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.

Here we report multiple parameters used to describe the diagenetic state of sediments, including total hydrolyzable amino acid (THAA), amino acid enantiomer, chlorin (CI) and amino acid degradation (DI, RI) indices, along a transect between the Upper St. Lawrence Estuary and the Gulf of St. Lawrence, Canada. The study area is characterized by gradients in water oxygen concentration, water depth, organic matter (OM) source, primary productivity, and sedimentation rate. Both Cl and DI indicate a decline in OM reactivity, with the transition from a more terrestrial to a more marine-dominated sedimentation regime as one moves from the shallow Upper Estuary (23-95 m) to the hypoxic, mid-depth Lower Estuary and to the deep (>400 m), well-oxygenated Gulf. Whereas the Cl more accurately reflected OM reactivity in surface sediments and sediments down to 5 cm, the amino acid-based degradation indices (DI and RI) better described degradation in sediments down to 35 cm. Systematic variations in the amino acid composition along the Laurentian Channel confirmed the increased diagenesis of OM with distance from the Upper St. Lawrence Estuary. The ratio of D/L-stereoisomers of alanine increased along the transect, and the co-variation between DI and the D/L-Ala suggest a close coupling between the extent of diagenesis and the accumulation and selective preservation of bacterially-derived cell wall material in the sediments. The same patterns that we observed along the estuarine transect were present down-core in two sediment cores, confirming the robustness of our reactivity indices. Oxygen exposure time of the sediments appears to strongly determine sediment OM reactivity in the St. Lawrence Estuary. The sediment oxygen regime itself is related to the interplay between water column depth, vertical OM flux, and reactivity of settling OM. (C) 2012 Elsevier Ltd. All rights reserved.

Erratum to “Benthic nutrient fluxes along the laurentian channel: Environmental controls and impact on the N-budget of the St. Lawrence marine system” [Estuar. Coast. Shelf Sci. 90 (4) (2010) 195e205] Benoit Thibodeau *, Moritz F. Lehmann , Jacqueline Kowarzyk , Alfonso Mucci , Yves Gelinas , Denis Gilbert , Roxane Maranger , Mohammad Alkhatib a Geochemistry and Geodynamics Research Center, GEOTOP, Universite du Quebec a Montreal, Montreal, Quebec, H3C 3P8, Canada b Institute for Environmental Geosciences, University of Basel, Bernoullistrasse 30, 4056 Basel, Switzerland Department of Biological Sciences, Universite de Montreal, Montreal, Quebec, H3C 3J7, Canada Department of Earth and Planetary Sciences, McGill University, Montreal, Quebec, H3A 2A7, Canada Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec, H4B 1R6, Canada f Fisheries and Oceans Canada, Institut Maurice-Lamontagne, Mont-Joli, Quebec, G5H 3Z4, Canada

The nitrogen (N) stable isotopic composition of pore water nitrate and total dissolved N (TDN) was measured in sediments of the St. Lawrence Estuary and the Gulf of St. Lawrence. The study area is characterized by gradients in organic matter reactivity, bottom water oxygen concentrations, as well as benthic respiration rates. N isotope effects on the water column associated with the benthic exchange of nitrate (epsilon(app)) and TDN (epsilon(sed)) during benthic nitrification-denitrification coupling were investigated. The sediments were a major sink for nitrate and a source of reduced dissolved N (RDN = DON + NH4+). We observed that both the pore water nitrate and RDN pools were enriched in N-15 relative to the water column, with increasing delta N-15 downcore in the sediments. As in other marine environments, the biological nitrate isotope fractionation of net fixed N loss was barely expressed at the scale of sediment-water exchange, with epsilon(app) values < 3 parts per thousand. The strongest under-expression (i.e. lowest epsilon(app)) of the biological N isotope fractionation was observed at the most oxygenated sites with the least reactive organic matter, indicating that, through their control on the depth of the denitrification zone, bottom water oxygen concentrations and the organic matter reactivity can modulate epsilon(app). For the first time, actual measurements of delta N-15 of pore water RDN were included in the calculations of epsilon(sed). We argue that large fractions of the sea-floor-derived DON are reactive and, hence, involved in the development of the delta N-15 of dissolved inorganic N (DIN) in the water column. In the St. Lawrence sediments, the combined benthic N transformations yield a flux of N-15-enriched RDN that can significantly elevate epsilon(sed) above epsilon(app). Calculated epsilon(sed) values were within the range of 4.6 +/- 2 parts per thousand and were related to organic matter reactivity and oxygen penetration depth in the sediments. epsilon(sed) reflects the delta N-15 of the N-2 lost from marine sediments and thus best describes the isotopic impact of fixed N loss from sediments on the oceanic fixed N pool. Our mean value for epsilon(sed) is larger than assumed by earlier work, questioning current ideas with regards to the state of balance of the modern N budget.

