Marine ecosystems

Th e Barents Sea is a high-latitude Arctic shelf sea comprising 1.6 million km² (Carmack et al., 2006) with a mean depth of 230 m. It is one of two major shallow and highly productive Arctic seas, the other being the Bering Sea in the North Pacifi c Region. Th e Barents Sea is bordered by the northern Norwegian and Russian coasts and by the Novaya Zemlya Islands. Th e 500-m depth contour is oft en used to delimit the Barents Sea towards the Greenland Sea, Norwegian Sea, and Arctic Ocean (see Figure 2.1).

Ocean circulation in this region is dominated by the Norwegian Atlantic Current, which brings warm saline Atlantic Water into the area from the south (e.g. Loeng, 1991). Atlantic Water extends throughout much of the western and central parts of the Barents

Figure 2.4 Distribution of permafrost in the circumpolar north (http://maps.grida.no/arctic/).

Sporadic permafrost Discontinuous permafrost Continuous permafrost

Isolated permafrost

Sea whereas cold fresher Arctic Water dominates the surface layer in the northern sectors. Atlantic Water that travels north along the west coast of Svalbard (the West Spitsbergen Current) infl uences ice cover in the region (Ivanov et al., 2012); inducing open water areas at very high latitudes, even in winter, in places such as Whalers Bay. Th e boundary between the two main water masses (Arctic water and Atlantic water) is delineated by the Polar Front. Th ere is also a coastal current running along the mainland shores carrying relatively warm and fresh water eastward. See Chapter 4 for further details.

Sea ice is one of the most important drivers of the Barents Sea system (see Chapter 4). Th e fl ow and interactions of the Atlantic Current in the south and the Arctic currents in the north have a signifi cant impact on the distribution and extent of sea ice in the Barents Sea (e.g. Vinje, 2001; Årthun et al., 2012).

Most of the sea ice in the Barents Sea is formed locally (Vinje and Kvambekk, 1991; Vinje, 2001) but a signifi cant amount is imported from adjacent regions of the Arctic Basin through the straits between Svalbard and Novaya Zemlya (e.g. Pavlov and Pavlova, 2008; Kwok, 2009). Arctic sea ice also makes its way south along the east Greenland coast via southward fl owing currents in Fram Strait. Th e Barents Sea ice cover has a strong seasonal variability. Th e spring melt stabilizes the upper water layers and is associated with a ‘spring’ plankton bloom that has traditionally followed the receding ice edge northward into the northern Barents Sea (Sakshaug and Skjoldal, 1989).

A significant part of the southern Barents Sea is ice-free throughout the year. Th e decline in sea-ice volume and extent in the Arctic Ocean is widely documented (e.g. Parkinson and Cavalieri, 2008; Comiso, 2012) and many studies have shown that the most dramatic changes have taken place in the Russian sector of the Barents Sea. (e.g. Overland and Wang, 2007; Pavlov and Pavlova, 2008; Rodrigues, 2008; Smedsrud et al., 2013).

An ecologically-focused study of seasonal changes in sea ice is provided by Laidre et al. (2015). Th is showed the Barents Sea region to have experienced four times the average rate of change in terms of seasonal sea-ice coverage compared to the Arctic in general, with a reduction of 20+ weeks in just the last few decades; these changes have already had impacts on the region’s biota (Figure 2.6). See Chapter 4 for more detail on sea-ice dynamics.

Figure 2.5 Mosaic of a Palsa mire (left ), demonstrating a collapsing pals (right).

Figure 2.6 Temporal patterns in ocean temperature, sea-ice extent, zooplankton biomass and fish biomass in the Barents Sea. Ocean temperature (50–200 m in the Atlantic Water in the Vardø-North section in August–September), September sea-ice extent, August–September zooplankton biomass (wet weight), and August–September pelagic fish biomass (capelin, polar cod, herring) and demersal fish biomass (cod, haddock).

