Zooplankton

Mesozooplankton biomasses

Challenges in covering the same area each year are inherent in such large-scale monitoring, and interannual variation in ice cover is one reason for this. This implies that the average biomasses estimated for the different years in some cases are not directly comparable. In 2016, like in the preceding years, the highest biomasses (>10 g m^-2) were in the west-ern and northern parts of the survey area, including northwest and north of Svalbard/Spitsbergen and south of Franz Josef Land. In addition, a subregion towards the southeast (ca. 73-75°N, 40-50°E) displayed elevated levels in 2016 as in earlier years. Comparatively lower biomasses (<3 g m^-2) were typical on the Svalbard Bank (northeast of Bear Island), in the central Barents Sea, and in easterly and south-easterly parts of the survey area the last years (Figure 3.3.1.)

Figure 3.3.1. Distribution of zooplankton biomass (dry weight, g m^-2) from bottom-0 m in autumn 2016 (upper panel). Data based on samples obtained during the joint Norwegian-Russian (IMR/PINRO) ecosystem survey in late August – early October. Interpolation made in ArcGIS v.10.3, module Spatial Analyst, using inverse data weighting (default settings).

In the Norwegian waters of the Barents Sea, mesozooplankton biomass is size-fractionated (180–1000 μm, 1000–2000 μm and >2000 μm) before weighing. For the smallest size-fraction, the 2016 average biomass was similar to the average for the last 20 years, while for the intermediate size-fraction, the 2016 average biomass was above the average for the last 20 years (Figure 3.3.2). In contrast, for the largest size-fraction, the area- averaged values have displayed a decreasing trend during the ca. 10 last years, and in 2016 the biomass for the largest size-fraction was in the lower part of the range for the time-series. Based only on Norwegian data, which represent the longest time-series, average zooplankton biomass for the sum of all size-fractions in August-October 2016 was estimated to be 7.7 g dry-weight m^-2 in the western-central Barents Sea. This is somewhat lower than measured in 2015, 8.7 g dry-weight m^-2, but still above the average for the last 20 years (1997–2016, 7.1 g dry-weight m^-2).

Combined Russian and Norwegian data covering the entire Barents Sea provided an estimated average zooplankton biomass of 6.6 g dry-weight m^-2 in 2016. This estimate is not directly comparable with those for 2015 (7.3 g m^-2) and 2014 (6.7 g m^-2), since the areas covered in 2016 differed from those in the two previous years. In the Russian sector alone, average biomass in 2016 was estimated to be 3.9 g dry weight m^-2, which again is difficult to compare with the values for earlier years due to an incomplete coverage in the southern part of the monitoring area in 2016.

Figure 3.3.2. Time-series of mean zooplankton biomass from bottom – 0m (dry-weight, g m^-2) for the western and central Barents Sea for the Norwegian part of the autumn ecosystem-survey, 1988– 2016. Data are shown for the three size-fractions (0.18-1 mm, 1–2 mm, >2 mm) based on wet sieving.

Zooplankton biomass can vary considerably between years and appears to be controlled largely by predation pressure, e.g. from capelin, although its yearly impact could also vary between regions. The capelin stock size was relatively high during 2008–2013, hence exerting a high predation pressure on zooplankton. In 2014, the capelin stock-size had decreased, and in 2015 and 2016 the stock declined further to very low levels, most likely easing the pressure on their prey. In addition, processes such as transport of plankton from the Norwegian Sea into the Barents Sea, primary production (see section above), and local production of zooplankton, are likely to contribute to the observed variability of the zooplankton biomass in the Barents Sea. Zooplankton biomass can vary considerably between years and appears to be controlled largely by predation pressure, e.g. from capelin, although its yearly impact could also vary between regions.

