Real-time, continuous, in situ water quality sensors were deployed on a fourth-order Iowa (U.S.) stream draining an agricultural watershed to evaluate key in-stream processes affecting concentrations of nitrate during a 24-day late summer (Aug-Sep) period. Overall, nitrate-nitrogen (NO3-N) concentrations declined 0.11 mg L-1 km-1, or about 1.9% km-1 and 35% in total across 18 km. We also calculated stream metabolic rates using in situ dissolved oxygen data and determined stream biotic N demand to be 108-117 mg m-2 day-1. From this, we estimate that 11% of the NO3-N concentration decline measured between two in-situ sensors separated by 2 km was a result of biotic NO3-N demand, while groundwater NO3-N data and estimates of groundwater flow contributions indicate that dilution was responsible for 53%. Because the concentration decline extends linearly across the entire 18 km of stream length, these processes seem consistent throughout the basin downstream of the most upstream sensor site. The nitrate-dissolved oxygen relationship between the two sites separated by 2 km, calculations of biotic NO3-N demand, and diurnal variations in NO3-N concentration all indicate that denitrification by anaerobes is removing less NO3-N than that assimilated by aquatic organisms unable to fix nitrogen for their life processes, and thus the large majority of the NO3-N entering this stream is not retained or removed, but rather transported downstream.
Keywords: Assimilation; Biotic N demand; Denitrification; In-situ sensor; Nitrate nitrogen.