Rotifers Stimulate The Specific N-No3-Uptake Rate In Lotic Phototrophic Biofilms

Stream biofilms can improve water quality (e.g. excessive nitrogen loads) because their microorganisms such as microphytobenthos and bacteria, are involved in self-depuration processes in the biofilm. In marine sediments, meiofauna can influence primary productivity of phototrophic biofilms, stimulate bacterial growth, and exert a significant role on nitrogen cycling. We hypothesised that in rivers biofilms, increased meiofauna abundance in combination with nutrient enrichment can lead to improve biofilm nutrient uptake ability.River biofilms (from the Garonne River, France) were incubated in flumes supplied with natural river water. The N - NO3- uptake rates of the biofilms as well as the response of microorganisms were studied under different conditions of meiofaunal density and N - NO3- concentration over a 10-day experiment.Rotifers dominated the meiofauna community in all biofilms. Under high nitrate conditions, the N - NO3- biofilm uptake rate was significantly higher in meiofauna-enriched than non-enriched biofilms over the first 2 days. This suggests that under high nitrate concentration conditions, high densities of rotifers enhanced the nitrogen uptake capacity of phototrophic biofilm despite a lower biomass. Densities of rotifers and bacteria in biofilms were positively correlated under N - NO3- enriched conditions suggesting that these two communities interacted over the experimental period. Signature pigment concentrations show that, under low concentrations of N - NO3-, biomasses of diatoms and green microalgae were higher in meiofaunal-enriched than non-enriched biofilms, indicating that rotifer activity favoured biomass accumulation of these algae.This study demonstrates that meiofauna inhabiting biofilms may contribute to the limitation of nitrogen loads in stream water and supports the thesis that the potential interactions between faunal groups and microbial communities of biofilms merits further investigation to improve our understanding of processes which regulate interactions between biofilms and the overlying water in rivers.