Ref ID : 3015
Birger Pejler; The zooplankton of Osbysjon Djursholm I. Seasonal and Vertical distribution of the Species. OIKOS 12(2):225-248, 1961
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Notes
On the whole the seasonal and vertical distribution of the individual species is best explained by the diagrams and tables. The following points require fuller comment, however: 1. At the period in late winter when the oxygen content is low we find a strong decrease in species as well as in individuals. The oxygen tension (and/or related factors) often also affects the vertical distribution. Only very few of the zooplankters are restricted to cold water, probably because of the briefness of the period when the water is cold and the oxygen content high. 2. During the summer the following three rotifer species mainly dwell in a narrow zone close to the bottom: Anuraeopsis fissa, Polyarthra dolichoptera and Filinia terminalis. 3. The species composition of the rotifer plankton indicates a relatively high trophic level. Several heleoplanktic species (typical of ponds and other small waters) are found in Osbysjon, whereas some of the commonest representatives of large lakes are missing. 4. The closely related species Polyarthra vulgaris and P. major exclude each other in the lake, presumably because of excessive competition. The extinction of the former species during 1957 is connected with a sporozoan parasite epidemic. 5. Bosmina longirostris has occurred in the lake since the time it was isolated from the Baltic.