Mesozooplankton play a key role in the Barents Sea ecosystem by transferring energy from primary producers to animals higher in the food web. Geographic distribution patterns of total mesozooplankton biomass show similarities over time, although some inter-annual variability is apparent. Challenges in covering the same area each year are inherent in such large-scale monitoring programs, and inter-annual variation in ice-cover is one of several reasons for this. This implies that estimates of average zooplankton biomasses for different years might not be directly comparable.
In 2017, relatively high biomass (> 10 g m-2) was observed in the Bear Island Trench (southwestern region), north of Svalbard/Spitsbergen; south of Franz Josef Land, and in large parts of the easterly survey-region including the South-eastern Basin. Relatively low biomass (< 3 g m-2) was observed: in the westernmost area bordering the Norwegian Sea; in regions both south and east of Svalbard/Spitsbergen, and in the south-eastern corner of the survey area (Fig. 3.3.1). Relative to 2016, the most notable difference in 2017 was enhanced biomass in easterly parts of the Barents Sea. However, a large area just north of the Kola Peninsula was not covered in 2016, which complicates comparison.
Figure 3.3.1. Distribution of total zooplankton biomass (dry weight, g m-2) from near bottom - 0 m in autumn 2017. Data based on 247 samples obtained during the joint Norwegian-Russian (IMR-PINRO) ecosystem survey in late August – mid-October. Interpolation made in ArcGIS v.10.3, module Spatial Analyst, using inverse data weighting (default settings).
In the Norwegian sector of the Barents Sea, mesozooplankton biomass was size-fractionated (180-1000μm, 1000-2000μm, and >2000μm) before weighing. For the smallest size-fraction, estimated average biomass was similar to average biomass the last 20 years (1987-2017). For the intermediate size-fraction, 2017 average biomass was slightly lower than the average level over the last 20 years (Fig. 3.3.2). For the largest size-fraction, the average values have shown a decreasing trend during the ca. last 13 years; in 2017 biomass for the largest size-fraction was well below the average level over the last 20 years. Based only on Norwegian data, which represent the longest time series, average zooplankton biomass combined all size-fractions in August-October 2017 was 6.4 g dry-weight m-2 in the western-central Barents Sea, this estimate is lower than in 2016 (7.7 g dry-weight m-2), and somewhat lower than the average for the last 20 years (7.0 g dry-weight m-2). The reduction in average total biomass from 2016 was mainly due to decreased biomass in the mid-size fraction (1000-2000 μm).
Combined Russian and Norwegian data (247 stations in total) covering the entire Barents Sea provided an estimated average zooplankton biomass of 7.2 (SD 5.7) g dry-weight m-2 in 2017. This estimate is not directly comparable with that for 2016 (6.6 g m-2), since areas covered these two years differed. In the Russian sector, average biomass in 2017 was 8.6 g dry weight m-2, not directly comparable to 2016 estimate (3.9 g dry-weight m-2) due to the above-mentioned incomplete 2016 coverage in the southern region.
Figure 3.3.2. Time-series of mean zooplankton biomass from bottom – 0 m (dry-weight, g m-2) for the western and central Barents Sea (Norwegian sector) of the autumn ecosystem-survey, 1988–2017. Data are shown for the three size-fractions; 0.18-1 mm (yellow), 1–2 mm (orange), >2 mm (red) based on wet-sieving.
Zooplankton biomass can vary considerably between years and appears to be controlled largely by predation pressure, e.g. from capelin, although the yearly predation impact is expected to vary between regions. Capelin stock size was relatively high during 2008-2013; thus, exerting high predation pressure on zooplankton. In 2014, the capelin stock-size decreased, and in 2015 and 2016 the stock declined further to very low levels; this likely easing pressure on their zooplankton prey. However, the 2017 estimate suggests a marked increase in capelin stock-size; this likely increasing pressure on their zooplankton prey. Predation from other planktivorous species (herring, polar cod and blue whiting) and pelagic juveniles of demersal fish (cod, haddock, saithe, and redfish) can also affect the state of the plankton in the Barents Sea. In addition, processes such as advective transport of plankton from the Norwegian Sea into the Barents Sea, primary production (see section above), and local production of zooplankton likely contribute to variability in zooplankton biomass. It should be noted that methodological factor,s such as differing spatial surveillance areas, also contribute to the reported variability between years. For a more direct comparison of inter-annual trends, that is less influenced by variable spatial coverages, we refer to the timeseries of biomass estimates for specific subareas of the Barents Sea (2018 WGIBAR Report, Annex 4).
