Periphyton-benthos interaction in acidified freshwaters
Many studies attribute the increased biomass of periphyton at low pH to a decrease in grazing by benthic invertebrates. Herbivorus larvae appear to be the most sensitive to acidification. However this hypothesis bas not received direct experimental verification. The present study analyses the evolution of periphytic biomass and production in the presence and absence of macrobenthos at two pH values, 6,3 ± 0,1 and 4,2 ± 0,2. The study was carried out in semi-natural conditions, using artificial troughs fed by a nearby stream. Unglazed ceramic tiles were placed in transparent plexiglass cylinders. Once acidification had begun, half the cylinders were closed with 160 μm mesh nytex screen. Although this exclusion device was not completely effective, notable differences were observed between the open and closed cylinders in the acidified conditions. After a month of acidification the closed cylinders showed an accumulation of biomass, a decrease in production and a very low P/B ratio; these conditions are characteristic of a reduced grazing pressure according to the Lamberti and Moore model. The evolution of periphytic biomass and production in acidified conditions would thus appear to be determined to some extent by reduced grazing.