Periphyton can contribute to maintaining oligotrophic conditions in restored shallow lakes and reservoirs. In two mesocosm experiments, we evaluated the periphyton responses to enrichment and nutrient dilution in a hypereutrophic reservoir. Effects of the isolated and combined N and P enrichment and dilution levels on periphyton biomass and structure on artificial substrate were investigated. For colonization of periphyton, glass slides were exposed for 14 days (enrichment experiment) and 21 days (dilution experiment) at 20 cm deep. We monitored the effects of N and P enrichment and dilution on the algal biomass, density, and species composition in the periphyton. P was a primary limiting factor for periphyton development. Periphyton algal density and biomass increased in response to combined and isolated P enrichment. Only the 75 % dilution had a positive and significant effect on biomass and algal density in the periphyton. In both mesocosm experiments, the periphyton structure changed, with Nitzschia palea (Kutz.) Smith responding positively to P enrichment and negatively to dilution. Achnanthidium minutissimum Complex becoming dominant following 50 % and 75 % eutrophic water dilution. Our results suggest that major environmental changes are necessary for a heterogeneous algal community structure in the periphyton in hypereutrophic con¬ditions. We conclude that, after a restoration process that reduces P-competition with phytoplankton and shading, periphyton can become an important primary producer in a hypereutrophic reservoir.

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