Nitrogen retention in a floodplain backwater of the upper Mississippi River (USA)

• Published in: Aquatic sciences 2010, Vol.72 (1), p.61-69
• Main Author: James, William F.
• Format: Article
• Language: English
• Subjects: Ammonia; Ammonium; Animal and plant ecology; Animal, plant and microbial ecology; Anoxia; Anoxic conditions; Aquatic ecosystems; Backwaters; Biogeochemistry; Biological and medical sciences; Biomedical and Life Sciences; Biota; Denitrification; Ecology; Floodplains; Fresh water ecosystems; Freshwater; Freshwater & Marine Ecology; Fundamental and applied biological sciences. Psychology; Life Sciences; Marine & Freshwater Sciences; Marine environment; Mineralization; Nitrates; Nitrification; Nitrogen; Oceanography; Research Article; Retention; Rivers; Sediments; Synecology
• ISSN: 1015-1621; 1420-9055
• DOI: 10.1007/s00027-009-0113-3

Backwaters connected to large rivers retain nitrate and may play an important role in reducing downstream loading to coastal marine environments. A summer nitrogen (N) inflow-outflow budget was examined for a flow-regulated backwater of the upper Mississippi River in conjunction with laboratory estimates of sediment ammonium and nitrate fluxes, organic N mineralization, nitrification, and denitrification to provide further insight into N retention processes. External N loading was overwhelmingly dominated by nitrate and 54% of the input was retained (137 mg m −2  day −1 ). Ammonium and dissolved organic N were exported from the backwater (14 and 9 mg m −2  day −1 , respectively). Nitrate influx to sediment increased as a function of increasing initial nitrate concentration in the overlying water. Rates were greater under anoxic versus oxic conditions. Ammonium effluxes from sediment were 26.7 and 50.6 mg m −2  day −1 under oxic and anoxic conditions, respectively. Since anoxia inhibited nitrification, the difference between ammonium anoxic–oxic fluxes approximated a nitrification rate of 29.1 mg m −2  day −1 . Organic N mineralization was 64 mg m −2  day −1 . Denitrification, estimated from regression relationships between oxic nitrate influx versus initial nitrate concentration and a summer lakewide mean nitrate concentration of 1.27 mg l −1 , was 94 mg m −2  day −1 . Denitrification was equivalent to only 57% of the retained nitrate, suggesting that another portion was assimilated by biota. The high sediment organic N mineralization and ammonium efflux rate coupled with the occurrence of ammonium export from the system suggested a possible link between biotic assimilation of nitrate, mineralization, and export.