Using in situ ultraviolet-visual spectroscopy to measure nitrogen, carbon, phosphorus, and suspended solids concentrations at a high frequency in a brackish tidal marsh

The collection of high frequency water quality data are key to making the next leap in hydrological and biogeochemical sciences. Commercially available in situ ultraviolet‐visual (UV‐Vis) spectrometers make possible the long‐term collection of absorption spectra multiple times per hour. This technology has proven useful for measuring nitrate, dissolved organic carbon, and total suspended solids in many environments, but has not been tested in tidal marsh conditions where upstream freshwater mixes with estuarine waters, resulting in rapid changes in concentrations and salinity. These three parameters encompass only a portion of the nutrients that are of interest in these systems. To test the potential of spectroscopy to measure these and other nutrient concentrations, spectrometers were installed in a constructed brackish tidal marsh and absorbance spectra were compared to lab analyses for coinciding discrete samples. Variable selection techniques, including partial least squares regression, lasso regression, and stepwise regression, were used to develop models with which nitrate, total kjeldahl nitrogen, dissolved organic carbon, phosphate, total phosphorus, total suspended solids, and salinity in brackish marsh waters can be predicted from UV‐Vis spectrometer measurements. Significant relationships between the absorption spectra and the laboratory measured concentrations were observed for all of the parameters. Phosphate and total phosphorus were the only nutrients which had R2 values less than 0.86 for their best calibrations. This study shows the potential to collect multiple water quality parameters at a high frequency in brackish waters using in situ spectrometers and gives the tools to replicate this analysis in all environments.