Marine and coastal ecosystems

The Barents Sea is a flow-through system with Atlantic Water entering from the Norwegian Sea in the southwest and leaving between Novaya Zemlya and Franz Josef Land in the northeast (see Chapter 2). This sets the stage for the Barents Sea being a biogeographical transition zone between a warmer boreal region in the southern part and an Arctic region in the north.

The Barents Sea has been monitored and investigated for more than 50 years in a collaborative effort between Norway and Russia. This has provided an extensive knowledge base for this sea area (Sakshaug et al., 2009; Jakobsen and Ozhigin, 2011).

6.2.2.1

Impacts of climate variability and change

Large climate and ecological variability is a key feature of the Barents Sea marine ecosystem. Climate variability is expressed on different time scales including multi-decadal and interannual fluctuations. The observed responses to climate variability of ecosystem components including plankton, fish, benthos, birds and marine mammals form a basis of reference for assessing the likely impacts of future climate change. With the warming over recent decades there has been a general increase in the overall abundance and spread of boreal species, and a decline and retreat of Arctic species. This ‘borealization’ with a northward shift in distribution is likely to continue under the warming projected for the next 50 years (Fossheim et al., 2015).

The cold waters of the Arctic Ocean are particularly vulnerable to the rapid and progressive process of ocean acidification (AMAP, 2014b and references therein). The pH of surface waters in the Norwegian Sea has decreased significantly over the past 30 years (Skjelvan et al., 2014). While not uniform across the area and demonstrating seasonal and interannual variability, ocean acidification has direct and indirect effects on Arctic marine life (Orr et al., 2005; AMAP, 2014b). While impacts vary significantly for different organisms (Secretariat of the Convention on Biological Diversity, 2014), they are likely to lead to significant changes in marine ecosystems, such as changes in species composition, leading to potential impacts on Arctic fisheries and economic and social impacts on livelihoods (AMAP, 2014b).

Continued decline in winter sea ice in the Barents Sea (see Chapter 4) is expected to lead to increased primary production by phytoplankton (Ellingsen et al., 2008; Skaret et al., 2014) and decreased primary production by ice algae. But because the contribution of ice algae to the total primary production in the Barents Sea has been low in recent years (<5%) despite extensive ice cover (Hegseth, 1998; Wassmann et al., 2006; von Quillfeldt et al., 2009), the effect on total production is low and compensated for by increased production by phytoplankton.

However, less ice algae as well as reduced occurrence of other ice biota represents a major qualitative change in the ecosystem in the northern Barents Sea.

Increased warming and little or no ice in the Barents Sea by 2070 (see Chapter 4), is expected to result in an expansion of boreal zooplankton and a reduction in Arctic zooplankton.

With warming there is an extension of the reproductive habitat for the dominant copepod Calanus finmarchicus in the southern and central Barents Sea, and increased production due to a greater role for a second generation of the copepods in the warmer Atlantic Water (Melle and Skjoldal, 1998; Skaret et al., 2014). However, this species is expected to continue to be expatriated and not able to occupy the still cold waters of the northern Barents Sea for its breeding habitat. For the closely related Arctic species C. glacialis, the impact of warming and little or no sea ice is unclear. It is possible that the overall effect of warming and less ice may be favorable for C. glacialis, allowing

it to sustain higher predation pressure from pelagic fish such as capelin (Mallotus villosus) and polar cod (Boreogadus saida).

With continued warming, krill are expected to expand their distribution and increase in the Barents Sea. The spawning habitat of Thysanoessa intermis may expand east and north with the warmer Atlantic Water in a similar manner as for C. finmarchicus, while the southwestern Barents Sea may become a regular part of the habitat for Meganyctiphanes norvegica. Predation from pelagic fish and other consumers will continue to be important, and the interaction between climate and predation will determine how the abundance and roles of the various krill species will develop in a future warmer climate (Eriksen and Dalpadado, 2011; ICES, 2015a).

It is expected that the Arctic Themisto libellula (Dalpadado, 2002; Dalpadado  et  al., 2002, 2008), which is one of the dominant hyperiid amphipod species of the genus Themisto, will be negatively impacted by warming, and its future role in the northern part of the Barents Sea ecosystem will diminish.

Jellyfish populations share the pelagic environment with many small planktivorous fishes (Brodeur et al., 2008; Eriksen, 2016), and further warming is likely to increase overlap and strengthen species interactions.

Climate may affect marine fish populations through many different pathways, operating at a range of temporal and spatial scales. Climate impacts may affect fish directly, or indirectly through bottom-up or top-down processes within the food web.

