Summary
Year 2003
- Because of budget constraints, Pools 4 and 26 of the Upper Mississippi River and La Grange Pool of the Illinois River were not sampled in 2003.
- Pools 8 and 13 were selected for continued monitoring to provide data
for modeling efforts that began in 2000 using data from these two pools.
Also, there was a poolwide water level drawdown in Pool 8 in 2001 and
2002. The drawdown was implemented to increase vegetation abundance,
which will in turn provide habitat for waterfowl and other aquatic life
forms. Because this type of hydrologic change could have an effect on
macroinvertebrate densities, we decided to continue the long-term data
string in Pool 8.
- The poolwide estimated mean densities of mayflies, fingernail clams,
and midges were all within the range of variation previously observed
in Pools 8 and 13 (Table 2;
Figures 2–4). The only substantial change
from 2002 was in Pool 13 where mayfly densities dropped from a 12-year
high of 221 m-2 to 77 m-2 in 2003, which matched
the 12-year low observed in 2001 (Figure
2).
- There was an extensive die-off in zebra
mussels in Pools 8 and 13
(Table 2) in 2003 (Figure
5). Large numbers of dead zebra
mussels were found in the samples. The few live zebra mussels found
were small (<1 cm). Although the die-off was extensive, there were
reports over the summer of small zebra mussels on aquatic vegetation,
boats, and docks; thus, 2003 may have been a good year for recruitment.
The die-off may be part of a demographic shift. Zebra mussels only live
4–5 years and our data from 1999 and 2000 indicate that large year
classes of zebra mussels were produced in these pools 4–5 years
ago. Thus, this may have been a natural die-off of older zebra mussels.
Other potential causes include parasites, water flows, or water temperature
changes in fall/winter 2002.
- The distribution of invertebrates among aquatic areas within Pools 8 and 13 was similar to previous years. The highest densities of mayflies occurred in contiguous backwaters and impounded areas (Table 3), fingernail clams were most abundant in the impounded areas (Table 4), and midges in the contiguous backwaters (Table 5).
Year 2002
- Overall, the high spring flood of 2001 did not seem to have an adverse effect on macroinvertebrate numbers in 2002. In fact, several aquatic areas in Pool 26 and La Grange study areas recorded the highest densities of fingernail clams and midges seen since sampling began in 1992 (see below).
- Macroinvertebrate sampling in 2002 produced a total of 2,411 mayflies,
3,983 fingernail clams, 3,514 midges, 1 Asiatic clam, and 3,700 zebra
mussels from 551 total samples. This is a 50% increase in the total
number of mayflies, 21% increase in the total number of fingernail clams,
and a 98% increase in midges from 2001, yet these numbers are still
within the range of numbers seen since sampling began in 1992. The total
number of zebra mussels collected in 2002, decreased 53% from 2001.
- The poolwide estimated mean densities of mayflies decreased between 2001 and 2002 in Pools 4 and 8, while Pools 13 and 26 and La Grange Pool showed increases (Table 2; Figures 2-4). All study areas, except for Pool 4, had increases in the poolwide estimated mean densities of fingernail clams and midges. Pool 13 was the only study area that had increases in all of the target taxa.
- In Pool 13, the poolwide estimated mean densities of midges dropped
precipitously from 574 m-2 in 2000 (the highest level ever
recorded by LTRMP) to 28 m-2 in 2001; however, in 2002 they
rebounded to 191 m-2.
- Densities of Asiatic clams were low in all study areas. Zebra mussel
densities were highest in Pool 13 showing a slight increase from 2001,
whereas Pools 4, 8, and 26 and La Grange Pool zebra mussel densities
decreased (Table 2).
- Over the last 9 years of sampling, Lake Pepin has consistently been the "hotspot" for mayflies in Pool 4 (Table 3). The impounded and backwater contiguous aquatic areas in Pools 8 and 13 supported the highest densities of mayflies (Table 3). Fingernail clam densities were highest in the impounded aquatic areas in Pools 4 (Lake Pepin), 8, 13, and 26 (Table 4) compared to other aquatic areas.
- The side channel aquatic area in La Grange Pool had the highest densities
of fingernail clams—this is the highest densities of fingernail
clams collected since sampling began in 1992 in this area (Table
4).
- The backwater contiguous and impounded areas had higher densities of midges than other aquatic areas (Table 5) in all study areas. In Pool 26, midge densities reached their highest recorded level of 313 m-2 in the impounded area compared to all other years and aquatic areas. While in La Grange Pool, the backwater contiguous area had the highest ever recorded density of midges since sampling began.
- In addition to the target taxa (mayflies, fingernail clams, midges, Asiatic clams, and zebra mussels, only Oligochaeta (aquatic worms and leeches) were present in samples more times than they were absent in all study areas except in Pool 26. In Pool 26, only midges were present in samples more times than they were absent.
Year 2001
- In 2001, the Open River Reach was dropped from the macroinvertebrate
monitoring design because the previous 9 years of sampling have consistently
indicated low densities of most taxa, especially mayflies (Hexagenia
spp.) and fingernail clams (Sphaeriidae). Within this reach, habitat
suitability for invertebrates in soft substrates is not expected to
increase in the foreseeable future. We are currently investigating other
methods for monitoring invertebrates in this river reach.
