Effects of chronic elevated nitrate concentrations on the structure and function of river biofilms

Biofilms play an important role in nutrient cycling and retention in streams and rivers, altering transport of nitrate ( NO 3 − ) to downstream ecosystems. However, increased nutrient availability may alter biofilm composition and function. We assessed effects of chronic N pollution on biofilm function by incubating initially thin (i.e., early stage) and well‐developed biofilms in laboratory chambers at four NO 3 − concentrations (0, 0.5, 5 and 25 mg N/L). For initially thin biofilms, elevated NO 3 − resulted in greater photosynthesis, higher chlorophyll contents and greater biomass accrual. Elevated NO 3 − also increased heterotrophic activity by increasing extracellular enzyme activity and heterotrophic respiration. However, well‐developed biofilms had lower growth and were less responsive to added elevated NO 3 − . Uptake kinetics showed a consistent decrease in cell‐specific maximum uptake ( U max ) with elevated NO 3 − treatments. As a result, ambient NO 3 − uptake at experimental NO 3 − concentrations was essentially constant per unit photosynthesis across the range of NO 3 − concentration and uptake was apparently driven by biofilm growth. Across treatments, initially thin biofilms shifted from a diatom‐dominated community to a community dominated by green algae and cyanobacteria. However, well‐developed biofilms were not affected by chronic NO 3 − in terms of biomass or enzyme activities. Our results show that chronic NO 3 − loading alters biofilm growth patterns, physiology, algal community composition and relationship between algae and bacteria. Taken together, these results provide a set of mechanisms to explain the efficiency loss of uptake as a function of stream nitrogen concentration seen in field studies.