2010 2000 1990 1980 1970 1960

1950 0

5 10

Fish biomass, million tonnes

Pelagic fish

Demersal fish 0

50 100

Zooplankton

Zooplankton biomass, million tonnes

0 100 200 300

Sea ice Sea ice extent, thousand km

3.0 3.5 4.0 4.5 5.0

Ocean temperature

Temperature, °CSusanne Backe Susanne Backe

2.2.2.1

Phytoplankton and zooplankton

Primary production and phytoplankton growth rates in the Barents Sea region are highly seasonal due to the extreme variation in light levels and temperature across the annual cycle at this high-latitude location (Sakshaug, 2004; Sakshaug et al., 2009). The Barents Sea has two main domains of phytoplankton production – the open-water domain and the seasonally ice-covered domain.

Total annual primary production in the Barents Sea is about 90 g C/m², with higher production in the open Atlantic water masses of the southern Barents Sea (100–150 g C/m²) than in the seasonally ice-covered northern Barents Sea (<50–70 g C/m²) (Sakshaug, 2004; Wassmann et al., 2006a,b; Hunt et al., 2013;

Dalpadado et al., 2014). New production is typically about 50 g C/m² in the Atlantic Water and less in the colder, northern water masses. Despite the seasonally ice-covered areas having lower overall production rates, the relatively predictable location of the pronounced nature of the short-lived spring bloom of phytoplankton (‘ice-edge bloom’) that sweeps across the northern Barents Sea as a more or less distinct band following the seasonal retreat of the sea ice, is an important source of food for zooplankton and other fauna (Sakshaug and Skjoldal, 1989;

Skjoldal and Rey, 1989). Studies in the 1980s revealed interannual variation of four to six weeks in the timing of the peak in the spring bloom in response to climatic variation between cold and warm years (Skjoldal et al., 1987; Skjoldal and Rey, 1989).

Modelling studies and remote sensing data suggest the less extensive sea-ice coverage of recent decades is likely to have increased the total annual primary production for the Barents Sea substantially (Slagstad and Wassmann, 1996; Wassmann et al., 2006a,b; Dalpadado et al., 2014). Diatoms are the predominant phytoplankton group during spring blooms in the Barents Sea, while other microalgal groups comprising a wide range of systematically different flagellates are important in the region at different times of the year (Sakshaug et al., 2009).

The Barents Sea zooplankton community is diverse and comprises many species of various taxonomic and trophic groups (Eiane and Tande, 2009). Monitoring has shown that large interannual variability in the mesozooplankton biomass, largely due to varying levels of predation by fish, is a normal condition in this ecosystem (Dalpadado et al., 2012; Johannesen et al., 2012a;

Stige et al., 2014). In addition to predation pressure from higher trophic levels, variable advective transport of plankton from the Norwegian Sea into the Barents Sea also contributes to biomass variability in the western/central Barents Sea (Skjoldal and Rey, 1989; Dalpadado et al., 2012; Orlova et al., 2014).

The zooplankton community can be broadly divided into a boreal group associated with the warmer Atlantic Water in the south and an Arctic group associated with the cold Arctic water in north.

Herbivorous ‘large’ Calanus copepods are dominant species among the mesozooplankton (Melle and Skjoldal, 1998a; Falk-Petersen et al., 2007, 2009), while several species of krill (mainly herbivores) and pelagic amphipods (mainly carnivores) are dominant macrozooplankton (Dalpadado, 2002; Dalpadado et al., 2002, 2008; Zhukova et al., 2009; Orlova et al., 2015).

Arctic copepod species include Calanus glacialis, C. hyperboreus, Metridia longa, and Pseudocalanus minutus. One of the most important of these northern species is C. glacialis, which thrives in the northern Barents Sea (Tande, 1991; Melle and Skjoldal, 1998a). It is considered a shelf species adapted to

living in the zone of seasonally ice-covered waters on the periphery of the central Arctic Ocean. It reproduces in spring or early summer with egg production fueled by the spring (ice-edge) phytoplankton bloom (Melle and Skjoldal, 1998b).

C. finmarchicus is the dominant copepod in the Atlantic Water in the southern Barents Sea. Egg production in this species also depends on the spring phytoplankton bloom (Melle and Skjoldal, 1998a; Niehoff, 2004, 2007). The development time of the new generation increases with decreasing temperature, from about one month at 10°C to five months at 0°C (Campbell et al., 2001).

Delayed and prolonged development limits the distribution of this species in more northerly waters within the Barents Sea (Melle and Skjoldal, 1998b); but its distribution has shifted northward over the last few decades (Skaret et al., 2014).

