Zooplankton

Mesozooplankton biomasses

By Espen Bagøien, Andrey Dolgov, Irina Prokopchuk, and Hein Rune Skjoldal

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 on a multiannual timescale, although some interannual variability is apparent. For logistical reasons, a large region in south-eastern and central-eastern parts of the Barents Sea was not sampled during the 2018 BESS (Figure 3.3.1). Differences in survey coverage between years may impact biomass estimates for territorial waters and the Barents Sea as a whole; particularly as the ecosystem is characterized by large-scale heterogeneous distribution of biomass. One way to address this challenge, is to make interannual comparisons of estimated biomass within well-defined and consistent spatial subareas (polygons).

During August-October 2018, relatively high biomass (>10 g m-2) was observed in and just south of Bear Island Trench, north and northeast of Svalbard/Spitsbergen, south of Franz Josef Land, and in basins of the south-eastern Barents Sea. Relatively low biomass (<3 g m-2) was observed on Great Bank, Central Bank, and in areas further west in the central region bordering the Norwegian Sea. Low biomass was also observed near the coast in the south-eastern corner of the Barents Sea (Figure 3.3.1).

The pattern of large-scale horizontal distribution for plankton during autumn 2018 resembled that of 2017, not considering large areas in south-eastern and central-eastern regions of the Barents Sea which were not surveyed in 2018.

Figure 3.3.1. Distribution of total mesozooplankton biomass (dry weight, g m-2) from surface to seafloor. Data based on 173 samples obtained during the BESS during late August – mid-October 2018. 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.10.3, 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 weighting . For the smallest- and middle size-fractions, estimated average biomass was slightly higher than the long-term (20-year) average (1998–2017) (Figure 3.3.2). For the largest size-fraction, average values have shown a decreasing trend during the ca. last 14 years; in 2018, biomass for the largest size-fraction was well below the long-term average. Based only on Norwegian data, which represent the longest time-series, average zooplankton biomass (summing all size-fractions) during August-October 2018 was 7.2 (SD 5.6) g dry-weight m-2 for the western part of the Barents Sea. This estimate is higher than in 2017 (6.4 g dry-weight m-2), and just above the long-term average (7.0 g dry-weight m-2) (Figure 3.3.2).

Combined Russian and Norwegian data (173 stations in total) (Figure 3.3.1), covering the entire area surveyed in the Barents Sea in 2018, provided an estimated average zooplankton biomass of 7.5 (SD 5.7) g dry-weight m-2. This estimate is not directly comparable with the 2017 estimate (7.2 g m-2), due to the above-mentioned lack of sampling in the south-eastern and south-central regions during 2018. In the Russian sector, average biomass for the area covered in 2018 was 9.1 (SD 6.1) g dry weight m-2; this value also is not directly comparable to the 2017 estimate (8.6 g dry-weight m-2) due to incomplete survey coverage in 2018.

Figure 3.3.2. 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 largely controlled by predation pressure, e.g. from capelin , however the annual impact of predation varies between regions . Capelin stock-size was relatively high during the 2008-2013 period and was expected to exert strong predation pressure on zooplankton. In 2014, capelin stock-size decreased; in 2015 and 2016 it declined further to very low levels, likely easing the pressure on their zooplankton prey. The 2017 estimate suggested a marked increase in capelin stock-size. The 2018 estimate again showed decrease, but was higher than the lows estimated for 2015 and 2016. Estimated stock size for capelin in 2018 (TSB ~ 1 600 thousand tonnes) was below the long-term average for the 1988-2017 period (TSB ~ 2 330 thousand tonnes), and likely exerted moderate predation pressure on the zooplankton community. Predation from other planktivorous pelagic fish (herring, polar cod, and blue whiting) and pelagic juvenile demersal fish species (cod, haddock, saithe, and redfish), 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 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 subareas of the Barents Sea are provided in the following section).

Mesozooplankton biomass in subareas of the Barents Sea

By Hein Rune Skjoldal and Padmini Dalpadado

IMR estimates of 2018 zooplankton biomass have been calculated as average values for each of 9 subareas. 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), and for Central Bank and Great Bank are shown in Figure 3.3.3.

Figure 3.3.3. Time-series estimates 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 survey data for the 1989-2018 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). Lower panel – two subareas in the central Barents Sea with colder and partly Arctic water conditions: Central Bank (CB) and Great Bank (GB). Results presented represent the total biomass 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 (up to 15 g dw m-2). The values for 2018 are at the same level as in the previous years. 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 the minim in 2013 (Figure 3.3.3 - Lower). Biomass at Central Bank has subsequently increased after this, although the 2018 estimate was relatively low (about 4 g dw m-2). In contrast, biomass at the Great Bank has remained at a very low level also in 2018 (about 2 g dw m-2).

