NUTRIENT DYNAMICS IN A HEADWATER STREAM: USE OF CONTINUOUS WATER QUALITY SENSORS TO EXAMINE DIURNAL, EVENT, AND SEASONAL PROCESSES

Stream water quality can change substantively over diurnal, event, and seasonal scales. Multiple biogeochemical processes operate on a 24-hour timescale, and also may be influenced by local weather and precipitation events. Increased sampling frequency, made possible by recent developments in continuous water quality monitors, allows us to better understand biogeochemical responses to these events. Multi-parameter (temperature, conductivity, pH, dissolved oxygen, turbidity) water quality sondes were deployed in the East Fork Jemez River and Jemez River (located in northern New Mexico) from 2010 to 2012. In addition, two real-time nutrient sensors (nitrate and phosphate) were deployed in the East Fork Jemez River from March to October 2011. Data indicate periods of substantial diurnal variability in dissolved oxygen (up to Δ 9 mg L^-1day^-1) and nutrient concentrations (up to Δ 3µM day^-1 nitrate and Δ1 μM day^-1 phosphate) in the East Fork Jemez River. In addition, local precipitation events are found to increase nitrate concentrations by up to 50%.

The year 2011 yielded a relatively small snowpack in this region, and the Jemez Mountains also experienced the largest fire on record in New Mexico. Sondes and nutrient sensors captured biogeochemical and nutrient responses both during the fire and during the post-fire monsoons. Approximately one month after the fire, monsoonal precipitation events resulted in large multi-day pulses in nitrate (>20µm), phosphate (>10μm), specific conductivity (>0.30 mS/cm @ 25°C), and turbidity (>1200 NTU). These periods correspond with significant sags in dissolved oxygen and pH. Sondes deployed downstream of the fire in the Jemez River and Rio Grande also show that water quality is significantly affected during this post-fire period. The short duration of these responses highlights the importance of continuous water quality monitoring to better understand event responses and biogeochemical cycling in headwater streams.