Acute changes in macronutrient stoichiometry alter nitrate uptake in benthic biofilms

Abstract. Benthic biofilms, located at the sediment-water interface, are hot-spots for macronutrient cycling in headwater streams. Here, the supply of dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P), affects nutrient cycling processes such as nitrate uptake. Flushing events can add short-term pulses of DOC, N and P to streams, changing the macronutrient ratios in the stream water, altering stoichiometric imbalances between water and microbial macronutrient ratios. However, there is little information on whether these short-term changes in macronutrient imbalances can alter biofilm nitrate uptake. To better understand how acute changes to DOC, N, and P stoichiometric imbalances affect nitrate uptake, we sampled stream biofilms from four different sites in Florida and incubated them in the lab in mesocosms after changing their macronutrient ratios by adding DOC and/or nitrate. Here we show that biofilms from anthropogenically less impacted streams with less N excess increased their nitrate uptake after 48 h of incubation in different macronutrient stoichiometric ratios, but biofilm structure remained mainly unaffected. Furthermore, nitrate uptake was positively related to biofilm metabolism, differentiating in sites with more autotrophic- or more heterotrophic-driven nitrate uptake. Our study reveals that acute changes in macronutrient stoichiometric imbalance between stream water and biofilm microorganisms changes nitrate uptake. This needs to be considered when assessing short-term nitrate uptake capacity of stream reaches.