Benthos is an essential component of the marine ecosystems. It can be stable in time, characterizing the local situation, and is useful to explain ecosystem dynamics in ret-rospect. It is also dynamic and shows pulses of new species distribution, such as the snow crab and the king crab, and changes in migrating benthic species (predatory and scavenger species such as sea stars, amphipods and snails with or without sea anem-ones).
The changes in community structure and composition reflect natural and an-thropogenic factors. There are more than 3000 species of benthic invertebrates regis-tered in the Barents Sea (Sirenko, 2001), but here we only present the megafaunal com-ponent of the benthos collected by trawl and registered (species, abundance and bio-mass) during the Barents Sea Ecosystem Survey (BESS). This includes mainly large bodied animals with long lifespans. This investigation was first initiated in 2005 re-sulting in a short timeline compared to investigations related to plankton and fish. Accordingly, interpretation of long-term trends for megabenthic data must be pursued with caution. Benthos collection. Benthos, collected with the standard demersal trawl gear during the BESS, have been registered annually by benthic taxonomic specialists since 2005 onboard Russian vessels; annual surveys have been conducted during 2007–2013 and 2015–2016 onboard Norwegian vessels. Species identification has been to the lowest possible taxonomic level. In cases where there were no specialist available onboard (2007–2008 in northern Barents Sea in the Norwegian sector), the benthos were identi-fied to major benthic group. Work is ongoing between IMR and PINRO specialists to standardize and improve species identification, as well as the catchability of benthos between different trawls and vessels. Several articles have been published based on the resulting megabenthic data (Anisimova et al., 2011; Ljubin et al., 2011, Jørgensen et al., 2015; Jørgensen et al., 2016, Zimina et al., 2015; Degen et al., 2016, Johannesen et al., 2016; Lacharité et al., 2016; Jørgensen, 2017; Jørgensen et al., 2017; Jørgensen et al., 2019). Megafauna description. The distribution of large benthos groups shows that Porifera (mainly species within the Geodiidae) dominate the biomass in the west, while Echino-dermata (mainly brittle stars) dominate in the central and northern part of the sea. In the Northeast, Cnidaria (where the biomass is mainly made up by species such as the sea pen Umbellula encrinus) dominates together with Echinodermata, while Crustacea dominates together with the Echinodermata in the Southeast (Figure 18.104.22.168).
Statistical analyses of monitoring data show that there are four distinct zones of benthos in the Barents Sea (Jørgensen et al., 2015, fig. 22.214.171.124). These four zones are characterized with temperate species in the southwestern zone, cold-water species in the eastern zone, arctic species in the northern and north-eastern zone, and an area in the eastern Barents Sea where the snow crab, a new non-indigenous large benthic species, are expanding. The period with warmer water entering the Barents Sea has led to migration eastwards and northwards of temperate species and groups (Jørgensen et al., 2015). The retreating ice front opens for new areas for human impact as well as imposing changes in the planktonic production and annual cycles, with possible impact on the benthic zones.
Figure 126.96.36.199 The baseline map of the Barents Sea mega‐benthic zones in 2011, based on fauna similarity (see Jørgensen et al., 2015 for methodology, results and discussion) with the northern (green and blue) and southern (yellow and red) region where the black full line is illustrating the “benthic polar front” in 2011. The grey full line is the approximately oceanographic Polar Front. Dotted line: Is partly illustrating a west‐east division. Red: South West subregion (SW) Yellow: Southeast, banks and Svalbard coast (SEW). Green: North West and Svalbard fjords (NW). Blue: North East (NE). Source: IMR.
Jørgensen et al. (2019) show a recent increase in community mean temperature ranks (P=0.0011), indicating an increased importance of species with affinity for warmer waters and a reduced importance of cold-water species. Commercial fish stocks expands northward (Fossheim et al., 2015) while and the snow crab expands toward the western part of the Barents Sea, thereby simultaneously increasing the exposure of both large immobile species to trawling and of small prey species to crab predation. Overall, we have found a high-level of vulnerability toward temperature increase, bottom trawling and snow-crab predation in the northwestern Barents Sea, because this might lead to alterations in community structure and diversity.