Figure 3.3.4. 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 1989-2018 period. Note: Size fractions are based on screen mesh size, not size of individual zooplankton.

Size composition of mesozooplankton is shown in Figure 3.3.4 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 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). Great Bank used to be the domain for the dominant Arctic species Calanus glacialis (Melle and Skjoldal, 1998); This area traditionally has been a core feeding area for capelin; decreased biomass of C. glacialis in this area may negatively impact the capelin stock.

Figure 3.3.5 shows a comparison of long-term average estimates of zooplankton biomass (1989–2016) for each subarea together with estimates for 2018. In most subareas, biomass estimates for 2018 are close to the long-term average. Note that due to lack of coverage in the eastern Barents Sea during 2018, number of observations (stations) in the South-East, South-East Basin, and Pechora was low (2-3 stations).

Figure 3.3.5. 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 sampled in 2018.

Mesozooplankton species-composition in the eastern Barents Sea

By Irina Prokopchuk

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 3.3.6). 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.

Figure 3.3.6. Abundance (ind. m-2) of C. finmarchicus, C. glacialis, and C. hyperboreus along the Kola section during May-June 1992 and the 2008-2018 period. Dashed line shows long-term average values. Note the gap in years between 1992 and 2008 to the left part of the figure.

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 3.3.6). 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 3.3.6). A gradual increase in C. hyperboreus abundance has been observed since 2015. Highest abundance of this species was observed northwards from 73º00′ N. The C. hyperboreus population was represented by copepodites CIV-CV.

Russian (PINRO) regional scale investigations of mesozooplankton conducted by the BESS during August-September of 2017 showed that copepods dominated both in terms of abundance (89.8%) and biomass (68.8%) (Figures 3.3.7 and 3.3.8). Total zooplankton abundance in the southern (south of ca. 75°N) and northern (north of ca. 75°N) Barents Sea was almost equal (1 477 and 1 458 ind. m-3, respectively); total zooplankton biomass was higher in the northern than the southern Barents Sea (192.3 and 145.8 mg m-3, respectively).

In the southern Barents Sea, copepods dominated both abundance and biomass (87.6 and 79.5%, respectively). Among other groups, chaetognaths comprised 9.9% of total zooplankton biomass. In 2017, total zooplankton abundance decreased by a factor of 1.4 compared to 2016; total zooplankton biomass decreased similarly. Considering species composition of copepods, the small Oithona similis and Pseudocalanus sp. and the larger Calanus finmarchicus were most abundant (~55.1 and 20 % of total copepod abundance, respectively), and the large Metridia longa comprised 4.9 % (Figure 3.3.7). However, in terms of copepod biomass, C. finmarchicus (59.9%), M. longa (14.3%), and Pseudocalanus sp. (9.8%) were the most important species; while O. similis comprised 5.6% of the total (Figure 3.3.8). In 2017, abundance and biomass of O. similis and Pseudocalanus sp. decreased compared to 2016. At the same time, both abundance and biomass of M. longa increased between 2016 and 2017 by a factor of 2, while these same parameters for C. finmarchicus remained at the 2016 level.

In the northern Barents Sea, copepods were the most abundant (91.5%) zooplankton group. Regarding total zooplankton biomass, copepods also represented the most important group (60.4%) during 2017, while chaetognaths and pteropods comprised 16.3 and 6.5%, respectively. In comparison to 2016, increases in total zooplankton abundance and biomass occurred primarily due to increased abundance of copepods and chaetognaths. In the northern Barents Sea, the small copepods O. similis and Pseudocalanus sp. contributed 33.3 and 33.2%, respectively; while larger species M. longa, C. finmarchicus, and C. glacialis contributed 11.6, 9.2 and 5.2% to total copepod numbers, respectively (Figure 3.3.7). Total copepod biomass consisted mainly of C. glacialis (28.0%), C. finmarchicus (24.5%), M. longa (20.7%), Pseudocalanus sp. (16.4%), and C. hyperboreus (6.2%). Compared to 2016, abundance of C. finmarchicus, C. glacialis, and Pseudocalanus sp. increased, while that of O. similis decreased in 2017. 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.

Figure 3.3.7. Abundance (ind. m-3) 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 2017).

Figure 3.3.8. Biomass (mg wet-weight m-3) 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 2017).

