The Mighty Midge
Fossil chironomid from the Andes
CHIRONOMIDAE AS A PALAEO-ECOLOGICAL TOOL
Chironomidae is a family of two-winged flies more commonly referred to as non biting midges. This diverse group of insects have been known for a long time to be sensitive environmental indicators. Early research in the field showed that the trophic status of lakes could be classified according to the characteristic chironomid assemblages found within them (Thienemann, 1922). Furthermore the head capsules of the larvae are well preserved within the sedimentary record. As a result palaeolimnological researchers became increasingly interested in the potential for using Chironomids to track the trophic development of a lake through time by examining the changing assemblages within the accumulated sediments. With geographically close lakes displaying significantly different midge faunas the potential for the insects being used as climatic indicators was dismissed and the following hypothesis became established: Chironomid assemblage composition reflects in-lake variables, e.g. lake depth, pH, dissolved oxygen, trophic status and substrate. However work by Walker and Matthews (1989) demonstrated that temperature was by far the most significant variable in controlling the broad scale distribution and abundance of midge fauna.
Walker and Matthews realised the potential for the non biting midge to be used as a palaeoclimatic indicator from two initial observations. Firstly within the fossil records, as climate began warming following the deglaciation of the northern hemisphere, the relative abundance of taxa associated with cold oligotrophic lakes (Heterotrissocladius) abruptly declined. Secondly they noticed the best analogues for late glacial assemblages were found in modern day arctic and alpine settings. Overall Walker and Matthews concluded that the northern limit of temperate taxon was controlled by cold summer air and/or water temperatures. The southern limit of Arctic species was instead driven by cold oxygenated refugia in the profundal zone of deep, temperate lakes. These temperatures were significant with respect to the insect’s life cycles as many species require critical temperature thresholds to complete pupation and emergence stages.
Since the pioneering work of Walker and Matthews (1989) and others the debate linking Chironomids to temperature has raged. Debate has centred upon what controls chironomid distribution and how suitable, if at all, the insects are in the context of palaeoecological studies. Recently Velle et al. (2010) discussed some key factors which must be considered when working on chironomid based temperature resonstructions.
Below I present some of the debate around the midge-environment-temperature debate; focusing on both midge distribution and identification and the potential of this proxy as a indicator of past environmental and climatic change.
Number crunching in palynology
Simulated pollen counts
HOW DO WE UNDERSTANT PAST VEGETATION CHANGE?
Our understating of vegetation in the past, and how it has changed through time, comes mainly from the examination of macrofossils (e.g. wood and leaves) and microfossils (e.g. pollen and spores) found in the sedimentary record. The potential for microscopic fossils to provide an insight into past vegetation change on a landscape scale was pioneered by von Post (Von Post, 1916, reprinted 1967) and has been subsequently used to understand changes in regional floras (Godwin, 1956), and address conservation issues (Willis et al., 2007). Analysis of fossil pollen and spores (palynology) is now widely used on late Quaternary timescales to answer ecological questions linking vegetation and wider environmental/climatic change; these include:
- Has there been a change in major vegetation type (biome)? For example a change between woodlands and grassland vegetation.
- How have the ecosystem dynamics altered? For example the presence or absence of fire.
- How has the diversity within the ecosystem changed? For example increase or decrease in sample richness.
Palynological analysis relies on obtaining a sub-sample of the pollen contained within the sediment at a specific depth (time) which allows the vegetation at that time to be reconstructed. This sub-sample is known as a pollen count. To build up a picture of vegetation change through time it is necessary to generate a sequence of pollen counts. The size of the sub-sample (pollen count) required from any particular depth (time period) is dependent on the nature of the vegetation association being investigated and the ecological question being addressed . For example, the amount of pollen analysed to determine if the vegetation was predominantly wooded or grassland is different to that required to provide information on the biological diversity within the vegetation assemblage.
Discussed below are some of the conventions related to choosing an appropriate pollen count size within palynology, with particular reference to the challenges of dealing with diverse tropical floras.
PCRG March
In March the Palaeoenvironmental Change Research Group (PCRG) have been involved with data collection in the labs, training, fieldwork planning (and un-planning) and outreach.
