AN ESTIMATE OF LIGHT AVAILABILITY FOR AQUATIC VEGETATION IN POOLS 4, 8, 13, AND 
26 OF THE UPPER MISSISSIPPI RIVER FROM 1993 to 1999.

Dennis M. Wasley1,2, James T. Rogala1, and David M. Soballe1
1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54603.  2University of Wisconsin-La Crosse, La Crosse, WI 54601

Dramatic fluctuations in abundance of submersed aquatic vegetation across the 
Upper Mississippi River System (UMRS) from the late 1980's to present have been 
the subject of several studies and wide speculation.  Management actions have 
been proposed and implemented (e.g. large-scale water level reductions in Pool 
8) in an attempt to enhance aquatic and semi-aquatic vegetation in the system, 
but the underlying causes for long-term changes in aquatic vegetation in the 
UMRS are not established.  Light availability from May through September, the 
primary growing season, is one factor that often limits the growth of submersed 
vegetation.  This is especially true in turbid river systems such as the UMRS 
where concentrations of plant nutrients appear to be present in excess, but 
light penetration is restricted by suspended materials in the water.  Detailed 
information on water clarity or light availability in broad areas of the UMRS 
was not collected prior to the implementation of the Long Term Resource 
Monitoring Program (LTRMP, ca 1990) and statistically reliable, pool-scale 
information on aquatic plant abundance across the system was not collected prior 
to1999.  Nonetheless, we believed that it might be possible to use existing 
information to estimate (model) the light regime in the UMRS for the past 10 - 
20 years. 

We used data from LTRMP stratified random sampling, LTRMP bathymetric surveys, 
and long-term stage and discharge records from the U.S. Army Corps of Engineers 
to estimate the areal extent of illuminated substrate for Pools 4, 8, 13 and 26 
of the UMRS.  Our estimates suggest an increase from 1993 to 1999 when aquatic 
plants were also increasing.  Among pools we found substantial differences in 
substrate illumination that were driven by differences in pool morphology and a 
general downstream increase in turbidity within the UMRS.  These differences in 
illumination seem to correspond well with among-pool differences in aquatic 
vegetation.  We found weak relations in the UMRS between water clarity and 
factors such as month, river stage, and tributary discharge and the relations 
among these variables may not be helpful for broad scale modeling of light 
availability in this system.

Keywords:  Mississippi River, submersed vegetation, turbidity, light, secchi 
depth