SUBMERSED AQUATIC VEGETATION MONITORING IN LAWRENCE LAKE, POOL 8, UPPER 
MISSISSIPPI RIVER SYSTEM, 1992-2001

Heidi A. Langrehr, James R. Fischer, and J. Therese Dukerschein
Wisconsin Department of Natural Resources, Onalaska Field Station, Onalaska, WI  
54650

Submersed aquatic vegetation (SAV) was monitored from 1992 to 2001 in Lawrence 
Lake, a contiguous 83-ha backwater in Pool 8, Upper Mississippi River System, as 
part of the Long Term Resource Monitoring Program.  Ten transects were 
established at 200-m intervals throughout the lake.  SAV was sampled at 15-m 
intervals along the permanent transects.  Physical and chemical water quality 
parameters were measured at one site every two weeks.  Fifteen species of SAV 
were recorded over the 10 years.  The five most common species recorded were 
coontail (Ceratophyllum demersum L.), Canadian waterweed (Elodea canadensis 
Michx.), Eurasian watermilfoil (Myriophyllum spicatum L.), curly pondweed 
(Potamogeton crispus L.), and sago pondweed (Potamogeton pectinatus L.).  In 
general, the frequency of sites that supported SAV increased from 1992 to 2000 
then decreased in 2001.  Although SAV declined in 1993 and 2001, both years of 
high water levels throughout June, different species contributed to the decline.  
Curly and sago pondweeds declined from 1992 to 1993 by 26% and 29% respectively 
while coontail, Canadian waterweed, and Eurasian watermilfoil increased by 26%, 
3%, and 12% respectively.  The opposite occurred from 2000 to 2001 when 
coontail, Canadian waterweed, and Eurasian watermilfoil declined by over 40% 
while curly and sago pondweeds increased by 10% and 16% respectively.  The 
highest percentage of vegetated sites occurred in 1998 (88%), 1999 (91%), and 
2000 (95%).  Volatile suspended solids and turbidity were at their lowest values 
in these three years indicating good water clarity.  Volatile suspended solids, 
turbidity, chlorophyll a, and total nitrogen all showed an overall downward 
trend from 1991 to 2000 then increased in 2001.  It is unknown whether the 
change in water quality parameters influenced the decline in the submersed plant 
community in 2001 or the decline in the submersed plant community influenced the 
water quality parameters.

Keywords:  submersed aquatic vegetation, water quality, Mississippi River, 
transect monitoring