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Reports and Publications
Macroinvertebrate Update Title Page

Yearly Summaries

Year 2004

• Because only Pool 8 and 13 were sampled in 2003, comparisons to previous year mean densities are not possible for Pools 4, 26, and La Grange. However, the poolwide estimated mean densities of mayflies, fingernail clams, and midges were all within the range of variation previously observed in all study areas over all years (Table 2; Figures 2–4).
  
  
• Pool 8 recorded the highest poolwide estimated mean densities of fingernail clams and midges seen in the pool over the period of record (Table 2). Pool 8 fingernail clam densities in 2004 continue the high levels seen beginning in 1999 (Figure 3). The mean density from 1992–1998 was 13 per square meter whereas from 1999–2004 was 347 per square meter. This represents a 25 fold increase in density. The reason for this substantial and persistent increase is unknown; but could be due to a decrease in inorganic suspended solids (Gray et al. 2005).
  

• Among all study reaches and aquatic area strata, the highest densities of mayflies (Table 3) and fingernail clams (Table 4) occurred in impounded areas in Pools 4 (Lake Pepin), 8, 13, and 26, which is similar to previous years.
  
  
• Midges were most abundant in the impounded areas of Pools 4, 8, and 26 (Table 5 ), and in the contiguous backwaters (Table 5) in Pool 13 and La Grange Pool.
  
  
• Zebra mussel densities (Table 2; Figure 5) continued to be low in all areas except for Pool 13, which reported the highest poolwide estimated mean densities among all study reaches since sampling began in 1995. Within Pool 13, highest densities occurred in the impounded area (Table 6).
  
  

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 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, over the summer there were reports 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 and 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 Pool 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.
  

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Content manager: Jennie Sauer

Page Last Modified: June 29, 2021