, the long-term temperature trend has been decreasing, with cold periods 1630—1450, 1150—1050, and around 250 cal. yr BP. Warm periods occurred between 1760 and 1650, 1420 and 1180, and 1000 and 830 cal. yr BP. Temperatures have been increasing during the last 250 years.

Inconsistent results should not be overlooked: A reply to Brooks et al. (2012)

Empirical data in the form of many chironomid-based temperature reconstructions give an excellent opportunity to assess the chironomid approach to temperature reconstruction by testing its reproducibility. Brooks et al. (The Holocene 22(12) 2012 (this issue)) offer a critique of points discussed in Velle et al. (The Holocene 20 (2010) 989–1002), but fail to explain the poor reproducibility found when Holocene chironomid-based temperature reconstructions are compared. We discuss the issues raised by Brooks et al. (2012) and cite studies that demonstrate the complexity involved. We are grateful to Brooks et al. (2012) for contributing to the discussion. However, they overly rely uncritically on transfer functions and the resulting reconstructions as representatives of true patterns in nature. A major source of bias involved when chironomids are used as a palaeoenvironmental proxy is the response to confounding gradients. Many of the challenges discussed in the Forum Article, in the comment, and in the reply are also valid for other research fields within palaeoecology. The challenges should still be properly addressed in chironomid research.

Testing intra-site transfer functions: an example using chironomids and water depth

Most calibration data sets used to infer past environmental conditions from biological proxies are derived from many sites. An alternative strategy is to derive the calibration data set from within a single site. Transfer functions derived from such intra-site calibration data sets are usually applied to fossil assemblages from the focal lake, but a recent development has been to apply these transfer functions to other sites. Transfer functions derived from intra-site calibration data sets can have impressive cross-validation performance, but that gives little indication of their performance when applied to other sites. Here, we develop transfer functions for lake depth from intra-lake chironomid calibration data sets in Norway and Alaska and test the resulting models by cross-validation and against known depth in external lakes. Lake depth is a statistically significant predictor of chironomid assemblages at all these lakes, and most intra-lake transfer functions perform reasonably well under cross-validation, but their performance against external data is erratic. Downcore reconstructions from transfer functions developed on different lakes are dissimilar. Ignoring the poorly performing transfer functions, only 3 of 14 downcore reconstructions are statistically significant. Few assemblages downcore had good modern analogues in the calibration data set, even when the core was from the same lake as the calibration data set. We conclude that intra-site calibration data sets can find site-specific rather than general relationships between species and the environment and thus should be applied with care and to external sites only after careful and critical validation.

Late-Holocene temperature and precipitation changes in Vindelfjällen, mid-western Swedish Lapland, inferred from chironomid and geochemical data

In this article, we present results from a palaeolimnological study from Lake Vuoksjávrátje in the mountain tundra region in the Vindelfjällen Mountains, northwest Sweden. We suggest that the influence of precipitation may be one of the factors causing discrepancies between chironomid-based late-Holocene July temperature (JulyT) reconstructions from Fennoscandia. We combine quantitative temperature reconstruction using chironomids for the last 5100 years with qualitative analysis of chironomid composition and geochemical analyses, such as x-ray fluorescence (XRF), total organic carbon (TOC) and C/N analysis. The studied sequence is dated by 210Pb, 137Cs and 11 14C datings from terrestrial macrofossils. The aim of the study was to use chironomids to reconstruct late-Holocene summer temperature variation on a multi-centennial to centennial timescale and to use geochemical data to identify periods during which the changes in chironomid composition might have been forced by environmental variables other than temperature, such as within lake processes or precipitation. Based on ordination techniques, and a comparison between chironomid-inferred JulyTs and changes in minerogenic sedimentation with regional temperature and wetness records, it is concluded that the JulyT signal was modulated by precipitation. The proxies indicate that both JulyT and annual precipitation have influenced the chironomid communities in Lake Vuoksjávrátje, and that catchment-related processes caused by enhanced precipitation have overridden the summer temperature signal between 3000 and 2200 cal. yr BP, and between 1050 and 100 cal. yr BP.

Microhabitat influence on chironomid community structure and stable isotope signatures in West Greenland lakes

Most functional feeding types are represented within the species rich group of aquatic chironomids. Thus, we hypothesized that different lake types and microhabitats within lakes would (1) host specific chironomid communities and (2) that the individual communities would show specific δ13C stable isotope signatures reflecting the prevailing origin of food source. To test our hypotheses, five lakes in southwest Greenland were investigated at a high taxonomic resolution and with detailed information on δ13C signature of the chironomids and of individual microhabitats (macrophytes, sediment, stones, and profundal). We found that there was a significant difference in δ13C between the chironomid assemblages of freshwater lakes and oligosaline lakes, while assemblages of the littoral microhabitats did not differ significantly. The δ13C of chironomids reflected the wide variety of habitat signals, particularly in the freshwater lakes. Our results indicate that many chironomid taxa are ubiquitous and are found in several microhabitats, suggesting that they can adjust their feeding strategy according to the habitat. The implication is that chironomid assemblage composition has only limited use as indicator of littoral microhabitats in the Arctic. On the other hand, the δ13C signature of fossil chironomids might have a potential as indicator of microhabitats in freshwater lakes.

Where the salmon roam: fjord habitat use of adult Atlantic salmon

Where the salmon roam: fjord habitat use of adult Atlantic salmon Shad K. Mahlum*, Knut W. Vollset, Bjørn T. Barlaup, Gaute Velle, and Tore Wiers Department of Biological Sciences, University of Bergen, Thormøhlensgate 53 A & B, 5006 Bergen, Norway Uni Research Environment, LFI – Freshwater Biology, Nygardsporten 112, 5006 Bergen, Norway *Corresponding author: tel: þ47 94114705; e-mail: shadmahlum@gmail.com