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JOURNAL OF PLANKTON RESEARCH | VOLUME 17 | NUMBER 9 | PAGES 1819-1833 | 1995
© Oxford University Press

research-article

Long-term changes in crustacean zooplankton—the effects of a mass removal of Arctic chan, Salvelinus alpinus (L.), from an oligotrophic lake

Geir A.P. Dahl-Hansen

Norwegian College of Fisheries Science, University of Tromsø N-9037 Tromsø, Norway

Received on June 8, 1994; accepted on May 13, 1995 The long-term effects of the removal of ~32 metric tons of stunted Arctic charr on the crustacean zooplankton community in the oligotrophic Lake Takvatn, north Norway, were investigated. The removal of charr resulted in a sharp decline in the abundance of this fish in the pelagic zone. The two copepods, Cyclops scutifer and Eudiaptomus graciloides, and the small cladoceran Bosmina longispina, were the only abundant plankton species in the limnetic zone prior to the stock reduction program. A slight increase in the density, biomass and relative importance of B.longispina, Holopedium gibberum and Daphnia longispina occurred from 1984 to 1989, reaching a maximum in 1989. An increased body length of ovigerous B.longispina was also observed. These changes seemed to coincide with the reduction in abundance of Arctic charr in the limnetic zone, suggesting a regulatory role of the charr. A decline in 1990, and a total collapse in the cladoceran compartment in 1991, occurred simultaneously with observations of high densities of threespine sticklebacks in Takvatn. After a mass die-off of this fish, B.longirostris contributed strongly to the zooplankton community as the only cladoceran in 1992. Along with the re-occurrence of B.longispina and H.gibberwn in 1993, the importance of B.longirostris diminished. The highest total zooplankton biomass ever registered was found this year.


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