ABSTRACT: Annual cycles of abundance of the ctenophore species Bolinopsis infundibulum and Beroe cucumis were studied in the Malangen fjord, northern Norway, during 1992. The seasonal dynamics were characterized by alternating abundances of the 2 species. A biomass increase of B. infundibulum occurred in April and May, reaching a peak of 2 to 7 ml m^-3 wet volume in the surface layers in summer. The species started to reproduce in the inner part of the fjord, and then propagated throughout the entire fjord system. Length frequency distributions indicated that B. infundibulum were reproducing continuously during the summer months. Biovolumes of B. infundibulum decreased in August, leaving a small number of individuals (<1 ind. per 5700 m³) to overwinter in the water column. Small individuals of B. cucumis occurred in the surface layers in spring. Biovolumes of this species were low during summer, but increased in the fall, and a large overwintering population(2 to 6 ml m^-3) was present in the deep basins of the fjord (below 200 m) from September to April. The calculated predatory impact of B. cucumis on B. infundibulum increased during summer (from 6 to 50%), and contributed to the autumn decrease in volume of B. infundibulum. The winter population of B. cucumis would be short of food if ctenophores were the only prey. Possible mechanisms contributing to the observed patterns are discussed.

KEY WORDS: Ctenophora · Seasonality · Zooplankton · Predation · Fjord · Northern Norway

Published in MEPS Vol. 140 (1996) on September 12
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996