『Abstract
In-stream nitrogen processing in the
Mississippi River has been suggested as one mechanism to reduce
coastal eutrophication in the Gulf of Mexico. Aquatic macrophytes
in river channels and flood plain lakes have the potential to
temporarily remove large quantities of nitrogen through assimilation
both by themselves and by the attached epiphyton. In addition,
rooted macrophytes act as oxygen pumps, creating aerobic microsites
around their roots where coupled nitrification-denitrification
can occur. We used in situ 15N-NO3-
tracer mesocosm experiments to measure nitrate assimilation rates
for macrophytes, epiphyton, and microbial fauna in the sediment
in Third Lake, a backwater lake of the upper Mississippi River
during June and July 2005. We measured assimilation over a range
of nitrate concentrations and estimated a nitrate mass balance
for Third Lake. Macrophytes assimilated the most nitrate (29.5
mg N m-2 d-1) followed by sediment microbes
(14.4 mg N m-2 d-1) and epiphytes (5.7 mg
N m-2 d-1). Assimilation accounted for 6.8%
in June and 18.6% in July of total nitrate loss in the control
chambers. However, denitrification (292.4 mg N m-2
d-1) is estimated to account for the majority (82%)
of the nitrate loss. Assimilation and denitrification rates generally
increased with increasing nitrate concentration but denitrification
rates plateaued at about 5 mg N L-1. This suggests
that backwaters have the potential to remove a relatively high
amount of nitrate but will likely become saturated if the load
becomes too large.
Keywords: Assimilation; Denitrification; Mississippi River; Nitrate
uptake; Vegetated backwater』
Introduction
Methods
Study site
N process analyses
15N chamber experiments
Sample processing
Whole lake vegetation and epiphyton mass estimates
δ15N uptake
Statistical analysis and whole lake uptake calculations
Results
Discussion
Management implications
Acknowledgments
References