Effects of a bacterial biofilm on intensive Daphnia culture

In aquatic systems microorganisms tend to adhere to solid-liquid interfaces. The role played by bacteria in the nutrition of zooplankton organisms has been well recognized. The purpose of this experiment was to know if a larger area of microbial film, resulting from the addition of extra walls (fourfold), would increase the production of Daphnia magna. Daphnid cultures were carried out in plexiglass tanks which received identical initial populations and were fed Scenedesmus grown on secondary urban effluent. The effect of the biofilm was further evaluated by including control tanks which were cleaned daily. Results showed that the presence of the biofilm and additional surfaces increased the density, biomass and biomass harvest of D. magna fourfold, while the biofilm had no observable effect on the biochemical composition of the Daphnia. The increased productivity in the tanks with the extra surface area can be explained by both their more extensive biofilm and a ‘wall effect’ acting on the spatial distribution of the organisms. The exact role of microorganisms is difficult to pinpoint; however their effect appears mostly qualitative and seems to be twofold, that is, in Daphnia nutrition and through the detoxification of the culture medium by nitrification. In view of the significant differences obtained at the bench-scale level (13 litres), there is little doubt that increasing the extent of biofilm, by the addition of surfaces in daphnid-rearing tanks, will result in higher productivities in large-scale installations.