The mesozooplankton biomass in autumn 2019 was at approximately the same level as in recent years for the Barents Sea as a whole. The biomass in the western inflow area of Atlantic water was higher in 2019, and also somewhat higher on the Central and Great Banks, than in preceding years. The biomass in the eastern and northern areas remained at a relatively high level comparable to the long-term mean. Krill biomass has shown an increasing trend in recent decades and remained high in 2019. Pelagic amphipods were nearly absent in 2012-2013 but have since shown increased abundances and expanded distributions in the area east of Svalbard. The plankton situation in 2019 indicates good feeding conditions for planktivorous consumers.
Mesozooplankton biomass and distribution
Mesozooplankton play a key role in the Barents Sea ecosystem by transferring energy from primary producers to animals higher in the foodweb. Geographic distribution patterns for mesozooplankton biomass show similarities over a multiannual time-scale, although some interannual variability is apparent.
Differing geographical survey-coverages between years will impact biomass estimates both for territorial waters and the Barents Sea as a whole, particularly as the ecosystem is characterized by large-scale heterogeneous distributions of biomass. One way to address this challenge, is to perform interannual comparisons of estimated biomass within well-defined and consistent spatial subareas (polygons).
During August-October 2019, relatively high biomass (>10 g m-2) was observed in the area of the Bear Island Trench, north and northeast of Svalbard/Spitsbergen, south of Franz Josef Land, and in the basin of the south-eastern Barents Sea. Relatively low biomass (<4 g m-2) was observed in mainly in the south-eastern corner of the Barents Sea, but also west and south-east of Svalbard incluing the Great and Central Banks (Figure 220.127.116.11). The large-scale horizontal distribution of plankton in the Barents Sea during autumn 2019 resembled that of 2018, not considering the large unsurveyed areas in south-eastern region in 2018 and in the north-eastern region in 2019.
Figure 18.104.22.168. Distribution of total mesozooplankton biomass (dry-weight, g m-2) from seafloor to surface. Data based on 229 samples collected during BESS in mid August – early-October 2019. A WP2 net was applied by IMR and a Juday net by PINRO; both nets with mesh-size 180 μm. Interpolation made in ArcGIS v.10.6.1, module Spatial Analyst, using inverse distance weighting (default settings).
In the Norwegian sector of the Barents Sea, mesozooplankton biomass was size-fractionated (180–1000 μm, 1000–2000 μm, and >2000 μm) before weighing. The biomass for the intermediate size-fraction in 2019 was above the 20-year (1999-2018) long-term average. In contrast, the biomasses for smallest and largest size-fractions were slightly lower than the long-term averages (1999–2018) (Figure 22.214.171.124). Regarding the largest size-fraction, average values have shown a decreasing tendency during the ca. last 15 years, even if the 2019 value was not particularly low.
Based only on Norwegian data, which represent the spatially most consistent time-series, average zooplankton biomass (summing all size-fractions) during August-October 2019 was 8.0 (SD 6.2) g dry-weight m-2 for the western part of the Barents Sea. This estimate is based on 171 observations and is higher than in 2018 (7.2 g dry-weight m-2), and also above the long-term (1999-2018) average (7.0 g dry-weight m-2) (Figure 126.96.36.199).
Combined Russian and Norwegian data (229 stations in total) (Figure 188.8.131.52), covering the entire area surveyed in the Barents Sea in 2019, provided an estimated average zooplankton biomass of 7.9 (SD 6.3) g dry-weight m-2, which is the arithmetic average for all stations shown in Figure 184.108.40.206. This estimate is not directly comparable with that for 2018 due to the above-mentioned differing sampling coverage in the eastern region in 2018 versus 2019. In the Russian sector, average biomass for the area covered in 2019 was 7.5 (SD 6.1) g dry-weight m-2. This value is based on 58 observations, and also not directly comparable to the 2018 estimate for the same reason as described above.
Figure 220.127.116.11. Time-series of average mesozooplankton biomass from surface to sea floor (dry-weight, g m-2) for western and central Barents Sea (Norwegian sector) during the autumn BESS (1988–2018). Data are shown for three size-fractions; 0.18–1 mm (yellow), 1–2 mm (orange), and >2 mm (red) based on wet-sieving.