Euphausiids (krill) can be important components of the system at times. Four species of krill are regular inhabitants of the Barents Sea (Thysanoessa inermis, T. raschii, T. longicaudata, and Megancytiphanes norvegica; Drobysheva, 1994; Dalpadado and Skjoldal, 1996; Orlova et al., 2015). T. raschii is a neritic species found predominantly in the shallow waters of the southeastern Barents Sea, while the other three species are associated with inflowing Atlantic Water. Their long lifespan makes krill sensitive to predation pressure from fish and other consumers such as the large baleen whales.

Analysis of time series going back to the 1950s shows a negative trend, due to warming, on T. raschii and positive effects on the other three species (Zhukova et al., 2009; Eriksen and Dalpadado, 2011; Dalpadado et al., 2012; Orlova et al., 2015;

Eriksen et al., 2016). Predation, particularly from capelin (Mallotus villosus), also has an influence on the standing stock as can be seen from the inverse relationship between T. inermis and the fluctuating capelin stock (Dalpadado and Skjoldal, 1991, 1996; Eriksen and Dalpadado, 2011). A krill index (based on an extensive joint Norwegian-Russian autumn survey) shows a marked increase in krill abundance after 2000, associated with the warming of the past few decades (Figure 2.7). The increase is associated with a northward expansion of krill in the northern Barents Sea, possibly augmented by increased transport onto the northern shelf via the West Spitsbergen Current (Eriksen et al., 2016).

Pelagic amphipods also play important roles in the food webs of the Barents Sea ecosystem and are represented by two dominant hyperiid amphipod species of the genus Themisto. T. abyssorum (~2 cm) is a boreal–Arctic species associated with the warmer Atlantic Water, while the larger T. libellula (~4.5 cm) is an Arctic species (Dalpadado, 2002; Dalpadado et al., 2002, 2008). The amphipods have shown declining trends over recent decades due to the reduction in Arctic Water within the region; the Arctic Water index explains 54% of the variation in amphipod abundance (Dalpadado et al., 2012, 2014).

Two species of scyphozoan jellyfish commonly occur in the Barents Sea: the lion’s mane jelly (Cyanea capillata) and the moon jelly (Aurelia aurita). They are mainly boreal species found in the temperature range 1–10°C in the Barents Sea, with peak abundance at about 4–7°C (Russel, 1970; Eriksen et al., 2012). Over the last two decades, jellyfish have showed a northern shift in distribution, partially explained by an increase in water temperature and increased areas of Atlantic and mixed waters (Prokhorova, 2013; Eriksen et al., 2014, 2015).

2.2.2.2

Fish and other harvested resources

More than 200 species of fish have been registered in the Barents Sea, although less than half are caught regularly (Stiansen and Filin, 2008; Dolgov et al., 2011a; Wienerroither et al., 2011). Some species complete all phases of their lifecycle within the Barents Sea, while others feed in the Barents Sea but spawn elsewhere.

Johannesen et al. (2012b) described six fish communities in the Barents Sea that were separated along depth and temperature gradients. Based on their geographical distribution and physiological adaptations, 166 of the fish species registered in the Barents Sea have been classified into zoogeographical groups (Andriashev and Chernova, 1995); 25% are Arctic or Arcto-boreal, half are boreal (or mainly boreal) and the rest are widely distributed or south-boreal species. However, shifts in distribution over recent decades and changing temperatures at depth are blurring the distinction among these assigned groupings. There has already been a marked ‘borealization’ of the fish community within the Barents Sea (Fossheim et al., 2015, see also Chapter 6).

From a trophic perspective there are three main groups of fish in the Barents Sea that each share fundamental life-history and habitat characteristics: species feeding on plankton, species feeding on benthos, and species feeding on other fish (Dolgov et al., 2011a). Planktivorous fish dominate in terms of biomass, but not in terms of the number of species (Dolgov et al., 2011b). Among the planktivorous species, capelin, polar cod (Boreogadus saida) and juvenile herring (Clupea harengus) are most abundant, although their biomass varies greatly from year to year. The three species have broadly divided the sea area among them with capelin in the north, herring in the south, and polar cod mainly in the east, although this species is also of key ecological importance within Svalbard. All three species are important to top trophic predators within their respective ranges. The events and conditions driving capelin cycles are clearly linked to climate variability but in a complex manner involving biological interactions with, for example, variable abundance of juvenile herring, zooplankton prey, and levels of cod predation. 0-group capelin are distributed further north in warm years (Eriksen et al., 2012). The distribution of immature capelin on their feeding migration in autumn is related to temperature conditions and this age group has a

more northerly distribution in warm years (Gjøsæter et al., 1998; Carscadden et al., 2013; Ingvaldsen and Gjøsæter, 2013).