Mesozooplankton species-composition in the western Barents Sea

Espen Bagøien and Padmini Dalpadado

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 1995–2018 period from four fixed locations at different latitudes (70.30°N, 72.00°N, 73.30°N, and 74.00°N) representing different water masses (Coastal, Atlantic, and mixed Atlantic/Arctic) have been analysed taxonomically. Average abundance is estimated annually, by pooling the four stations and summing copepodite stages I-VI of C. finmarchicus, C. glacialis, and C. hyperboreus (Figure 3.3.9). C. finmarchicus is, by far, the most common of these three species, and displays high interannual variation in abundance. C. finmarchicus tends to be most abundant at the three southernmost stations. Particularly high abundance was recorded during 2010 along most of the transect, except at the northernmost station (74.00°N). After registering very low abundances at all stations in 2013, C. finmarchicus has generally been abundant along most of the transect during the last 4 years (2014–2018).

Figure 3.3.9. Abundance (ind. m-2) of Calanus finmarchicus, C. glacialis, and C. hyperboreus along the Fugløya-Bjørnøya transect during the 1995–2018 period. Each bar represents the annual average for 4 stations and 5–6 coverages per year (except for 4 and 3 station coverages in 2012 and 2013, respectively).

C. glacialis has typically been most abundant at the two northern-most stations where Atlantic and Arctic waters mix. This species also shows large interannual variations in abundance (Figure 3.3.9). Abundance of C. glacialis along the FB transect has decreased since 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. Abundance of the largest species, C. hyperboreus, along the FB transect has been low relative to the abundance of C. finmarchicus, but also compared to C. glacialis in general throughout the study period. Few individuals of this species were observed during 2008-2010, 2013, and 2016. (Figure 3.3.9). The FB time-series of C. hyperboreus abundance shows strong interannual variability.

Calanus helgolandicus, a more southern species, has been observed regularly at Fugløya-Bear Island transect, particularly during the December-February period (Dalpadado et al., 2012). In recent years, it has been observed more frequently in the North Sea and in southern regions of the Norwegian Sea (Svinøy transect). C. helgolandicus is similar in appearance to C. finmarchicus and taxonomic separation of these two species is time-consuming. Hence, the routine is to examine a limited number of C. helgolandicus individuals belonging to the later stages – up to 20 copepodites of stage V and up to 20 adult females – to establish the species-proportions in each FB sample. During winter, the ratio of C. helgolandicus to C. finmarchicus along the Fugløya–Bear Island transect has been observed to increase and C. finmarchicus is normally overwintering in deeper waters. Our FB time-series provides no evidence of an increase over the years in either the proportion or absolute abundance of C. helgolandicus at the entrance to the Barents Sea.

Macroplankton biomasses and distributions

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 associated with Atlantic water in the western and central Barents Sea, Meganytiphanes norvegica and Thysanoessa longicaudata associated with the inflowing Atlantic water, particularly during warm periods, and Thysanoessa raschii found mainly in shallow waters in the southeastern Barents Sea. M. 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. T. longicaudata is the smallest species – not exceeding 1.8 cm.

Winter distribution and biomass

By Anna Mikhina, Valentina Nesterova, Andrey Dolgov

Euphausiids were collected in the Barents Sea during the PINRO winter survey (November-December 2017) with the trawl-attached plankton net. Note: The PINRO long-term data series on euphausiids was initiated in 1959 and stopped in 2016 (Figure 3.3.12). In 2017, only a part of this survey was conducted; results from only one cruise are presented, covering the southern part of the Barents Sea (Figure 3.3.10). These data are not comparable with the previous years.

Results indicate that in 2017, the trend of increasing euphausiid abundance continued in the southern Barents Sea. Compared to 2015 (no sampling in 2016), average euphausiid abundance in the southern Barents Sea in 2017 increased. This increase was mainly observed in central and coastal areas. Euphausiid abundance in eastern and western areas decreased in 2017 relative to 2015. Euphausiid concentrations were formed mainly by local species (T. inermis and T. raschii) and 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. At the same time, euphausiid abundance decreased in the northwestern Barents Sea. The overall average abundance of euphausiids in the Barents Sea decreased in 2017 relative to 2015.

Figure 3.3.10. Distribution of euphausiids (ind. 1000 m-3) in the near-bottom layer of the Barents Sea based on data from the Russian winter survey during November-December 2017.

Euphausiids were also collected in the southern Barents Sea during the Russian-Norwegian winter survey (February 2018) with the trawl-attached plankton net. Euphausiid sampling in this survey was initiated in 2015 onboard a Russian research vessel (no sampling occurred in 2017), but different areas were covered in different years. These results are very preliminary, and comparison with previous years requires caution. Results indicate that in 2018, euphausiid abundance was high in the southern Barents Sea in comparison to November-December 2017. Average abundance of euphausiids in all areas was much higher in 2018 than in 2016. However, in 2018 the average abundance of euphausiids in western and coastal areas decreased compared to 2015 but increased considerably in the central and eastern areas. As during November-December 2017, euphausiid concentrations were formed mainly by local species (T. inermis and T. raschii) and Atlantic species (M. norvegica and T. longicaudata). 

