Organic matter distribution and reactivity in the waters of a large estuarine system

Abstract

Dissolved and particulate organic matters from different stations and depths in the St. Lawrence estuarine system (Canada) were collected and extracted using humic substance (HS) fractionation protocols. The origin, composition, and fate of organic matter/HS were assessed by distribution characteristics, elemental and isotopic compositions, and infrared diffuse reflectance spectroscopy (DRIFTS). HS represented over 50% of dissolved organic matter in the Saguenay Fjord's surface waters but this proportion decreased to 9–19% in the St. Lawrence Lower Estuary (SLLE). Fulvic acids were the dominant component (68–92%) of dissolved HS, especially in high salinity waters having very low humic acid content. Dissolved organic carbon (0.8–7.0 mg L − 1 ) and dissolved HS concentrations steadily decreased with downstream distance and depth. Isotopic and elemental data indicated that dissolved HS were mostly terrigenous, even in the SLLE deep waters. HS represented 62–100% of total particulate organic matter (POM) collected by sediment traps. Humin, the POM associated with the mineral matrix, was the dominant fraction of HS followed by humic acids and fulvic acids. The POM and its HS were mostly terrigenous in the fjord but mostly marine in the SLLE. Unlike its dissolved counterpart, a large part of the POM appeared to be mineralized in the water-column, even at relatively shallow (< 200 m) locations receiving large terrigenous inputs. At the upstream portion of the SLLE, it was estimated that 290 μmolC cm − 2 y − 1 was lost during sinking and shortly after deposition, corresponding to an O 2 demand of more than 360 μmolO 2 cm − 2 y − 1 assuming aerobic respiration. Distribution profiles indicated a rapid particulate HS removal during sinking (16–29%) and at the water–sediment interface (up to 70% total removal). These losses were especially important for N-containing molecules, aliphatic components, and marine non-humic POM. In contrast, humin was the most stable fraction. The interactions between POM and the mineral matrix appeared mediated by carboxylate groups. Differences in the composition of dissolved HS and particulate HS may explain their contrasting reactivities. Dissolved HS were nitrogen-poor, more oxidized, and depleted in aliphatic structures as compared to POM. The aliphatic content of dissolved and particulate HS was greater in marine samples (downstream) and decreased during diagenesis. The proportion of CH 3 groups relative to CH 2 groups was surprisingly high in dissolved fulvic acids, an important fraction of DOM. DRIFTS was particularly useful in providing information on HS structures and transformations.