However, the size of the stock also plays a role with a less northerly distribution being the norm when the stock is low, presumably because of lower food demand (Ingvaldsen and Gjøsæter, 2013).

Juvenile herring of strong year-classes of the Norwegian spring spawning herring stock grow up in the southern Barents Sea.

They leave after three to four years to join the adult stock in the Norwegian Sea (Krysov and Røttingen, 2011).

Polar cod spawn in association with sea ice and young age classes of this small fish species tend to remain close to sea ice, often living in interstitial spaces within the ice, which provides some protection against predators. The polar cod stock has shown large fluctuations in abundance; from high levels during the early 1970s to a dramatic decline in the 1980s, followed by a recovery during the 1990s and then high levels in the early-mid 2000s. Since 2007, the stock size has again decreased, apparently driven by poor recruitment related to warming and associated reductions in sea ice and the area containing Arctic Water (ICES, 2014b; Eriksen et al., 2015). Expansion of Atlantic cod (Gadus morhua) into the northern Barents Sea has also played a role, leading to increased spatial overlap between the two species and increased predation pressure from Atlantic cod on polar cod.

All three planktivorous fishes are or have been harvested;

capelin being the most important commercially (Figure 2.8).

The harvest of Barents Sea polar cod has been very limited since the 1970s. The herring fishery targets adult fish, which are actually taken outside the Barents Sea.

The most important commercial species among the benthic-feeding and fish-benthic-feeding species include Atlantic cod, haddock (Melanogrammus aeglefinus), saithe (Pollachius virens) and Greenland halibut (Reinhardtius hippoglossoides). It is well established that climate variability is a major factor causing large variability in recruitment to the commercial fish stocks in the Barents Sea, expressed as alternating strong and weak year classes (Sætersdal and Loeng, 1987; Ottersen and Loeng, 2000).

Strong and weak year classes drive fluctuations in the stocks, and strong year classes in particular have marked ‘snowballing’

effects as the cohort develops over time, with impacts on prey and predators throughout food webs.

Recruitment of Atlantic cod and haddock (as well as herring) is positively related to high inflows of Atlantic water and the accompanying higher temperatures in the Barents Sea (Sætersdal and Loeng, 1987; Ottersen and Loeng, 2000). During the last decade the cod stock has covered most of the Barents Sea shelf in autumn (August-September) and has also expanded northward during winter (Johansen et al., 2013; Prokhorova 2013; also see Figure 2.9). The cod distribution area increased from 2004 to 2013, expanding into the northern and northeastern part of the Barents Sea. In recent years a major part of the stock has been found on the northern shelf (north of 78°N) with some cod moving to the shelf edge at the rim of the Arctic Ocean at around 82°N. Increased temperature from sub-zero to positive may have removed a threshold barrier, now allowing cod to enter this northern area (Lind and Ingvaldsen, 2012). The northward expansion during the main feeding season in late summer

0

Figure 2.7 Mean biomass of krill recorded during joint Norwegian-Russian autumn surveys by trawl sampling in the upper 60 m of the Barents Sea.

Based on Eriksen and Dalpadado (2011) with updates for 2010–2015 (Institute of Marine Research, Norway, unpubl. data).

appears to be determined by more old and large cod in the stock and the northward shift in the distribution of capelin following its recovery to high abundance (2008–2013). Such trends have been seen in the past; both the cod and herring stocks increased significantly between 1920 and 1940 when water temperatures increased (Toresen and Østvedt, 2000; Hylen, 2002). This increase in stock size was probably an effect of enhanced recruitment, because catches also increased over this period. The northern expansion of cod is a prime example of the ‘borealization’ of the Barents Sea ecosystem.

