The response of organisms in a chronically polluted lake supports the hypothesized link between stress and size

Most of the available support for the hypothesis that pollutants' effects depend on size comes from shortterm experiments. We appried a size-based approach to the analysis of the fossil remains of Lago d'Orta in Northern Italy, a lake that has been polluted with copper, other metals, and acid for more than 50 yr. Once this pollution began, the size distribution of diatoms, thecamoebians, and cladocerans shifted to smaller individuals. These changes in size reflected shifts in the taxonomic composition of the assemblages, but we also observed reduction of body size within a single taxon (the diatom Achnanthes minutissima). These reductions in the average size across three communities from different kingdoms and trophic levels provide strong evidence that chronically stressed environments select for smaller organisms.