Plankton community composition and vertical migration during polar night in Kongsfjorden

The polar night in the Arctic is characterized by up
to six months of darkness, low temperatures and limited food
availability. Biological data on species composition and
abundance during this period are scarce due to the logistical
challenges posed when sampling these regions. Here, we
characterize the plankton community composition during
the polar night using water samplers and zooplankton net
samples (50, 64, 200, 1500 lm), supplemented by acoustics
(ADCPs, 300 kHz), to address a previously unresolved
question–which species of zooplankton perform diel vertical
migration during the polar night? The protist community
(smallest plankton fraction) was mainly represented by ciliates
(Strombidiida). In the larger zooplankton fractions (50,
64, 200 lm) the species composition was represented primarily
by copepod nauplii and small copepods (e.g., Microcalanus
spp., Pseudocalanus spp. and Oithona similis). In
the largest zooplankton fraction ([1500 lm), the euphausiid,
Thysanoessa inermis, was the most abundant species
followed by the chaetognath Parasagitta elegans. Classical
DVM was not observed throughout the darkest parts of the
polar night (November–mid-January), although, subtle vertical
migration patterns were detected in the acoustic data.
With the occurrence of a more distinct day–night cycle (i.e.,
end of January), acoustical DVM signals were observed,
paralleled by a classical DVM pattern in February in the
largest fractions of zooplankton net samples.Wesuggest that
Thysanoessa spp. are main responsible for the acoustical
migration patterns throughout the polar night, although,
chaetognaths and copepods may be co-responsible.