Hydrologic Control of Nitrate Loading and Transformation in Backwater Lakes of the Upper Mississippi River
Abstract
Floodplain backwater lakes (BWL) are biogeochemically active with potential to remove large quantities of transported nitrate (NO3-) from the Upper Mississippi River. We measured nitrate transformations in BWL receiving high NO3- water under natural flooding and controlled inflow conditions to determine: 1) patterns of NO3- loss; 2) biogeochemical processes affecting NO3- transformation; and 3) effect of loading rate on removal capacity. In a large (300 ha) BWL, floodwater NO3- concentrations dropped from 6.5 to < 0.5 mg-N L-1 in 12 d, with a loss of >18 tons-N. Under controlled inflow another BWL (Third Lake, 15 ha) exhibited high rates denitrification (22 μg-N cm-2 d-1), limited by NO3- loading and tightly coupled with nitrification. Nitrate retention was linear with load (r2=0.95), with greatest retention occurring in late June. An average of 48 kg - N d-1 NO3- was removed from Third Lake (43 % of total inflow load, 32 % via denitrification). These results show NO3- removal from backwater lakes is directly related to river-flood plain connectivity, river discharge, and NO3- loading rate. Engineered reconnection of backwaters to main channels could reduce downstream flux of NO3- while also restoring other ecologic functions and meeting multiple management goals.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2005
- Bibcode:
- 2005AGUSMNB31D..01R
- Keywords:
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- 0400 Biogeosciences;
- 1806 Chemistry of fresh water;
- 1821 Floods;
- 1845 Limnology;
- 1871 Surface water quality