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 (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.