Mesozooplankton biomass in subareas of the Barents Sea
The 2017 IMR zooplankton biomass estimates have been calculated as mean values for each of 9 subareas (polygons). A 1989-2016 time series biomass estimates for these subareas described in a background document (2018 WGIBAR Report, Annex 4). Time series of estimates for four Atlantic water subareas, Central Bank, and Great Bank are shown in Fig. 3.3.3, and are updated with 2017results.
Figure 3.3.3. Time series of mean zooplankton biomass (g dw m-2) for stations within subareas of the Barents Sea (see WGIBAR 2018 report, Annex 4) based on autumn surveys in the years from 1989 to 2017. Upper panel – four subareas in the southwestern Barents Sea covered mainly with Atlantic water: Bear Island Trench (BIT), South-West (SW), Hopen Deep (HD), and Thor Iversen Bank (TIB). Lower panel – two subareas in the central Barents Sea with colder and partly Arctic water conditions: Central Bank (CB), and Great Bank (GB). The results presented in these figures represent the total biomass as collected with WP2.
Biomass estimates in the ‘Atlantic’ subareas have fluctuated between 5 to 10 g dw m-2 since about year 2000, with generally higher values for the Bear Island Trench. Biomass in the three other subareas has tended to increase since 2012-2013, while biomass in the Bear Island Trench has decreased since 2015. It should be noted that sampling variance is high, with coefficient of variation (CV = SD/mean) of about 0.5 for mean values per subarea (see WGIBAR 2018 report, Annex 4). This translates into confidence intervals (95 %) of +/- 20-25 % around the mean for n observations of 16-25 (which is a typical number of stations within a subarea).
Biomass estimates at Central Bank and Great Bank have shown declining trends since the 1990s to minima in 2013 (Fig. 3.3.3 Lower). Biomass at Central Bank has subsequently increased after this, the 2017 estimate was relatively high (about 7 g dw m-2). In contrast, biomass at Great Bank was very low in 2017 (about 2 g dw m-2).
Fig. 3.3.4 shows a comparison of the long-term mean values (1989-2016) for each of the subareas along with the values for 2017. The biomass values for 2017 are in most cases close to the long-term mean, with a lower value for the Great Bank, as already pointed out, and lower values also for the Svalbard-South and Franz-Victoria Trough subareas.
Figure 3.3.4. Mean zooplankton biomass (g dw m-2) for nine subareas of the Barents Sea, comparing long-term means for the 1989-2016 period with mean values for the stations collected in 2017. SS – Svalbard-South, SN – Svalbard-North, FVT – Franz-Victoria Trough. for other abbreviations, see legends to Figs. 3.3.3. and 3.3.
Russian investigations along the Kola section in early June 2017 showed that copepods were the dominant zooplankton group at that time, comprising on average 75% in abundance and 72% in biomass; Calanus finmarchicus was the dominant species. The mean abundance of C. finmarchicus in 2017 was 159 761 ind. m-2, less than half of the last years mean value (395 941 ind. m-2), and comparable to that in 2013 (133 814 ind. m-2), and somewhat lower than the long-term mean (Fig. 3.3.5). The abundance of C. finmarchicus in the southern part of the section was lower than in the northern part, and the highest values, as in 2016, were observed at 72º00′ and 73º30′N. In the C. finmarchicus population, individuals of all stages were present, but while CI-CIV stages dominated at most stations, the portion of the young individuals СI-CII stages was higher at the northern stations.
