Recovery potential of periphytic communities in a river impacted by a vineyard watershed

Vineyard areas are important causes of water contamination, especially by pesticides and residues. These compounds can markedly disturb aquatic communities particularly photosynthetic organisms that are targeted by herbicides. Biofilms and diatoms were used as bioindicators for quality assessment in the Morcille watershed, an area impacted by Beaujolais vineyards (SE France), during the pesticide spreading period (April–May 2008). Biofilms were allowed to settle on glass slides for 4 or 8 weeks at three sites along a 7-km long gradient of trophic (mainly orthophosphate) and pesticide pollution. After a 4-week colonization, samples from the two contaminated downstream sites were transferred upstream to the clean site for 4 weeks while others were left in the same place.In vivo fluorescence measurements indicated that the periphytic communities were dominated by diatoms. Going downstream, biofilm biomass and diatom species richness decreased; normalized diatom indices (including the French standard BDI) expressed the increase in trophic status quite well. The species composition of the assemblages was used to discriminate between the effects of nutrients and toxicants, which increased simultaneously as the river continued downstream.The way in which the biofilm samples transferred upstream recovered was quite different depending on the location of the original site in the contamination gradient. Most of the quantitative descriptors reached a level comparable to that of the reference communities, but the diatom assemblages (cell density per surface unit, taxonomic composition) varied between dates and along the gradient. These communities did not entirely recover a reference structure but the increase in diversity, associated with the settlement of sensitive species, suggested an elevated recovery potential.