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What can midges tell us about climate change? Ted Nield at the BA finds out more than he ever thought possible about the flies that bite.

Geoscientist Online 8 September 2008

Chironomidae are a family of two-winged flies (Diptera) that are closely related to mosquitoes and biting midges. However, unlike their cousins, who can be a scourge for any one taking a holiday in Scotland or elsewhere in the north of Europe, adult chironomids do not require a blood meal.

Britain boasts about 600 species of chironomids, and there are about 10,000 world-wide. The larvae of most species are aquatic, and abundant in lakes and rivers. The familiar 'blood-worms', which can occur in huge numbers in nutrient-enriched waters (because of their ability to tolerate low oxygen concentrations) are the larvae of one of the subfamilies of Chironomidae. Although they have a worm-like appearance and wriggle when they swim, they are in fact insects. A full-grown blood worm may reach 2cm long - but many species never reach more than 5mm.

Steve Brooks, an entomologist working at the Natural Histry Museum in London, told the BA: “Chironomidae are sensitive indicators of environmental change and recently have become pre-eminent in reconstructing past climate change.”

This is because the heads of chironomid larvae are well-preserved in lake sediments and can be used to identify the midge species. By taking a sediment core from a lake and sieving out the heads, which may be less than one tenth of a millimetre in size, changes in chironomid assemblages through time can be analysed and related to climatic change.

“We know from the modern distribution of chironomids that some species are typical of warm climates whereas others only occur in cold regions” said Brooks. “We can estimate the summer air temperature optimum of European chironomid species and use this information to reconstruct past summer temperatures from fossil chironomid assemblages in our lake sediment cores. By applying the chironomid-temperature inference model to modern species assemblages (collected from the surface sediments of lakes where we know the present-day mean July air temperature) we know that our 'chironomid thermometer' has an accuracy of plus or minus one degree Celsius! We also know that the chironomid larvae have developed in the lakes that are sampled so they must be responding to local climatic conditions.”

This is a distinct advantage over pollen as a climate indicator, because pollen may have been blown over long distances before it was deposited. Because chironomids are so abundant in lakes, only a few grams of mud need to be sampled to obtain enough chironomid heads for a robust temperature reconstruction. This means that the lake sediments can be sliced at fine intervals (1 cm or less) which often equates to one sample every 10 years or fewer, and provides scientists with detailed estimates of the size and rapidity of climate change in the past.

Says Brooks: “Beetles are also frequently used as climate indicators, but they are relatively rare in sediments and so kilograms of sediment are required for analysis. This means that studies at high temporal resolution are not usually possible and short-term climate oscillations may not be detected. Adult chironomids are weak fliers but they are carried by the wind so they can travel hundreds of kilometres very short amount of time and so they respond very quickly to abrupt climate change - much more quickly than trees can, for example, which may take hundreds of years to migrate across the landscape!”

Context

The quantitative evidence that chironomids provide about past climate helps to put modern climate change in context. For example, is the scale of modern global warming unprecedented, or have natural fluctuations of this kind occurred before? These chironomid-inferred temperature estimates can be used to test the reliability of global climate models by seeing how well the models perform in reconstructing past climate. Reconstructions of past climate also help provide scientists with a better understanding of climate dynamics, global climate teleconnections, feedback mechanisms and tipping points.

Brooks told the BA: “With Professor John Birks from the University of Bergen, Norway, I have developed a chironomid-mean July air temperature inference model that has been applied widely to fossil chironomid assemblages from throughout northern Europe covering the last 15,000 years - the period since the last ice age, known as the late-glacial and Holocene. We already know something of the rapidity and relative magnitude of climate change during this period from oxygen isotope analysis of the ice cores collected from Greenland. However, it can be problematic to convert these oxygen isotope records into actual temperature estimates. Also, we cannot be sure how representative the Greenland record is of the past European climate. This is where the midge-inferred temperatures from European lakes are useful.”

Chironomid-inferred summer air temperatures from lakes in northern Europe show broad similarities to the temperature trends found in Greenland ice cores. All these records show that there was rapid warming towards the end of the last ice age, which reached a peak in the early Interstadial after about 15,000 years ago. Temperatures then began a general downward decline, punctuated by two sudden short-lived cold oscillations about 13,800 and 13,000years ago. Then about 12,800 years ago ice age conditions returned to northern Europe for a period of about 1000 years. This episode is known as the Younger Dryas because of the return of tundra vegetation including the plant Dryas (the Mountain Aven).