In contrast to 2016, when C. glacialis was not found in the plankton samples along the Kola section, the 2017 mean abundance of this species was 15.8 ind. m-2. This value was slightly lower than in 2015 (18.6 ind. m-2) as well as the long-term mean (Figure 3.3.5). C. glacialis was found only north of 72º00′ N. In the C. glacialis population, individuals of stages CIV-V were observed, with a dominance of CV copepodites.
Abundance of the arctic C. hyperboreus in 2017 was higher than in 2016 (112.6 and 77.2 ind. m-2, respectively) and very close to the long-term mean (111.3 ind. m-2) (Figure 3.3.5). The highest abundance of C. hyperboreus was found north of 72º00′ N. Copepodites CIV-CV of C. hyperboreus represented its population.
Russian (PINRO) investigations of mesozooplankton during the 2016 BEES survey (August-September) showed that copepods dominated both abundance and biomass. Copepods comprised on average 88% of total zooplankton numbers and 75% of total zooplankton biomass (Figs. 3.3.6 and 3.3.7). Total zooplankton abundance and biomass in the southern Barents Sea (south of ca. 75°N) were both considerably higher than in the northern Barents Sea (north of ca. 75°N): averaging 2 056 ind. m-3 and 1 263 ind. m-3 and 145.8 ind. m-3 and 102.0 mg m-3, respectively.
In northern Barents Sea, copepods (87%) were the most abundant zooplankton group, while heteropods (9.8%) were less numerous. Estimates of total zooplankton biomass, indicated that copepods represented the most important group (71%), while chaetognaths and heteropods comprised 12% and 11% respectively. Increase in total zooplankton abundance and biomass were observed relative to 2015; primarily due to more copepods, chaetognaths and heteropods. In the northern Barents Sea, the small copepods Oithona similis and Pseudocalanus minutus were numerous; comprising 60 and 26% of total copepod abundance. While, the larger species Calanus finmarchicus, C. glacialis, and Metridia longa represented 7.5%, 5.7%, and 1.1% of total copepod abundance, respectively (Fig. 3.3.6). Total copepod biomass consisted mainly of C. glacialis (45%), C. finmarchicus (20%), M. longa (12%), P. minutus (11%), and C. hyperboreus (8%). Relative to 2015, the 2016 observed abundance of most copepods had increased: particularly C. finmarchicus (by a factor 2.6) and C. glacialis (by a factor 1.8); while abundance of P. minutus decreased slightly. Increased biomass of C. finmarchicus and O. similis was also observed in 2016 relative to 2015, while biomass of C. glacialis, M. longa, and C. hyperboreus remained at the same level.
In the southern Barents Sea, copepods also dominated both abundance and biomass (90% and 79%, respectively). Among copepods, the small species O. similis and P. minutus were most abundant (66% and 23% of total copepod abundance, respectively); while larger species C. finmarchicus and M. longa contributed 10.3% and 1.2%, respectively (Fig. 3.3.6). However, in terms of copepod biomass, C. finmarchicus (71%), M. longa (12.7%), and P. minutus (11.7%) were the dominant species (Fig. 3.3.7). In 2016, both abundance and biomass of O. similis were at the same level as in 2015, but relative to 2014, these parameters increased by factors of 2.3 and 2.4, respectively. Abundance and biomass of P. minutus and C. finmarchicus in the southern Barents Sea in 2016 increased (by factors of 2.3 and 2.2 and 1.9 and 2.6, respectively) relative to 2015. In 2016, abundance and biomass of M. longa had increased only by factors 1.1 and 1.2, relative to 2015.
Figure 3.3.7. Biomass (mg wet-weight m-3) of the most numerous copepod species (bottom-0 m) in the Barents Sea (based on the PINRO samples from the PINRO/IMR ecosystem survey in August-September 2016).