Zooplankton biomass varies between years and is believed to be partly controlled by predation pressure, e.g. from capelin. However, the annual impact of predation varies geographically. Predation from other planktivorous pelagic fish (herring, polar cod, and blue whiting) and pelagic juvenile demersal fish species (cod, haddock, saithe, and red-fish), and larger plankton forms (e.g. chaetognaths, krill, and amphipods) can also impact mes-ozooplankton in the Barents Sea. In addition, processes such as advective transport of plankton from the Norwegian Sea into the Barents Sea, primary production, and local production of zooplankton are likely to contribute to the variability of zooplankton biomass. As mentioned above, methodological factors such as differing spatial survey coverage also contribute to the variability of biomass estimates between years. For a more direct comparison of interannual trends, that is less influenced by variable spatial coverages, time-series of biomass estimates for specific sub-areas of the Barents Sea are provided in the following section).
Figure 18.104.22.168. Time-series for mean zooplankton biomass (g dw m-2) for stations within subareas of the Barents Sea (see WGIBAR 2018 Report, Annex 4) based on autumn survey data for the 1989-2019 period. 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). Middle panel – two subareas in the central Barents Sea with colder and partly Arctic water conditions: Central Bank (CB) and Great Bank (GB). Lower panel – two subareas in the eastern Barents Sea: Southeast Basin (SEB) and North East (NE). Results represent total biomass collected with WP2 or Juday plankton nets.
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. In this subarea, the biomass showed a marked increase to 16.5 g dw m-2 in 2019. The biomass of the Hopen Deep (HD) and Thor Iversen Bank (TIB) subareas have shown an increase from low values of about 4 g dw m-2 in 2013-14 to about 9 g dw m-2 in 2019. 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 the typical number of stations within a subarea).
Biomass estimates at Central Bank and Great Bank showed declining trends since the 1990s to minimum values around 2013 (Figure 22.214.171.124 - Middle). The biomass at these two subareas have subsequently increased, with a marked jump from 2018 to a relatively high value (6.8 g dw m-2) for the GB in 2019. The zooplankton biomass in the eastern polygons, South-East Basin and North-East, has shown fluctuations with no clear trends in the last 10 years. The biomass values were relatively high in 2019, around 9-10 g dw m-2 for both polygons.
Figure 126.96.36.199. Zooplankton biomass (g dry-weight m-2) in three size fractions (small 2 mm) for Bear Island Trough (upper panel) and Great Bank (lower panel) subareas for the 1989-2019 period. Note: Size fractions are based on screen mesh size, not size of individual zooplankton.
Size composition of mesozooplankton is shown in Figure 188.8.131.52 for two subareas: Bear Island Trench - a region of Atlantic water inflow; and Great Bank. These two subareas have had different temporal development. Bear Island Trench has had a more consistent pattern, with relatively high biomass of the medium size fraction since 2005. This fraction contains older stages of Calanus spp. which dominate mesozooplankton biomass in the Barents Sea (Aarflot et al. 2017). The increase in total biomass in 2019 reflected an increase of the medium size fraction. The recent situation likely reflects high influx of Calanus finmarchicus with Atlantic inflow to the Barents Sea, possibly related to a second generation within a single spawning season under warmer climate conditions (Skjoldal et al., unpublished manuscript).
Decline in biomass for Great Bank has been associated with a shift in dominance from the medium size fraction to the small fraction over the last decade. During this same period, the large size fraction declined to a very low level. The decline and shift from large to small zooplankton could reflect a combination of warming and predation from capelin (Dalpadado et al., unpublished manuscript). The Great Bank used to be part of the domain for the dominant Arctic species Calanus glacialis (Melle and Skjoldal, 1998) and has traditionally been a core feeding area for capelin. The increase in biomass for the Great Bank subarea in 2019 was due to an increase of the medium size fraction.
