High-resolution temporal variations of nitrate in a high-elevation pond in alpine tundra (NW Italian Alps)

High-resolution temporal measurements in remote, high-elevation surface waters are required to better understand the dynamics of nitrate (NO3? ) in response to changes in meteoclimatic conditions. This study reports on the first use of a UV-Vis submersible spectrophotometric probe (UV-Vis probe) to measure the hourly concentration of nitrate nitrogen (NO3? -N) in a pond located at 2722 m a.s.l. in an alpine tundra area (NW Italian Alps), during two snow-free seasons (July-October) in 2014 and 2015. Weekly analyses of NO3? -N and stable isotopes of water (?18O and ?2H), together with continuous meteorological, water temperature, and turbidity measurements, were performed over the same period. The integration of in-situ UV-Vis spectrophotometric measurements with weekly samples allowed depicting the role of summer precipitation, snow melt, and temperature (air and water) in influencing NO3? dynamics. Short-duration meteorological events (e.g., summer storms and rain-on-snow events) produced rapid variations of in-pond NO3? concentration, i.e., fivefold increase in 18 h, that would not be detectable using the traditional manual collection of discrete samples. The observed seasonal variability of NO3? concentration, negatively correlated with water temperature, highlighted the important role of in-pond biological processes leading to an enhanced N uptake and to the lowest NO3? concentration in the warmer periods. The occurrence of heavy rainfall events critically altered the expected seasonal NO3? trends, increasing the N supply to the pond. The comparison of N dynamics in two years characterised by extremely different meteoclimatic conditions allowed us to obtain insights on the potential effects of climate changes (e.g., high air temperature, heavy rainfalls, and rain-on-snow events) on sensitive aquatic ecosystems as high-elevation ponds.