Seasonal vertical strategies in a high-Arctic coastal zooplankton community

Abstract

We studied the larger (>1000 µm) size fraction of zooplankton in an Arctic coastal water community in Billefjorden, Svalbard (78°40' N), Norway, in order to describe seasonal ver- tical distributions of the dominant taxa in relation to environmental variability. Calanus spp. numerically dominated the herbivores; Aglantha digitale, Mertensia ovum, Beroe cucumis, and Parasagitta elegans were the dominant carnivores. Omnivores and detritivores were numerically less important. Descent to deeper regions of the water column (>100 m) between August and October, and ascent to the shallower region (<100 m) between November and May was the overall seasonal pattern in this zooplankton community. In contrast to other groups, P. elegans did not exhibit pronounced vertical migrations. Seasonal vertical distributions of most species showed statistical associations with the availability of their main food source. The vertical distribution of later developmental stages of Calanus spp. was inversely associated with fluorescence, indicating that they descended from the shallower region while it was still relatively productive, and ascended before the primary production had started to increase. Strong associations between the vertical distributions of secondary consumer M. ovum and Calanus spp., and tertiary consumer B. cucumis and M. ovum indicated that these carnivores seasonally followed their prey through the water column. We conclude that seasonal vertical migrations are a widespread trait in the high Arctic community studied, and predatorprey interactions seem particularly central in shaping the associations between the seasonal vertical strategies of adjacent trophic levels.