NUTRIENTS, CHLOROPHYLL, AND SUSPENDED SOLIDS IN THE UPPER 
MISSISSIPPI RIVER:  TEMPORAL AND SPATIAL VARIABILITY

Jeffrey N. Houser, Brian R. Gray, and James T. Rogala
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI  
54603

Substantial variability in time and space is a defining characteristic of large, floodplain 
rivers.  We investigated temporal and spatial variation in a group of limnological 
variables related to river primary production using data from the Upper Mississippi River 
(UMR) Long Term Resource Monitoring Program (LTRMP).  We included nutrient, 
chlorophyll a, and suspended sediment concentrations from 6 study reaches of the UMRS 
measured quarterly from 1994 to 2005.  We found that total phosphorus (TP) and total 
suspended solids (TSS) increased moving downriver among the study reaches.  Total 
nitrogen (TN) and chlorophyll varied among study reaches but did not exhibit consistent 
downstream increases. Chlorophyll and TP were generally higher in backwater areas than 
in the main channel, indicating higher algal standing stock and potentially higher rates of 
production in these backwater areas. TN was usually higher in the main channel than in 
backwater areas, probably reflecting the relatively high rates of denitrification that occur 
in off-channel areas.  Backwater TSS was higher than main channel TSS during low 
discharge conditions and lower than main channel TSS during high discharge conditions.  
Contrasting seasonal patterns occurred for TN (highest in spring) and TP (lowest in 
spring) in some reaches.  Seasonal patterns in chlorophyll varied among study reaches.  
These spatial and temporal patterns in limnological characteristics of the UMR provide 
important context for understanding the physical and biological processes that structure 
the UMRS and similar large, floodplain rivers.

Keywords: nitrogen, phosphorus, chlorophyll, suspended solids, water quality