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

[摘要] Estuaries and rivers are important biogeochemicalreactors that act to modify the loads and composition of nutrients in thecoastal zone. In a case study during July 2013, we sampled an 80 km transectalong the Elbe Estuary under low-oxygen conditions. To better elucidatespecific mechanisms of estuarine nitrogen processing, we tracked theevolution of the stable isotopic composition of nitrate, nitrite,particulate matter, and ammonium through the water column. This approachallowed assessment of the in situ isotope effects of ammonium and nitriteoxidation and of remineralization at the reach scale. The isotope effects ofnitrite oxidation and ammonium oxidation were consistent with pure-cultureassessments. We found that the nitrogen budget of the Elbe Estuary isgoverned by settling, resuspension, and remineralization of particulatematter, and we further used our stable isotope data to evaluate sources andsinks of nitrogen in the Elbe Estuary via an isotope mass-balanceapproach. We find that the reactivity of particulate matter, through itsremineralization in the estuary, is the main control on the isotope dynamics ofinorganic nitrogen species. Moreover, while underscoring this role ofparticulate matter delivery and reactivity, the isotope mass balance alsoindicated additional sinks of reactive nitrogen, such as possibledenitrification of water column nitrate in the intensively dredged and deepHamburg Harbor region.

[学科分类] 大气科学