Intraregional variability in chironomidinferred t

Authors: S Engels S J P Bohncke O Heiri M Nyman

Publish Date: 2007/10/02

Volume: 40, Issue: 1, Pages: 129-142

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Abstract

Floodplain lakes are rarely analysed for fossil chironomids and usually not incorporated in modern chironomidclimate 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 lossonignition 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 laketypes The variability in chironomid assemblages is also best explained by LOI Application of a chironomidtemperature 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 chironomidbased temperature reconstructionThe Chironomidae Insecta Diptera is a very diverse family of aquatic insects occurring in a multitude of habitats and chironomids are frequently among the most abundant invertebrates found in lakes and rivers eg Armitage 1995 Cranston 1995 Chironomids have been used in palaeoecological studies as a proxy for a range of different environmental variables For instance Gandouin et al 2006 2007 explored the ratio between lotic and lentic chironomid taxa as a tool to qualitatively reconstruct flowregimes in dead branches of two French rivers Rhône and Garonne Other research projects used fossil chironomid assemblages in lake sediments to quantitatively infer past changes in July air temperature eg Heiri and Lotter 2005 Brooks 2006 Walker and Cwynar 2006 salinity eg Heinrichs and Walker 2006 Eggermont et al 2006 or trophic conditions in lakes Lotter et al 1998 Brooks et al 2001Chironomidbased temperature reconstruction has received increasing attention in recent years and a large number of chironomidbased July air temperature records are now available from formerly glaciated regions in Europe and North America eg Walker et al 1991 Brooks and Birks 2001 However lacustrine sediments deposited in floodplain environments such as palaeochannels and oxbow lakes have to our knowledge not previously been used for quantitative temperature reconstruction based on fossil chironomidsFloodplain sediments have great potential for chironomidbased temperature reconstructions They can be found in landscapes beyond the maximum extent of late Quaternary glaciations Gandouin et al 2005 2006 and in time windows for which other lacustrine sediment records are rare For example in the opencast lignite mines in eastern Germany lacustrine sediments dating back to Oxygen Isotope Stage OIS—5a and early OIS3 ca 80 and 55 ka BP respectively have been retrieved eg Mol 1997 Bos et al 2001 Kasse et al 2003 The fossil chironomid and botanical assemblages together with the sedimentological record all suggest that these palaeolakes were situated on a river floodplainModern lakes where highdischarge events of nearby rivers or streams can reach the lake are usually not included in the modern calibration sets used to develop chironomidtemperature inference models because of the complex ecological processes that can be the result of such floods As a consequence lacustrine floodplain sediments potentially present nonanalogue situations in respect to most modern chironomidtemperature transfer functions and it is presently unclear to what extent this influences quantitative temperature reconstructionsStudies on modern lakes have shown that certain lakes and their associated chironomid faunas are sensitive recorders of smallscale climate changes whereas other lakes are less likely to respond to climate variability depending on their relative location eg Velle et al 2005 Heegaard et al 2006 Withinregion discrepancies are also recorded for downcore analysis of chironomidassemblages and their associated inferred temperatures eg Bigler et al 2002 Korhola et al 2002 Velle et al 2005 Presently it is unclear how large intraregional variability of chironomidinferred temperature estimates can be or whether this variability changes between floodplain lakes and lakes that are unaffected by inundationsIn this study we investigate the differences between the physical and chemical properties of lakes on a river floodplain that are annually inundated and lakes that are isolated from such riverine influence The subfossil chironomid assemblages of these lakes are compared and the influence of river inundations on the chironomid concentrations taxon richness and relative chironomid abundances is explored The intraregional variability of chironomidinferred temperature estimates is determined and compared for the two classes of lakes with the aim of assessing whether chironomidtemperature inference models developed for lakes unaffected by riverine influence can be applied to floodplain sediment records

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