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Summary

Longitudinal distribution

• Vegetation stratified random sampling was conducted in Pools 4, 8, 11, 13, 26, and La Grange Pools in 2001.

• The estimated percent frequencies of submersed aquatic vegetation in the shallow water areas in Pools 4, 8, 13, 26, and La Grange Pool were 36.9%, 47.5%, 41.7%, 0.3%, and 0%, respectively (Tables 2 , 3, 5, 6, and 7). The longitudinal pattern of submersed aquatic vegetation is the same as revealed in the previous three years from 1998 to 2000. Pool 11 (16.1%) had less submersed vegetation than the upper three pools (Table 4).

• This pattern is also consistent with the longitudinal pattern displayed in the aerial photos of 1989 that submersed aquatic vegetation was abundant in the Upper Mississippi River reaches upstream of Lock and Dam 13, but rare or negligible elsewhere in the Upper Mississippi River System (UMRS; Rogers and Theiling 1999). A deviation from this longitudinal pattern was observed after the 1987-89 drought and in 1993 after an unusually high flood disturbance, when little submersed aquatic vegetation occurred in the entire UMRS. We did not sample the entire UMRS in 2001, but we have no reason to suspect a deviation from the normal pattern occurred in 2001.

• The estimated percent frequencies of rooted floating-leaf vegetation in Pools 4, 8, 11, 13, and 26, and La Grange Pools were 7.9%, 15.2%, 7.5%, 22.7%, 4.2%, and 0.3%, respectively ( Tables ).

• Rooted floating-leaf species shifted in dominance from white water lily (Pools 4 and 8) to American lotus (Pools 13 and 26). This same longitudinal pattern has occurred since 1998.  Neither American lotus (5.8%) nor white water lily (4.4%) was dominant in Pool 11.

Within-pool Distribution

• The within-pool distribution patterns of submersed aquatic vegetation were highly heterogeneous between pools but remained little changed since 1998.

• Submersed aquatic vegetation was sparse and species-poor in upper Pool 4 above Lake Pepin compared with the lower Pool 4 below Lake Pepin ( Figure 2 ). Rooted floating-leaf vegetation followed the same general pattern. The most common submersed species included coontail and water stargrass.

• Submersed aquatic and rooted floating-leaf vegetation was distributed widely throughout Pool 8 except in the lower end where water depth generally exceeded 1 m  ( Figure 4). The most common submersed species included coontail, Canadian waterweed, and water stargrass.

• Submersed aquatic vegetation was found throughout Pool 11 ( Figure 6). Rooted floating–leaf was found mostly in the middle portion of the pool.  The most common submersed species included sago pondweed and coontail.

• A considerable amount of submersed aquatic vegetation was recorded in Pool 13, most of which occurred in the contiguous backwaters and impounded areas at the lower half of the pool ( Figure 8). Most of the rooted floating–leaf vegetation was found in contiguous backwaters and along the shoreline in impounded areas in the lower half of the pool. The most common submersed species included coontail and sago pondweed.

• An insignificant amount of submersed aquatic and rooted floating–leaf vegetation was found in Pool 26, in the isolated backwater areas of the Illinois River ( Figure 10).

• In La Grange Pool, submersed aquatic vegetation was found to exist in the lakes on the Illinois River floodplain and was absent in the river's backwater areas ( Figure 12). Most rooted floating-leaf vegetaApril 14, 2006l was the most common submersed species recorded.

• The distribution of submersed aquatic vegetation appears to be correlated with the physical parameters of water depth, current velocity, and fetch.

Status and Trend

• The spatial extents of submersed aquatic vegetation in the five key pools have remained stable since 1998 based on the frequencies of the sites sampled that supported submersed aquatic vegetation .
  
  
• However, the amount of vegetation per site, based on the abundance index, has displayed a trend of steady decline in Pool 8 and a steady increase in Pool 13 since 1999.  Factors responsible for the two opposite trends are yet to be determined.

• Pool 13 has shown a slight increase in the frequency of rooted floating-leaf vegetation from 1998 to 2001 while the other pools have remained stable.
  
  
• Two exotic submersed species have been recorded, Eurasian watermilfoil and curly pondweed.  Both were found in Pools 4, 8, 11, 13, and islolated lakes of the La Grange Pool.  Curly pondweed has also been recorded in Alton Pool.  Other than Eurasian watermilfoil in the isolated lakes of the La Grange Pool, neither species has been dominant, occuring in 15% or less of the sites in all pools and years.

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