Impacts of Cd and Zn on the development of periphytic diatom communities in artificial streams located along a river pollution gradient.

Many field and experimental studies have been conducted to reveal modifications of periphytic diatom communities under metal pollution, but most of the approaches presented drawbacks: variability of environmental factors for field studies and lack of representativity for experimental ones. An original approach that allows growth of periphytic communities under conditions more controlled than in the field and more realistic than experimental investigations was developed to try conclusions being drawn on the real impact of metallic pollution. Artificial streams were placed on the river bank at three sites along the pollution gradient (Cd/Zn) of the River Lot (SW France): reference (upstream pollution discharge), slightly polluted (around 1.5 microg Cd x L(-1) and 50 microg Zn x L(-1)), and polluted (around 6 microg Cd x L(-1) and 400 microg Zn x L(-1)) sites. Structural characteristics of diatom communities growing in these artificial streams, on glass substrates, and under homogenized current velocity and water depth were compared after four durations (1, 2, 3, and 4 weeks) to study effects of metal on first steps of community development. Marked perturbations of diatom community development were observed overall in the polluted site: after 1 week, early biofilms differed already by taxonomic composition; after 4 weeks, cell density was significantly lower than in the reference site and taxonomic composition was very different. These results, generated by a methodology intermediate between field and experimental approaches, compare well with literature findings based on more traditional approaches, strongly attesting that metallic pollution affects diatom community architecture and induce rising of abnormal cells.