- The spring flood of 2001 made macroinvertebrate sampling exciting
this year! River crests were the second or third highest on record for
many areas of the Upper Mississippi River. Fighting high water and strong
currents, the LTRMP field stations were able to complete all sampling.
Macroinvertebrate sampling in 2001 produced a total of 1,610 mayflies,
3,289 fingernail clams, 1,773 midges, 5 Asiatic clams, and 7,838 zebra
mussels from 557 total samples. This is a 47% decline in the total number
of mayflies and a 32% decline in midges from 2000, yet these numbers
are still within the range of numbers seen since sampling began in 1992.
The total number of fingernail clams collected in 2001 increased 22%
from 2000, whereas zebra mussel numbers increased by 64%. This is the
highest total number of zebra mussels collected since sampling of zebra
mussels began in 1995.
- The poolwide estimated mean densities of mayflies decreased between
2000 and 2001 in all study areas (Table
2; Figures 2-4). Pools 4 and 8 had the
highest estimated mean density of mayflies, whereas in Pool 13 mean
densities reached their lowest recorded level at 77 m-2.
Changes in densities of fingernail clams and midges were variable among
study areas with no consistent patterns. However, in Pool 13, density
of midges dropped precipitously from 574 m-2 in 2000 (the
highest level ever recorded by LTRMP) to 28 m-2 in 2001.
- Zebra mussel densities were highest in Pools 4, 8, and 13 all of which
increased in 2001, whereas Pool 26 and La Grange Pool decreased
(Table 2). Pool 8's estimated
density of 882 zebra mussels m-2 was the highest poolwide
density ever recorded by LTRMP sampling.
- The impounded aquatic areas in Pools 4 (Lake Pepin), 8, 13, and 26
supported the highest densities of mayflies (Table
3) and fingernail clams (Table
4 ) compared to other aquatic areas. The impounded and backwater
contiguous aquatic areas have consistently been the most productive
areas for mayflies and fingernail clams over the years.
- The backwater contiguous areas in Pools 8, 13, and 26 and La Grange Pool had higher densities of midges than other aquatic areas (Table 5). In Pool 4, the highest density of midges was found in Lake Pepin. In Pool 26, midge densities reached their highest recorded level of 267 m-2 in backwater contiguous areas compared to all other years and aquatic areas.
- Densities of zebra mussels were highest in the impounded areas in
Pools 4, 8, and 13 (Table
6 ). Extremely low numbers of zebra mussels were found in Pool 26
(33 individuals collected) and La Grange Pool (1 individual collected).
- Overall, the high spring flood of 2001 did not affect LTRMP macroinvertebrate sampling. The flood's immediate effect on invertebrate abundance is unclear, but high flows with associated sediment scouring and deposition could have contributed to reduced densities of mayflies and midges. However, sampling in 2002 will provide the first opportunity to assess the long-term effects of the flood and of the experimental reduction in water levels conducted in Pool 8 during summer 2001.
Year 2000
- Macroinvertebrate sampling (n = 659) in 2000 produced a total
of 3,446 mayflies, 2,694 fingernail clams, 5,503 midges, 8 Asiatic clams,
and 4,771 zebra mussels.
- Mean densities of target taxa were weighted by strata selected for
macroinvertebrate sampling (Sauer
1998) to estimate pool- or reach-wide means (Table
2; Figures 2-4). Pool 8 had the highest
estimated mean numbers of mayflies and fingernail clams. Pool 13
had the highest densities of midges. The estimated mean number of mayflies
increased between 1999 and 2000 in Pools 4 and 8. All study areas showed
slight declines in fingernail clams. No fingernail clams were found
in the Open River Reach. Midge densities increased in Pools 13 and 26,
the Open River Reach, and La Grange Pool.
- Mayfly densities have remained relatively stable in Pool 13 since
the beginning of the monitoring program (Table
2).
- Zebra mussel densities were highest in Pools 8, 13, and 26 with some
Ponar grab samples containing more than 200 individual zebra mussels
(equivalent to 3,846 m-2). Zebra mussel densities increased
tremendously in Pool 26 (Table
2).
- The impounded aquatic areas in Pools 4 (Lake Pepin) and 8 supported the highest numbers of mayflies (Table 3).
- Mean densities of fingernail clams were highest in Lake Pepin (Pool
4) and the impounded areas of Pools 8 and 26. The highest densities
of fingernail clams in Pool 13 and La Grange Pool were found in
the backwater contiguous aquatic areas (Table
4).
- The backwater contiguous areas in Pools 4, 8, 13, and 26 and La Grange
Pool had higher densities of midges than other aquatic areas (Table
5). In the Open River Reach, the highest density of midges were
found in the side channel aquatic areas.
- The impounded areas had the highest densities of zebra mussels in
Pools 4, 8, and 13 (Table
6). Low numbers of zebra mussels were found in La Grange Pool—only
four individuals were found during stratified random sampling (n
= 99).
- Visual classification of sediments indicated that sample sites in
Pools 4, 8, 13, and 26 and La Grange Pool were predominantly silt clay.
Sampled substrates in the Open River Reach were mostly sand.
Content manager: Jennie Sauer
Page Last Modified: April 17, 2018