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

Nitrate (NO3 (-)) and ammonium (NH4 (+)) are the two major dissolved inorganic nitrogen (DIN) species available in streams. Human activities increase stream DIN concentrations and modify the NO3 (-):NH4 (+) 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 NO3 (-) or NH4 (+). We incubated nutrient diffusing substrata (NDS) bioassays with three treatments (DIN-free, +NO3 (-) and +NH4 (+)) in five streams. Biomass-specific uptake rates (U (spec) ) of NO3 (-) and NH4 (+) were then measured using in situ additions of N-15-labeled NO3 (-) and NH4 (+). Biomass (estimated from changes in carbon content) and algal accrual rates, as well as U (spec) -NO3 (-) 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 (spec) -NO3 (-) was one order of magnitude greater and more variable than U (spec) -NH4 (+), 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., +NO3 (-) and +NH4 (+)) 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 (spec) -NO3 (-) and for the +NH4 (+) than for the +NO3 (-) treatments. In particular, enrichment with NH4 (+) reduced biofilm U (spec) -NO3 (-) uptake, which has important implications for N cycling in high NH4 (+) streams.