Because most laboratory experiments on the ecological effects of chemical pollutants on aquatic species have focussed on single-generation impacts, the longer term consequences of contamination are much less well understood. German researchers reared four populations of midges (Chironomus riparius) in the lab over 12 generations. Two populations of midge larvae were exposed to sediment contaminated with the pesticide tributyltin, while the other two populations were experimental controls. In each generation, a range of life history parameters, including larval mortality and egg hatchability, were measured, and genetic diversity was assessed using microsatellite markers (variable repeating sequences of DNA). Relative to the controls, the pollutant-exposed midges had lower reproductive output, delayed larval development, higher larval mortality and lower genetic diversity. There were no time trends in any of the life history traits, which suggested that there had been no selection for stress-tolerant midges. However, the lower rates of survival and fertility in the exposed groups reduced the effective population size, the number of individuals contributing to the next generation, and thus genetic variation. The results have conservation implications because they suggest that small, isolated populations are particularly vulnerable to the rapid erosion of genetic diversity as a result of chemical exposure.
Reference: Nowak, C., Vogt, C., Pfenninger, M., Schwenk, K., Oehlmann, J., Streit, B. & Oetken, M. 2009. Rapid genetic erosion in pollutant-exposed experimental chironomid populations. Environmental Pollution 157, 881–886.
Freshwater Research News 
Posted on June 26, 2010