Figure 184.108.40.206 shows a comparison of long-term average estimates of zooplankton biomass (1989–2016) for each subarea together with estimates for 2019 and the previous year 2018. The 2019 biomass was considerably higher than the long-term average for the Bear Island Trench (by >50 %), Thor Iversen Bank, Hopen Deep, and Great Bank subareas. The biomass in 2019 was also higher than in 2018 for these subareas, except Thor Iversen Bank.
Figure 220.127.116.11. Average zooplankton biomass (g dry-weight m-2) for twelve subareas of the Barents Sea, comparing long-term averages for the 1989–2016 period with average values for stations sampled in 2018 and 2019.
Mesozooplankton species-composition along the Fugløya - Bear Island and the Kola transects
The Fugløya - Bear Island (FB) transect, spanning the western entrance to the Barents Sea, is generally monitored by IMR 5-6 timer per year, covering the different seasons. Up to eight stations with fixed positions are sampled during each coverage, although the number may vary depending on weather conditions. Zooplankton samples collected each year during the 1995–2019 period from four fixed locations at different latitudes (70.30°N, 72.00°N, 73.30°N, and 74.00°N) and representing different water masses (Coastal, Atlantic, and mixed Atlantic/Arctic) have been analysed taxonomically. Average annual abundance for each of the species C. finmarchicus, C. glacialis and C. hyperboreus is estimated by pooling the four stations throughout the sesonal cycle and summing up the copepodite stages I-VI (Figure 18.104.22.168, left). The arcto-boreal species C. finmarchicus is, by far, the most common of these three species, and displays some interannual variation in abundance. C. finmarchicus tends to be most abundant at the three southernmost stations. A particularly high abundance was recorded during 2010 along most of the transect, except at the northernmost station. After registering very low abundances at all stations in 2013, C. finmarchicus has generally been abundant along most of the transect during the last 6 years (2014–2019). The Arctic species C. glacialis has typically been most abundant at the two northern-most stations, representing Atlantic and mixed Atlantic-Arctic waters, respectively. This species also shows some interannual variation in abundance, particularly in the late nineteen-nineties (Figure 22.214.171.124, left). Abundance of C. glacialis along the FB transect has decreased since the initial years of this time-series (1995–1998), with very low abundance recorded in 2005, 2008, and during the 2012-2014 and 2017-2018 periods. The abundance of the large and Arctic species, C. hyperboreus, along the FB transect has been low relative to the abundance of C. finmarchicus, but generally also compared to C. glacialis throughout the study period. Few individuals of this species were observed during 2008-2010, 2013, and 2016. The FB time-series of C. hyperboreus abundance shows a clear interannual variability, and the abundances were not low in 2018 and 2019 (Figure 126.96.36.199, left). Calanus helgolandicus, a more southerly species, is observed regularly at the Fugløya-Bear Island transect, particularly during the December-February period (Dalpadado et al., 2012). Even in winter, the abundance of C. helgolandicus along the FB transect seldom surpasses a few hundred individuals per square meter. In spring and summer, this species is more or less absent at the entrance to the Barents Sea. In recent years, C. helgolandicus has become more abundant in the North Sea, and it is also observed in the Norwegian Sea off mid-Norway, particularly in autumn (Continuous Plankton Recorder data, Espen Strand, IMR, pers. comm.). C. helgolandicus is similar in appearance to C. finmarchicus and taxonomic separation of these two species is time-consuming. Hence, the IMR-routine is to examine a limited number of individuals belonging to the later stages of the C. finmarchicus/helgolandicus assemblage – up to 20 copepodites of stage V and up to 20 adult females – to establish the species-proportions in each FB sample. Our FB time-series provides no evidence of an increase over the years, neither of the proportion or absolute abundance of C. helgolandicus at the entrance to the Barents Sea.
Figure 188.8.131.52. Time series of abundances (ind. m-2) of Calanus finmarchicus, C. glacialis, and C. hyperboreus along the Fugløya-Bjørnøya (1995-2020) (left) and Kola (1992, 2008-2018) (right) transects. For the FB transect, each bar generally represents the annual average for 4 stations and 5–6 coverages per year. For the Kola transect the data show early-summer abundances. Note strongly differing scales for abundances between the two transects and the species. The right-hand sides from the vertical green lines show the same years for the two transects.
Russian (PINRO) investigations along the Kola section in June 2018 showed copepods as the dominant group of zooplankton at that time, comprising on average 64% in abundance and 85% in biomass, and Calanus finmarchicus as the dominant species. Average abundance of C. finmarchicus in 2018 was 308 309 ind. m-2, almost twice the 2017 value, but lower than for 2016 and long-term average (Figure 184.108.40.206, right). The highest abundance of C. finmarchicus was observed at the most southerly station of the section at 69º30′N and further north at 72º30′N, while its lowest abundance was observed at 70º00′N. In the C. finmarchicus population, individuals at all life stages were present: CIII-CIV stages dominated at the southern stations, and СI-CIV individuals were represented at the northern stations.
Average abundance of the arctic species C. glacialis in 2018 was 121 ind. m-2, which is 7.6 times higher than the 2017 estimate, and 4.2 times higher than the long-term average (Figure 220.127.116.11, right). C. glacialis mainly occurred from 73º00′ N and northwards; only copepodites CV were observed for this species.
Average abundance of the arctic species C. hyperboreus, the largest Calanus species in the Barents Sea, was higher in 2018 than in 2017 (182 and 113 ind. m-2, respectively) and exceeded the long-term average (117 ind. m-2) (Figure 18.104.22.168, right). A gradual increase in C. hyperboreus abundance has been observed since 2015. The highest abundance of this species was observed northwards from 73º00′ N, and the population was represented by copepodites CIV-CV.
Species composition from the autumn ecosystem cruise
PINRO investigations of mesozooplankton conducted by the BESS during August-September 2018 showed that in the Russian part, copepods dominated both in terms of abundance (86.7%) and biomass (63.2%) (Fig. 22.214.171.124). Total zooplankton abundance in the southern (south of ca. 75°N) Barents Sea was lower than in the northern part (north of ca. 75°N) of the sea (1 324 and 2 012 ind. m-3, respectively). Total zooplankton biomass was higher in the northern than the southern Barents Sea (243.5 and 134.0 mg m-3, respectively). However, the results from the southern Barents Sea are not quite comparable with previous years as the number of stations in the southern part of the sea in 2018 was very low (only 9 stations).
Figure 126.96.36.199. Abundance (ind. m-3) (left) and biomass (mg wet-weight m-3) (right) of the most numerous copepod species (surface to sea floor) in the Barents Sea (based on PINRO samples from the BESS during August-September 2018)
In the southern Barents Sea, total zooplankton abundance and biomass in 2018 had decreased slightly (by factor 1.1) compared to 2017. Copepods dominated both abundance and biomass (78.5 and 65.9%, respectively). Among other groups, the most important were chaetognaths comprising 25.5% of total zooplankton biomass. Their abundance had increased by a factor of 3.0 and the biomass increased by factor of 2.0 compared to 2017. Considering species composition of copepods, the small Oithona similis and Pseudocalanus sp. and the larger C. finmarchicus were the most abundant (48.6, 17.2 and 17.5% of total copepod abundance, respectively), and the large Metridia longa comprised 8.0% (Figure 188.8.131.52). However, in terms of copepod biomass, C. finmarchicus (70.7%), M. longa (10.2%), and Pseudocalanus sp. (8.1%) were the most important species, while O. similis comprised only 2.1% (Figure 184.108.40.206). In 2018, abundance of Pseudocalanus sp. and M. longa had increased compared to 2017, while abundance of other important copepods had decreased. Biomass of the main copepods (with the exception of Pseudocalanus sp.) had also decreased in 2018.
In the northern Barents Sea, total zooplankton abundance and biomass in 2018 increased by factors of 1.4 and 1.3, respectively, in comparison to 2017. The main increase of abundance and biomass was observed in the populations of copepods and chaetognaths. Copepods were the most abundant (90.0%) zooplankton group. Regarding total zooplankton biomass, copepods also represented the most important group (63.1%) during 2018, while chaetognaths, pteropods and hydrozoans comprised 24.5, 4.3 and 4.2%, respectively. In the northern Barents Sea, the small copepods Pseudocalanus sp. and O. similis and the larger M. longa contributed 45.6, 22.4 and 14.4%, respectively; while, C. finmarchicus, and C. glacialis contributed only 7.2 and 5.0% to total copepod numbers, respectively (Figure 220.127.116.11). Total copepod biomass consisted mainly of C. glacialis (36.9%), M. longa (20.4%), C. finmarchicus (17.5%) and Pseudocalanus sp. (18.6%). Abundance of C. finmarchicus, C. glacialis, and M. longa have been increasing since 2015, and Pseudocalanus sp. since 2016. At the same time, abundance of O. similis have been decreasing since 2016. The same trends were observed in biomass of these copepod species. The most prominent increases in both abundance and biomass were observed for M. longa in 2017-2018.
Macroplankton biomass 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 primarily associated with the Atlantic boreal western and central regions, whereas the neritic Thysanoessa raschii mainly occurs in the southeastern Barents Sea. These two species can reach 30 mm in length. Meganytiphanes norvegica, the largest species (up to 45 mm) is mainly restriced to typical Atlantic waters. The smallest of the species, the oceanic Thysanoessa longicaudata (up to 18 mm), is associated with the inflowing Atlantic water.
Winter distribution and abundance
The PINRO long-term data series on euphausiids was initiated in 1959 and stopped in 2016 (Figure 18.104.22.168). In 2017, only a part of this survey was conducted; results from only one cruise covering the southern part of the Barents Sea were presented in the previous WGIBAR Report. In November-December 2019, the survey was conducted in western and north-western areas only and the samples are being processed now. These data are not comparable with the previous years.
Figure 22.214.171.124. Abundance-indices of euphausiids (log10 of number of individuals per 1000 m-3) in the near–bottom layer of the Barents Sea based on data from the Russian winter survey during October-December for the 1959-2015 period. Based on trawl-attached plankton net catches from the bottom layer in: a) Southern Barents Sea; and b) Northwestern Barents Sea. Note that these data-series were stopped in 2016 but are presented here to show the general trends since the early 1950s and 1960s.
Euphausiids were collected in the southern Barents Sea during the Russian-Norwegian winter survey (February-March 2019) with the trawl-attached plankton net (Figure 126.96.36.199). Euphausiid sampling in this survey was initiated in 2015 onboard a Russian research vessel, but different areas were covered in different years. These results are very preliminary, and comparison with previous years requires caution. Results indicate that in 2019, distribution of euphausiids in the southern Barents Sea was similar to 2018. Average abundance of euphausiids in all areas (866 ind. 1000 m-3) was lower than in 2015 and 2018 (1255 and 1214 ind. 1000 m-3), but higher than in 2016 (561 ind. 1000 m-3). In 2019 the average abundance of euphausiids in the central and coastal areas decreased considerably compared to 2018, was similar in the western areas but increased considerably in the eastern areas. As during previous years (2015-2018), euphausiid concentrations were formed mainly by local species (T. inermis and T. raschii) and Atlantic species (M. norvegica and T. longicaudata). It should be noted that one more warmwater species Nematoscellis megalops has occurred in the coastal, western and central areas since 2003. The abundance of this species started to increase since 2012 (up to 1-3% of total euphausiids abundance), and in 2019 their average abundance consisted of 0.3%. Probably it was related to intensive inflow of Atlantic waters from the Norwegian Sea.
Summer-autumn distribution and biomass
In 2019, krill (euphausiids) were caught by standard pelagic «Harstad» trawl and 39% of all samples were identified to species level. The data here reported on krill represent bycatches from trawling on the 0-group fish. In 2019, krill were widely distributed in the BESS area (Figure 188.8.131.52). The biomass values in the report are given as grams of wet weight per square m (g m-2). Larger catches (more than 50 g m-2) were made around Svalbard/Spitsbergen and in the western and southeastern Barents Sea. About one third of the stations during the survey in 2019 were sampled during night (Table 184.108.40.206). The total krill biomass was estimated on basis of night catches only. During the night, most of the krill migrate to upper layers to feed and are therefore more accessible for the trawl. Both the day and night catches in 2019 (means of 8.2 g m-2 and 18.5 g m-2 respectively) were higher than the long-term means (2.5 g m-2 and 8.0 g m-2 respectively).
Species identification of euphausiids took place on the Norwegian vessels only. M. norvegica and T. inermis were widely observed in the Norwegian samples, while T. longicaudata were mostly observed in the western areas (Figure 220.127.116.11).
During the survey, length measurements of krill onboard the Norwegian vessels were made. Length distribution of two common species (M. norvegica and T. inermis) is shown in Figure 18.104.22.168. The length of M. norvegica varied from 10 to 46 mm (with an average of 30.1 mm), and T. inermis from 13 to 33 mm (with an average of 22 mm).
In 2019, the total biomass of krill was estimated as 22.3 million tonnes for the whole Barents Sea. It is the highest biomass since 2011, and much higher than long-term mean of 9.3 million tonnes (Fig. 22.214.171.124).
Figure 126.96.36.199. Krill biomass (wet-weight, million tonnes) estimated for upper layers of the whole Barents Sea during 1980-2019, based on night catches with standard pelagic «Harstad» trawls covering the upper water layers (0-60 m)
Amphipods (mainly hyperiids)
By Tatyana Prokhorova, Elena Eriksen and Irina Prokopchuk Figures by Pavel Krivosheya, Tatyana Prokhorova and Irina Prokopchuk
The data here reported on pelagic amphipods represent bycatches from trawling on the 0-group fish, using the standard pelagic «Harstad» trawl in th 60-0 m layer in autumn. During 2012 and 2013, amphipods were absent from pelagic trawl catches, while in 2014 some limited catches were taken north of Svalbard/Spitsbergen. Several large catches were made east and north of Svalbard/Spitsbergen during 2015-2017. In 2018, amphipods were caught east of the Svalbard/Spitsbergen Archipelago. In 2019, amphipods were found mainly in the northern part of surveyed area (Figure 188.8.131.52). The largest catches were dominated by Arctic Themisto libellula, and made north and east of Svalbard/Spitsbergen (Figures 184.108.40.206 and 220.127.116.11).
In 2019, the mean day-time catches were higher than the night-time catches (1.1 g m-2 and 0.8 g m-2, respectively), and the same was the case for the maximum catches (39.8 g m-2 during day and 17.0 g m-2 during night). This year, the estimated amphipod biomass for the upper 60 m of the whole Barents Sea was high (1.23 million tonnes), and about twice as high as in 2015-2016 (close to 570 thousand tonnes) and more than 20 times higher than in 2017. The higher biomasses in 2019 were most likely related to lower temperatures in the northern area, which was covered by Arctic water masses (close to 0°C and below).
T. libellula dominated in the catches, while only two catches of Themisto abyssorum were taken during the survey. In addition, to Themisto sp., low catches of Hyperia galba, which associates with jellyfish, were found in the northern part of the central area, where jellyfish were abundant.
Figure 18.104.22.168. Distribution of amphipods of genus Themisto (biomass, g wet-weight m- 2), based on standard pelagic «Harstad» trawls covering the upper layers (0-60 m) of the Barents Sea in August-October 2019.
The length of the most common and abundant T. libellula varied from 11.0 to 39.0 mm with an average length of 20.0 mm (Figure 22.214.171.124).
The estimated biomass of gelatinous zooplankton here presented represents bycatches from trawling on the 0-group fish, using the standard pelagic «Harstad» trawl in th 60-0 m layer in autumn during BESS. The biomass of gelatinous zooplankton for the entire Barents Sea was not estimated for 2018 due to an incomplete spatial coverage that year and has not yet been possible for 2019 for logistical reasons. Therefore, we here only present the time series on estimated biomass for the Barents Sea as a whole for the years 1980-2017 (Fig. 126.96.36.199).
Figure 188.8.131.52. Estimated total biomass of the jellyfish, mainly constituting Cyanea capillata, in the BESS sampling area during August-October for the 1980-2017 period. Based on catches by Harstad trawl in the upper 0-60 m layer - 95% confidence interval indicated by grey lines.