Seasonal changes in the gelatinous zooplankton community and hydromedusa abundances in Korsfjord and Fanafjord, western Norway

Abstract

Quantitative seasonal studies on gelatinous zooplankton in Norwegian fjords are scarce. We recorded the quantitative composition of the gelatinous zooplankton community in Korsfjord and Fanafjord during 1 yr. Thirty-six species or genera of hydromedusae, 7 species of siphonophores, 4 species of ctenophores and 2 species of scyphomedusae were recorded. Aglantha digitale was numerically dominant in both fjords. A separate video-profiling study on the vertical distribution of fully grown specimens of this species was made in Korsfjord and the adjacent Bjørnafjord. Our data suggest 2 A. digitale generations yr–1, with relatively low importance of the latter generation. The overwintering strategy includes autumn growth to full size and distribution at intermediate depth, mainly between 200 and 300 m. Siphonophores were prominent in the more oceanic Korsfjord, while Fanafjord was characterized by meroplanktonic hydromedusae. More species were recorded from Korsfjord, which may be partly attributed to the larger volume sampled and the higher probability of encountering oceanic visitors in this fjord. Korsfjord also harbored a community of deep-water hydromedusae absent from Fanafjord. The gelatinous community of both fjords was most numerous and species-rich from April to June. Spring maximum densities were higher in Fanafjord. During winter, gelatinous zooplankton was more abundant in Korsfjord, with eudoxids of Dimophyes arctica and Lensia conoidea forming the bulk of the community. Hydromedusa species appeared sequentially and differed in terms of maximum abundance attained and length of their presence in the plankton. Multivariate analyses revealed a clear seasonal succession for the gelatinous community of both fjords.