Diurnal variability in riverine dissolved organic matter composition determined by in situ optical measurement in the San Joaquin River (California, USA)

Abstract

Abstract:Dissolved organic matter (DOM) concentration and composition in riverine and stream systems are known to vary with hydrological and productivity cycles over the annual and interannual time scales. Rivers are commonly perceived as homogeneous with respect to DOM concentration and composition, particularly under steady flow conditions over short time periods. However, few studies have evaluated the impact of short term variability (<1 day) on DOM dynamics. This study examined whether diurnal processes measurably altered DOM concentration and composition in the hypereutrophic San Joaquin River (California) during a relatively quiescent period. We evaluated the efficacy of using optical in situ measurements to reveal changes in DOM which may not be evident from bulk dissolved organic carbon (DOC) measurement alone. The in situ optical measurements described in this study clearly showed for the first time diurnal variations in DOM measurements, which have previously been related to both composition and concentration, even though diurnal changes were not well reflected in bulk DOC concentrations. An apparent asynchronous trend of DOM absorbance and chlorophyll-a in comparison to chromophoric dissolved organic matter (CDOM) fluorescence and spectral slope S 290 -350 suggests that no one specific CDOM spectrophotometric measurement explains absolutely DOM diurnal variation in this system; the measurement of multiple optical parameters is therefore recommended. The observed diurnal changes in DOM composition, measured by in situ optical instrumentation likely reflect both photochemical and biologically-mediated processes. The results of this study highlight that short-term variability in DOM composition may complicate trends for studies aiming to distinguish different DOM sources in riverine systems and emphasizes the importance of sampling specific study sites to be compared at the same time of day. The utilization of in situ optical technology allows short-term variability in DOM dynamics to be monitored and serves to increase our understanding of its processing and fundamental role in the aquatic environment.

High‐frequency monitoring of catchment nutrient exports reveals highly variable storm event responses and dynamic source zone activation

“…In contrast, photochemical degradation of DOC [e.g., Spencer et al, 2007;Lu et al, 2013] is unlikely to have had a substantial impact on diel DOC dynamics because the stream channel was incised and shaded by riparian vegetation for much of its length.…”

Linking diel patterns in community respiration to bacterioplankton in an oligotrophic high‐elevation lake

“…Diel patterns in DOC, especially the labile fraction associated with PER, are difficult to measure because of rapid uptake in the plankton and because the signal is so small relative to higher background DOC concentrations. Although overnight drawdown in DOC has been documented in lotic (Spencer et al 2007) and productive marine (Johnson et al Fig. 6.…”

Impact of seasonality and anthropogenic impoundments on dissolved organic matter dynamics in the Klamath River (Oregon/California, USA)

“…DOM composition was examined using a suite of chromophoric dissolved organic matter (CDOM) parameters that can be linked to molecular weight and aromaticity [Helms et al, 2008;Spencer et al, 2012]. Since several CDOM parameters can be measured in situ at a high temporal resolution [Spencer et al, 2007a[Spencer et al, , 2007bPellerin et al, 2012], a further aim of this study was to assess the utility of different parameters for tracking DOC export and DOM composition in impounded systems. Specifically, this work will help inform restoration activities associated with removal of the four lower dams on Klamath River (Oregon/California, USA) scheduled for the next decade.…”

“…The spectral slope (S) parameter varies with CDOM source and biological and photochemical processes that lead to changes in molecular weight or aromaticity [Spencer et al, 2007a[Spencer et al, , 2007bHelms et al, 2008]. Spectral slope values were calculated for wavelength ranges 275-295 nm (S 275-295 ) and 350-400 nm (S 350-400 ) as previous studies have shown that these wavelength ranges are sensitive to changes in the chemical composition of CDOM and track processes that differentially influence S [Helms et al, 2008].…”