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

Abstract

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 R 2 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.

Interstorm Variability in the Biolability of Tree-Derived Dissolved Organic Matter (Tree-DOM) in Throughfall and Stemflow

“…Further information is also needed to scale spatially, including data on the interstorm variability in tree-BDOM in other forest types and whether the temporal drivers vary geographically. Perhaps optical metrics, many of which are now able to be automatically collected in the field [35], can progress large-scale data collection efforts on tree-DOM and its biolability.…”

Short-Term Dissolved Organic Carbon Dynamics Reflect Tidal, Water Management, and Precipitation Patterns in a Subtropical Estuary

“…High-frequency FDOM data provide the resolution necessary to fill in knowledge gaps beyond the scope of conventional monthly sampling frequencies. For instance, high-frequency FDOM measurements have facilitated the development of ecosystemspecific proxies for mercury , nutrients (Wilson et al, 2013;Etheridge et al, 2014), and DOC lability (Wilson et al, 2013). Likewise, high-resolution characterization of DOC proxies have provided new insight into processes controlling diel carbon cycles (Watras et al, 2015), rapid changes in hydrologic export of DOC Wilson et al, 2013) and the role of DOC in light attenuation within optically complex waters (Ganju et al, 2014).…”

Comparison of UV–VIS and FDOM sensors for in situ monitoring of stream DOC concentrations

“…Measurements of stream [DOC] with a low temporal resolution (e.g., weekly or monthly) cannot fully capture DOC changes that typically last for just a few hours in small streams during a storm event, resulting in large uncertainty when estimating DOC loads (Jollymore et al, 2012). Thus, optical sensors have been used to achieve high-resolution in situ monitoring of [DOC] (Etheridge et al, 2014;Jollymore et al, 2012;Koehler et al, 2009;Strohmeier et al, 2013).…”

“…Two types of optical sensors have been used frequently for this purpose; the ultraviolet-visible (UV-VIS) spectrophotometer (Etheridge et al, 2014;Jeong et al, 2012;Jollymore et al, 2012;Strohmeier et al, 2013) and the fluorescent dissolved organic matter (FDOM) sensor Saraceno et al, 2009;Watras et al, 2011). UV-VIS sensors use the range of ultraviolet and visible light wavelengths (e.g., 220 to 720 nm) to rapidly scan the absorbance of UV-VIS light by molecules in the water and estimate the concentration of the molecules based on the Beer-Lambert law.…”