Dryas

It is thought to have been caused by a shutdown of the Thermohaline Circulation in the north Atlantic (the Gulf Stream) due to the release of huge quantities of cold freshwater from the melting Scandinavian ice cap. The Younger Dryas came to an equally abrupt end when the Gulf Stream returned, marking start of the Holocene and the period of human expansion. Chironomid-inferred temperature estimates indicate that summer temperatures fell by about five degrees Celsius over a period of about a decade at the start of the Younger Dryas and increased at a similar rate and magnitude at the end of this period.

Chironomid-inferred temperatures for the first and second cold oscillations during the Interstadial are about 1.5 and 0.8 degrees Celsius respectively. There are subtle but consistent differences between the oxygen isotope records from Greenland and chironomid-inferred temperatures in northern Europe which suggest that the Greenland ice core record cannot be assumed to indicate late-glacial temperature trends for the whole region. North European chironomid-inferred temperature records indicate that summer temperatures declined much less strongly during the Interstadial than those inferred from the Greenland ice cores. Also, it appears that the first Interstadial cold oscillation was cooler than the second - which is the reverse of what is indicated in the Greenland ice core records.

The consistency of north European chironomid-inferred temperature reconstructions is well-illustrated by a recent study carried out by researchers at the Natural History Museum and the Universities of Liverpool and Exeter on five lakes in the English Lake District. At all five sites, similar summer temperature trends and estimates were inferred by chironomid analysis, and these estimates also agreed closely with climate inferences derived from oxygen isotope analysis of the carbonate sediments in the lakes that were studied. While showing remarkable consistency between each other and other northern European estimates there were consistent differences in details between these records and the oxygen isotope records from Greenland.

Recently, chironomid-inferred summer air temperatures have been used to infer, for the first time, precipitation gradients in Scotland at the end of the Younger Dryas. The equilibrium-line altitude of glaciers is determined by the balance between the amount of winter precipitation and the summer temperature. If the summer temperatures and the maximum extent of the Younger Dryas glacial moraines are known, then the winter precipitation can be calculated.

Chironomid assemblages have recently been analysed from sediment cores taken from a lake in the Cairngorms and a lake on the Isle of Skye and temperatures for the late-glacial were estimated. The equilibrium line altitudes of Younger Dryas terminal moraines from nearby sites were calculated and these data were used to estimate winter precipitation. This study showed that winter precipitation was similar to modern values in the eastern Highlands but about 400mm per year higher in the Western Isles of Scotland at the end of the Younger Dryas than today and that the west-east precipitation gradient was also greater at that time than today.

Chironomid analysis has also been used recently for the first time in Europe to estimate temperatures from periods before the late-glacial. Until now temperature estimates from these earlier periods were only available from beetle analysis. A recent study of midges from a lake sampled in northern Finland, which contained sediments dating to the height of last ice age about 50,000 years ago, revealed that summer temperatures at that time were about the same as they are today.

Despite this, the surrounding vegetation was typical of arctic tundra, even though it would have been warm enough to support birch and pine forest. Presumably the warm period did not last long enough for trees to migrate into the area from further south. These results indicate that northern Scandinavia was not continually covered in ice during the last ice age but that the ice sheet fluctuated in response to climatic oscillations.

Future midge research

Brooks told the BA that past climate changes in Russia are relatively poorly known at present, and no chironomid-inferred temperature estimates are currently available. To address this, a new project is currently underway to develop a chironomid-temperature inference model for arctic Russia. Current modern chironomid distributional data has been assembled from over 100 lakes from the north Urals, the Putoran Plateau, Yakutia and the Lena River delta, and the relationship with summer temperature has been used to create a chironomid-temperature inference model for the region.

The model has been used to reconstruct summer temperatures during the late Holocene from three lakes on the Putoran Plateau. The study has confirmed the results of climate models, which suggest that there has been little increase in summer temperatures during this period. However, recent significant changes in the chironomid assemblages suggest a response to increased lake productivity - which may be a result of spring ice melt occurring earlier in the year.

Conclusion

Analysis of chironomid midges preserved in lake sediments can produce low error summer temperature estimates at high temporal resolution over tens of thousands of years, and are one of the best indicators of past climate change.

Comparison of midge-inferred temperature trends with those from the Greenland ice cores show that, although there are similarities in the records, the ice core record does not provide an accurate summary of climate change in northern Europe. These midge-inferred temperature estimates, when used in combination with evidence from glaciers, can also provide information on past changes in rainfall.

Analysis of midges in arctic Russia has confirmed evidence that the high artic has already been seriously impacted by modern climate change. Midge-inferred temperature estimates from ice age lake sediments has revealed that even in northern Finland there were periods when temperatures were similar today and the ice retreated.

