TRACE-METAL LEVELS IN WATER, SEDIMENTS AND CHIRONOMUS GR THUMNI, FROM DIFFERENT WATER COURSES IN FLANDERS (BELGIUM)

Fourth instar larvae of the midge, Chironomus gr. thumni were collected at 12 sampling stations, belonging to 8 watercourses of different types. Trace metal levels (cadmium, lead, copper and zinc) were measured in water, sediment and chironomids. Although high metal levels were measured in water and/or sediment at many of the sample sites,no sediment-larvae, nor water-larvae relationship was found, except for cadmium. Only at one sampling site with very high surface water trace metal levels, elevated metal concentrations were measured in the chironomids. Even when sampling sites of the same water quality composition and same substrate are considered independently from the other sites, no relationship was found. The results indicate that metal analysis of insect-larvae is not only invaluable in the quality assessment of surface water, but is even more usefull as a means of monitorring heavy metal contamination in freshwater ecosystems than water or sediment analysis.