Zooplankton biomass in subareas of the Barents Sea

Zooplankton biomass data from the late 1980s up to 2016 have been processed by gridding, and mean biomass values have been calculated for each of the subareas (or polygons) described previously (Figure 3.3.2). IMR has monitored zooplankton biomass (dry-weight) in three size-fractions since the 1980s, while the joint IMR-PINRO coordinated monitoring of total zooplankton biomass started in 2002. The IMR- sampling included many stations in the eastern Barents Sea during the 1990s. The total IMR dataset from 1989–2016 comprises nearly 4000 stations sampled with vertical net (mainly WP2) and in addition there are almost 1000 stations taken with 1-m2 MOCNESS. We have combined the IMR data from 1989 with the joint IMR-PINRO data from 2002 to provide time-series of zooplankton biomass also for the eastern Barents Sea.

The zooplankton biomass in the subareas in the southwestern Barents Sea influenced by inflowing Atlantic water has shown stable or declining trends since year 2000, following a pronounced peak in 1994 (Figure 3.3.3). The Bear Island Trench subarea has shown an increase after 2004, which appears to reflect an increase in Calanus finmarchicus with evidence of a second generation predominant in autumn in the Atlantic water (Aarflot et al., manuscript in preparation, Skjoldal et al., manuscript in preparation). The biomass in the ‘downstream’ subareas, Thor Iversen Bank to the south of Central Bank and Hopen Deep, has not reflected this increase.

Figure 3.3.3. Time-series of total zooplankton biomass for four subareas located in the inflow region of Atlantic water in the western Barents Sea.

The zooplankton biomass in the Central Bank and Great Bank subareas has shown declining trends since the peak in 1995 (one year later than the 1994 peak in the Atlantic water) (Figure 3.3.4). The bio-mass showed some increase in the period when the capelin stock was low in 2002–2005 with biomass levels around 6–7 g dw m^-2. It is noticeable that after the last capelin collapse in 2014–2015 there was little sign of recovery of the zooplankton biomass in 2016 in these central areas that are part of the traditional feeding areas of capelin.

Figure 3.3.4. Time-series of total zooplankton biomass (dry weight, g m^-2) for the Central Bank and Great Bank based on combined IMR-PINRO data, 1989–2016.

Zooplankton biomass showed an increase in the South-East Basin after 2005 (Figure 3.3.5), similar to the trend showed ‘upstream’ in the Bear Island Trench subarea (Figure 3.3.3). The trend for the Thor Iversen Bank, which lies in between these two subareas, did not show a similar increase. The biomass in the South-East region has typically been lower than in other subareas and has shown a decrease in the most recent years (Figure 3.3.5). The incomplete spatial coverage in 2016 may have influenced the mean values for the subareas in the southeastern part of the Barents Sea (see map in Figure 3.3.1).

Figure 3.3.5. Time-series of total zooplankton biomass for the Thor Iversen Bank, South-East Basin and South-East subareas based on combined IMR-PINRO data, 1989–2016.

The mean biomass values for 2016 for the various subareas are shown in Figure 3.3.6 together with the mean biomass for the 2002-2016 period based on the joint PINRO- IMR survey results. Figure 3.3.7 shows a similar plot for the size-fractioned results from the IMR surveys where the long-term mean values are for the 1989-2016 period.

Figure 3.3.6. Zooplankton biomass (total) in 2016 compared to the mean biomass (2002–2016) for the various subareas of the Barents Sea.

Figure 3.3.7. Zooplankton biomass in three size-fractions in 2016 compared to the long-term mean biomass (1989–2016) for various subareas of the Barents Sea surveyed by IMR.

The biomass in 2016 for the SW subarea was higher than the mean (by >50%). The biomass in 2016 was close to, or slightly higher than, the mean for the 2002-2016 period for the Bear Island Trench, Hopen Deep, and Svalbard subareas. The biomass was lower than the 2002–2016 mean for the Central Bank and Great Bank, and for the sub- areas in the eastern Barents Sea being up to 50% lower or more for the North-East and South-East subareas (Figure 3.3.7). The IMR results are in general agreement, showing higher biomass in the South-West subarea, and lower biomass for the Thor Iversen, Central Bank and Great Bank subareas. One notable feature is a consistently lower contribution by the largest size fraction (>2 mm) in 2016 compared to the long-term mean. This effect is particularly pronounced for the Central and Great Bank areas.

