The fate of terrigenous dissolved organic carbon in a river‐influenced ocean margin

Abstract

The mineralization of terrigenous dissolved organic carbon (tDOC) discharged by rivers can impact nutrient and trace metal cycling, biological productivity, net ecosystem metabolism, and air‐sea CO2 exchange in ocean margins. However, the extreme heterogeneity of river‐influenced ocean margins represents a major challenge for quantitative assessments of tDOC transformations and thereby obscures the role of tDOC in biogeochemical cycles. Here a lignin‐based optical proxy for tDOC and a shelf‐wide mass balance approach were used to quantitatively assess the fate of tDOC discharged from the Mississippi‐Atchafalaya River System (M‐ARS) to the Louisiana shelf. The mass balance revealed that ~40% of the tDOC discharged by the M‐ARS during March 2009–2010 was mineralized to CO2 on the Louisiana shelf, with two thirds of the mineralization taking place in the mixed layer. A strong seasonality in tDOC mineralization was observed, with mineralization rates severalfold higher during summer than during winter. Independent assessments of specific mineralization processes indicated biomineralization accounted for ~94% of the tDOC mineralization on an annual basis and suggest that photochemical transformations of tDOC enhanced biomineralization by ~50% in the mixed layer. Direct photomineralization accounted for a relatively small fraction (~6%) of the tDOC mineralization on an annual basis. This quantitative assessment directly confirms that ocean margins are major sinks of the tDOC discharged by rivers and indicates that tDOC mineralization rates in the shelf mixed layer are sufficiently large to influence whether the Louisiana shelf is a net sink or source of atmospheric CO2.

Deciphering associations between dissolved organic molecules and bacterial communities in a pelagic marine system

“…The general correlation between active BC composition and DOM was weaker than the DNA/DOM correlation (Table 1). One factor strengthening the correlation between total BC and DOM along the salinity gradient while diluting the active BC-DOM correlation may be the type of DOM that is affected: the terrigenous DOM driving the DNA/DOM relationship is only consumed by the BC (Fichot and Benner, 2014), whereas the more labile compounds affecting the active BC-DOM relationship are both consumed and produced. The aforementioned methodological constraints favoring the more refractory DOM compounds additionally weaken the active BC-DOM correlation.…”

“…The decomposition pathways of terrigenous DOM in the ocean are still not fully constrained. Especially in coastal areas, which are the first receivers of terrigenous DOM, remineralization is a possible source of nutrients for primary production (Moran and Hodson, 1994;Opsahl and Benner, 1997;Fichot and Benner, 2014).…”

Microbially-Mediated Transformations of Estuarine Dissolved Organic Matter

“…Riverine input can be a large source of DOM of terrigenous origin to estuaries and coastal areas (Hedges et al, 1997;Fichot and Benner, 2014;Medeiros et al, 2016). As riverine DOM is transported toward the ocean, its quantity and composition is often altered (Cole et al, 2007) by a variety of chemical, biological and physical processes (Tzortziou et al, 2008), making estuaries hot spots of DOM cycling.…”

Photomineralization and photomethanification of dissolved organic matter in Saguenay River surface water

“…Photomineralization alone or combined with photochemically stimulated biomineralization has been suggested as a significant sink of DOC in many rivers and lakes (e.g. Bertilsson and Tranvik, 2000;Vähätalo and Wetzel, 2004;Cory et al, 2014) and a major sink of terrigenous DOC in coastal and shelf waters (Miller and Zepp, 1995;Aarnos et al, 2012;Fichot and Benner, 2014). Many trace gases produced from CDOM-involved photoprocesses are supersaturated in natural waters (e.g.…”