Suspended particulate matter drives the spatial segregation of nitrogen turnover along the hyper-turbid Ems estuary

[摘要] Estuaries are nutrient filters and change riverine nutrient loads beforethey reach coastal oceans. Their morphology have been extensively changed byanthropogenic activities like draining, deepening and dredging to meeteconomic and social demand, causing significant regime changes like tidalamplifications and in some cases to hyper-turbid conditions. Furthermore,increased nutrient loads, especially nitrogen, mainly by agriculture causecoastal eutrophication. Estuaries can either act as a sink or as a source ofnitrate, depending on environmental and geomorphological conditions. Thesefactors vary along an estuary, and change nitrogen turnover in the system.Here, we investigate the factors controlling nitrogen turnover in thehyper-turbid Ems estuary (Northern Germany), which has been stronglyimpacted by human activities. During two research cruises in August 2014 andJune 2020, we measured water column properties, dissolved inorganicnitrogen, dual stable isotopes of nitrate and dissolved nitrous oxideconcentration along the estuary. We found that three distinct biogeochemicalzones exist along the estuary. A strong fractionation ( ∼26  ‰) of nitrate stable isotopes points towards nitrateremoval via water column denitrification in the hyper-turbid tidal river,driven by anoxic conditions in deeper water layers. In the middle reaches ofthe estuary nitrification gains importance, turning this section into a netnitrate source. The outer reaches are dominated by mixing, with nitrateuptake in 2020. We find that the overarching control on biogeochemical nitrogen cycling,zonation and nitrous oxide production in the Ems estuary is exerted bysuspended particulate matter concentrations and the linked oxygen deficits.

[学科分类] 大气科学