Mesozooplankton species-composition

The Russian investigation along the Kola section in May 2016 showed that copepods were the dominant group of zooplankton at this time, comprising on average 87% in abundance and 85% in biomass, and with Calanus finmarchicus as the dominant species. The abundance of C. finmarchicus in 2016 (395 941 ind. m^-2) was much higher than in 2015 (23 864 ind. m^-2) and similar to in 2014 (381 417 ind. m^-2), and somewhat lower than the long-term mean (Figure 3.3.8). In the southern part of the section the abundance of C. finmarchicus was lower than in the northern part, and the highest values were observed at 72º00′and 73º30′N. In the C. finmarchicus population, individuals of all stages were present, but while CI-CIV stages dominated on most stations, individuals of the early stages СI-CII prevailed on the northern stations.

Abundance of the arctic C. hyperboreus in 2016 was slightly higher than in 2015 (77 and 68 ind. m^-2 respectively) and much lower than in 2014 (210 ind. m^-2), as well as lower than the long-term mean (Figure 3.3.8). The highest abundance of C. hyperboreus was observed the northernmost stations of the Kola section. Older copepodites CIV-CV of
C. hyperboreus represented its population. The other arctic species, C. glacialis, was completely absent from the Kola section in 2016 (Figure 3.3.8).

Figure 3.3.8. Interannual mean abundances of three copepod species along the Kola section in May/June in 1992 and 2008-2016, ind. m^-2.

Russian investigations of mesozooplankton communities in the northern Barents Sea (north of approximately 75°N) in the joint ecosystem survey in August-September 2015 showed continued tendencies revealed in previous years. Copepods were the most abundant group of zooplankton (88% of total zooplankton numbers), while the second most abundant group were heteropods (8.6%). In 2015, total abundance of copepods decreased, while abundance of heteropods increased compared to in 2014. Copepods and chaetognaths represented the most important groups in terms of biomass (74 and 11% of total zooplankton biomass, respectively), and both these decreased from 2014 to 2015. The heteropods displayed a similar biomass both years, which in any case was very low compared to copepods and chaetognaths. The biomasses of other groups generally decreased, but their contribution to the total biomass was very low compared with the dominant groups. The small species Oithona similis and Pseudocalanus minutus were the most abundant among copepods (53 and 37% of total copepod abundance) in the northern Barents Sea, while the larger species Calanus glacialis, C. finmarchicus and Metridia longa represented only 1.4–3.9% (Figure 3.3.9). Abundances as well as biomasses of all these copepod species declined from 2014 to 2015. In 2015 the total abundance and biomass of zooplankton decreased by a factor of 1.4 and 1.9, respectively, in the northern Barents Sea compared to 2014. C. glacialis was the dominant species among copepods in terms of biomass (48% of total copepod biomass) (Figure 3.3.10).

Figure 3.3.9. Abundance (ind. m-3) of the most abundant copepod species without Oithona similis (bottom-0m) in the eastern Barents Sea (based on the PINRO samples from the PINRO/IMR ecosystem survey in August-September 2015).

In the southern Barents Sea, copepods were the dominant group in terms of both abundance and biomass (94 and 77%, respectively). Among copepods, small O. similis and P. minutus were the most abundant species (79 and 13% of total abundance of copepods, respectively), while the larger species C. finmarchicus contributed only 7% (Figure 3.3.9). However, the biomass of copepods was mainly formed by C. finmarchicus (65% of total copepod biomass), M. longa (22%) and P. minutus (10%) (Figure 3.3.10). In 2015, the total zooplankton abundance increased by factor of 2, while the total biomass decreased slightly compared to 2014 in the southern Barents Sea.

Figure 3.3.10. Biomass (mg wet-weight m-3) of the most abundant copepod species without Oithona similis (bottom-0m) in the eastern Barents Sea (based on the PINRO samples from the PINRO/IMR ecosystem survey in August-September 2015).

