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Effects of chronic elevated nitrate concentrations on the structure and function of river biofilms
Author(s) -
Koch Ryan,
Kerling David,
O'Brien Jonathan M.
Publication year - 2018
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/fwb.13126
Subject(s) - biofilm , algae , photosynthesis , nutrient , cyanobacteria , extracellular polymeric substance , nitrate , autotroph , environmental chemistry , periphyton , heterotroph , diatom , biomass (ecology) , chlorophyll a , biology , botany , chemistry , ecology , bacteria , genetics
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.