Effects of atrazine and nicosulfuron on periphytic diatom communities in freshwater outdoor lentic mesocosms

One of the agro-environmental characteristics of Brittany is the extensive use of pesticides, which leads to high concentrations of atrazine in contin ental water systems. Periphytic algae play a basic part in aquatic ecosyste ms and some of them, diatoms, are highly sensitive to environmental changes . Analyses of diatoms can thus indicate the impact of a herbicide on aquati c systems. We compared the effects of two herbicides (atrazine and nicosulf uron) on an aquatic ecosystem in 1998. This study was conducted in 15 ident ical ponds, each containing a holder for installing artificial substrates. The periphytic diatoms were analysed for cell density, biomass, species div ersity and morphology. The general specific diversity was poor, and the dia tom community was essentially dominated by Achnanthidium minutissimum. The first results showed a strong similarity between the different categories o f mesocosms. Chlorophyll a concentration only seems to be affected by high herbicide concentrations.