The Fugløya-Bear Island (FB) transect is located at the western entrance to the Barents Sea. Normally, 5 to 8 stations with fixed positions are sampled depending on weather conditions, and the transect is generally covered 5-6 timer per year. Zooplankton samples collected between 1995 and 2016, from four locations representing different water masses (coastal, Atlantic, and mixed Atlantic/Arctic), have been analysed taxonomically. Annual averages including all seasons for the abundance of the species Calanus finmarchicus, C. glacialis and C. hyperboreus are shown for each of the 4 stations in Figure 3.3.11. C. finmarchicus, which is by far the most common of these species, displays large interannual variations in abundance. High abundances were recorded during 2010 along most of the transect except at the northernmost position (74°00’N). Despite some exceptions, C. finmarchicus tends to be most abundant at the station located at 73°30’N, and following very low abundances at all stations in 2013, it has been present in considerable numbers along the transect during the last 3 years 2014- 2016.

Figure 3.3.11. Copepodite abundances for 3 Calanus species recorded along the Fugløya-Bjørnøya section during the period 2006–2016. The bars represent the annual averages of the 5-6 coverages per year (except for 4 and 3 coverages in 2012 and 2013, respectively). Each station is shown separately.

As expected, C. glacialis displays its highest abundance at the two northern-most stations (Figure 3.3.11), where Atlantic and Arctic waters mix. The abundance of this species is subject to large interannual variations. The numbers of C. glacialis along the FB transect seem to have decreased during the period 2006–2014, with a very low abundance recorded in 2008, and with low levels also during 2012–2014. However, the registered abundances were much higher again in 2015 and 2016. The abundance of the larger species C. hyperboreus along the FB transect has generally been low throughout the time-series, and very few individuals of this species were recorded in 2016 (Figure 3.3.11). The time-series for C. hyperboreus shows a strong year-to-year variability, but no unidirectional trends.

Calanus helgolandicus, a more southerly species which spawns during autumn, has regularly been observed at the Fugløya – Bear Island transect, particularly during the period from December to February (Dalpadado et al., 2012). This species is similar in appearance to C. finmarchicus. In recent years, it has been observed more frequently in the North Sea and southern parts of the Norwegian Sea (Svinøy transect). During winter, the ratio of C. helgolandicus to C. finmarchicus along the Fugløya – Bear Island transect has been found to increase. At this time of the year, however, C. finmarchicus is normally overwintering in deeper waters. There is no evidence of an increase in the relative proportion C. helgolandicus over the years along the FB transect, which suggests that this species has not increased in absolute abundance at the entrance to the Barents Sea.

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 three species of krill in the Barents Sea; Thysanoessa inermis associated with Atlantic water in the western and central Barents Sea, Thysanoessa raschii found mainly in the shallow waters in the southeastern Barents Sea, and Meganytiphanes norvegica associated with the inflowing Atlantic water, particularly during warm periods. Meganyctiphanes norvegica is the largest species reaching a maximum length of about 4.5 cm, while Thysanoessa inermis and T. raschii reach lengths of about 3 cm.

Krill were collected in the Barents Sea during the PINRO winter survey with a zooplankton net attached to the bottom trawl. The Russian investigation of euphausiids during the Russian winter survey in October-December 2015 showed a continued rather high abundance of euphausiids (Figure 3.3.12). Still, compared to in 2014, the estimated abundance of euphausiids in the Barents Sea in 2015 had decreased – from 1637 to 803 ind. 1000 m-3 in the southern part and from 1656 to 1638 ind. 1000 m-3 in the northwestern part. These estimates are much higher than the long-term means – 568 and 939 ind. 1000 m-3, respectively. The distribution of the euphausiid species was typical for warm years (Figure 3.3.13). Thysanoessa inermis (most areas) and T. raschii (south-eastern areas) were typically the most abundant species, T. longicaudata occurred mainly in the south-western areas and Meganyctiphanes norvegica was distributed widely in the Barents Sea.

Figure 3.3.12. Abundance-indices of euphausiids (log10 of number of individuals per 1000 m3) in the near–bottom layer of the Barents Sea based on data from the Russian winter survey during October-December, 1959-2015. Based on trawlnet catches in bottom layer. a) Southern Barents Sea. b) Northwestern Barents Sea.

Figure 3.3.13. Distribution of euphausiids (ind. per 1000 m^-3) in the near-bottom layer in the Barents Sea based on data from the Russian winter survey during October-December 2015.

