During August and September of 2013 in the Norwegian sector of the Barents Sea, the average mesozooplankton biomass was clearly below the long-term average. During 2008-2012 in this region, estimates of average biomass were relatively stable and slightly above 6.0 g dry weight m-2, although the 2012 Norwegian data was less certain than in previous years. In 2013, highest mesozooplankton biomass in the Norwegian sector was observed north-east of Svalbard and in Atlantic Water masses in the south-west — where transport of zooplankton from the Norwegian Sea into central and western parts of the Barents Sea occurs.
The continued lower-than-average biomass of mesozooplankton in the Norwegian sector of the Barents Sea suggests that also in 2014 initial conditions for local production there will largely be suboptimal; also, the overwintering population will likely be lower than observed in previous years.
In retrospect, there was considerable decline in abundance of euphausiids in the southern Barents Sea during 2009-2010, probably associated with increased consumption by capelin. The abundance of pre-spawning euphausiids by early 2011 is estimated to be 1.2 times above the long-term mean in the southern Barents Sea and 1.3 times above the long-term mean in the north- western Barents Sea. From 2011 to 2012 there has been a consistent increase in Meganyctiphanes norvegica in the western-, central and coastal areas, while Thysanoessa inermis account for most of the increase in the northwestern area. Hence, it is likely that during 2013-2014, advection and population abundance for M. norvegica — an Atlantic warmth-loving euphausiid species — will remain at a levels comparable to those observed during 2012. A similar pattern is predicted for the T. inermis population. The short term prediction for water temperatures in the Kola section is a slight decrease during 2014, which may help maintain a reasonable population level for arcto-boreal T. raschii in eastern areas — as this species seems to prefer shallow shelf regions and colder, less saline coastal water.
The long-term general warming trend, and further decrease in the extent of winter sea ice, will continue to facilitate expansion of warm-water species towards northern and eastern regions of the Barents Sea. Evidence of this expansion is seen in finding considerable amounts of euphausiids in the stomach contents of capelin north of Svalbard in 2007, and in the stomachs of both capelin and polar cod in the central and eastern Barents Sea during recent years. Recent findings of juvenile euphausiids north of 78ºN, and the regular occurrence of high krill biomass in north-west and south-east regions of the Barents Sea, support the belief that krill are expanding their range of in the Barents Sea, either due to local recruitment (T. inermis and Thysanoessa raschii), or due to the intrusion of Atlantic Water masses and the invection of more southerly species (M. norvegica, Thysanoessa longicaudata and Nematocelis megalops). The increasing occurrence of Atlantic krill species during the last 10 years illustrates their expansion northward into the Barents Sea. It is less certain, however, just how these species will interact with other more firmly-established species, and whether they will be able to reproduce successfully and complete their life cycles in the new areas they populate.
The current below-average level of mesozooplankton biomass in the Barents Sea is probably linked to high capelin biomass. Other plankton consumers such as 0-group herring, cod, haddock, and redfish also have an important influence on zooplankton biomass. This was likely the case during 2013 when 0-group cod, herring, and haddock all had strong year classes; whereas capelin year-class size was closer to the long-term average. Total biomass of the four most abundant 0-group fish stocks (cod, haddock, herring, and capelin) reached 2.7 million metric tons. Capelin biomass alone was estimated to be 3.9 million metric tons during August-September 2013. It follows that predation pressure on zooplankton from numerous 0-group plankton consumers was considerable during autumn 2013. It is possible that conditions for lower-trophic-level production were above average, despite the low levels of mesozooplankton biomass. If so, this may have prevented mesozooplankton biomass from being reduced to even lower levels.
Gelatinous zooplankton, such as medusa (jellyfish) and ctenophores (comb jellies) are also believed to be important predators on mesozooplankton in the Barents Sea, but their influence is difficult to assess quantitatively. It should be noted, however, that the low mesozooplankton abundance in the central Barents Sea during August-September to a large extent coincided with high abundance of gelatinous zooplankton; this has been observed each year from 2010 to 2013, but was particularly evident during 2013. How this may link to the distribution of capelin and its consumption of mesozooplankton is not known. Gelatinous zooplankton and capelin may prefer different size spectra of zooplankton and fish larvae as prey items. If so, their diet overlap would be smaller, and their impact on each other as competitors may be smaller. Nonetheless, for gelatinous zooplankton in the Barents Sea, there is limited information on their preferred prey or the size spectrum of organisms they prey upon. Also of note, there are a range of carnivorous zooplankton competing with pelagic fish and jellyfish to prey on the basically herbivorous mesozooplankton. Their impact is largely uncertain, but the samples from the Norwegian WP2 >2000 um size fraction during 2013 (not shown) could be useful to help indicate the biomass of this carnivorous component. Current biomass of this size fraction is the lowest in the 1988-2012 time series, most likely due to high predation from pelagic fish, poor recruitment, or unfavorable feeding conditions, i.e., low availability of preferred prey.
Based on our current understanding of hydrographic conditions and long-term dynamics of zooplankton development, we expect spawning of copepods and euphausiids to begin in mid April in the south-western areas of the Barents Sea. Having overwintered, these groups of crustaceans, along with the warm-water species which have been transported from the Norwegian Sea, will create a zone with high density of zooplankton in north-western and western parts of the Barents Sea. In recent years a region with elevated zooplankton biomass, extending north- and southward, has been observed west of Novaja Zemlja in the Russian sector. This region had high mesozooplankton biomass during 2009-2011, albeit these levels were lower than observed during 2008. Levels again increased in 2013. This seems to be an area where herbivorous zooplankton (in certain situations or during certain years) are able to sustain viable populations and avoid excessive predation during summer and autumn, making it an important area for overwintering and re-establishing the population the following spring.
The high biomass of mesozooplankton found south to south-east of Franz Josef Land in 2009 and 2010 appears to have been reduced by 2011. During 2013, however, this region regained its high biomass, extending beyond what has been observed earlier. This was caused, in part, by an extended area of survey coverage in 2013. This area partially overlaps with distributions of capelin and polar cod in the north-eastern part of the Barents Sea, suggesting that predation from these two species on zooplankton could be large. At the time of the 2013 survey, however, 2013 the effect of such predation seemed insignificant. Relatively low zooplankton biomass in central parts of the Barents Sea appears to be a recurring phenomenon. This may result from heavy predation by capelin stock and other key 0-group fish species; gelatinous zooplankton could also be important predators. Since the central Barents Sea is among the more shallow regions, mesozooplankton there have limited potential to reduce predation through vertical migration to deeper waters.