Benthic nutrient fluxes along the Laurentian Channel: Impacts on the N budget of the St. Lawrence marine system

Thibodeau, Benoît; Lehmann, Moritz F.; Kowarzyk, Jacqueline; Mucci, Alfonso; Gélinas, Yves; Gilbert, Denis; Maranger, Roxane; Alkhatib, M.

doi:10.1016/j.ecss.2010.08.015

Cited by 36 publications

(49 citation statements)

References 50 publications

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“…Sedimentary respiration was plausibly pointed out as the most important sink of oxygen in the St. Lawrence estuarine system , and yet the relative importance of benthic versus pelagic oxygen demand in the St. Lawrence estuary is an ongoing debate (Bourgault et al, 2012). In previous work, we have shown that POM source, oxygen concentration, benthic nutrient and DO fluxes, and POM reactivity vary along the Laurentian Channel (Alkhatib et al, 2012a;Thibodeau et al, 2010). Chlorin-and amino acid-based degradation indices show that sedimentary OM in the lower estuary is more reactive than in the gulf (Alkhatib et al, 2012a;see Table 1).…”

Section: Study Areamentioning

confidence: 91%

“…Higher POM reactivity induces the production and activity of bacterial hydrolytic enzymes (Boetius and Lochte, 1994;Wilczek et al, 2005), and hydrolysis proceeds under both oxic and anoxic conditions, although not necessarily always at the same rates (Hansen and Blackburn 1991;Kristensen and Holmer, 2001). In this regard, Thibodeau et al (2010) recently reported that OM remineralization rates along the LC are highest in the lower estuary and decrease eastwards. Alkhatib et al (2012a) also found a significant correlation between OM reactivity and the OM remineralization rates along the LC as reported by Thibodeau et al (2010;r 2 = 0.76, n = 8).…”

Section: Doc and Don Fluxesmentioning

confidence: 98%

“…Water samples collected for DIN and total dissolved N (TDN, the sum of NO Data from: (A) Thibodeau et al (2010), (B) Alkhatib et al (2012a), (C) Silverberg et al (1987), (D) Colombo et al (1996), (E) Muzuka and Hillaire-Marcel (1999), (F) Silverberg et al (2000).…”

Section: Samplingmentioning

confidence: 99%

“…Likewise, most benthic studies have mainly focused on net DIN fluxes when assessing total denitrification rates and the N elimination capacity of the estuarine sediments, neglecting the reflux of N as DON from particulate OM hydrolysis/lysis. For example, Thibodeau et al (2010) have shown that sediments of the Laurentian Channel (LC) function as an efficient sink for nitrate, especially in the lower St. Lawrence estuary section, but the role of sediments as a potential source of DON and the biogeochemical role of benthic DON fluxes have not been investigated yet.…”

Section: Introductionmentioning

confidence: 99%

“…Hence it is reasonable to assume that general sediment characteristics (i.e., amount, composition and reactivity/age of the POM) and redox conditions in estuarine and coastal-area sediments may influence not only the overall rates of benthic inorganic and organic solute exchange (Alkhatib et al, 2012a;Thibodeau et al, 2010), but also the element partitioning during remineralization. However, the effect of POM reactivity on the fluxes and fate of remineralized DOM remains uncertain.…”

Section: Introductionmentioning

confidence: 99%

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Benthic fluxes of dissolved organic nitrogen in the lower St. Lawrence estuary and implications for selective organic matter degradation

et al. 2013

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Abstract. The distribution of dissolved organic nitrogen (DON) and carbon (DOC) in sediment porewaters was determined at nine locations along the St. Lawrence estuary and in the gulf of St. Lawrence. In a previous manuscript (Alkhatib et al., 2012a), we have shown that this study area is characterized by gradients in the sedimentary particulate organic matter (POM) reactivity, bottom water oxygen concentrations, and benthic respiration rates. Based on the porewater profiles, we estimated the benthic diffusive fluxes of DON and DOC in the same area. Our results show that DON fluxed out of the sediments at significant rates (110 to 430 µmol m −2 d −1 ). DON fluxes were positively correlated with sedimentary POM reactivity and varied inversely with sediment oxygen exposure time (OET), suggesting direct links between POM quality, aerobic remineralization and the release of DON to the water column. DON fluxes were on the order of 30 to 64 % of the total benthic inorganic fixed N loss due to denitrification, and often exceeded the diffusive nitrate fluxes into the sediments. Hence they represented a large fraction of the total benthic N exchange, a result that is particularly important in light of the fact that DON fluxes are usually not accounted for in estuarine and coastal zone nutrient budgets. In contrast to DON, DOC fluxes out of the sediments did not show any significant spatial variation along the Laurentian Channel (LC) between the estuary and the gulf (2100 ± 100 µmol m −2 d −1 ). The molar C / N ratio of dissolved organic matter (DOM) in porewater and the overlying bottom water varied significantly along the transect, with lowest C / N in the lower estuary (5-6) and highest C / N (> 10) in the gulf. Large differences between the C / N ratios of porewater DOM and POM are mainly attributed to a combination of selective POM hydrolysis and elemental fractionation during subsequent DOM mineralization, but selective adsorption of DOM to mineral phases could not be excluded as a potential C / N fractionating process. The extent of this C-versus N-element partitioning seems to be linked to POM reactivity and redox conditions in the sediment porewaters. Our results thus highlight the variable effects selective organic matter (OM) preservation can have on bulk sedimentary C / N ratios, decoupling the primary source C / N signatures from those in sedimentary paleoenvironmental archives. Our study further underscores that the role of estuarine sediments as efficient sinks of bioavailable nitrogen is strongly influenced by the release of DON during early diagenetic reactions, and that DON fluxes from continental margin sediments represent an important internal source of N to the ocean.

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