The information on krill presented below is based the joint Norwegian-Russian Barents Sea Ecosystem surveys in autumn 2016, and is modified from Eriksen et al. (2012). Based on catch-data from pelagic trawls covering the upper layers (0–60 m) of the Barents Sea during August-October 2016, krill were more widely distributed than in the previous years (Figure 3.3.14). For comparison, the highest catches in 2013 were generally made in the central area, in 2014 in the western area, and in 2015 mostly south and southeast of Svalbard/Spitsbergen). In August-October 2016, Meganyctiphanes norvegica and Thysanoessa inermis were widely distributed in the Barents Sea, Thysanoessa longicaudata occurred mostly in the western areas, and Thysanoessa raschii mainly in the eastern areas. (Figure 3.3.15). The average night-catches in 2016 (13.5 g wet-weight m^-2) and 2015 (14.2 g m^-2) were similar, and higher than the long-term average (7.5 g m^-2) (Figure 3.3.16). During night, krill typically migrate to upper water layers to feed, and are then in general caught in larger numbers than during day when trawling in the uppermost 60 m.

Figure 3.3.14. Krill distribution based on sampling with pelagic 0-group fish trawl in the 60-0 m layer during the joint ecosystem survey in August–October 2016. The unit is g wet-weight m^-2.

Figure 3.3.15. Distribution of krill species, based on pelagic trawl stations both day and night, covering the upper waters (0-60 m) in the Barents Sea in August-October 2016. The proportions are based on wet-weights. The unit is grammes of wet-weight m-2. Figure from Eriksen et al. (2017).

Figure 3.3.16. Mean biomass of krill (g wet-weight m^-2) sampled with pelagic 0-group fish-trawl within the 60-0 m layer in the Barents Sea (based on night catches) from 1991 to 2016. Based on data from the joint autumn ecosystem-survey.

Hyperiid amphipods

Hyperiid amphipods are the second most important group of macrozooplankton in the Barents Sea. During the Russian winter survey in the Barents Sea in 2015, hyperiid abundance had increased sharply, and concentrations were estimated to 56 ind. 1000 m^-3. In previous years, the concentrations of hyperiids were estimated to 23 ind. 1000 m^-3 in 2012, and to 13 and 12 ind. 1000 m^-3 in 2013 and 2014, respectively. As in previous years, Themisto libellula was the dominant species. The distribution during the PINRO winter survey in 2015 is shown in Figure 3.3.17.

Figure 3.3.17. Distribution of hyperiids (ind. per 1000 m³) in the near bottom layer in the Barents Sea based on data from the Russian winter survey in October-December 2015.

The following information is based on the joint Norwegian/Russian Barents Sea Ecosystem survey in autumn 2016, and the text and Figures below are extracted and partly modified from Eriksen et al. (2017). The catches from pelagic trawls covering the upper layers (0-60 m) of the Barents Sea in August-October 2016 revealed the presence of amphipods in the northern Barents Sea (Figure 3.3.18). In 2012 and 2013, amphipods were lacking in the pelagic catches. In 2014 some restricted catches were taken north for Svalbard/Spitsbergen, and in 2015 several large catches were made east of Svalbard/Spitsbergen. In 2016, the largest catches were made north and east of Svalbard/Spitsbergen, and mainly consisted of the Arctic species Themisto libellula. In 2016, the estimated biomass of amphipods was 430 thousand tonnes for the covered area, which was slightly lower than estimated in 2015. In addition to Themisto sp., small catches of Hyperia galba, which is biologically associated with jellyfish, were made in the northern part of the central area. Other hyperiids (from genuses Hiperia and Hyperoche) also occurred, but their abundances were very low.

Figure 3.3.18. Hyperiid amphipod distribution, based on trawl stations covering the upper water layers (60-0 m), in the Barents Sea in August-October 2016. Figure from Eriksen et al. (2017).

