The zooplankton community of the Barents Sea consists mainly of typical phytophages feeding on phytoplankton. However, there are also representatives of predatory plankton including Chaetognatha, most Amphipoda (Hyperiidae), Pteropoda, and also “jellyfish” – Scyphozoa (genus Aurelia, Cyanea) and Ctenophora. Large-scale predation of Calanus by ctenophores was observed in the Barents Sea in 1971 and 1983 (Fomin, 1985). Chaetognatha prefer rather large prey, such as small and large Copepoda, Cirripedia larvae, Euphausiacea and their larvae, also Amphipoda, fry of Chaetognatha and Oikopleura.
The daily ratio in adult Chaetognatha of 10 %, equivalent to one meal in every 3-6 days, found in the North Sea, is probably representative for the situation in the Barents Sea also. Among two Pteropoda species, the smallest– Limacina helicina eats various planktonic organisms – Crustacea (Copepoda, nauplii), larvae of Bivalvia, Tintinnidae, Dinoflagellata and Diatomea (Foster 1987, Gilmer and Harbison, 1991). Migrating to the surface, L. helicina can form big concentrations attracting predators. These molluscs are food item for some fishes (capelin, herring), as well as seabirds and whales. Hyperiidae prey intensively on large and small Copepoda, Chaetognatha, Euphausiacea and even larvae of fish. The mentioned groups of predatory zooplankton are usually very abundant and can their abundance influence the structure of the plankton community in the Barents Sea.
Jellyfish feed on zooplankton but have few enemies, leading to “dead-end” pathways to higher trophic levels. The most striking example is the disastrous outburst of the ctenophore Mnemiopsis leidyi in the Black and Azov Seas, completely suppressing the development of fish biomass in these areas (Kidey, 1994). In the Barents Sea such dead-ends are unlikely to be important due to interspecies predation among jellyfish. The planktivorous ctenophore Bolinopsis infundibulum are rarely consumed by fish predators like cod and haddock, but it is consumed by the other ctenophore Beroe cucumis, which is also a year-round food item for lumpsucker Cyclopterus lumpus and a seasonal food item (in winter) for cod (Kamshilov et al.,1958; Kamshilov,1960; 1961). However, increasing of abundance of other jelly fish (Scyphozoa) might slow or even stop energy transfer from zooplankton to planctivors and predatory fishes.
Zooplankton is important food for several commercially important fish species in the Barents Sea. Important predators are immature herring, capelin, polar cod, as well as juveniles (especially 0-group) cod, haddock, saithe and redfish. In addition, other fish species have in recent years extended their distribution into the Barents Sea. For one of these species, blue whiting, the phenomenon appears to be transient as the species have now retracted from the area. The main pelagic fish stocks include a large proportion of zooplankton in their diet (Figure 2.6.2).
Zooplankton is also important for some adult demersal fishes in the Barents Sea, like cod, in years with low abundance of their preferred prey, and there is an inverse relationship between the amount of zooplankton in cod diet and the abundance of the preferred prey. Cod stomach content analyses showed that the 0 and 1 group cod fed mainly on crustaceans with krill and hyperiid amphipods comprising up to 70% of their diet. Krill (Thysanoessa spp. and M. norvegica) and hyperiid amphipods (Themisto spp.) were mainly found in cod stomachs sampled in the central and close to the Polar Front region in the Barents Sea where these prey organisms are reported to be abundant in summer. A shift in the main cod diet from zooplankton to fish is observed from age 1 to age 2.
A lot of the zooplankton production that is transferred to higher trophic levels in the Barents Sea ecosystem, is transferred through capelin. From the early 1980s till today the capelin stock has fluctuated significantly and the variations have profound impact on the zooplankton biomass and production. Even if many other factors influence the abundance and production of zooplankton, it seems to be close to an inverse relationship between capelin and zooplankton biomass (Figure 2.6.3).
The effect of capelin on zooplankton abundance is also reflected in the spatial distribution of zooplankton. Figure 2.6.4 shows that the areas of high densities of capelin (high Sa values) and high TFI (total fullness index) reflected by low densities of zooplankton.
Zooplankton is also important prey for seals, baleen whales and some species of sea birds, such as little auk. Pelagic feeding marine mammals, including both seal and whale species, feed primarily on schooling prey including both fish and zooplankton (e.g Themisto libellula, Gonatus fabricii, Thyssanoessa spp. and Meganyctephanes norvegica). Interactions between pelagic marine mammals and their pelagic prey in the Barents Sea appear to be both strong and complex. Skern-Mauritzen and colleagues have done a detailed study from 2003-2007 of baleen whale distribution in years with low capelin abundance. Baleen whales in arctic waters were restricted to a narrow band along the northern and eastern rim of the pelagic fish distributions, suggesting that they target zooplankton rather than pelagic fish.
Little auk is the most specialized planktivorous sea birds, feeding exclusively planktonic crustacean, with calanoid copepods (Calanus species) accounting for 84-96% of the energetic content of chick meals across their range (e.g., Pedersen and Falk 2001). Northern fulmars also forage mostly zooplankton including jellyfishes, while for most fish-eating sea birds zooplankton become important alternative food in the cases of low fish availability.