COMPOSITION OF THE SEEDBANK IN NAVIGATION POOL 8, UPPER MISSISSIPPI
RIVER.

Kevin P. Kenow and James E. Lyon.  
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603.

In an effort to enhance aquatic plant production and habitat diversity on the Upper Mississippi
River (UMR), the U.S. Army Corps of Engineers St. Paul District has scheduled a pilot water
level reduction on Navigation Pool 8 of the UMR.  The water level reduction is expected to dry
and consolidate bottom sediments and, thereby, increase the area of emergent and submersed
aquatic vegetation by natural seed germination.  To quantify the availability of seed, we assessed
the potential seed bank of selected areas of Navigation Pool 8 of the UMR from substrate
samples collected in spring 2000.  Samples from each sample site were divided into 4 equal
portions, spread over sterile soil, and reared in containers under prescribed conditions.  We tested
these samples for viable seed content under 4 sets of hydrologic conditions; substrate exposed
and allowed to dry (dry), substrate exposed but kept moist (moist), substrate covered with 2-3 cm
of water (shallow flooded), and substrate covered with about 60 cm of water (submerged).  The
experiment was initiated on 1 May and containers were monitored for plant growth on a regular
basis (i.e., 2-3 times per week) through the end of August.  Fifty species were identified in the
seedbank, including 10 submersed aquatic, 6 emergent, 13 obligate wetland, 11 facultative
wetland, and 10 upland species.  Dominant taxa within the seedbank included Sagittaria spp. ((
density = 508 seedlings/m2; frequency of occurrence = 85%), Lindernia dubia (378; 60%),
Zosterella dubia (146; 70%), Cyperus spp. (122; 53%), Eragrostis spp. (110; 40%), and Leersia
oryzoides (73; 43%).  Of the 4 hydrologic treatments, moist substrate flats had the greatest
species diversity and were the most productive, yielding an average density of 1,420
propagules/m2.  Propagule density generally did not differ (P > 0.05) between sites with sand or
silt substrates.  Emergent and submersed aquatic species were widely distributed, occurring in
more than 90% of the samples.  Invasive species (Phalaris arundinaceae, Lythrum salicaria, and
Potamogeton crispus) were present in 35% of the samples.  Pool 8 seedbank results are compared
to seedbank assessments conducted on selected areas of Pool 5 and 9 of the UMR. 
Experimentally determining the composition of seedbanks is an effective way of predicting the
types of vegetation that may develop on substrates exposed during water level reductions.  The
information is useful to resource managers in determining whether a specific water level
management action has the potential of achieving desired vegetation response. 

Keywords:  aquatic plant, Navigation Pool 8, propagule, seedbank, Upper Mississippi River