CRUSTACEAN PLANKTON IN HOYLANDET

Crustacean plankton was studied in 12 lakes in the Hoylandet area in 1 986-87. Basic lake characteristics are elevation 134-415 m, surface ar ea 9-530 ha, pH 5.9-7.1, conductivity (25 degrees C) 12-40 mu S cm(-1) and Secchi depth 4-9 m. Number of species present varied between 3 an d 11. Population numbers between 4000 and 400 000 per m(2) and biomass es were within the range 30 to 1800 mg m(-2) dry weight. Cladocera dom inated over Copepoda in lakes with allopatric brown trout (Salmo trutt a L.), on the contrary to lakes also populated by Arctic charr (Salvel inus alpinus (L.)). These variations are caused by differences in elev ation, lake morphometry, water quality, fish predation and the general distribution of the species. The largest lakes at lowest elevation we re richest in species. The acid sensitive genus Daphnia was represente d by 3 species. The lakes Storgronningen (530 ha) and Royrtjonnna (27 ha) were sampled monthly in the ice-free seasons of 1986-89, and Storg ronningen more intensively from June to November in 1987 and 1988. The same 6 species of Cladocera and 5 of Copepoda were present in both la kes. Their life cycles were traditional or known from several other Sc andinavian lakes. Mean seasonal biomasses were of the range 600-750 mg m(-2). At the species level, there were considerable variations betwe en years in Storgronningen and particularly in Royrtjonna. No effects of human impacts on the crustacean plankton were found. The Hoylandet lakes are representative for Scandinavian oligotrophic to almost ultra -oligotrophic lakes. Storgronningen is well qualified as a reference s ystem. The between year variations in Royrtjonna are so extreme, that any human impact could only be traced at a level causing the extinctio n of species.