Nitrogen isotopes reveal a particulate-matter-driven biogeochemical reactor in a temperate estuary

Abstract

Estuaries and rivers are important biogeochemical reactors that act to modify the loads and composition of nutrients in the coastal zone. In a case study during July 2013, we sampled an 80 km transect along the Elbe Estuary under low-oxygen conditions. To better elucidate specific mechanisms of estuarine nitrogen processing, we tracked the evolution of the stable isotopic composition of nitrate, nitrite, particulate matter, and ammonium through the water column. This approach allowed assessment of the in situ isotope effects of ammonium and nitrite oxidation and of remineralization at the reach scale. The isotope effects of nitrite oxidation and ammonium oxidation were consistent with pure-culture assessments. We found that the nitrogen budget of the Elbe Estuary is governed by settling, resuspension, and remineralization of particulate matter, and we further used our stable isotope data to evaluate sources and sinks of nitrogen in the Elbe Estuary via an isotope mass-balance approach. We find that the reactivity of particulate matter, through its remineralization in the estuary, is the main control on the isotope dynamics of inorganic nitrogen species. Moreover, while underscoring this role of particulate matter delivery and reactivity, the isotope mass balance also indicated additional sinks of reactive nitrogen, such as possible denitrification of water column nitrate in the intensively dredged and deep Hamburg Harbor region.