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 nutrient loads and composition in the intensively used coastal zone. In a case study during July 2013, we sampled an 80 km transect along the Elbe estuary under unusually 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.

We used this exceptional summer situation to constrain 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 are consistent with pure culture assessments. We found that estuarine biogeochemistry is governed by settling, resuspension, and remineralization of particulate matter. We used the stable isotope data to quantify sources and sinks of nitrogen in the Elbe estuary. An isotope mass balance box-model was developed to reproduce internal N-cycling and associated isotope dynamics. The model underscores the role of the delivery and reactivity of particulate matter, but it also allowed us to pinpoint additional sinks of reactive nitrogen, such as the denitrification of water column nitrate in the intensively dredged and deep Hamburg harbour basin.