Fugløya-Bear Island (FB) transect
Fugløya-Bear Island (FB) transect, located at the western entrance to the Barents Sea, is typically monitored by IMR 5-6 timer per year, covering different seasons. Up to 8 stations with fixed positions are sampled during each coverage, though the number may vary depending on weather conditions. Zooplankton samples taken during the 1995-2017 period from four locations representing different water masses (Coastal, Atlantic, and mixed Atlantic/Arctic), have been analysed taxonomically. Annual averages (sum of all copepodite stages I-VI) of the species C. finmarchicus, C. glacialis and C. hyperboreus are shown in Figure 3.3.8 for the period 2007-2017. C. finmarchicus, is by far the most common of the
3 species, and displays large inter-annual variations in abundance. C. finmarchicus tends to be most abundant at the station located at 73º30’N; high abundances was recorded during 2010 along most of the transect except at the northernmost position (74º00’N). Following very low abundances at all stations in 2013, C. finmarchicus has been abundant along the transect during the last 4 years (2014-2017).
Figure 3.3.8. Abundance of Calanus finmarchicus, C. glacialis and C. hyperboreus along the Fugløya-Bjørnøya transect during 2007-2017. Note that only a portion of the time series is shown in this figure. The bars represent the annual averages of the 5-6 coverages per year (except for 4 and 3 coverages in 2012 and 2013, respectively). Each station is shown separately.
As expected, C. glacialis was most abundant at the two northern-most stations (Fig. 3.3.8), where Atlantic and Arctic waters mix. Abundance of this species also showed large inter-annual variations. Numbers of C. glacialis along FB transect seem to have decreased in later years of the time-series (1995-1998), with very low abundances recorded in 2005, 2008, during 2012-2014, and in 2017 (data for years before 2007 not shown in Fig. 3.3.8). Abundance of the largest species, C. hyperboreus, along the FB transect has generally been low relative to C. finmarchicus and C. glacialis throughout the study period. Few individuals of this species were recorded during 2008-2010 and in 2016. (Fig. 3.3.8). The FB time-series for C. hyperboreus shows a strong year-to-year variability in abundance. Still, abundances during 2008-2017 tended to be lower than during 1995-2007 (data before 2007 not shown in Fig. 3.3.8).
Calanus helgolandicus, a more southerly species which spawns during autumn, has been observed regularly at the Fugløya – Bear Island transect, particularly during the period from December to February (Dalpadado et al. 2012). This species is similar in appearance to C. finmarchicus. In recent years, it has been observed more frequently in the North Sea as well as in the southern parts of the Norwegian Sea (Svinøy transect). Since taxonomic separation of C. finmarchicus and C. helgolandicus is time-consuming, limited numbers of individuals of the later stages up to 40 copepodites of stage V females were examined in each sample to establish the species-proportions. During winter, the ratio of C. helgolandicus to C. finmarchicus along the Fugløya – Bear Island transect has been observed to increase. At this time of the year C. finmarchicus is normally overwintering in deeper waters. Our FB time series provides no evidence of an increase in the relative proportion nor absolute abundance of C. helgolandicus over the years at the entrance to the Barents Sea.
Macroplankton biomasses and distribution
Krill (euphausiids) represent the most important group of macrozooplankton in the Barents Sea, followed by hyperiid amphipods. Krill play a significant role in the Barents Sea ecosystem, facilitating transport of energy between different trophic levels. There are mainly four species of krill in the Barents Sea; Thysanoessa inermis associated with Atlantic water in the western and central Barents Sea, Thysanoessa raschii found mainly in shallow waters in the southeastern Barents Sea, while Meganytiphanes norvegica and Thysanoessa longicaudata are associated with the inflowing Atlantic water, particularly during warm periods. Meganytiphanes norvegica is the largest species reaching a maximum length of about 4.5 cm, while T. inermis and T. raschii reach lengths of about 3 cm and T. longicaudata is the smallest species – not exceeding 1.8 cm.
