Fluctuating life-history parameters indicating temporal variability in metal adaptation in riverine chironomids

Adaptation to toxicants in animal populations is influenced primarily by tw o counteracting forces. First, the intensity and duration of peak concentra tions of toxicants is responsible for the actual level of selection pressur e on the population. Second, the process of adaptation can be disrupted by gene flow as a result of crossings with nontolerant individuals. These coun teracting forces were analyzed in riverine insects in which we expected tha t the level of metal adaptation is subject of considerable fluctuations, du e to variable dilution of metals and a variable transport of nontolerant in dividuals in river water. To this purpose, the stability of metal adaptatio n in different Chironomus riparius populations was analyzed during a 5-mont h period in a heavily polluted lowland river. This was examined by measurin g mortality, larval dry weight, smd accumulation of zinc under laboratory c onditions. The results showed that in midge populations originating from me tal-contaminated field sites several life-history parameters (like control mortality and growth response under cadmium exposure) of the laboratory rea red F1 generations showed considerable temporal variation. In addition, the presence of metal-adapted midge populations was indicated on several occas ions on the metal-exposed field sites. Reference populations on the other h and, showed stable life history patterns throughout the sampling period, an d no signs of metal adaptation were found. These observations showed that t he actual level of metal adaptation varies considerably, both in time and s pace. Adaptation to metals in riverine chironomids, therefore, should be lo oked on as a highly dynamic process.