Adaptation Actions for a Changing Arctic: Perspectives from the Barents Area

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Adaptation Actions for a Changing Arctic: Perspectives from the Barents Area

2.2.2.1 Phytoplankton and zooplankton Primary production and phytoplankton growth rates in the Barents Sea regionarehighly 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 twomain 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/m 2 , with higher production in the open Atlantic water masses of the southern Barents Sea (100–150 g C/m 2 ) than in the seasonally ice-covered northern Barents Sea (<50–70 g C/m 2 ) (Sakshaug, 2004; Wassmann et al., 2006a,b; Hunt et al., 2013; Dalpadado et al., 2014). New production is typically about 50 g C/m 2 in the AtlanticWater 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 andRey,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 andWassmann,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 andTande,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 fromhigher 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 associatedwith thewarmerAtlanticWater 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 dominantmacrozooplankton (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 AtlanticWater 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 ecosystemand 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 AtlanticWater,while the larger T. libellula (~4.5 cm) is anArctic 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 ArcticWater 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 andmixed waters (Prokhorova, 2013; Eriksen et al., 2014, 2015).

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