Mitigation for the Construction and Operation of Libby Dam

Libby Reservoir Zooplankton Monitoring
Zooplankton species composition and abundance within Libby Reservoir has
remained relatively stable during the past several years (Appendix Tables A7-A13). Since
1997, Cyclops and Daphnia have been the first and second most abundant genera of
zooplankton present in the reservoir (Figure 41). Other lesser abundant genera in decreasing
order of abundance include Diaptomus, Bosmina, Diaphanosoma, Epichura and Leptodora
(Figure 41). Zooplankton abundance within the reservoir varies by season (Table 12; Figure
42). The results from 7 analysis of variance procedures that tested for differences in monthly
zooplankton abundance (by species) indicated that at least one month was significantly
different from other months in 2003 for the most abundant 7 species of zooplankton (Table
12). We did not perform multiple comparisons required to determine pairwise comparisons.
Although zooplankton abundance varies within a season, seasonal peaks in abundance over
the past six years (Figure 42) have remained relatively consistent across years. For example,
Daphnia abundance has peaked during July each year except 2003 (June peak) since 1997,
Diaphanosoma abundance has peaked in September during 6 of the last 7 years, Diaptomus
has peaked during October during 4 of the last 7 years, and Cyclops has peaked in June
during 4 of the last 7 years. In most cases when the annual peak differed from the mean
peak, the difference was not more than several weeks.
Our sampling design stratified the reservoir into thirds, and although each stratum
was long (> 58 km), we found only weak evidence that zooplankton abundance differed
between the three sampling areas (Tenmile, Rexford, and Canada) in 2003 (Table 12). For
the 7 most abundant species of zooplankton in the reservoir at the three sites, we only found
significant differences (by species) for 8 out of the possible 21 comparisons. When
significant differences did occur between sampling location, there was no clear trend in terms
of whether zooplankton abundance was always highest for most downstream site. During
2003, abundance estimates of Daphnia, Diaptomas, Cyclops, Leptodora, and Epischura
differed between at least one of possible three comparisons between sampling areas.
Subsequent multiple comparisons indicated that Daphnia densities were significantly higher
at the Rexford and Canada sites than the Tenmile site and Diaptomas densities were
significantly higher at the Rexford site than the Tenmile site, Cyclops abundance at Rexford
was significantly higher than the Canada and Tenmile sites, Leptodora abundance was
significantly higher at the Canada site than Tenmile and Rexford sites, and Epischura
densities were significantly higher at the Rexford site than the Canada site. The month and
area interaction term was significant for Bosmina, Cyclops, and Leptodora in 2003 (Table
12).
Although the abundance of zooplankton of the genus Daphnia in 2003 was the
highest during the previous 8 years, the trend has remained particularly stable in terms of
abundance (Figure 41) and size (Figures 43 and 44) during the past several years. Mean
annual Daphnia densities in Libby Reservoir from 1997 through 2003 have averaged 1.94
Daphnia /liter (standard deviation = 0.59/liter; Figure 43). Mean Daphnia length has also
varied relatively little since 1991, averaging 0.90 mm (standard deviation = 0.05; Figure 44).
Most Daphnia since 1993 are between 0.5 – 1.5 mm, with majority of Daphnia being
represented in the smaller size class 0.5 – 0.99 mm (mean annual proportion = 0.61, standard
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