Zooplankton in Lake Atnsjøen 1985–1997

• Published in: Hydrobiologia 2004-06, Vol.521 (1-3), p.149-175
• Main Authors: Halvorsen, Gunnar, Dervo, Børre K., Papinska, Katarzyna
• Format: Article
• Language: English
• Subjects: Anthropogenic factors; Arctodiaptomus laticeps; Bosmina longispina; Bythotrephes longimanus; Cladocera; Conochilus unicornis; Copepoda; Crustaceans; Cyclops scutifer; Daphnia longispina; Eggs; Environmental factors; Freshwater; Heterocope saliens; Holopedium gibberum; Hypolimnion; Kellicottia longispina; Life cycles; Plankton; Polyarthra vulgaris; Polyphemus pediculus; Population density; Rotifera; Species composition; Thermocline; Vertical distribution; Winter; Zooplankton
• ISSN: 0018-8158; 1573-5117
• DOI: 10.1023/B:HYDR.0000026357.80231.d2

Issue Title: The Atna River: Studies in an Alpine-Boreal Watershed The aim of this paper is to study long-term changes in the zooplankton of a subalpine locality unaffected by direct anthropogenic disturbances. The material has been collected during the period 1985-1997; since 1988 a standardised sampling program has been followed, with five sampling dates during the ice free period (June-October) at three fixed stations. Altogether 17 species of Rotatoria, 9 species of Copepoda and 11 species of Cladocera were recorded. Of these 10 species of Rotatoria, two species of Copepoda and five species of Cladocera occur regularly in the plankton. Polyarthra vulgaris dominate among the rotifers together with Kellicottia longispina and Conochilus unicornis. The crustacean community is dominated by the copepod Cyclops scutifer, and the cladocerans Bosmina longispina, Holopedium gibberum and Daphnia longispina. The cladocerans Bythotrephes longimanus, Polyphemus pediculus,and the copepodsArctodiaptomus laticeps and Heterocope saliens all occur regularly, but at low densities. The zooplankton density is low in May/June and peaks in August and September, but the timing of maximum densities varies from year to year. By numbers, the rotifers strongly dominate with densities 10-15 times higher than the crustaceans. The annual maximum density (rotifers plus crustaceans) ranged from 50 ind. l^sup -1^ (1985, 1988) to 450 ind. l^sup -1^ (1995). Mean density is about 140-150 ind. l^sup -1^. By biomass (dry weight), the cladocerans constitute 60%, while the copepods and rotifers constitute 30% and 10% of the zooplankton, respectively. The annual maximum has varied between 70 and 260 μg dw l^sup -1^, with 170 μg dw l^sup -1^ as the mean level. C. scutifer usually has a one-year lifecycle without diapause, but a small fraction of the population has a two-year lifecycle. The life cycle in 1989 and 1990 differed strongly from the other years. The life cycles of B. longispina, D. longispina and possibly also H. gibberum and A. laticeps, indicate two generations during the summer. H. gibberumandH. saliens pass the winter as resting eggs. The other crustacean species, except C. scutifer, pass the winter mainly as resting eggs, but all have a small winter population in the water mass. C. scutifer is the only species without resting eggs. The vertical distribution normally shows highest density between 5 and 10 m depth. However, during some periods maximum density is observed close to t