Haddock has also recently reached a historic high in abundance and has increased its distribution range over the past few decades (1950–2013; Mehl et al., 2013; McBride et al., 2014).

This is related to large stocks, an increasing proportion of large individuals in the stocks and higher water temperatures, similar to the situation for Atlantic cod.

2.2.2.3

Benthos

More than 90% of the invertebrates in the Arctic belong to the benthic community (Sirenko, 2001; Gradinger et al., 2010).

Animals that live on (epifauna) or in (infauna) the sediments are collectively referred to as benthos. Most species of benthos are largely stationary. The composition of the bottom fauna of a region reflects prevailing environmental conditions including large-scale oceanography (Carroll et al., 2008; Cochrane et al., 2009, Jørgensen et al., 2015). For example, infauna (mostly worms and bivalves) density and species richness in the Barents Sea area are 86% and 44% greater at stations near the Polar Front than at stations in either Atlantic- or Arctic-dominated water masses (Carroll et al., 2008). In Arctic Water north of the Polar Front, sea ice suppresses water column

productivity and infaunal abundances are significantly lower than in open-water areas south of the Polar Front, while the numbers of taxa present are similar (Cochrane et al., 2009).

Epifauna biomass (mostly brittle stars, sponges, shrimps) is over five times greater in the north-eastern Barents Sea influenced by Arctic Water than at stations in Atlantic-Water influenced regions, with the exception of areas in the south-western Barents Sea where sponge fields dominated by a large biomass of Geodia-sponges prevail (Jørgensen et al., 2015). Areas in the southwest, the central Barents Sea and north of 80°N have a high biomass of species easily taken by bottom trawls (Jørgensen et al., 2016), including large-bodied Arctic species such as seapens and cephalopods, sponges and ophiuriods (Jørgensen et al., 2015).

In the Pechora Sea, despite its southerly location, Arctic species are common in its northern parts, which are influenced by cold-water currents. Boreal species predominate in areas of the Pechora Sea that are affected by warmer coastal waters, showing that this area functions as a transitional zone between the boreal and Arctic biogeographic regions (Denisenko et al., 2003).

Temperature (Lüning, 1990) and substrate characteristics (Saher et al., 2012) are important in the distribution of benthic algae, and areas exposed to the mechanical effects of sea ice or icebergs are generally devoid of macroalgae (Gutt, 2001; Wulff et al., 2011). Marked changes in surface salinity due to melting of sea ice and freshwater input from rivers have affected algae distribution, and an abrupt increase in macroalgal presence has been recorded in Arctic fjords together with changes in the abundance of benthos that are thought to be indicative of a climate-driven ecological regime shift (Kortsch et al., 2012).

Figure 2.8 Total catches of the most important fish stocks in the Barents Sea since the mid-1960s. The data include catches in all ICES areas: I, IIb and IIa (i.e. along the Norwegian seas and the Norwegian coast south to 62°N). Redfish refers to Sebastes mentella (ICES, 2014a).

0 750 1500 2250 3000 3750 4500

Catch, thousand tonnes

Saithe Herring Shrimp Greenland halibut Haddock Redfish Polar cod Atlantic cod Capelin

1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

In coastal areas of Svalbard, recent warming with less sea ice has been associated with a two-fold increase in the number of species found intertidally on rocky shores, and a three-fold increase in macrophyte biomass. Subarctic boreal species occupied new areas, while Arctic species retreated (Weslawski et al., 2010). In Svalbard fj ords, rapid and extensive structural changes in rocky-bottom communities have occurred along with an abrupt increase in macroalgal cover (Kortsch et al., 2012). Simultaneous changes in the abundance of benthic invertebrates suggest that macroalgae play a key

In coastal areas of Svalbard, recent warming with less sea ice has been associated with a two-fold increase in the number of species found intertidally on rocky shores, and a three-fold increase in macrophyte biomass. Subarctic boreal species occupied new areas, while Arctic species retreated (Weslawski et al., 2010). In Svalbard fj ords, rapid and extensive structural changes in rocky-bottom communities have occurred along with an abrupt increase in macroalgal cover (Kortsch et al., 2012). Simultaneous changes in the abundance of benthic invertebrates suggest that macroalgae play a key

Im Dokument Impact analysis and consequences of change (Seite 28-36)