High‐frequency in situ optical measurements during a storm event: Assessing relationships between dissolved organic matter, sediment concentrations, and hydrologic processes

Abstract

Dissolved organic matter (DOM) dynamics during storm events has received considerable attention in forested watersheds, but the extent to which storms impart rapid changes in DOM concentration and composition in highly disturbed agricultural watersheds remains poorly understood. In this study, we used identical in situ optical sensors for DOM fluorescence (FDOM) with and without filtration to continuously evaluate surface water DOM dynamics in a 415 km2 agricultural watershed over a 4 week period containing a short‐duration rainfall event. Peak turbidity preceded peak discharge by 4 h and increased by over 2 orders of magnitude, while the peak filtered FDOM lagged behind peak turbidity by 15 h. FDOM values reported using the filtered in situ fluorometer increased nearly fourfold and were highly correlated with dissolved organic carbon (DOC) concentrations (r2 = 0.97), providing a highly resolved proxy for DOC throughout the study period. Discrete optical properties including specific UV absorbance (SUVA254), spectral slope (S290–350), and fluorescence index (FI) were also strongly correlated with in situ FDOM and indicate a shift toward aromatic, high molecular weight DOM from terrestrially derived sources during the storm. The lag of the peak in FDOM behind peak discharge presumably reflects the draining of watershed soils from natural and agricultural landscapes. Field and experimental evidence showed that unfiltered FDOM measurements underestimated filtered FDOM concentrations by up to ∼60% at particle concentrations typical of many riverine systems during hydrologic events. Together, laboratory and in situ data provide insights into the timing and magnitude of changes in DOM quantity and quality during storm events in an agricultural watershed, and indicate the need for sample filtration in systems with moderate to high suspended sediment loads.

Fluorescence spectroscopy opens new windows into dissolved organic matter dynamics in freshwater ecosystems: A review

“…Methods for fluorescence DOM characterization combined with PARAFAC are also becoming more standardized and widely available (tutorial by Stedmon and Bro 2008). Moreover, the potential for in situ monitoring of DOM fluorescence will allow for highresolution collection of data in the field (Spencer et al 2007;Saraceno et al 2009). With the recent technological advances and the availability of bench-top spectrofluorom- Hood et al 2007).…”

“…For example, research from an urban catchment in the U.K. that receives wastewater inputs showed that protein-like fluorescence was significantly correlated with biological oxygen demand and inorganic N and P concentrations (Baker and Inverarity 2004). With technological advances that now enable in situ collection of fluorescence and absorbance measurements (Spencer et al 2007;Saraceno et al 2009), spectroscopic techniques have the potential to be a useful tool for water-quality monitoring in urban and agricultural catchments.…”

Recent Advances in Understanding Flow Dynamics and Transport of Water-Quality Constituents in the Sacramento–San Joaquin River Delta

“…Importantly, these variations are not necessarily in concert with variation in other parameters such as total suspended sediment. Saraceno et al (2009), working in Willow Slough, a tributary of the Sacramento River, found that the peak in dissolved organic carbon associated with the storm did not align with either the peak in the hydrograph or the peak in TSS, consistent with their different sources (Florsheim et al 2011;Oh et al 2013).…”

Diel fluctuations of viscosity-driven riparian inflow affect streamflow DOC concentration

“…With increasing stream orders, autochthonous sources become more important (Dawson et al, 2001;Nimick et al, 2011). While DOC from allochthonous sources is characterized by fulvic and humic acids with high molecular weight and aromaticity, DOC from autochthonous sources has a lower molecular weight and is less aromatic (Hood et al, 2006;Saraceno et al, 2009;Spencer et al, 2012). 10 Different techniques have been used to gain information on the composition and the concentration of DOC.…”

“…Dissolved organic matter (DOM) is a major constituent of the carbon cycle and aquatic biogeochemistry, eventually linking terrestrial and aquatic ecosystems (Battin et al, 2008;Lee et al, 2016;Saraceno et al, 2009). The largest component of DOM in forested stream ecosystems is dissolved organic carbon (DOC) (McLaughlin and Kaplan, 2013).…”