Sigleo, A.C., and Frick, W.E. 2001. Estimation of total dissolved nitrate load in natural stream flows using an in-stream monitor. Pages 220-227 in J.A. Ramirez, editor. Proceedings of the 21st Annual American Geophysical Union Hydrology Days, Colorado State University, Fort Collins.
Estuaries respond rapidly to rain events and the nutrients carried by inflowing rivers such that discrete samples at weekly or monthly intervals are inadequate to catch the maxima and minima in nutrient variability. To acquire data with sufficient sampling frequency to realistically model the Yaquina River, Oregon, USA, data were collected hourly for over 18 months using an in-stream monitor. The results indicate that for the Yaquina River dissolved nitrate nitrogen averages 0.70 ▒ 0.4 mg l-1 (50 ÁM ▒ 30 ÁM) throughout the year. The lowest values occur during late summer and low river flow, whereas the higher values occur during winter periods of higher rainfall. During late summer, nitrate, specific conductance, temperature and pH co-vary, suggesting a down-river source for part of the nitrate at that time. During winter, the high influx of low conductivity, fresh water indicates that the source of nitrate is from the surrounding watershed. Nutrient load calculations indicate that the annual nitrate load is directly related to river flow. Thus in high rainfall years, more nitrate is brought into the estuary from the river, relative to low rainfall years.