Winter distribution and biomass
Euphausiids were collected in the Barents Sea during the PINRO winter survey (November-December 2017) with the trawl-attached plankton net. Note, that results from only one cruise are presented here, covering the southern part of the Barents Sea; these data are not quite comparable with the previous years. Preliminary results indicate that in 2017, the trend of increasing euphausiid abundance continued, at least in the southern Barents Sea. Compared to 2015 (no sampling in 2016), mean euphausiid abundance in the southern Barents Sea in 2017 increased by a factor of 1.7 — from 803 to 1338 ind. 1000 m-3. The main increase in euphausiid abundance was observed in central (from 266 to 535 ind. 1000 m-3) and coastal areas (from 616 to 2290 ind. 1000 m-3) of the Barents Sea. Euphausiid abundance in the eastern Barents Sea decreased in 2017 relative to 2015 by a factor of 2.3 (from 2695 to 1159 ind. 1000 m-3). Euphausiid concentrations were formed mainly by local species (T. inermis andT. raschii) as well as Atlantic species (M. norvegica and T. longicaudata). The proportion of Atlantic species decreased in 2017 relative to 2015, but remained at a quite high level.
Summer-autumn distribution and biomass
The following information on krill is based on the joint Norwegian-Russian Barents Sea Ecosystem survey conducted during autumn 2017. Euphausiids (krill) caught by standard pelagic trawl were identified to species level at 92% of all stations. Some parts of the northeastern Barents Sea were not covered in 2017, in contrast to most previous years.
Krill were widely distributed in the Barents Sea in 2017 (Fig. 3.3.10), with very low abundances in the southeast. Biomass values in the upper 60 m are presented in wet weight grams per square meter (g/m2). The center of distribution varies between years. In 2013, largest catches were made mostly in the central area. In 2014, largest catches were made in the western area. In 2015, largest catches were made in the south and southeast of Svalbard/Spitsbergen. Whereas in 2016 and 2017, a wider distribution pattern was observed. The mean night-catch in 2017 (15.35 g/m2) was higher than the long-term mean (7.7 g/m2), and the highest observed since 2012. Note that areas covered may vary between the years.
The number of night stations in 2017 was approximately half that of day stations. During night, most krill migrate to upper layers of the water column to feed, and are therefore more available to the trawl. Larger catches (> 50 g/m2) were observed in the central area.
Based on euphausiid species identification in 2017, Meganyctiphanes norvegica and Thysanoessa inermis were widely observed in the Barents Sea. M. norvegica was mostly restricted to Atlantic waters in the south, with a few additional catches in west and north of Svalbard/Spitsbergen, in areas influenced by the northward-flowing West Spitsbergen Current. Some samples from additional pelagic stations also showed occurrence of M. norvegica off south-eastern Franz Josef Land and between Svalbard/Spitsbergen, and Franz Josef Land (up to 79-79˚N and 40-50˚E) (not shown in Fig. 3.3.11). In contrast, T. inermis was mainly found in the central and northern Barents Sea, with a few additional catches of low abundance in the southeastern region. Two catches with Thysanoessa longicaudata were made in the northern area, and one catch with Thysanoessa raschii was made in the eastern area (Fig. 3.3.11). Smaller T. longicaudata and juvenile euphausiids are not sampled representatively by the pelagic trawl due to escapement through the mesh.
In 2017, total krill biomass was estimated to be approximately 16 million tonnes wet-weight. This is higher than in 2016 and above the long-term (1980-2017) mean (9.0 million tonnes). The 2017 value is high considering heavy predation by capelin and other planktivorous fish during the summer season.
Figure 3.3.11. Krill species distributions based on trawl stations both day and night, covering the upper water layers (0-60 m) of the Barents Sea in August-October 2017. The proportions are based on wet-weights.
Information on amphipods (mainly Hyperiids) presented here is based the BEES survey in autumn 2017. In 2017, amphipods were observed in the northern Barents Sea (Fig. 3.3.12) close to Svalbard/Spitsbergen region. In 2012 and 2013, amphipods were absent from the pelagic trawl catches, while in 2014 some limited catches were made north of Svalbard/Spitsbergen. During 2015-2017, several large catches were made east of Svalbard/Spitsbergen.