The status of megabenthos in 2019
The area covered in 2019 are given in figure 188.8.131.52. Ten Russian and Norwegian experts were involved in the megabenthos by-catch processing across the four BESS vessels. The main results are given in table 184.108.40.206 while the megabenthos distribution are given in figure 220.127.116.11
The results of 2019 showed a high number of taxa (fig. 18.104.22.168 A) in the western part of the sea (Norwegian area of the survey) and low number in the east (Russian area of the survey). This might include a low level of taxonomical processing of the by-catches in the Russian vessel “Vilnus” due to the lack of benthic experts onboard. The general distribution of abundance and biomass are resemble the distribution in 2018. The abundance (fig. 22.214.171.124 B) showed high numbers of individuals in the northeast and middle part of the sea, including coastal waters of the Novaya Zemlya archipelagoe. But in the south and along the slope in the west the abundance was low. The biomass (fig. 126.96.36.199 C) were high in the northeast, in the southeast (near Novaya Zemlya coast, Kanin peninsula and in North-Kanin Bank), and particularly in the southwest.
Long-term trends in distribution of the megabenthic biomass
The monitoring time-series of the megabenthos biomass distribution shows relative stable large-scale patterns, with high biomass particularly in the southwest; biomass is also stable in the northeast, but more variable. In the central Barents Sea, biomass has a high level of spatial and temporal variability which is difficult to characterize due to the relatively short data time-series (Fig. 188.8.131.52).
Figure 184.108.40.206. also illustrates the deficiency and fault in the megabenthos assessment for several years. In 2005, 2014 and 2018 there were a lack of station coverage in the Norwegian or Russian areas. In 2019, the north-eastern part of the sea was incompletely covered. In 2014, 2015, 2016 and 2019 the Loophole area was not sampled because a commercial snow crab fishery. In 2012 and probably 2017 the biomass was overestimated in the Russian zone due to technical issues with the trawl tuning. Such lack of coverage and non-standardised processing should be taken into account when analyzing the megabenthic time series.
Inter-annual fluctuation of the mean megabenthos biomass
To estimate long-term dynamics of the megabenthos, inter-annual changes of the mean biomass were calculated for the total Barents Sea and separated for the four sectors – northeast, northwest, southeast and southwest part of the sea (Fig. 220.127.116.11 A, B, ICES, 2018, 2019). In 2018, the eastern part of the Barents Sea was only partly covered (Fig. 18.104.22.168). This made it impossible to estimate the annual biomass-trend for the “total Barents Sea” (Fig. 22.214.171.124 A) and for the “easten part” of the Barents Sea (Fig. 126.96.36.199 D). But the data from the western part of the sea shows moderate increase of the megabenthic biomass in 2019 comparing to 2018 (Fig. 188.8.131.52.C). The fourteen years of monitoring shown a moderate linear positive trend of increasing megabenthic biomass for the “Total Barents Sea” (Fig. 184.108.40.206 B). At the same time, interannual biomass fluctuations show positive correlation (r = 0.59 for "total") with the water temperature on the Kola Sections (0-200 m laer, st. 3-7, Fig. 220.127.116.11 A) with a time lag of 7 years (except NW) (Table 18.104.22.168).
Table 22.214.171.124. Correlation (r) between water temperature on the Kola Section (average annual water temperature in the lyer 0-200 m in the st. 3-7) and mean biomass for the Barents Sea within 68-80° N and 15-62° E (Total) and it four sectors (NW – 74-80° N, 15-40° E; NE – 74-80° N, 40-62° E; SW – 68-74° N, 15-40° E; SE– 68-74° N, 40-62° E) when shifting back temperature time-series from 1 to 10 years
Similar response to change of environmental conditions was documented for the Bar-ents Sea macrobenthos, but with a delay in approximately four years (Lubina et al., 2012, 2016; Denisenko, 2013). Difference in duration of the tomelag between macro- (grab’s) and mega- (trawl’s) benthos is caused by different mean size and longevity of the lifespan of these size groups of benthic organisms.
Figure 126.96.36.199 Variations of the average annual temperature in the water layer 0-200 m in the 3-7 stations of the Kola Section (A) (http://www.pinro.ru) and the inter-annual fluctuation of the mean megabenthos biomass in total Barents Sea (B) and in it western (C) and eastern (D) sec-tions. “Total” – Barents Sea within 68-80° N, 15-62° E, “NE” – north-eastern sector (74-80° N, 40-62° E), “NW” – north-western sector (74-80° N, 15-40° E), “SE” – south-eastern sector (68-74° N, 40-62° E), “SW” – south-western sector (68-74° N, 15-40° E). Biomass of Pandalus borealis and all catches more than 1t are excluded. Red circles in the plot A show cold years which could cause decrease of the biomass in 2010, 2015 and probably in 2018 (are shown by gray arrows). The dotted line in the plot B is linear trend.
