A novel boat-based field application of a high-frequency conductometric ammonium analyzer to characterize spatial variation in aquatic ecosystems

Documenting dissolved inorganic nitrogen (DIN) concentrations and forms at appropriate temporal and spatial scales is key to understanding aquatic ecosystem health, particularly because DIN fuels primary productivity. In addition to point and nonpoint source nutrient inputs, factors such as hydrology, geomorphology, temperature, light, and biogeochemical transformations influence nutrient dynamics in surface waters, allowing for the formation of steep spatial gradients and patchiness. Documenting nutrient variability is also necessary to identify sources, quantify transformation rates, and understand drivers. Because of logistical and cost constraints, it is often unfeasible to measure concentrations of nutrients in surface waters using discrete sampling followed by laboratory analysis at a resolution high enough to identify steep spatial gradients and patchiness. Because of these constraints, data generated from discrete sampling are limited in space and time, often missing key variabilities. Recent advancements of in situ nitrate plus nitrite (${\mathrm{NO}}_3^{-}$ and ${\mathrm{NO}}_2^{-}$) sensor technology have enabled highly temporally and spatially resolved ${\mathrm{NO}}_3^{-}$ concentration measurements in aquatic ecosystems. However, comparable information about ammonium (${\mathrm{NH}}_4^{+}$) concentrations remains unavailable. To address this need, US Geological Survey collaborated with Timberline Instruments to modify their commercially available benchtop TL-2800 ammonia analyzer to operate in flow-through mode, enabling rapid continuous ${\mathrm{NH}}_4^{+}$ concentration measurements at a micromolar (0.5 μM) resolution while receiving water pumped from a moving boat. Although the utility of this method is described for spatial surveys, we anticipate that it would be adaptable to installation at a fixed station for continuous monitoring of ${\mathrm{NH}}_4^{+}$ concentration.