Diel Variability in Dissolved Organic Matter Composition Determined by in-situ Optical Measurements.

Annual and interannual variability in DOM concentration and composition in rivers has been well documented however few studies have evaluated changes in DOM over short time scales such as diel cycles. Recent research has shown that concentrations of dissolved oxygen, inorganic nitrogen and trace metals for example vary considerably over diel cycles in streams and rivers due to a combination of biological, physical and chemical processes. In this study DOM variability was investigated over the diel cycle under stable riverine summer flow conditions in the San Joaquin River (California, U.S.A.) to evaluate if high resolution in-situ optical measurements revealed changes in DOM concentration and composition. Bulk dissolved organic carbon (DOC) concentrations showed no clear trend over diel cycles but in contrast the absorption coefficient of chromophoric DOM (CDOM) measured in situ at 350nm (a350 m-1 showed a clear diurnal pattern with CDOM maxima in the late evening and minima in the early morning. Similar diurnal patterns were recorded at a254 m- 1 and a440 m-1, absorbance wavelengths which have also been referred too as proxies for DOC quality and quantity. Chlorophyll-a fluorescence showed the same diurnal pattern as CDOM absorbance with early evening maxima and early morning minima in concentrations and previous studies indicate that phytoplankton is the primary source of organic matter in the San Joaquin River during summer months. In situ DOM fluorescence (measured at excitiation 370 nm, emission 450 nm) and spectral slope (S) calculated using a non-linear fit of an exponential function to the absorption spectrum in the range of 290 - 350 nm both showed clear diurnal patterns. With respect to DOM fluorescence and S290-350 maxima were recorded in the morning and minima in the late evening and so were out of phase with CDOM absorbance and chlorophyll-a fluorescence. This asynchronous pattern in DOM fluorescence and absorption coefficients over diel cycles in our study likely reflects both photochemical and biological processes and potential mechanisms to reconcile the apparent CDOM divergent patterns are discussed. This study highlights diel variations in DOM composition in a riverine system even though diel changes are not reflected in bulk DOC concentrations. The ability to monitor such sort term variability in DOM dynamics serves to increase our understanding of its behaviour in aquatic ecosystems and shows future potential for delineating its multifunctional role in the environment.