The aim of this subchapter is to summarise key features of the state of the Barents Sea ecosystem and discuss what aspects of the ecosystem are likely to be influenced by anthropogenic impacts. A discussion is also undertaken regarding possible developments of the ecosystem in the future. The chapter takes the ecosystem perspective, and will consequently focus on ecosystem status, function and processes. The anthropogenic driver that currently has the largest documented impact on the functioning of the Barents Sea ecosystem is harvesting.
In addition, the ecosystem is affected significantly by climate changes. and the interaction between harvesting and climate change. Special emphasis is therefore given to these factors in the discussion below. The climate changes that have been observed in recent years are in part due to the effect of anthropogenic emissions of CO2 and other greenhouse gases, but they also represent natural variation in the system on long-time scales. Development of oil and gas production (see chapter Current and expected state of the ecosystem - Human activities /impact - Pollution.), increased maritime transport (chapter Current and expected state of the ecosystem - Human activities /impact - Biotic components.) and ocean acidification due to climate warming (see chapter Current and expected state of the ecosystem - Human activities /impact - Pollution.) may become additional factors that probably will affect the system in the future.
The Barents Sea is a shelf ecosystem situated at the border between the Arctic and North Atlantic Oceans where water moves from the North Atlantic into the deep Arctic Ocean basin. From the Arctic Ocean perspective, the Barents Sea is a highly productive, deep, inflowing shelf sea (Carmack and Wassmann, 2006). Compared to the other North Atlantic shelf ecosystems, however, the Barents Sea has relatively low productivity and low biodiversity (Frank et al., 2007). However, south of the polar front, high primary production and advection from the Norwegian Sea translate into high biomass of zooplankton and large stocks of small pelagic fish that support one of the largest fisheries in the world. The Norwegian coastal current carries fish larvae into the Barents Sea and the southern part of the region is the nursery area for important commercial species such as the NEA (North East Atlantic) cod and the NSS (Norwegian Spring Spawning) herring The vast areas north of the polar front are characterized by highly variable and seasonal ice cover. Primary production is generally low but ice melting during summer stratifies the water masses and initiates a concentrated, short-lived phytoplankton bloom that supports high concentrations of zooplankton. These areas are targets for the northbound feeding migrations of capelin , cod, seabirds and marine mammals in late summer and early autumn.
The Barents Sea has been harvested by humans for centuries (Chapter General background description of the ecosystem - Ecosystem interactions - Human impact). Similar to other continental shelf areas (Jackson et al., 2001; Lotze and Worm, 2009), hunters and fishermen have targeted the high-value/low-cost catch. The targeted species have often been slow growing, large animals, on the top of the food chain (Lotze and Worm, 2009). The result has been a sequential extirpation of large predatory fish, seabirds and sea mammals. This has almost certainly resulted in fundamental changes in the ecosystem (see e.g. Jackson et al., 2001; Lotze et al., 2005). For example, the ecosystem that once supported vast baleen whale and harp seal populations in the Barents Sea (see Weslawski et al., 2000; Skaug et al., 2007) was certainly different from the present one. Today, cod is the dominant predator in the Barents Sea. This is similar to the other North Atlantic shelf ecosystems (Link et al., 2009). However, high fishing pressure has, in several of these systems, reduced the populations of cod to very low levels. There has been a subsequent increase in the populations of small pelagic forage fish species such as capelin, herring and sprat (Sprattus sprattus) in systems such as the North Sea, the Baltic Sea and the Scotian Shelf (e.g. Frank et al., 2005; Casini et al., 2009). Large stocks of small pelagic fish might be responsible for reducing the recruitment of cod through either predation on eggs and larvae (Swain and Sinclair, 2000; Bakun, 2006), or through competition for larval food (Casini et al., 2009). Such positive feedback mechanisms result in stable ecosystem shifts from a predator (cod) dominated state to a prey (herring or capelin) dominated state (Bakun, 2006).
Fluctuations in ocean climate have profound effects on northern shelf ecosystems (Ottersen et al., 2009). The main mechanisms work through effects on the recruitment of major fish stocks with large consequences for fisheries, and through changes in the large-scale distribution of species which may influence community structure dramatically (Beaugrand et al., 2008; Ottersen et al., 2009). In the Barents Sea, climate change will in addition affect the distribution of sea ice, with large consequences for primary production (Ellingsen et al., 2008) and for ice-dependent flora and fauna (e.g. Kovacs and Lydersen, 2008). Perturbations from climate anomalies propagate through the food web and generate more or less abrupt changes in the ecosystem (de Young et al., 2008). For example, in the North Sea, climate influences have profound effects on the plankton community through changes in phenology (Edwards and Richardson, 2004) and large-scale biogeography (Beaugrand et al., 2002). Recent warming has therefore resulted in a mismatch between the timing of cod spawning and the peak in the abundance of food for larval cod. Combined with a small parent stock, this has severely impaired the recruitment of North Sea cod in recent years (Beaugrand et al., 2003).
Through complex effects on life-histories, individual behaviour and interactions between species, perturbation from harvesting and climate changes can have subtle effects on the ecosystem, resulting in more or less unpredictable changes. By canalizing ecosystem interactions through alternative pathways, the ecosystem might compensate for the perturbations and thus be quite resistant. However, when changes occur they might be more or less abrupt, and through positive feedback mechanisms, the ecosystem can be locked in a new alternative state even if the causes of the perturbations cease (see e.g. Scheffer et al. 2001, Willis 2007, de Young et al. 2008). Predictable, resistant, and resilient ecosystems are generally associated with high biodiversity (Worm et al. 2006), high productivity and bottom-up regulation (Frank et al. 2006, 2007). Compared to the other North Atlantic shelf ecosystems the Barents Sea has low productivity, low biodiversity and is top-down regulated (Frank et al. 2007, Petrie et al. 2009). Consequently, the Barents Sea is likely to be more vulnerable to perturbations, and a careful harvesting strategy is strongly recommended (Petrie et al. 2009). However, contrary to the Baltic Sea, the North Sea and the Scotian Shelf, the Barents Sea has not gone through any major system changes in recent years, and at present the system seems to be quite resistant to the current level of anthropogenic drivers.