These direct and indirect effects can act simultaneously but with complex patterns involving non-linearity and time lags, and are not mutually exclusive (Rijnsdorp et al., 2009). The many drivers and pathways through which climate affects marine fish stocks can often make it difficult to establish unequivocal connections between climate forcing and the ecological responses of fish populations, let alone quantify them (Ottersen et al., 2004, 2010; Vilhjálmsson and Hoel, 2005). This is even more the case for exploited fish populations where the effects of fishery exploitation interact with effects of climate forcing and the two can be difficult to separate (Skjoldal, 2004; Perry et al., 2010;

Planque et al., 2010).

What will happen to three species of plankton-feeding fish: capelin, herring (Clupea harengus) and polar cod with continued warming is a key issue due to their importance in the ecosystem (see Chapter 2). The complex biological interactions involved make it difficult to develop predictions. Occupation of new spawning grounds on banks off Novaya Zemlya is a possibility that may shift the spatial distribution and ecological role of capelin in the Barents Sea ecosystem under a warmer climate (Huse and Ellingsen, 2008). Norwegian spring spawning herring is expected to continue to thrive under the warming projected for the next 50 years, but with fluctuations driven by fluctuations in the future climate. The loss of sea ice may have led to a loss of spawning habitat and thus have contributed to the dramatic recent recruitment failures and stock decline.

Further, the expansion of Atlantic cod (Gadus morhua) into the northern Barents Sea has led to increased spatial overlap between the two species and increased predation pressure from Atlantic cod on polar cod. The decline in the polar cod stock may cause structural reorganization of the Arctic food web in the future (Hop and Gjøsæter, 2013). The projected warming

may lead to a permanently reduced polar cod stock in the Barents Sea with consequences for the ecology of the northern and southeastern Barents Sea.

With further warming, the Barents Sea will continue to be a favorable habitat for commercially important cod (see Chapter 2; Fossheim et al., 2015). The stock will probably not be able to increase further due to restrictions in space and productivity. It is likely that there will continue to be large fluctuations in the ecosystem, as is now being seen with the ongoing collapse of the capelin stock and which is likely to affect the cod stock as well as other species in the ecosystem.

How the ecosystem dynamics will develop is difficult to predict, however, due to the complexity of climate forcing and food web interactions. The northern expansion of cod is a prime example of the borealization of the Barents Sea ecosystem under warming (Fossheim et al., 2015) (see also Box 6.1).

As is the case for the fish communities (Fossheim et al., 2015), continued warming is expected to lead to a further borealization of megabenthos (and probably also benthic infauna) with an increase in boreal species and a decrease in Arctic species along the southwest-northeast axis. The ecological processes thought to drive the observed changes are likely to promote the borealization of Arctic marine communities in the coming years (Kortsch et al., 2012).

Climate change is expected to affect all marine mammal species (see Table 2.1 for an overview) in the Barents Sea through impacts on the productivity of plankton, benthos and fish.

The ice-associated species are very likely to be negatively affected by the loss of sea ice (Laidre et al., 2015), while open water species such as the large baleen whales are very likely to benefit from the warming trend. Ringed seal (Pusa hispida), harp seal (Pagophilus groenlandicus), hooded seal (Cystophora cristata) and bearded seal (Erignathus barbatus) depend on ice as a substrate for breeding, lactation, molting and resting, and are therefore particularly vulnerable to the decline in Arctic sea ice (Laidre et al., 2015). Some bearded seals follow the marginal ice zone and may therefore be negatively affected by increased migration distances and possible changes in prey composition and availability. If sea ice retreats to deep water north of Svalbard, it can no longer serve as a feeding platform for bearded seal. Reduced availability of ice habitat over the continental shelf is therefore a concern for this species (Kovacs et al., 2011). A general concern with respect to Arctic warming is the replacement of Arctic species of zooplankton and fish by less energy-rich southern species. These species may not allow sufficient accumulation of body reserves for capital breeding animals like seals (Grebmeier et al., 2006;

Dalpadado et al., 2012).

Owing to low abundance, crowding in haul-out areas or food limitation close to haul-outs do not currently appear to be a problem for walrus (Odobenus rosmarus) in the Barents Sea area in contrast to large parts of the Pacific Arctic (Laidre et al., 2008). However, continued sea-ice retreat may become a problem for Barents Sea walrus over the long term.

Continued retraction of the sea ice will almost certainly lead to large reductions in the abundance of all ice breeding seals and thereby to a reduction in the prey base for polar bears (Ursus maritimus) (Wiig  et  al., 2008; Kovacs  et  al., 2011;

Box 6.1 Observed changes in Barents Sea fi sh and benthic species Th e Barents Sea is home to roughly 100 species of fi sh that are regularly recorded during surveys (Bogstad et al., 2008; Wienerroither et al., 2011). Of these, just over half are considered boreal species while about one third are Arctic species (Andriyashev and Chernova, 1995; Bogstad et al., 2008, 2014). Th e species are distributed in patterns of fi sh communities that shift in composition and distribution with changing climatic conditions (Fossheim et al., 2006;

Johannesen et al., 2012; Aschan et al., 2013). Th e general increase in overall abundance and expansion of boreal species that has accompanied the warming of the past few decades, referred to as ‘borealization’ (Figure 6.2), is likely to continue under the projected warming over the next 50 years (Fossheim et al., 2015).

A decrease in total benthic biomass between surveys in 1924–

1935 and 1968–1970 through almost the entire Barents Sea (Figure 6.3) has been attributed to climate change by many researchers. However, this situation changed in the period 1991-1994 with biomass shift ing and showing a considerable increase in the central region. Th e mechanisms underlying the changes in biomass are not clear. Some studies have suggested that this was due to a change in faunal distribution during the cold period between the 1960s and 1980s (Bochkov and Kudlo, 1973; Bryazgin, 1973; Antipova, 1975), while others have invoked declining biomass of resident boreal-Arctic species during the warm period from the 1930s to the 1960s (Galkin, 1987; Kiyko and Pogrebov, 1997, 1998). Th e dominant boreal-Arctic species have an optimum temperature range that is positioned within the long-term mean temperature measured for the region. According to the latter theory, any deviations from the long-term mean have negative impacts on the reproduction, abundance, and biomass of boreal-Arctic species (Anisimova et al., 2011 and references therein).

Monitoring of benthos at the Kola transect, which was started in 1994 by the Murmansk Marine Biological Institute, revealed an increase in the relative number of boreal species following the historical maximum

temperature anomaly recorded in 2006. Benthic biomass increased through the entire 17-year monitoring period and peaked in 2010. Th is is believed to have been caused by the long period of warming and abnormally high bottom temperatures between 2006 and 2012 (Olga Ljubina, Murmansk Marine Biological Institute, pers. comm.).

Figure 6.2 Comparison of the abundance of fi sh communities in the Barents Sea between 2004 and 2012 (Fossheim et al., 2015).

Figure 6.3 Distribution of benthic biomass in the Barents Sea for three survey periods (aft er Brotskaya and Zenkevich, 1939; Antipova, 1975; Kiyko and Pogrebov, 1997) (Institute of Marine Research).

1991-1994 1968-1970

1924-1935

10-25 25-50 50-100 100-300 300-500

<10 >500

Total biomass, g/m

2012 2004

Fish communities Atlantic Central Arctic

Box 6.2 Cumulative impacts and consequences for seabird populations Many seabird populations in the Barents area have shown a

signifi cant and steady decline (see Chapter 2). Studies have been attributing this not just to one factor (such as loss of prey base due to human infl uence or natural variation, disease, or increase in contaminant loads), but rather to a cumulative eff ect of multiple stressors.

Th e breeding population of glaucous gull (Larus hyperboreus) at Bjørnøya, which is home to the largest colony in the Barents area, has drastically declined; by 65% over a 30-year period. Autopsy and chemical analyses of dead and dying birds showed very high levels of chlorinated pollutants in their brain and liver (Sagerup et al., 2009). Th e elevated chlorinated pollutant levels are likely to have aff ected the gulls directly (physiological) or indirectly (suppression of condition) (Sagerup et al., 2009) and could be one of the main causes of mortality in glaucous gull.

According to Erikstad et al. (2013), the Bjørnøya glaucous gull population is currently declining at 8% per year. Th is indicates a median time to quasi-extinction of 19 years for this species. However, a third of the population decline is estimated to be due to the eff ect of pollutants in the adult population. In the absence of pollution, median time to population quasi-extinction is 50 years (Erikstad et al., 2013).

See also Figure 6.4. In 1980 and 2006, total counts indicated population sizes of 2000 and 650 breeding pairs, respectively.

Temporal trend assessment suggests that although several organochlorines are declining in Svalbard glaucous gull samples (Verreault et al., 2010), environmental factors such as atmospheric variability may modulate the infl ux and thus the food chain transfer of these compounds in the Arctic ecosystem (Verreault et al., 2010). Th e eff ects of pollutants are more severe in years when the environmental conditions are worse (Bustnes et al., 2006). Even low levels of pollution in combination with other stress factors, such as food shortage or increased competition for nesting sites, can be critical (Erikstad et al., 2013).

As well as pollution, other explanations for the decline in the breeding population of glaucous gull include reduced prey availability, increasing predation from Arctic foxes (Vulpes lagopus) and competition from a growing number of great skua (Stercorarius skua) (Erikstad et al., 2013). Th ere is agreement that climate change may result in reduced food availability and in an increase in adverse weather events. Th e eff ects of these factors on the glaucous gull population, in combination with pollutants, are not yet clear.

All monitored colonies of Brünnich’s guillemot (Uria lomvia) at Bjørnøya and Svalbard (Descamps et al., 2013; Fauchald et al., 2014) began to decline during the same period (1994–1998).

Th e annual rate of decline has since varied from 2–5%, and during the past decade Brünnich’s guillemot colonies have decreased by about 15-45%. If this trend continues at the same rate, the Svalbard population has an almost one in two chance (43%) of becoming quasi-extinct within the next 50 years and extinct in the next 100 years (Descamps et al., 2013). Further, because there is high synchrony between colonies at west

Svalbard, the risk of extinction increases as all of these colonies may crash concurrently (Heino et al., 1997). Th is decline in population has coincided with a major shift in oceanographic conditions (Descamps et al., 2013). Th e 1995 shift in the sub-polar gyre and consequent changes in the subarctic waters of the North Atlantic are very likely to have played an important role (Descamps et al., 2013).

Brünnich’s guillemots may be vulnerable to pollution and to human impact on the availability of their prey through climate change and overfi shing (Fauchald et al., 2014). Since the mid-1980s, levels of polychlorinated biphenyls (PCBs) have been reported for Brünnich’s guillemot from Svalbard (Norheim and Kjoshanssen, 1984; Mehlum and Daelemans, 1995). Levels correspond to those for other auk species and are lower than for glaucous gull (Norheim and Kjoshanssen, 1984; Mehlum and Daelemans, 1995; Borgå et al., 2005;

Letcher et al., 2010; Verreault et al., 2010). Although lower pollutant concentrations have been measured in Brünnich’s guillemot, negative effects on vitamin status have been observed (Murvoll et al., 2007). However, taken in context the fi ndings for glaucous gull indicate that pollutants, even at low levels, act as a stressor enhancing the negative eff ect of other stressors (Bustnes et al., 2006). It is possible that pollutants are playing a role in the decline of the Brünnich’s guillemot population, or are making these birds more vulnerable to other changes related to climate and food availability.

Ivory gull (Pagophila eburnea) is the least studied species in the Arctic, with an estimated global population of 14,000 pairs (de Wit et al., 2003). Th is species has a strong and year-round association with pack ice and its scavenging habits, and thus is vulnerable to changes in sea ice cover and exposure and the accumulation of high levels of organic pollutants, including mercury (Braune et al., 2006, 2007; Miljeteig et al., 2009; Lucia et al., 2015). Global warming and pollution have been identifi ed as the major threats to ivory gull and how this species will progress in the future is unknown.

Figure 6.4. Nests occupied by glaucous gull at Bjørnøya between 1987 and 2010 at a study plot. No monitoring was undertaken in 1989, 1990, 1994, 1996 and 1997. Open symbols indicate explorations based on the PROC EXPAND procedure which is a tool to work with time series.

Erikstad et al. (2013).

Number of breeding pairs

2010 2005

2000 1995

1990 01985

20 40 60 80 100 120 140 160

McKinney et al., 2013). Following the seasonally retreating ice edge with much open water north of Svalbard may also be associated with increased mortality, particularly of young cubs that are less able to endure long swims in cold water (Aars and Plumb, 2010; Pagano et al., 2012).

Finally, most seabird species (see Chapter 2) are susceptible to changes in the marine ecosystem, including changes in prey availability related to ocean climate change, and it is likely that these changes will be even more significant in the future. An increase in boreal species and a decrease in Arctic and subarctic species in Norwegian waters are anticipated. According to Fauchald et al. (2015), ecosystem specific changes, possibly initiated by past and present fisheries in combination with climate change, are the major indirect drivers of the observed seabird declines. While human impacts cannot alone explain the recent population declines, they are an important contributor to declining and threatened seabird populations and are therefore especially important to control (Box 6.2).

Patterns of species change in the marine ecosystem are complex because different species are affected differently by warming waters and decreasing ice cover. It is expected that the marine

ecosystem of the Barents Sea will exhibit borealization with northward shifts in species over the next few decades. These changes are overlaid by impacts from the oil and gas industry, shipping, and fisheries with further consequences for the fisheries and aquaculture sectors (Sections 6.3.1.4 and 6.3.1.6).

6.2.2.2

Impacts of non-climatic factors

A wide range of industrial sectors are represented in the Barents Sea region, including fisheries, oil and gas production, mining, and shipping (see Chapters 2 and 4, and Section 6.3). Fish products are a major source of animal protein for a significant fraction of the world’s population, and large-scale oil and gas development, new mining, and the promotion of the Northern Sea Route as a major

A wide range of industrial sectors are represented in the Barents Sea region, including fisheries, oil and gas production, mining, and shipping (see Chapters 2 and 4, and Section 6.3). Fish products are a major source of animal protein for a significant fraction of the world’s population, and large-scale oil and gas development, new mining, and the promotion of the Northern Sea Route as a major

Im Dokument Impact analysis and consequences of change (Seite 146-155)