Nitrogen retention in a floodplain backwater of the upper Mississippi River (USA) |
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Authors: | William F James |
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Institution: | (1) U.S. Army Corps of Engineers, Engineer Research and Development Center, Eau Galle Aquatic Ecology Laboratory, W500 Eau Galle Dam Road, Spring Valley, WI 54767, USA |
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Abstract: | 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. |
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