Number of replicates per treatment varied from two to five depending on the amount of test sediment available. Control sediments constituted from six replicates. Sediments were conditioned in test vessels for one week after which the conditioning water was removed in order to remove potential ammonia (Moore et al., 1997) and sediment pH was measured. Test water (M7) was added and it was gently aerated with glass Pasteur pipettes for two days prior to start of the test. Evaporated water was compensated with deionised water (Milli-Q®) periodically. The 1st instar (2–3 d old) C. riparius larvae were introduced with glass Pasteur pipettes to the test vessels. Aeration of overlying water was resumed one day after introducing the larvae to the test vessels. Fromthe 12th to the 28th day test vessels were observed daily and emerged adults were counted, gender determined and removed. The larvae were fed with 0.5 ml of ground TetraMin® suspension (10 mg/ml) 3 times per week and after 10 days the amount of food suspension was increased to 1.0 ml. At the end of the test organic matter was eliminated with 10% KOH and sediments were sieved through 250 μm mesh. Head capsules of emerged adults and 4th instar larvae that failed to emerge were collected and mounted to glass microscope slides with Euparal mounting medium. Mentum deformities were inspected under light microscope with 40-fold magnification and deformities were classified either as mechanical damage (broken teeth) or as developmental deformities (extra or missing teeth and gaps). Deformity index (DI, Hämäläinen, 1999) was calculated as follows: