Zooplankton

Photo: Alison Bailey, NP.

Zooplankton 2020
Typography
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Mesozooplankton biomass in autumn 2020 averaged about 7 g dry-weight m-2 for the parts of the Barents Sea that were covered. The spatial distribution of biomass across the Barents Sea displayed a typical pattern with high levels in southwestern and northern regions, and relatively low levels in central areas. Compared to the preceding 5 year averages, mesozooplankton biomass in 2020 was lower in the western, central, and eastern Barents Sea and slightly higher south and east of Svalbard. In the western subareas of the Barents Sea, including the region influenced by the inflowing North Atlantic Current, and where information on size-fractions is available, the decreased biomass in 2020 was mainly ascribed to the intermediate fraction (1000-2000 µm). This fraction mainly includes relatively large copepods such as older developmental stages of Calanus finmarchicus. Krill indices for biomass and abundances in the Barents Sea have shown increasing trends over recent decades.

Zooplankton

Mesozooplankton biomass and species abundances

Mesozooplankton biomass – large scale distributions

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 for mesozooplankton biomass show similarities across years, although some interannual variability is apparent. Differing geographical survey-coverages from year to year will impact biomass estimates both for territorial waters and the Barents Sea as a whole, particularly since the region 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) – see separate section below. During August-November 2020, relatively high biomass levels (>10 g m-2) were observed just north of the Norwegian mainland, in the area of the Bear Island Trench, west and northeast of Svalbard/Spitsbergen, around Franz Josef Land, and northwest of Novaya Zemlya. Relatively low biomass levels (

Figure 3.4.1.1. Distribution of total mesozooplankton biomass (dry-weight, g m-2) from seafloor to surface. Data based on 293 samples collected during BESS from 8. August – 11. Nov 2020. 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). Figure 3.4.1.1. Distribution of total mesozooplankton biomass (dry-weight, g m-2) from seafloor to surface. Data based on 293 samples collected during BESS from 8. August – 11. Nov 2020. 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 by filtering the wet samples through a series of 3 sieves with decreasing mesh-size (2000 μm, 1000 μm, and 180 μm) before weighing (see time series in Fig. 3.4.1.2). The biomass for the intermediate size-fraction in 2020 was below the 20-year (2000-2019) long-term average, 3.1 vs. 3.5 g dry-weight m-2. The biomass for the smallest size-fraction was a little above the long-term average (2000–2019), 2.8 vs. 2.5 g dry-weight m-2. Regarding the largest size-fraction, the values during the last ca. 12 years have typically been lower than during preceding years, and the level in 2020 was somewhat below the 20-year (2000-2019) long-term average (0.9 vs. 1.0 g dry-weight m-2).

Based only on Norwegian data, average zooplankton biomass (sum of all size-fractions) during August-October 2020 was 6.8 (SD 6.0) g dry-weight m-2 for the western part of the Barents Sea. This estimate is based on 178 observations and is lower than in 2019 (8.0 g dry-weight m-2), and also somewhat below the long-term (2000-2019) average (7.0 g dry-weight m-2) (Fig. 3.4.1.2). Note that the density of stations west and northwest of Svalbard in 2020 was higher than usual in earlier years, and also higher than in the rest of the Norwegian sector in 2020. All the Norwegian stations shown in Fig. 3.4.1.1 are included in the 2020 biomass average presented above. Since the area west and northwest of Svalbard had a comparatively high biomass compared to the Norwegian sector as a whole (c.f. Fig. 3.4.1.1), this implies that the estimated average was somewhat biased towards a higher level. To put this in perspective, an experimental removal of 6 “excess” stations west and northwest of Svalbard was made, with these 6 removed stations chosen so as to obtain a more regular spacing. This exercise indicated that the biomass estimate would then be reduced with about 2% (corresponding to ca. 0.14 g dry-weight m-2), while the effect on the spatially interpolated level of biomass was very subtle.

Note that the Russian samples were collected markedly later in autumn (29. September - 9. November) than the Norwegian samples (13. August – 1. October), and that the station density was variable also within the Russian sampling area. 6 of the 115 Russian sampling stations in the northern area were deeper than 500 m, with a maximum of 670 m. In the Russian sector, average biomass for the area covered in 2020 was 6.9 (SD 4.8) g dry-weight m-2. This value is based on 115 observations, and not comparable to the average for the Russian area covered in 2019. This is because the northeastern part of the Russian sector was unsurveyed in 2019, while data for the southeastern part in 2020 are lacking at present.

Figure 3.4.1.2. Time-series of average mesozooplankton biomass from seafloor to surface (dry-weight, g m-2) for the western and central Barents Sea (Norwegian sector) during the autumn BESS (1988–2020). Biomass is shown for three size-fractions representing sieves with mesh-sizes of 0.18–1 mm (yellow), 1–2 mm (orange), and >2 mm (red) (wet-sieving). Figure 3.4.1.2. Time-series of average mesozooplankton biomass from seafloor to surface (dry-weight, g m-2) for the western and central Barents Sea (Norwegian sector) during the autumn BESS (1988–2020). Biomass is shown for three size-fractions representing sieves with mesh-sizes of 0.18–1 mm (yellow), 1–2 mm (orange), and >2 mm (red) (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 mesozooplankton 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, and to some extent also variable spatial sampling density, also contribute to the variability in the estimated average biomass between years. For a more direct comparison of interannual trends, that are less influenced by variable spatial coverage and station density, time-series of biomass estimates for specific sub-areas of the Barents Sea are provided in the following section.

Mesozooplankton biomass in subareas of the Barents Sea

Fig. 3.4.1.3. Zooplankton sampling stations at the ecosystem survey in autumn 2020. Fig. 3.4.1.3. Zooplankton sampling stations at the ecosystem survey in autumn 2020.

The spatial coverage was relatively good for western and central polygons but was only partial for the Southeastern Basin (SEB) and Franz Joseph Land (FJL) polygons and lacking for the South East (SE) and Pechora polygons. Samples were collected from these polygons in the southeastern Barents Sea and will hopefully be added later.

Total biomass – combined results IMR and PINRO

Fig. 3.4.1.4. Time series 1989-2020 for mean zooplankton biomass (dry-weight, g m-2) for stations within subareas of the Barents Sea. 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. Fig. 3.4.1.4. Time series 1989-2020 for mean zooplankton biomass (dry-weight, g m-2) for stations within subareas of the Barents Sea. 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.

Zooplankton biomass data for 2020 have been calculated as average values for each of 13 subareas or polygons. Time-series of biomass estimates for these subareas for the 1989-2016 period were described in a background document (2018 WGIBAR Report, Annex 4). Time-series estimates for four Atlantic water subareas (Bear Island Trench, South-West, Thor Iversen Bank, and Hopen Deep), two central subareas (Central Bank and Great Bank), and two subareas in the eastern Barents Sea (South-East Basin and North-East) are shown in Fig. 3.4.1.4.

The biomass showed a pronounced peak in 1994 and 1995 for subareas in the southern and central Barents Sea. After year 2000, the biomass for the ‘Atlantic’ subareas in the southwestern Barents Sea has shown fluctuations between about 5-10 g dw m-2, being generally higher for the BIT polygon (up to 15 g dw m-2, Fig. 3.4.1.4, upper panel). The biomass tended to be high in 2019 but was lower in 2020 for three of the four Atlantic subareas. The biomass for the Central Bank and Great Bank has shown a declining trend since the 1990s to a minimum around 2013 (Fig. 3.4.1.4, middle panel). The biomass increased after this to maxima in 2017 (CB) and 2019 (GB) but is lower again in 2020 for both subareas. The biomass for the two eastern subareas has fluctuated at a relatively high level and is low in 2020 compared to most previous years for both the Southeastern Basin and North East subareas.

We note that sampling variance of zooplankton is relatively high, which reflects intrinsic patchy distribution more than procedures of sampling. The coefficient of variation (CV = SD/Mean) was from 0.44 to 0.89 with an average of 0.69 for the 11 polygons with data (n from 9 to 46). This degree of variance translates into confidence intervals of roughly +/- 40-20% for n between 10 and 40, which can serve as a rough ‘yardstick’ when considering the ‘significance’ of the observed variations.

The 2020 results for polygons have been plotted in Fig. 3.4.1.5 along with 5-year mean values from 2005 (2005-2010, 2011-2014, 2015-2019). The sequence of polygons is arranged broadly from SW to NE along a gradient from general dominance of Calanus finmarchicus in Atlantic water to dominance of Calanus glacialis in Arctic water. There is a consistent and persistent spatial pattern with highest biomass in the deeper areas and lower biomass in shallow areas like CB and PE. Given the dynamics of the zooplankton with from one to multiple generations per year for dominant species, and with water residence time of months for each polygon, the apparent stability of the spatial distribution pattern is remarkable.

Fig. 3.4.1.5. Zooplankton biomass (dry-weight, g m-2) by polygons with mean values for 2020 shown along with mean values for three preceding 5-year periods. Note that data for the SE and PE (Pechora) polygons for 2020 are missing (will hopefully be provided later). Fig. 3.4.1.5. Zooplankton biomass (dry-weight, g m-2) by polygons with mean values for 2020 shown along with mean values for three preceding 5-year periods. Note that data for the SE and PE (Pechora) polygons for 2020 are missing (will hopefully be provided later).

The 2020 biomass data are generally lower compared to the previous years (Fig. 3.4.1.5). In comparison with the 5-year means for the 2015-2019 period, 2020 is lower by 1-3 g dw m-2 for 8 out of 11 polygons (Fig. 3.4.1.6). Three polygons showed either no change or a slight increase.

Fig. 3.4.1.6. Difference between zooplankton biomass in 2020 compared to the 5-year mean for the 2015-2019 period. Fig. 3.4.1.6. Difference between zooplankton biomass in 2020 compared to the 5-year mean for the 2015-2019 period.

Zooplankton biomass in size fractions

IMR uses a size-fractionation method to determine zooplankton dry weight biomass in three size fractions: large (>2 mm), medium (1-2 mm), and small (

Time series of biomass in the three fractions are shown for four selected polygons in Fig. 3.4.1.7. The Bear Island Trench (BIT) polygon lies in the inflow region of Atlantic water to the Barents Sea, and the zooplankton (which is dominated by C.finmarchicus) is dominated by the medium size fraction (the ‘Calanus fraction’) (Fig. 3.4.1.7 A). The Thor Iversen Bank (TIB) polygon located further into the Barents Sea has a somewhat different size composition with more equal proportions of the medium and small fractions (Fig. 3.4.1.7 B). The Great Bank (GB) polygon located further north in the Arctic domain of the Barents Sea, has shown a declining trend in biomass over the last decades with general dominance of the small fraction (Fig. 3.4.1.7 C). The Franz-Victoria Trough (FVT) polygon located in the northern Barents Sea has a biomass dominated by the medium size fraction (Fig. 3.4.1.7 D). In this case it reflects a dominance of C. glacialis in the cold Arctic water mass, or a combination of C. glacialis and C. finmarchicus in Atlantic water from the West Spitsbergen Current.

Fig. 3.4.1.7. Time series of zooplankton biomass (dry-weight, g m-2) in three size fractions from 1989 to 2020 for four selected polygons: A – BIT, Bear Island Trench, B – TIB, Thor Iversen Bank, C – GB, Great Bank, and D – FVT, Franz-Vitoria Trough. Fig. 3.4.1.7. Time series of zooplankton biomass (dry-weight, g m-2) in three size fractions from 1989 to 2020 for four selected polygons: A – BIT, Bear Island Trench, B – TIB, Thor Iversen Bank, C – GB, Great Bank, and D – FVT, Franz-Vitoria Trough.

A summary of the biomass time series for 8 polygons is shown in Fig. 3.4.1.8, where the 2020 data are plotted together with decadal means for the 1990s and 2000s and 5-year means for the 2010s. In this figure, the total biomass (similar as that shown in Fig. 3.4.1.5) is shown as stacked columns with contributions by the three fractions. The biomass showed a peak for many polygons in 1994-95, driven by the small size fraction. This was reflected in generally high decadal mean for the 1990s. The 5-year mean for 2015-19 also tended to be high for the ‘Atlantic’ polygons as well as some of the central polygons.

Fig. 3.4.1.8. Zooplankton dry weight biomass (dry-weight, g m-2) in three size fractions shown as mean values for 8 polygons – four ‘Atlantic’ polygons (upper panel) in the inflow region of Atlantic water, and four central and northern polygons (lower panel). Values are decadal means for the 1990s and 2000s, 5-year means for 2010-14 and 2015-19, and mean values for 2020. Fig. 3.4.1.8. Zooplankton dry weight biomass (dry-weight, g m-2) in three size fractions shown as mean values for 8 polygons – four ‘Atlantic’ polygons (upper panel) in the inflow region of Atlantic water, and four central and northern polygons (lower panel). Values are decadal means for the 1990s and 2000s, 5-year means for 2010-14 and 2015-19, and mean values for 2020.

The decrease in biomass in 2020 compared to the previous five years (2014-19) for many of the polygons (see Fig. 3.4.1.6) was associated with a decrease in the medium size fraction and a corresponding increase in the small fraction. This can be seen for the ‘Atlantic’ polygons (less red and more grey column sections) as well as for the CB and SS polygons (Fig. 3.4.1.8). The ratio of biomass of the small to medium fractions was often substantially higher in 2020 compared to the 2015-19 average, except for TIB and the two northern polygons SN and FVT (Fig. 3.4.1.9). The increase of the small fraction was especially large for the SW, BIT and SS polygons in the western Barents Sea, from about half (ratio 0.5) to 25-50% larger than the medium fraction.

Fig. 3.4.1.9. Ratio of biomass between the small and medium size fractions in 2020 compared to the mean for the five preceding years (2015-19). Fig. 3.4.1.9. Ratio of biomass between the small and medium size fractions in 2020 compared to the mean for the five preceding years (2015-19).

In summary, 2020 can be characterized as a year with lower biomass of zooplankton in the western and central Barents Sea compared to the previous 5 years. The lower biomass was associated with a shift from the medium to the small size fraction, most pronounced in the Atlantic inflow region in the western Barents Sea. Since zooplankton biomass is generally dominated by older copepodite stages of C. finmarchicus (and C. glacialis in Arctic water) (Aarflot et al. 2018), the shift can be interpreted to reflect a decrease of C. finmarchicus in the Atlantic water.

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 1995–2020 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 seasonal cycle and summing up the copepodite stages I-VI (Fig. 3.4.1.10, 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 until 2019. However, in 2020 an exceptionally low average abundance of C. finmarchicus was registered along the Fugløya-Bjørnøya section (Fig. 3.4.1.10). The 2020 average was the lowest since the time-series was started in 1995. This year comprised only 4 coverages of the transect. Both the May and August abundances of this species in 2020 were very low compared to what is typical for those parts of the season in earlier years. January and December 2020 showed very low abundances, which is the usual for winter (note that only two samples were available in January). The March cruise was cancelled due to the COVID-19 situation. The above-mentioned low C. finmarchicus abundances were supported by a remarkably FB zooplankton biomass (8 stations) in May 2020 compared to this month in other years, while also in August 2020 the biomass was much lower than typical for that month. The FB abundances of C. finmarchicus during the next year(s) will be followed closely to see if this was a one-off year, or if low levels become a recurring feature.

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 (Fig. 3.4.1.10, 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. (Fig. 3.4.1.10, 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. This species is also observed in the Norwegian Sea off mid-Norway, particularly in autumn (Continuous Plankton Recorder survey, Strand et al. 2020). 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 3.4.1.10. 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 mostly 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. Figure 3.4.1.10. 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 mostly 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 C. 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 (Fig. 3.4.1.10, right). The highest abundance of C. finmarchicus was observed at the most southerly station of the section at 69º30′N and further north at 72°30′N, while its lowest abundance was observed at 70°00′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 (Fig. 3.4.1.10, 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) (Fig. 3.3.1.10, right). A gradual increase in C. hyperboreus abundance has been observed since 2015. The highest abundance of this species was observed northwards from 73°00′ 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 2019 showed that in the Russian part, copepods dominated both in terms of abundance (79.5%) and biomass (69.5%) (Fig. 3.4.1.11). Total zooplankton abundance in the southern (south of ca. 75°N) Barents Sea was higher than in the northern part (north of ca. 75°N) of the sea (2 190 and 1 725 ind. m-3, respectively), while the total zooplankton biomass was almost twice higher in the northern than the southern Barents Sea (286.9 and 143.9 mg m-3, respectively). However, the results from the northern Barents Sea are not quite comparable with previous years as only 13 stations were conducted in the northern part of the sea in 2019.

Figure 3.4.1.11. Abundance (ind. m-3) (left) and biomass (wet-weight, mg m-3) (right) of the most numerous copepod species (surface to sea floor) in the Barents Sea (based on the PINRO samples from the BESS during August-September of 2019) Figure 3.4.1.11. Abundance (ind. m-3) (left) and biomass (wet-weight, mg m-3) (right) of the most numerous copepod species (surface to sea floor) in the Barents Sea (based on the PINRO samples from the BESS during August-September of 2019)

In the southern Barents Sea, total zooplankton abundance and biomass in 2019 had increased by factors 1.7 and 1.1, respectively, in comparison to 2018. Copepods dominated both abundance and biomass (76.8 and 70.6%, respectively). Among other groups, the most important were chaetognaths comprising 16.6% of total zooplankton biomass, while their abundance was very low (0.2%). Considering species composition of copepods, Oithona similis, C. finmarchicus and Pseudocalanus sp. were the most abundant (43.4, 23.2 and 15.9% of total copepod abundance, respectively), and Metridia longa comprised 3.5% (Fig. 3.4.1.11). However, in terms of copepod biomass, C. finmarchicus (63.7%), Pseudocalanus sp. (14.4 %) and M. longa (9.4 %) were the most important species, while O. similis comprised only 3.0 % (Fig. 3.4.1.11). In 2019, abundance of C. finmarchicus, O. similis and Pseudocalanus sp. had increased compared to 2018, while increase of biomass was observed only for Pseudocalanus sp. and O. similis. It is necessary to point out, in 2019 plankton was collected on 46 stations, while in 2018 only on 9 stations, so that the results for 2018 and 2019 should be compared with caution.

In the northern Barents Sea, total zooplankton abundance decreased by factor 1.2, and total biomass increased by factors of 1.2 in 2019 in comparison to 2018. Copepods were the most abundant (89.2% of total zooplankton abundance) zooplankton group. Regarding total zooplankton biomass, copepods (64.7%) also represented the most important group during 2019, while chaetognaths and pteropods comprised 15.6 and 11.2%, respectively. In the northern Barents Sea, the small copepods Pseudocalanus sp. and O. similis were the most abundant (42.6 and 31.7% of total copepod abundance, respectively) (Fig. 3.4.1.11). Total copepod biomass consisted mainly of larger C. glacialis (56.8%) and M. longa (15.8%), and of small Pseudocalanus sp. (12.8 %) (Fig. 3.3.1.11). Abundance and biomass of C. glacialis have been increasing since 2015. Abundance and biomass of C. finmarchicus, M. longa and Pseudocalanus sp. increased in the period from 2016 to 2018 and in 2019 it decreased. At the same time, abundance and biomass of O. similis have been decreased from 2016 to 2018, and it increase was observed in 2019.

Macroplankton biomass and distribution

Krill

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 restricted 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 (Fig. 3.4.2.1). A new time-series on euphausiids has been launched in February-March 2015 in the course of the Joint Barents Sea Russian-Norwegian winter survey.

Figure 3.4.2.1. 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. Figure 3.4.2.1. 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 southeastern Barents Sea in February 2020 during the Russian-Norwegian winter survey using the trawl-attached plankton net. Since in course of the survey different areas were covered in different years (2015-2019), comparison with previous years requires caution.

Distribution of euphausiids in the southeastern Barents Sea in February 2020 is presented in Fig. 3.4.2.2. Results indicate that total abundance of euphausiids increased on the average by factor 2.0 in 2020 in comparison to 2019. Average abundance of euphausiids in 2020 (1860 ind. 1000 m-3) was higher than in previous years (2015-2019 period: 561–1255 ind. 1000 m-3). In 2020 an increase of average euphausiid abundance was observed in all subareas and was especially considerable in the central and coastal areas, in comparison to 2019. At 6 stations of three different subareas an extremely high total euphausiids abundance of 5000–15000 ind. 1000 m-3 was observed.

Euphausiid species composition was traditionally presented by local species Thysanoessa inermis and T. raschii, as well as by Atlantic species Meganictiphanes norvegica and T. longicaudata. The two last-named species penetrate the Barents Sea with warm waters from the Norwegian Sea. Another warmwater species Nematoscelis megalops, which has been regularly registered in the coastal, western and central areas since 2003, was not found in 2020. T. inermis is the most numerous species and comprised 60-70% of total euphausiids abundance in the southeastern Barents Sea. High euphausiids abundance in 2020 was formed mainly by T. inermis. In 2020 abundance of T. inermis, T. raschii and T. longicaudata considerably increased, compared to 2019, while that of M. norvegica decreased.

 width= Figure 3.4.2.2. Distribution of euphausiids (abundance, ind. 1000 m-3) in the near-bottom layer of the Barents Sea based on data from the Russian-Norwegian winter survey during February 2020.

The following subchapters (from summer-autumn distribution of krill to jellyfish) were not updated due to challenges with estimation software.

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 (Fig. 3.4.2.3). 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 south-eastern Barents Sea. About one third of the stations during the survey in 2019 were sampled during night (Table 3.4.2.1). 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).

Figure 3.4.2.3. Krill distribution (biomass, wet-weight, g m-2), based on pelagic trawl stations covering the upper water layers (0-60 m), in the Barents Sea in August-October 2019. Figure 3.4.2.3. Krill distribution (biomass, wet-weight, g m-2), based on pelagic trawl stations covering the upper water layers (0-60 m), in the Barents Sea in August-October 2019.

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 3.4.2.4).

Figure 3.4.2.4. Krill species distribution (biomass, wet-weight, g m-2), based on trawl stations covering the upper water layers (0-60 m), in the Barents Sea in August-October 2019. Figure 3.4.2.4. Krill species distribution (biomass, wet-weight, g m-2), based on trawl stations covering the upper water layers (0-60 m), in the Barents Sea in August-October 2019.

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 Fig. 3.4.2.5. 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).

Figure 3.4.2.5 Length distribution of T. inermis and M. norvegica from catches with standard pelagic trawl in the upper layers (0-60 m) of the Barents Sea in August-October 2019. Figure 3.4.2.5 Length distribution of T. inermis and M. norvegica from catches with standard pelagic trawl in the upper layers (0-60 m) of the Barents Sea in August-October 2019.

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. 3.4.2.6).

Table 3.4.2.1 Day and night total catches (g m-2) of krill taken by the pelagic trawl in the upper water layers (0-60 m). Table 3.4.2.1 Day and night total catches (g m-2) of krill taken by the pelagic trawl in the upper water layers (0-60 m).

* – 2018 is not included due to the poor coverage.

Figure 3.4.2.6. 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) Figure 3.4.2.6. 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 (PINRO), Elena Eriksen (IMR), and Irina Prokopchuk (PINRO)

Figures by Pavel Krivosheya (PINRO), Tatyana Prokhorova (PINRO), and Irina Prokopchuk (PINRO)

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 (Fig. 3.4.2.7). The largest catches were dominated by Arctic Themisto libellula, and made north and east of Svalbard/Spitsbergen (Fig. 3.4.2.7 and 3.4.2.8).

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).

Figure 3.4.2.7 Amphipods distribution (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. Figure 3.4.2.7 Amphipods distribution (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.

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 3.4.2.8. 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. Figure 3.4.2.8. 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 (Fig. 3.4.2.9).

Figure 3.4.2.9 Length distribution of T. libellula from catches with standard pelagic trawl in the upper layers (0-60 m) of the Barents Sea in August-October 2019. Figure 3.4.2.9 Length distribution of T. libellula from catches with standard pelagic trawl in the upper layers (0-60 m) of the Barents Sea in August-October 2019.

Jellyfish

By Elena Eriksen (IMR)

The estimated biomass of gelatinous zooplankton presented here are based on bycatches from trawling on the 0-group fish, using the standard pelagic «Harstad» trawl in the 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 and 2020 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. 3.4.2.10).

Figure 3.4.2.10. 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. Figure 3.4.2.10. 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.

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