Chironomid sample preparation, extraction and identification
By: J. Bunbury
Processing sediment for chironomid extraction is a fairly simple process. Much variation between methods exists, and those outlined here have worked well for calcareous and non-calcareous sediment and either surface or core samples (modified after Hofmann, W., 1986. In Berglund (ed.) Handbook of Holocene Palaeoecology and Palaeohydrology, pp. 715-727 and Walker, I. 2002. In Smol, Birks & Last (eds) Tracking Environmental Change Using Lake Sediments, Vol. 4, pp. 43-66).
Identifications are performed at 100 to 400x magnification under bright-field illumination. Standard taxonomic references can be found in our library.
- Subsample sediment in 1 or 2 cm 3 increments. When dealing in forested regions it is good to start with small increments. Often between 50 and 100 head capsules can be picked for 2 cm 3 (sometimes far more ie. 200+). In Arctic regions, you may need up to 10 cm 3 . It is a good idea to start with 1 cm 3 to evaluate how much sediment is required, otherwise you could be picking for awhile!
- Add sample to a 250 ml beaker and add 10% KOH (Potassium hydroxide) up to the 100 ml mark. If you need to rinse a vial or beaker, do so with the KOH, but don't exceed 100 ml. If you still need to rinse more to get all of the sediment, use deionized water.
- Put sample on a hot plate and heat for 30 minutes. Do not bring to a boil, have it just below boiling. Stir the samples frequently.
- Sieve sample using a 90 µm Nitex ® mesh into a beaker. Once picked the sample can be stored in a vial. We usually use a 20ml vial with a screw top cap.
- The sample is transferred to a Bogorov chamber and chironomids are hand-sorted using forceps under a 20x stereomicroscope. Picked head capsules are transferred to a coverslip where they air dry. It is very important to record the number of whole and half chironomids picked per sample. For quantitative analysis you will need at least 50 head capsules (two halves make a whole) (Heiri, O.A.F. & G. Lotter, 2001. Journal of Paleolimnology 26: 343-350 and Quinlan, R. & J.P. Smol, 2001. Journal of Paleolimnology 26:327-342)
- Cover slips are mounted onto slides using Entellan ® .