The nitrogen isotope effect of benthic remineralization-nitrification-denitrification coupling in an estuarine environment
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 (ε app) and TDN (ε 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 + NH 4+). We observed that both the pore water nitrate and RDN pools were enriched in 15N relative to the water column, with increasing δ 15N 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 ϵ app values <3‰. The strongest under-expression (i.e. lowest ε 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 ε app. For the first time, actual measurements of δ 15N of pore water RDN were included in the calculations of ε sed. We argue that large fractions of the sea-floor-derived DON are reactive and, hence, involved in the development of the δ 15N of dissolved inorganic N (DIN) in the water column. In the St. Lawrence sediments, the combined benthic N transformations yield a flux of 15N-enriched RDN that can significantly elevate ε sed above ε app. Calculated ε sed values were within the range of 4.6 ± 2‰ and were related to organic matter reactivity and oxygen penetration depth in the sediments. ϵ sed reflects the δ 15N 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 ε sed is larger than assumed by earlier work, questioning current ideas with regards to the state of balance of the modern N budget.