Seasonal variability of dissolved organic matter in the Columbia River: In situ sensors elucidate biogeochemical and molecular analyses

Abstract. The in situ detection of fluorescent dissolved organic matter (FDOM) at high temporal resolution is a powerful proxy to follow dissolved organic matter (DOM) dynamics and DOM flux to coastal oceans when FDOM measurements and dissolved organic carbon (DOC) are highly correlated. Here, we investigated the relationship between FDOM sensors and DOC concentration in the lower Columbia River, USA in spring and summer 2013. Furthermore, we studied the seasonal variability of FDOM and chromophoric DOM (CDOM) optical indices, as well as the seasonal and spatial variability for the molecular characteristics of DOM using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The fieldwork was conducted concurrently with the operation of in situ sensor platforms that recorded physical and biogeochemical data at hourly intervals. In situ FDOM and DOC concentration was highly correlated and the relationship was used to quantify the river's DOC flux between March and August 2013. The average flux was 0.93 Gg d⁻¹, which included over three-fold temporal variability (0.45 to 1.62 Gg d⁻¹) associated with seasonal biogeochemical variability. Spectrofluorometry measurements demonstrated that FDOM parameters correlated with major seasonal biogeochemical shifts in the river associated with phytoplankton blooms and river discharge and thus revealed predictable seasonal patterns in DOM quality. FT-ICR-MS analyses elucidated these shifts on the molecular level: the relative abundance of 561 formulas, most of which contained N and S, correlated significantly with chlorophyll a, while 417 formulas (mostly CHO) correlated with CDOM absorbance at 254 nm.