AQUATIC VEGETATION CHANGES DUE TO A DRAWDOWN IN A BACKWATER POND
ON THE UPPER MISSISSIPPI RIVER.  

Heidi A. Langrehr 1 and Joseph H. Wlosinski 2.   1 Wisconsin Department
of Natural Resources, 575 Lester Avenue, Onalaska, WI 54650; 2
U.S. Geological Survey, Upper Midwest Environmental Sciences
Center, 2630 Fanta Reed Road, La Crosse, WI 54603  

Water level management is used by the U.S. Army Corps of
Engineers (Corps) as well as other state and federal agencies to
stimulate vegetative growth.  To date, however, little
opportunity existed for this management action on the Upper
Mississippi River National Wildlife and Fish Refuge.  In 1997 the
Corps St. Paul District, as part of a Habitat Rehabilitation and
Enhancement Project, lowered water levels at Lizzy Pauls Pond
(LPP), a backwater located in Pool 5 near the Wisconsin bluffs. 
One of the project goals was to increase vegetation.  Aquatic
vegetation surveys were conducted during the summers of 1996
(pre-drawdown) and 1998 (post-drawdown).  Transects were
established at 40-m intervals on a north/south grid.  Presence or
absence of aquatic species was recorded at each grid intersection
(sample site) using both visual and a rake drag in three
quadrats.  The survey was also performed at an adjoining control
site that was not dewatered.  Relative abundance was calculated
for each sample site, representing presence or absence in each of
the three quadrats, with a paired comparisons t-test used to test
for significance.  In LPP, 15 aquatic plant species were recorded
in 1996 and 20 in 1998.  In the control location, there were 14
aquatic plant species in 1996 and 1998.  Species richness for
submersed plants increased the same amount in both LPP and the
control (9 in 1996; 12 in 1998).  The percent frequency of
submersed species was greater than 95% in all years and
locations.  Coontail (Ceratophyullum demersum L.) was the
dominant submersed species in all years and locations (>80%). 
For emergents, species richness and percent frequency increased
in LPP (5 species, 17% in 1996; 7 species, 35% in 1998) but
decreased in the control (4 species, 21% in 1996; 1 species, 4%
in 1998).  Stiff arrowhead (Sagittaria rigida Pursh.), the
dominant emergent species in both years (14% in 1996; 25% in
1998), as well as rice cutgrass (Leersia oryzoides (L.) Swartz;
0% to 16%) and wild rice (Zizania aquatica L.; 2% to 15%)
increased from 1996 to 1998 in LPP.  Smartweed (Polygonum spp.)
was the dominant emergent in the control site in 1996 (18%).  The
only emergent recorded in 1998 was wild rice (4%).  White water
lily (Nymphaea odorata Ait.), the only rooted floating-leaved
species recorded, increased in percent frequency in both LPP and
the control.  Submersed and floating-leaved species showed no
difference in response between the drawdown and the control site. 
Emergents, however, increased in species richness and percent
frequency in the drawdown site, but decreased in both in the
control site.  The increase in relative abundance at Lizzy Pauls
Pond was significant (à = 0.05) but the decrease at the control
was not.  

Keywords:  aquatic vegetation, drawdown, Mississippi River, water
level management