Biofilm growth and nitrogen uptake responses to increases in nitrate and ammonium availability.

Nitrate (NO) and ammonium (NH) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO:NH ratio. However, few studies have examined biofilm responses to enrichment of both DIN species. We examined biofilm responses to variation in ambient concentrations and enrichments in either NO or NH. We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO and +NH) in five streams. Biomass-specific uptake rates ( U) of NO and NH were then measured using in situ additions of N-labeled NO and NH. Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U-NO of biofilms in DIN-free treatments varied among the streams in which the NDS had been incubated. Higher ambient DIN concentrations were only correlated with enhanced biofilm growth rates. U-NO was one order of magnitude greater and more variable than U-NH, however similar relative preference index (RPI) suggested that biofilms did not show a clear preference for either DIN species. Biofilm growth and DIN uptake in DIN-amended NDS (i.e., +NO and +NH) were consistently lower than in DIN-free NDS (i.e., control). Lower values in controls with respect to amended NDS were consistently more pronounced for algal accrual rates and U-NO and for the +NH than for the +NO treatments. In particular, enrichment with NH reduced biofilm U-NO uptake, which has important implications for N cycling in high NH streams. [ABSTRACT FROM AUTHOR]