Water column concentrations and benthic fluxes of dissolved inorganic nitrogen (DIN) and oxygen (DO) were measured in the Gulf of St. Lawrence and the Upper and Lower St. Lawrence Estuary (USLE and LSLE, respectively) to assess the nitrogen (N) budget in the St. Lawrence (SL) system, as well as to elucidate the impact of bottom water hypoxia on fixed-N removal in the LSLE. A severe nitrate deficit, with respect to ambient phosphate concentrations (N*∼−10 μmol L−1), was observed within and in the vicinity of the hypoxic bottom water of the LSLE. Given that DO concentrations in the water column have remained above 50 μmol L−1, nitrate reduction in suboxic sediments, rather than in the water column, is most likely responsible for the removal of fixed N from the SL system. Net nitrate fluxes into the sediments, derived from pore water nitrate concentration gradients, ranged from 190 μmol m−2 d−1 in the hypoxic western LSLE to 100 μmol m−2 d−1 in the Gulf. The average total benthic nitrate reduction rate for the Laurentian Channel (LC) is on the order of 690 μmol m−2 d−1, with coupled nitrification-nitrate reduction accounting for more than 70%. Using average nitrate reduction rates derived from the observed water column nitrate deficit, the annual fixed-N elimination within the three main channels of the Gulf of St. Lawrence and LSLE was estimated at 411 × 106 t N, yielding an almost balanced N budget for the SL marine system.

The biogeochemistry of the Dumai River estuary in eastern Sumatra, Indonesia, was studied in order to obtain information on the sources, transformation, and fate of organic matter. Between October and December 2003, water, total suspended matter (TSM), and sediments were sampled along a salinity gradient during four campaigns, and plants and soils were collected from the catchment. Water samples were analyzed for dissolved inorganic nutrients and dissolved organic carbon (DOC). The concentrations of organic carbon (Corg) and total nitrogen (N) and the stable carbon (δ13Corg) and nitrogen (δ15N) isotope distributions were determined in TSM, sediments, plants, and soils. The pH as well as the concentrations of dissolved inorganic nutrients and TSM were very low in the river and increased toward the sea. A maximum DOC concentration of 5,050 µmol L−1 was measured in the river, and concentrations decreased toward the sea. Low‐gradient relief and a dense vegetation cover, and hence little weathering and erosion, appear to be responsible for low river loads of dissolved nutrients and TSM in this black‐water river. Leaching from extensive peat soils in its catchment may account for the high DOC content of the Dumai River. Peat swamps drained by numerous small rivers are estimated to cover 3.3 X 104 km2 in eastern Sumatra, suggesting that leaching of DOC may be a significant source of carbon to the adjacent coastal seas. A comparison with “normal” rivers shows that black‐water rivers can export similar amounts of DOC from catchments that are orders of magnitude smaller. Thus, export from small black‐water rivers may be quantitatively more significant for the global DOC input into the ocean than previously thought.