Midge-inferred temperature estimates, when used in combination with evidence from glaciers, can also provide information on past changes in rainfall. Analysis of midges in arctic Russia has confirmed evidence that the high artic has already been seriously impacted by modern climate change. Midge-inferred temperature estimates from ice age lake sediments has revealed that even in northern Finland there were periods when temperatures were similar today and the ice retreated.

Suggested further reading

• Brooks, S.J., Langdon, P.G. and Heiri, O. 2007. The identification and use of Palaearctic Chironomidae larvae in palaeoecology. Quaternary Research Association Technical Guide no. 10. 276 pp.
• Marshall, J.D., Lang, B., Crowley, S.F., Weedon, G.P., van Calsteren, P., Fisher, E.H., Holme, R., Holmes, J.A., Jones, R.T., Bedford, A., Brooks, S.J., Bloemendal, J., Kiriakoulakis, and K., Ball, J.D. 2007. Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK. Geology 35: 639-642.
• Helmens, K.F., Bos, J.A.A., Engels, S., Van Meerbeeck, C.J., Bohncke, S.J.P., Renssen, H., Heiri, O., Brooks, S.J., Seppä ˆ., Birks, H.J.B. & Wohlfarth, B. 2007. Present-day temperatures in northern Scandinavia during the Last Glaciation. Geology 35: 987-990.
• Brooks, S.J. 2006. Fossil midges as palaeoclimatic indicators of the Eurasian region. Quaternary Science Reviews 25: 1894-1910
• Antonsson, K., Brooks, S., Seppä ˆ., Telford, R.J. and Birks, H.J.B. 2006 Quantitative palaeotemperature records inferred from fossil pollen and chironomid assemblages from Lake Gilltjä²®en, northern central Sweden. Journal of Quaternary Science 21: 831-841.
• Smol, J.P., Wolfe, A.P., Birks, H.J.B., Douglas, M.S.V., Jones, V.J., Korhola, A., Pienitz., R., R., K., Sorvari, S., Antoniades, D., Brooks, S.J., Fallu, M.A., Hughes, M., Keatly, B.E., Laing, T.E., Michelutti, N., Nazarova, L., Nyman, M, Paterson, A., Perren, B., Quinlan, R., Rautio, M., Saulnier-Talbot, E., Siitonen, S.I., Solovieva, N., & Weckstrom. 2005. Climate-driven regime shifts in the biological communities of arctic lakes. Proceedings of the National Academy of Sciences 102: 4397-4402.
• Velle, G., Brooks, S.J., Birks, H.J.B, & Willassen, E. 2005. Chironomids as a tool for inferring Holocene climate: an assessment based on six sites in southern Scandinavia. Quaternary Science Reviews 24: 1429-1462.
• Dalton, C., Birks, H.J.B., Brooks, S.J., Cameron, N.G., Evershed, R.P., Peglar, S.M., Scott, J.A. and Thompson, R. 2005. A multiproxy study of lake-development in response to catchment changes during the Holocene at Lochnagar, north-east Scotland. Palaeogeography, Palaeoclimatology, Palaeoecology 221: 175-201.
• Brooks, S.J. & Birks, H.J.B. 2004. The dynamics of Chironomidae populations in response to environmental change during the past 300 years in Spitsbergen. Journal of Paleolimnology 31: 483-498.
• Bedford, A., Jones, R.T., Lang, B & Brooks, S.J. 2004. A Late-glacial chironomid record from Hawes Water, N.W. England. Journal of Quaternary Science 19: 281-290.
• Brooks, S.J. & Birks, H.J.B. 2001. Chironomid-inferred air temperatures from late-glacial and Holocene sites in north-west Europe: progress and problems. Quaternary Science Reviews 20: 1723-1741.
• Benn, D.I. & Ballantyne, C.K. 2005. Palaeoclimatic reconstruction from Loch Lomond Readvance glaciers in the West Drumochter Hills, Scotland. Journal of Quaternary Science 20: 1-16.
• Coope, G.R. 2006. Insect faunas associated with Palaeolithic industries from five sites of pre-Anglian age in central England. Quaternary Science Reviews 25: 1738-1754.
• Engels, S., Bohncke, S.J.P., Bos, J.A.A., Brooks, S.J., Heiri, O. and Helmens, K.F. 2008. Chironomid-based palaeotemperature estimates for northeast Finland during the early Middle Weichselian. Journal of Paleolimnology 40: 49-61.