Decreased influx of Calanus spp. into the south-western Norwegian Sea since 2003

The copepods Calanus hyperboreus and C. finmarchicus are the ecologically most important copepod species in the
Nordic Seas. The larger C. hyperboreus, of polar origin, is advected into the south-western Norwegian Sea with
the East Icelandic Current (EIC), and can therefore be considered an indicator species of these waters. Using large
spatio-temporal hydrographic and zooplankton data sets, we show that an abrupt reduction in abundance of C.
hyperboreus around the year 2003 in the south-western Norwegian Sea coincided with a reduction in the volume
of Modified East Icelandic Water (MEIW) in the intermediate layers (R = 0.64, lagged one year). Abundance of
C. hyperboreus and the volume of MEIW have since remained lower compared to the previous years (MannWhitney U tests, p < 0.05). The total zooplankton biomass in spring within the EIC, east of Iceland, was also
much reduced around 2003 and has also since remained low. A comparable post-2003 reduction is observed in
the abundance of overwintered stages of C. finmarchicus in May. Our results suggest that the reduction of
zooplankton in the south-western Norwegian Sea around 2003 was caused by reduced influence of subarctic
waters from the western region. This may in turn impact the extensive feeding migration of herring to this
region.