Effects of a diel oxygen cycle on nitrogen transformations and greenhouse gas emissions in a eutrophied subtropical stream

Abstract.

Rivers and streams constitute a major pathway for material transport from land to sea and an important source of greenhouse gases. Short-term variation in stream chemistry due to day-night oxygen (O₂) fluctuations could significantly affect a stream’s capacity to transport nutrients and produce greenhouse gases. Though it is well established that diel cycles are important in controlling O₂ and CO₂ dynamics, the study of diel dynamics of other redox-sensitive elements has been fairly limited. Here we show that a subtropical stream can undergo complete reduction and oxidation sequences in just a few hours. This redox fluctuation has important consequences for downstream transfer of nutrients and for greenhouse gas production. Among these consequences are decreased denitrification rates relative to estimates made using only daytime measurements (17–38% during our observation period), increased downstream nitrogen (N) transfer, a striking change in form of transported N (decreased nitrate and increased ammonium), and decreased estimates of in-stream nitrous oxide (N₂O) flux (36–38% during our observation period). Additionally, methane (CH₄) concentrations doubled and carbon dioxide (CO₂) concentrations rose from 4.6 to 30.7 mg L⁻¹ during the 24-hour period of our observations. Diel shifts like the one we describe have implications for our understanding of nutrient transport and greenhouse gas production in eutrophied streams.