Figure 3.3.11. Distribution of euphausiids (ind. 1000 m-3) in the near-bottom layer of the Barents Sea based on data from the Russian Norwegian winter survey during February 2018.

Figure 3.3.12. 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-net catches from the bottom layer in: a) Southern Barents Sea; and b) Northwestern Barents Sea. Note: this data-series was stopped in 2016, but is presented here to show general trends since the early 1950s and 1960s.

Summer-autumn distribution and biomass

By Espen Bagøien, Elena Eriksen, Tatiana Prokhorova and Andrey Dolgov

The following results are based on autumn 2018 BESS data; krill catches were made using standard pelagic trawls. Estimation of krill biomass for the entire Barents Sea was not possible in 2018 due to a lack of spatial coverage. As result, only krill distribution is presented.

During 2018, krill were widely distributed in the western Barents Sea (Figure 3.3.13). Biomass estimates in the upper 60 m are presented as grams of wet weight per square meter (g m-2). Night catches in the west averaged 5.8 g m-2 in 2018 and were lower than the 1980-2017 long-term average for the entire Barents Sea (7.7 g m-2, Figure 3.3.14).

Figure 3.3.13 Krill distribution, based on pelagic trawl stations covering upper layers of the water column (0-60 m) in the Barents Sea during August-October 2018.

The number of the night stations in 2018 was 74, while there were 104 day stations. During night, most krill migrate to upper water layers to feed and are therefore more accessible to the trawl. Larger catches (more than 50 g m-2) were observed in the central area of the sea.

Figure 3.3.14. Average krill biomass (g wet-weight m-2) sampled during BESS autumn night catches with pelagic 0-group fish-trawls within the 0-60m layer during the 1991-2016 period.

Amphipods

Information on amphipods (mainly hyperiids) presented here is based on catches with standard pelagic trawl in the 0-60 m layer during the BESS in autumn. Estimation of amphipod biomass in 2018 for the entire Barents Sea was not possible due to lack of survey coverage. Therefore, only amphipod distribution is presented here.

During 2018, amphipods were caught east of Svalbard/Spitsbergen Archipelago (Figure 3.3.15). 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.

Figure 3.3.14. . Average krill biomass (g wet-weight m-2) sampled during BESS autumn night catches with pelagic 0-group fish-trawls within the 0-60m layer during the 1991-2016 period.

Jellyfish

Geographic distribution and estimated biomass of gelatinous zooplankton here presented are based on the BESS during autumn and using a standard pelagic trawl for 0–60 m depth. Estimation of the biomass of gelatinous zooplankton for the entire Barents Sea was not possible in 2018 due to lack of coverage. Therefore, only the geographic distribution of jellyfish is presented for 2018.

During August-October 2018, lion’s mane (Cyanea capillata; Scyphozoa) was the most common jellyfish species, with respect to both weight and abundance (average catch of 39 kg, corresponding to 5.1 tonnes per sq nmi), and was widely distributed over the covered area (Figure 3.3.16). Large catches (> 10 tonnes per sq nmi) were made in the northcentral Barents Sea.

Individual blue stinging jellyfish (Cyanea lamarckii) were observed at three stations close to the northern Norwegian coast (Figure 3.3.17). C. lamarckii has been observed regularly in the Barents Sea in recent years and the presence of this warm-temperate species may be linked to the inflow of Atlantic water masses.

Individual helmet jellyfish (Periphylla periphylla) were observed at two stations only (Figure 3.3.17). P. periphylla were caught in seven pelagic trawl (27 specimens) and one bottom trawl (2 specimens) station during 2014, and in six pelagic trawl- (9 specimens) and six bottom trawl (6 specimens) stations during 2016. The presence of this deep-water species may be linked to pelagic feeding during the night.

Figure 3.3.16. Distribution of jellyfish, mainly Cyanea capillata, catches (wet weight; kg per sq nmi) in the Barents Sea, during August-October 2017. Catches presented for both day and night from standard pelagic trawl stations at 0–60 m depth.

Figure 3.3.17. Jellyfish catches (wet weight; kg per sq nmi) of Cyanea lamarckii and Periphylla periphylla in the Barents Sea during August-October 2018. Catches are presented for both day and night from standard pelagic trawl stations in the upper 0–60 m layer.

Long-term trends

Total biomass of jellyfish, mainly C. capillata, in upper 60 m for the entire Barents Sea during autumn 2018 could not be estimated due to incomplete coverage, but is shown for the years 1980-2017 (Figure 3.3.18). The estimated long-term average is 1.3 million tonnes. Interannual variation is considerable.

Figure 3.3.18. Estimated total biomass of the jellyfish, mainly 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.