NITRATE UPTAKE IN A VEGETATED BACKWATER LAKE IN THE UPPER MISSISSIPPI RIVER: 
RESULTS OF IN SITU CHAMBER EXPERIMENTS WITH ADDITIONS OF 14NO3- AND 15N-NO3- 
TRACER.

William Richardson1, 1Rebecca Kreiling, 1Lynn Bartsch, 2Jennifer Cavanaugh, 
3William James and 4Eric Strauss

1US Geological Survey Upper Midwest Environmental Sciences Center, 2630 Fanta 
Reed Rd., La Crosse, WI
2US Department of Agriculture Natural Resource Conservation Service, St. 
Peter, MN
3US Army Corps of Engineers Eau Galle Limnological Research Center, Spring 
Valley, WI
4University of Wisconsin, Department of Biology, Rivers Studies Center, La 
Crosse, WI

Backwater lakes of floodplain rivers are active sites for nitrate removal  
through bacterial denitrification, assimilation by bacteria, periphyton, and 
macrophytes, and possibly burial.  Recent research has shown bacterial 
denitrification to be an important contributor to nitrate retention (loss) in 
the carbon-rich backwaters of the Upper Mississippi River (UMR) – 
particularly in carbon-rich backwaters.  Denitrification in backwaters tends 
to be  limited by the delivery of nitrate from nitrate-rich main channels;  
high, post flood nitrate concentrations in backwaters decline to near non-
detectable levels in a matter of weeks.  The role of macrophytes and 
associated periphytic communities in the mass balance of nitrogen is poorly 
understood in these habitats.

To improve our understanding of the role of macrophytes in nitrogen cycling, 
we conducted two sets of in situ chamber experiments (n=8) in Third Lake of 
the Pool 5 Finger Lakes complex.  We added both 14NO3-  (4 concentrations 
from 0 to 7.5 mg ·L-1) and a stable isotope of nitrate (15N-NO3-, chambers 
enriched to ?5000) to determine nitrate uptake rate and the biological uptake 
compartment (sediment, macrophyte, epiphyton, or water column).  Experiments 
were conducted in late June and late July in 2005, under contrasting water 
temperature and rates of nitrate loading.   Independent estimates of sediment 
denitrification and nitrification were conducted just prior to the chamber 
experiments.

We estimated NO3- uptake by vegetation (primarily Ceratophyllum) as 168,  
epiphyton as 16.5, and sediment 4.2 mg-N ·m-2 ·d-1.  NO3- uptake rates in 
control chambers were then averaged over the two experiments and extrapolated 
over the area of backwater covered with macrophytes to determine lake-wide  
NO3-  uptake by macrophyte beds.   The total uptake by macrophyte beds in 
Third Lake was 206.2 mg-N ·m-2 ·d-1, and was extremely close (74 %) to that 
estimated by James et al. 2007 using mass-balance techniques.  These two 
independent estimates of nitrate retention in floodplain river backwaters 
show  1) the importance of macrophytes for nitrate retention relative to 
sediments, and 2) that aquatic vegetation is critical for N retention during 
the growing season when much of the N transport occurs.  What is less well 
understood are the N loss processes occurring during macrophyte decomposition 
and remineralization following autumnal senescence. 

Keywords:  nitrate uptake, biogeochemistry, macrophytes, periphyton, 
sediment, backwater lakes, 15N-NO3- tracer, chamber experiment, mass-balance