Biomass varies between years, but no trends have yet been detected.
The high-latitude ecosystem of the Norwegian Sea consists of areas with different physical regimes, and the length of productive season and intensity of biological production varies among these areas. In the east–west direction the Norwegian Sea can be divided into Norwegian coastal, Atlantic, and Arctic habitats, which is reflected in the zooplankton species composition. One of the most important zooplankton groups in the Norwegian Sea is the genus Calanus, both in numbers and biomass. In the Norwegian coastal and Atlantic habitats C. finmarchicus dominates the zooplankton biomass in spring and summer, and C. helgolandicus is also found in southern and eastern parts of these habitats. In the Arctic habitat C. hyperboreus is important. Of other species, the krill Thysanoessa inermis, T. longicaudata, and Meganyctiphanes norvegica are widespread, the latter especially in the warmer Atlantic and coastal habitats. The amphipod Themisto libellula is abundant in the Arctic, and T. abyssorum in the Atlantic habitats. The seasonal pulse of zooplankton production starts in southern and eastern parts of the Norwegian Sea, with a time delay towards the colder areas in the western and northern parts.
Zooplankton biomass has shown considerable fluctuations over the period 1995-2016 (Figure 11). The plankton index was relatively high from 1995 to 2002, decreased steadily from 2003 to 2006 and has since remained at relatively low levels.
Figure 11: Indices of zooplankton dry weight sampled in May in and near the Norwegian Sea, from 1995 to 2016.