Organisms' response in a chronically polluted lake supports hypothesized link between stress and size

Most of the available support for the hypothesis that pollutants' effects d epend on size comes from shortterm experiments. We appried a size-based app roach to the analysis of the fossil remains of Lago d'Orta in Northern Ital y, a lake that has been polluted with copper, other metals, and acid for mo re than 50 yr. Once this pollution began, the size distribution of diatoms, thecamoebians, and cladocerans shifted to smaller individuals. These chang es in size reflected shifts in the taxonomic composition of the assemblages , but we also observed reduction of body size within a single taxon (the di atom Achnanthes minutissima). These reductions in the average size across t hree communities from different kingdoms and trophic levels provide strong evidence that chronically stressed environments select for smaller organism s.