The horizontal distribution of mesozooplankton in 2008 is shown at the Figure 4.3.4. According to the joint ecosystem survey, the average zooplankton biomass was clearly below the long-term mean, and the spatial coverage revealed very low zooplankton biomass in the central parts of the Barents Sea – but with some scattered higher values recorded in the central part of the Barents Sea and along the border of the Russian zone.
In the western part of the Barents Sea, a well defined area of higher zooplankton abundance was observed south of Bear Island, but its areal extension was much less pronounced compared to previous years. It has to be noticed that both Norwegian and Russian sampling coverage is poor north of 78°N, except for the area between Novaya Zemlya and Franz Josef Land. Compared to the situation in 2007, it seems that areas of high mesozooplankton biomass are extended eastward into the Russian zone in 2008.
The average zooplankton biomass in the western and central Barents Sea in 2008 was 6.48 g dry weight m-2 compared to 7.13 g in 2007 and 8.63 g in 2006. These values are based on WP2 hauls (Norwegian data) covering the whole water column and depths less than 500 m (Figure 4.3.7). However, the average value for 2008 was based on only 98 stations which are considerably lower than the 145 stations covered in 2007. Combining both Russian (Juday) and Norwegian data (WP2), the average zooplankton biomass for a total of 171 stations was 7.15 and 7.7 g m-2 dry weight in 2008 and in 2007 respectively. These values are less than what was observed in 2006 (8.4 g m-2). However, in the Russian sector alone, the average biomass in 2008 was estimated to 8.05 g m-2dry weight (N=81 stations from bottom-0m).
The examination of the zooplankton composition showed a predominance of the three species Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus), but euphausiids, chaetognaths, and in some cases pteropods, had high biomass estimates. C. finmarchicus was the main species in terms of biomass in the western parts of the Barents Sea, whereas C. glacialis dominated in the northeastern parts. At the same time there were local differences in biomass distribution.
The importance of water mass characteristics on zooplankton abundance for western and central parts of the Barents Sea is shown in Table 4.3.1. It is again apparent that the average zooplankton abundance was highest in the Atlantic water masses (8.5 g dry-weight m-2), and in mixed water of Atlantic and coastal origin (6.0 g dry weight m-2). Quite low biomass was found in pure coastal water masses in 2008 (3.9 g dry weight m-2). This was significant lower compared to 2007 (6.6 g dry weight m-2), but not as low as what was observed in 2006 (1.6 g dry-weight m-2). This year to year biomass variability in coastal waters can be understood from the high horizontal heterogeneity of the zooplankton biomass (Figure 4.3.4), and the relatively low number of sampling stations.
The observations showed that the eastern part of the Frantz Josef Land had more advanced development of C. finmarchicus in 2008 compared to 2007. It was concurrently observed a significant increase in the population of juvenile C. glacialis, and the reproduction in the first half of September 2008 occurred farther north in 2008 (79-82°N) than in 2007 (79°N).
The biomasses in 2008 in the bottom-0 m layer differed from 2007 in the smaller amplitude equivalent to 5.5-13.7 g m-2 versus 3.2-24.9 g m-2 in 2007. This was caused by the high abundance of juveniles in 2008. In the eastern parts of the Barents Sea (Admiralteistvo Peninsula, Jelaniya Cape) the situation was similar to what was observed off the Novaya Zemlya Shallows – except for the high proportion of Calanoida eggs and nauplii as well as juvenile C. glacialis. The total biomasses were here not higher than 3-8.5 g m-2, except for the southern areas of Jelaniya Cape where the biomass reached 13.4 g m-2 due to high abundance of C. finmarchicus of older stages.
The general distribution pattern of copepod species south of Franz Josef Land in 2007 is shown in Figure 4.3.5. High concentration of the Arctic species С. glacialis, M. longa and P. minutus were observed in the whole studied area. In September, individuals of all life stages (nauplii, younger copepodites to adults) were found in the populations of C. finmarchicus, С. glacialis, and M. longa. This may indicate a prolongation of their breeding period, which is also observed in other Arctic species (Pertsova, Kosobokova, 1996; Orlova et al., 2008, Melle, Skjoldal 1998). Furthermore, it was conspicuous to find higher concentration of C. finmarchicus in eastern and central parts of the Barents Sea in 2008 compared to 2007. The exception was for Franz Josef Land where C. glacialis was more prevalent. Due to the high abundance of juvenile C. glacialis, the total number of small crustaceans reached 150 000-320 000 individuals per m2 in samples in 2008. This was significantly higher numbers than in 2007 (c.f. Figure 4.3.4 and 4.3.6).
The study of age structure of the most abundant species showed that the population of C. finmarchicus north of 78°N consisted of copepodites CIV (more seldom - CV), and the proportion of juveniles CI-III increased to the south. In the eastern parts of the study area, C. finmarchicus was mainly found as CIV-V. Among the adults, mainly females occurred. In the western areas of FJL and the Persey Elevation, where the species abundance was maximal high, the population of C. glacialis was mainly represented by juveniles belonging to the stages CI-III, and the portion of juveniles increased eastward. In most of the areas, overwintered crustaceans in stadium CIV (more seldom CV) were present. Mature individuals – primarily females – occurred in great numbers right south of FJL. Intensive reproduction of C. glacialis and C. hyperboreus was found in broad areas between 32-66°E.
C. glacialis in stages CIII-VI was found to constitute roughly 50-60% of the total biomass in the north and northeastern parts of the study area. C. hyperboreus, M. longa, as well as the representatives of Pteropoda (Clione limacina) and Sagitta were common, whereas C. finmarchicus was less important. The total biomass varied between 1.2-11 g m-2, and the portion of Euphausiidae and jellyfish were quite high in some parts (0.5-8.8 g m-2).
The state of zooplankton in the Barents Sea in 2007-2008 was highly affected by two important factors: a) the weakening of the total discharges of Atlantic water into the North Cape Current, into the northern branch of the North Cape Current and into the Bear Island Current, and b) the very dynamical state of seaice during the summer period. The first factor caused a reduced transportation of C. finmarchicus from the Norwegian Sea to the Barents Sea. The second factor caused a predominance of C. glacialis and P. minutes (Orlova et al., 2008). However, it is expected that C. finmarchicus gradually will accumulate and become more abundant in the Barents Sea as the ice is retreating.