Chaetognaths

During the Russian winter surveys, chaetognath concentrations increased from 734 ind. 1000 m^-3 in 2012 to 1022-1198 ind. 1000 m^-3 in 2013–2014, reaching 1225 ind. 1000 m^-3 in 2015. Such high abundances of predatory chaetognaths may affect the abundances and biomasses of other groups of mesozooplankton. Distribution of chaetognaths in late 2015 is shown in Figure 3.3.19.

Figure 3.3.19. Distribution of chaetognaths (ind. per 1000 m3) in the near bottom layer in the Barents Sea based on data from the Russian winter survey in October-December 2015.

Jellyfish

The geographic distributions and estimated abundances (biomass) of gelatinous zooplankton presented here are based on data collected during the joint Norwegian/Russian Barents Sea Ecosystem survey in autumn 2016, using the standard pelagic trawl for the depth-stratum of 0-60 m. The Figures and text below are modified from Falkenhaug et al., 2017. Trawling is a harsh sampling method for gelatinous zooplankton, and the data presented here should be considered as semi-quantitative data. The trawl does not sample the entire water column, the filtered volume of water is not known, and small and fragile species will may pass through the meshes of the trawl and are easily destroyed in the codend. The Harstad trawl probably has a higher catchability for large, robust scyphozoans (Periphylla periphylla, Cyanea capillata) than for the smaller Aurelia aurita, and even more so than for fragile taxa such as ctenophores and small medusa (Falkenhaug et al., 2017). Nevertheless, we here consider that the error in catchability is constant for each taxon, allowing taxon-specific comparisons between years and between stations.

Figure 3.3.20. Distribution of Cyanea capillata (wet-weight; kg per sq nm) in the Barents Sea, August-October 2016. Catches from standard pelagic trawl, 0-60 m depth, both day and night. Figure by Elena Eriksen – see Falkenhaug et al. (2017).

In 2016, lion’s mane jellyfish (Cyanea capillata; Scyphozoa) was the most common jellyfish species both regarding catch-weight and occurrence (average catch of 21.9 kg per nm), and it was distributed across the entire survey area (Figure 3.3.20). The biomass per station was generally lower than in 2015. The horizontal distribution of C. capillata in 2016 was similar to in 2014 and 2015, with the highest biomasses located in the central and south-eastern area.

Figure 3.3.21. Distribution of five taxa of gelatinous plankton (wet-weight; kg per sq nm) in the Barents Sea, August-October 2016. Catches from standard pelagic trawl 0-60 m depth, both day and night. Figure from Falkenhaug et al. (2017).

The moon jellyfish (Aurelia aurita; Scyphozoa) was the second most abundant jellyfish species by total weight (average catch of 1.2 kg per nautical mile), and mainly distributed in the southern part of the survey area (Figure 3.3.21). The whitecross jellyfish (Staurostoma mertensii; Hydrozoa) is a common arctic species. In August-October 2016, this species was distributed from the northern to the south-eastern part of the survey area, with maximum abundances in the central Barents Sea (Figure 3.3.21). The blue stinging jellyfish (Cyanea lamarckii; Scyphozoa) is considered a more temperate species than C. capillata, but in recent years this species has been indicated to increase its distributional range northward. During the Barents Sea Ecosystem cruise 2016, C. lamarckii was registered at 32 stations in the western area, with an average catch of 0.1 kg per nm. It is believed that C. lamarckii is unable to reproduce in the Barents Sea, and that the presence of this warm-temperate species may be linked to the inflow of Atlantic Water.

The estimated total biomass of C. capillata in upper water layers (0–60 m) of the Barents Sea in August-October 2016 was 1.6 million tonnes (Figure 3.3.2). This was less than in 2015 (2.5 million tonnes), and close to the long-term mean 1980–2016 (1.2 million tonnes). During the last 6 years (2011–2016), the estimated total biomass of jellyfish has been above the long-term mean.

Figure 3.3.22. Estimated jellyfish biomass for the Barents Sea, in million tonnes with 95% confidence interval (grey line) for the period 1980–2016. Estimates based on autumn trawl-catches covering the upper layer (60–0 m). Note that until 2013, all jellyfish are included in estimates presented in the Figure, while from 2014 onwards only Cyanea capillata is represented (Eriksen, 2012)