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Crustacean plankton was studied in 12 lakes in theHoylandet area in 1986–87. Basic lake characteristicsare elevation 134–415 m, surface area 9–530 ha, pH 5.9–7.1,conductivity (25 °C) 12–40 µS cm-1 andSecchi depth 4–9 m. Number ofspecies present varied between 3 and 11. Populationnumbers between 4000 and 400 000 per m2 andbiomasses were within the range 30 to1800 mg m-2 dry weight. Cladocera dominated overCopepoda in lakes with allopatric brown trout (Salmo trutta L.), on the contrary to lakes also populatedby Arctic charr (Salvelinus alpinus (L.)). Thesevariations are caused by differences in elevation,lake morphometry, water quality, fish predation andthe general distribution of the species. The largestlakes at lowest elevation were richest in species. Theacid sensitive genus Daphnia was represented by 3species. The lakes Storgronningen (530 ha) andRoyrtjonna (27 ha) were sampled monthly in theice-free seasons of 1986–89, and Storgronningen moreintensively from June to November in 1987 and 1988. The same6 species of Cladocera and 5 of Copepoda were presentin both lakes. Their life cycles were traditional orknown from several other Scandinavian lakes. Meanseasonal biomasses were of the range600–750 mg m-2. At the species level, there wereconsiderable variations between years inStorgronningen and particularly in Royrtjonna. Noeffects of human impacts on the crustacean planktonwere found. The Hoylandet lakes are representative forScandinavian oligotrophic to almost ultra-oligotrophiclakes. Storgronningen is well qualified as a referencesystem. The between year variations in Royrtjonna areso extreme, that any human impact could only be traced at alevel causing the extinction of species. |
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