State of selected benthic species
The snow crab (Chionoecetes opilio) is a non-indigenous species in the Barents Sea and was first recorded in 1996 on the Goose Bank area (Strelkova, 2016). It is not known whether it is introduced by accident or if it has expanded its distribution area. Preliminary results show that the latter is the most probable explanation. The establishing of the snow crab in the Barents Sea is believed to have occurred during 1993–1996. Regular annual monitoring of the snow crab population began with BESS in 2004. This survey is, currently, the most important source of information on snow crab population status. Assessments of snow crab dynamics based on BESS data (Table 188.8.131.52 and Figures 184.108.40.206 and 220.127.116.11) indicate that in the Barents Sea the snow crab population is still developing (spreading, population increase).
Figure 18.104.22.168. The dynamic of the snow crab population in the Barents Sea given as the total number of crabs (blue bars) and the number of trawl hauls with crabs (red line) during the BESS 2004–2017.
In 2018, as in previous years, the densest aggregations of snow crabs (more than 1000 ind/nml) were concentrated in the central part of the Barents Sea in the Loophole area and near Novaya Zemlya archipelago within the Russian Economic Zone. In 2017, the snow crab was for the first time recorded at Svalbard. One record was made in Storfjorden at 162 m depth (two immature males with 47 mm and 48 mm carapace widths); the other was northwest of Svalbard archipelago at 506 m depth (juvenile male 14 mm carapace width) (fig. 22.214.171.124).
Studies of snow crab population size structure indicate that abundant generations appear periodically, and that this affects the overall population dynamics in the Barents Sea. During the ecosystem survey period, abundant generations were recorded with 3 years’ interval - in 2009, 2012, and 2015–2016 (fig. 126.96.36.199, Bakanev & Pavlov 2016).
Figure 188.8.131.52 The sex and size structure of the snow crab population from 2006–2017 (Bakanev & Pavlov 2016, with editions). On vertical axes: 2006–2015 – number of individuals; 2016–2017 – abundance, %.
Compared with previous year, the mean abundance of snow crab, standardized to nautical mile, has increased 2.7 times while biomass 1.2 times only. It can be results of preferential increasing of juvenile part of population that is agreeing with size structure of the crab catches in 2018 (Fig. 184.108.40.206).
Since 2003, snow crabs in the eastern part of the Barents Sea, have been recorded in stomachs of bottom fish species (cod, haddock, catfish, American dub, and starry ray).
In recent years, snow crabs have become one of the most important prey species for cod. In 2011–2012 it made up about 2% of the cod stomachs examined, in 2013–2014 it made up 4–7%, and in 2015–2016 it made up 5–6%. All size categories of snow crab (up to 120 mm carapace width) are eaten by cod. Cod feeding on snow crabs was most intensive (up to a quarter of total stomach content) during autumn at Novaya Zemlya, Great Bank, Central Banks.
Northern shrimp (Pandalus borealis) is common and widely distributed in the Barents Sea above the deep (250-350 m) muddy flats of the Barents Sea and in temperatures between 1 - 2°C (Fig. 220.127.116.11).
During the 2018 BESS survey, it was recorded at 160 of the 2018 trawl stations with a biomass that varied from a few grams to 128.9 kg per nautical mile, with an average catch of 10.2±1.4 kg/nml across 218 station. The densest concentrations of shrimp were registered in central Barents Sea, around Spitsbergen, and in Franz Victoria Trough (fig. 18.104.22.168).
During the BESS 2006-2018 average catches of the shrimp varied from 4 to 11 kg (Figure 22.214.171.124), all stayed stable around the average level. The increase of biomass in 2017-2018 may be connected with the investigations in northeastern were large biomasses of shrimp were recorded.
Biological analyses of the northern shrimp population in the eastern part of the BESS were conducted in 2018 by Russian scientists. Similar to 2017, the bulk of the population consisted of younger individuals: males of 12–27 mm carapace length; and females of 17–30 mm carapace length (fig. 126.96.36.199).
In the western survey area, as in the eastern part of the Barents Sea, smaller shrimp (males 11–23 mm carapace length, and females 18-28 mm carapace length) were most abundant; comprising up 64% of catches (fig. 188.8.131.52)