In 2017, the largest catches were made north and east of Svalbard/Spitsbergen, and were mostly represented by the Arctic species Themisto libellula (Figure 3.3.13). In 2017, day-time catches were generally larger than night-time catches. Smaller amphipods such as T. abyssorum were not sampled representatively using pelagic trawls. Estimated 2017 amphipod biomass in the upper 60 m was 30 thousand tonnes for the area covered. Catches in 2017 were substantially lower than in 2015 and 2016.
Geographic distributions and estimated biomass of gelatinous zooplankton presented in this report are based on data collected during the Joint Norwegian-Russian Barents Sea Ecosystem Survey conducted in autumn 2017, using the standard pelagic trawl for the 0-60 m depth-stratum. Gelatinous zooplankton was sorted from all trawl catches, identified to the lowest possible taxonomic level, and recorded as total wet weight per taxon.
Trawling is a harsh sampling method for gelatinous zooplankton, and data presented here should be considered semi-quantitative. The trawl used does not sample the entire water column, the filtered volume of water is not known, and small and fragile species may pass through the trawl mesh and are easily destroyed in the cod-end. The trawl likely has a higher catchability for large, robust scyphozoans (Periphylla periphylla, Cyanea capillata) than for the smaller Aurelia aurita. Catchability may be even lower for fragile taxa such as ctenophores and small medusa. Nevertheless, we consider the error in catchability is constant for each taxon, enabling taxon-specific comparisons between years and stations.
In August-October 2017, lion’s mane jellyfish (Cyanea capillata; Scyphozoa) was the most common jellyfish species, both with respect to weight and occurrence (average catch of 15 tonnes per sq nmi), and was widely distributed in the entire survey area (Fig. 3.3.14). Catch per station was higher than in 2015-2016, and ranged between 155 kg and 224 tonnes per sq nm. Large catches (> 10 tonnes per sq nmi) were taken at half of the stations; a higher frequency than observed during the previous two years.
Cyanea capillata was observed throughout the entire Barents Sea, with the highest concentrations (> 15 tonnes per sq nmi) in the central area, the southeastern area, and along the western Svalbard coast. Blue stinging jellyfish (Cyanea lamarckii; Scyphozoa) whichusually occurs mainly outside the Barents Sea have, most likely, been transported into the Barents Sea by Atlantic waters from the Norwegian Sea and Norwegian coast. The first observation of C. lamarckii in the Barents Sea was recorded during the 2014 BESS survey. In 2017, C. lamarckii distribution was similar to that in 2016 (Figure 3.3.15). C. lamarckii was recorded at 21 stations (~ 9% of standard pelagic trawl stations) in western and southwestern regions of the Barents Sea. Single specimens were observed in pelagic catches, with the average catch being 0.04 kg per nmi. Single specimens of the helmet jelly Periphylla periphylla, a deep-water jellyfish, were caught at two stations in the western Barents Sea in 2017 (Figure 3.3.15). Distribution of the helmet jelly in 2017 was similar to that in 2016. Only standard pelagic trawl stations are reported here, however, helmet jellies were caught by bottom-trawl. Other species are not presented in this report due to technical challenges.
Figure 3.3.15. Distribution and catches (wet weight; kg per nmi) of Cyanea lamarckii and Periphylla periphylla in the Barents Sea, August-October 2017. Catches both day and night from standard pelagic trawl in the upper 0-60 m layer.
Total biomass of C. capillata in upper layers of the water column (0-60 m) of the Barents Sea during August-October 2017 was estimated to be 4.6 million tonnes (Fig. 3.3.16). This is the third highest estimate thus far, and much higher than the estimated long-term mean for 1980-2017 (1.3 million tonnes). Inter-annual variation in estimated total biomass of gelatinous zooplankton (dominated by C. capillata) based on data from Barents Sea Ecosystem Surveys (1980-2017) is considerable, with peaks also observed in 2001 and 2014 (5 million tonnes); the lowest estimate was in 1997 (0.02 million tonnes).