Two notable pieces of pollen data collection have made significant progress this month: 1) Hayley has been working at collecting data to establish what is a suitable pollen count size to assess vegetation change within her highly diverse Amazonina samples, and 2) Lottieis on to about the last dozen samples to complete the overview of 500,000 years of pollen from Lake Bosumtwi (Ghana); an amazing pollen record and an excellent research effort which will be the cornerstone of her PhD thesis! More soon on both these pollen stories as they unfold… In addition, I am pleased to report that the list of taxa within our pollen reference collection has finally been fully digitized – Thank you Jason; details of the >3000 taxa collection will soon be available on the lab web pages.
Gigantic Prasinophytes (>100 microns)
Also in the lab: Alice Kennedy, working on ‘deep time’ palaeoecology, has identified a bloom in the foraminifera Reinholdella macfadyeni and gigantic Prasinophytes associate with marine annoxia in sediments from Yorkshire. Will be interesting to find out what these all mean at the next lab meeting!
At the beginning of March four of us (Frazer, Hayley, Lottie and myself) attended a First Aid for field work training course run by Mediact. The course was excellent with plenty of useful information and the opportunity to practice techniques such as cardiopulmonary resuscitation (on dummies) and bandaging (on each other). Unfortunately we will have to wait to practice any of the techniques in the field as our planned trip to Ecuador looks likely to be postponed due to injury to one of our members! Get well soon Frazer 
On the up side this should allow me to catch up with the piles of papers I should be writing.
The month finished with an exciting outreach event. I was asked to present our research to the Oxford Geology Group. The event was hosted at the Oxford University Museum of Natural History. It was an excellent day of talks and it was fun to discuss our research with interested people.
PCRG February
Laboratory activity has continued through February with progress on pollen counts (Lottie and Hayley) and chironomid (non-biting midges) picking (Frazer). Hayley also managed to escape the microscope lab for a short period: 1) to commence work on selecting samples from tephras for Ar-Ar dating, and 2) to counduct loss-on-ignition analysis of organic samples to identify the constituents of her sediment. I did not make it on to the microscope 
I was however very pleased to welcome Macarena Cardenas back into the lab as a visiting Research Fellow. Maca will be working on the pollen reference collection, assisting with PhD student analysis and continuing to write papers during her renewed association.
Frazer, Hayley and I have also begun planning for field work in Ecuador for April-May. We will be working in collaboration with the Instituto Geophisico in Quito and the plan is to visit the Mera site which Hayley is working on, and to collect lake surface samples for Frazer to examine the midges. In preparation for the collection of midges samples expert, and project co-supervisor, Steve Brooks (Natural History Museum) visited for a day to brief us on how best to do this.
Away from research I have been working on writing exam questions and tutor marked assignments for the level 3 module The geological record of environmental change (S369, to those familliar with OU codes!). Hopefully, I have managed to set some interesting and challenging tasks for our students. . .
Relay team palaeo
The 21st The Open University Les Irvine Memorial Relay was held yesterday (29/02/2012) and four teams from the Environment, Earth & Ecosystems department were entered amoung the 40 which took part. Run over four legs and covering a 1.1 mile course at Walton Hall it was another fun event.
For the third year running a team of palaeoecologists took part. This year “Team Palaeo” comprised Hayley Keen, myself, Frazer Bird and Lottie Miller (left to right on photo) and we were the fastest finishing team from the department!
Nitrogen isotopes in lake sediments
COMMENT ON THE USE OF NITROGEN ISOTOPES IN PALAEOLIMOLOGICAL STUDIES
As a component of my doctoral research, I am examining nitrogen (N) isotopes within sediments obtained from Lake Bosumtwi (West Africa). Below I review and comment on the key uses and limitations of using N isotopes to interpret past environmental change with particular reference to lake sediments. Discussion is based on the key text by Talbot (2001).
REFERENCE
Talbot, M.R. 2001. Nitrogen isotopes in palaeolimnology. Tracking environmental change using lake sediments. Volume 2. Physical and geochemical methods (ed. by W.M. Last and J.P. Smol), pp. 401-439. Kluwer Academic Press, Dordrecht.
NOTE: This text is avaliable to Open University students as an ebook via the library
