Zooplankton Data

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Since 1985, two crustacean taxa and 11 rotifer taxa where collected in Crater Lake. In winter, most of the taxa were distributed from the lake surface to the depth of mixing (200-250 m). During periods of thermal stratification the taxa were spatially segregated within the water column (Figure 8). Polyarthra was the dominant taxon in the upper 40 m of the lake, but it occurred in low density. Between 40 m and 80 the dominant taxa were Bosmina, Polyarthra, Kellicottia and Asplanchna. From 80 to 120 m the dominant taxa were Daphnia, Keratella, Synchaeta, Filinia and Polyarthra. The dominant taxa from 120 to 200 m were Philodina, Conochilus, Keratella and Collotheca.

Some taxa were not present every year from 1985 to 1990. Philodina was present between 1985 and 1988, Conochilus was present in 1985, and Asplanchna was present in 1990. Daphnia was not present in quantitative subsamples in 1985, and was in low density in 1986 (Figure 9). The population reached its maximum abundance in 1988, declined to low density by 1990, and was absent in 1993. The taxon returned in abundance in 1998-99.

Figure 8: Depth profiles of selected zooplankton taxa. Data presented as averages per depth. Bars represent 1 standard deviation.

Figure 9: Densities of Daphnia pulicaria integrated to 200 m from 1985 to 1999.