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TRAINING MIDGES: CONTEMPORARY CHIRONOMID-ENVIRONMENT RELATIONSHIPS FROM NW ICELAND
LANGDON, PETER G University of Exeter, UK.
Attempts to reconstruct the Holocene terrestrial climate of Iceland have utilised a range of proxies, although as yet little progress has been made on quantification of this record. Recent research has, however, shown that chironomid communities were sensitive to climate change throughout deglaciation and the early Holocene (Caseldine et al., 2003), and current research is assessing the potential of using them for reconstructing late Holocene climate (Holmes et al. 2004). Although chironomids have been identified as a key proxy for detecting past climate change, it is apparent that they are also influenced by a range of other environmental and limnological variables including food-web interactions as well as allogenic factors. This can be illustrated from the abundance of chironomid transfer functions for use with palaeolimnological data which have recently been developed. Depending on the environmental gradient of interest a range of factors have been shown to influence chironomid communities, notably air and water temperature, hypolimentic dissolved oxygen, total phosphorus, total nitrogen, lake depth, treeline, and salinity.
Much depends on the subset of lakes chosen for environmental training sets as to which environmental variable will best explain the distribution of chironomid communities. In the Arctic, where the majority of lakes are relatively low in productivity, temperature can account for significant variation in chironomid communities (e.g. Larocque et al., 2001), although the organic content of lake sediment, substrate and certain nutrients can also affect chironomid distribution (e.g. Brodersen and Anderson, 2002). Lake type is thus important, coupled with catchment vegetation, sediment supply and food-web dynamics.
In NW Iceland 54 lakes have been sampled for surface sediments using a modified Kajak-gravity corer (Renberg, 1991). Water samples were also taken and a range of geochemical analyses undertaken. The lakes surveyed were mainly in the NW peninsula, but also extended west towards the Snæfellsnes peninsula, including the region around Stykkishólmur, and covered a mean July temperature gradient of 6.5-11.0ºC. NW Iceland was initially chosen due to the relative lack of geothermal activity which could potentially compromise any thermal interpretations of chironomid distribution, although the training set will in the future be extended throughout Iceland increasing the temperature gradient.
A range of geochemical and lithological analyses have been undertaken on the water and sediment samples. The surface sediments from 32 of the lakes have been analysed for chironomid head capsules and some exploratory multivariate numerical techniques undertaken in order to assess the main factors affecting chironomid distribution. The species data comprised of 52 chironomid taxa, although this was based mainly on ‘splitting’ rather than ‘lumping’ taxa. As these data are based on subfossil larval head capsules, rather than the larvae themselves, key taxonomic issues will be discussed. The results from canonical correspondence analysis and two-way indicator species analysis (TWINSPAN) will also be discussed, as well as the potential for this research within the context of Holocene climate reconstruction from Iceland.
Brodersen, K.P. and Anderson, N.J. 2002. Distribution of chironomids (Diptera) in low arctic West Greenland lakes: trophic conditions, temperature and environmental reconstruction. Freshwater Biology 47: 1137-1157.
Caseldine, C.J., Geirsdóttir, Á. and Langdon, P.G. 2003. Efstadalsvatn – a multi-proxy study of a Holocene lacustrine sequence from NW Iceland. Journal of Paleolimnology 30: 55-73.
Holmes, N. 2004. The use of chironomids in reconstructing Holocene palaeoclimate in Northwest Iceland. Arctic Workshop Abstracts, Boulder.
Larocque I., Hall R.I. and Grahn E. 2001. Chironomids as indicators of climate change: a 100-lake training set from a subarctic region of northern Sweden (Lapland). Journal of Paleolimnology 26: 307-322.
Renberg, I. 1991. The HON-Kajak sediment corer. Journal of Paleolimnology 6: 167-170.
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