Winter Tales: the dark side of planktonic life

Abstract

This paper presents the first comprehensive insight into the structure of planktonic communities, from nanoprotists to mesozooplankton in the waters north of Svalbard during the dark Arctic winter. Our investigations were carried out in two contrasting locations comparing a shallow fjord, Rijpfjorden, and a deep ice-covered region, Sofiadjupet, in January 2012. They revealed low chloro- phyll a concentrations (\0.05 mg m -2 ), as a result of the low protist biomass (0.54-2.04 mg C m -2 ). Overall, 27 plastidic protist taxa were observed including 14 auto- trophic taxa and one typical early Arctic spring bloom species (Entomoneis sp.). The presence of these photo- trophic organisms, mostly dinoflagellates, showed that some pelagic autotrophs are able to survive the dark winter period. The high mesozooplankton biomass (6-8 g C m -2 ) indicated a healthy community despite the scarcity of pri- mary producers. Three Calanus species dominated the mesoplanktonic biomass (79 %) in Rijpfjorden, whereas in Sofiadjupet carnivorous Chaetognatha (72 %) were domi- nant. Vertical distribution/profiles of the different trophic categories showed that herbivores were concentrated in the surface water layers (20-0 and 100-10 m) at both sites, whereas predators were occupying the deeper waters (600-100 m) of Sofiadjupet. Our results indicate that high mesozooplankton biomass is concentrated in surface waters. This contradicts the paradigm that mid-January, Arctic zooplankton communities overwinter in deep waters in a dormant state. Moreover, this work fills an evident gap in existing knowledge of Arctic plankton standing stocks and the relationships between its various size fractions during the polar night.