AQUATIC MACROINVERTEBRATE RESPONSES TO ENVIRONMENTAL POOL MANAGEMENT AND 
EMERGENT VEGETATION IN BACKWATERS OF MISSISSIPPI RIVER POOL 25.

Michael B. Flinn1, Matt R. Whiles1, S. Reid Adams1,2
1Department of Zoology, Southern Illinois University, Carbondale, IL 62901
2Fisheries and Illinois Aquaculture Center, Southern Illinois University, 
Carbondale, IL 62901

The St. Louis District of the Army Corps of Engineers implemented Environmental 
Pool Management (EPM) on Mississippi River Pools 24, 25, and 26 in 1994.  The 
operational goal of EPM is to maintain relatively low, stable water levels in 
the lower portion of the pools following maximum drawdown in the spring to 
better simulate a natural hydrograph without inhibiting commercial navigation.  
When possible, water levels are held up to 2.0 feet below the target pool 
elevation at the lock and dam for at least 30 days.  Previous investigations of 
mudflats exposed via EPM showed substantial production of emergent vegetation 
consisting primarily of millet, chufa, and smartweeds.  However, little is known 
about the response of other biota to EPM.  From 2000-2002, we studied 
macroinvertebrate responses to EPM by examining communities in recently flooded 
vegetation in the fall.  Six paired (400 m2) vegetated and experimentally de-
vegetated plots were established to examine benthic organic matter and 
macroinvertebrate responses.  Sites represented a range of backwater types 
(e.g., island slough, island tip, and shallow bay).  Invertebrates were sampled 
using a stove pipe coring device (312cm2) during fall of each year.

In 2000, there was no vegetation response because a water level drawdown could 
not be sustained, whereas in 2001 there was a strong vegetation response. In 
2001 there was marginally higher total organic matter in vegetated plots 
(p<0.06), primarily because of differences in coarse material. During both 
years, there were no significant differences in total macroinvertebrate 
abundances between plot types. Total biomass was significantly higher in the 
vegetated plots in 2001 (p<0.001). Taxonomic differences were evident, for 
example, Dicrotendipes (p<0.01) and Physa/Physella  (p<0.03) abundances were 
significantly higher in the vegetated plots in 2001, whereas abundances of total 
Chironomidae (p<0.003), Chironomus (p<0.04), Cryptochironomus (p<0.02), 
Polypedilum (p<0.01) and the leech Helobdella (p<0.05) were significantly higher 
in de-vegetated plots. Total Chironomidae biomass was significantly higher in 
the devegetated plots (p<0.01). Results show that EPM and the associated 
vegetation responses influence the distribution of benthic organic matter and 
macroinvertebrate communities.  Although total macroinvertebrate abundance and 
biomass did not differ between vegetated and devegetated plots, individual taxa 
exhibited differential responses to these treatments.

Key Words:  Mississippi River, Environmental Pool Management, 
macroinvertebrates, vegetation, wetlands