Progress in Oceanography Progress in Oceanography 71 (2006) 182–231

Progress in Oceanography Progress in Oceanography 71 (2006) 182–231 www.elsevier.com/locate/pocean

Physical and biological characteristics of the pelagic system across Fram Strait to Kongsfjorden

Haakon Hop a,*, Stig Falk-Petersen a, Harald Svendsen a,b, Slawek Kwasniewski c, Vladimir Pavlov a, Olga Pavlova a, Janne E. Søreide d

a Norwegian Polar Institute, N-9296 Tromsø, Norway b Geophysical Institute, University of Bergen, Allegt. 70, N-5007 Bergen, Norway c Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy St. 55, 81-712 Sopot, Poland d Akvaplan-niva, N-9296 Tromsø, Norway

Abstract

The Fram Strait is very important with regard to heat and mass exchange in the Arctic Ocean, and the large quantities of heat carried north by the West Spitsbergen Current (WSC) influence the climate in the Arctic region as a whole. A large volume of water and ice is transported through Fram Strait, with net water transport of 1.7–3.2 Sv southward in the East Greenland Current and a volume ice flux in the range of 0.06–0.11 Sv. The mean annual ice flux is about 866,000 km2 yr1. The Kongsfjorden–Krossfjorden fjord system on the coast of Spitsbergen, or at the eastern extreme of Fram Strait, is mainly affected by the northbound transport of water in the WSC. Mixing processes on the shelf result in Transformed Atlantic Water in the fjords, and the advection of Atlantic water also carries boreal fauna into the fjords. The phytoplankton production is about 80 g C m2 yr1 in Fram Strait, and has been estimated both below and above this for Kongs- fjorden. The zooplankton fauna is diverse, but dominated in terms of biomass by calanoid copepods, particularly Calanus glacialis and C. finmarchicus. Other important copepods include C. hyperboreus, Metridia longa and the smaller, more numerous Pseudocalanus (P. minutus and P. acuspes), Microcalanus (M. pusillus and M. pygmaeus) and Oithona similis. The most important species of other taxa appear to be the amphipods Themisto libellula and T. abyssorum, the euphausiids Thysanoessa inermis and T. longicaudata and the chaetognaths Sagitta elegans and Eukrohnia hamata. A comparison between the open ocean of Fram Strait and the restricted fjord system of Kongsfjorden–Krossfjorden can be made within limitations. The same species tend to dominate, but the Fram Strait zooplankton fauna differs by the presence of meso- and bathypelagic copepods. The seasonal and inter-annual variation in zooplankton is described for Kongsfjorden based on the record during July 1996–2002. The ice macrofauna is much less diverse, consisting of a handful of amphipod species and the polar cod. The ice-associated biomass transport of ice-amphipods was calculated, based on the ice area transport, at about 3.55 · 106 ton wet weight per year or about 4.2 · 105 tCyr1. This represents a large energy input to the Green- land Sea, but also a drain on the core population residing in the multi-year pack ice (MYI) in the Arctic Ocean. A continuous habitat loss of MYI due to climate warming will likely reduce dramatically the sympagic food source. The pelagic and sympagic food web structures were revealed by stable isotopes. The carbon sources of particulate organic matter (POM), being Ice-POM and Pelagic-POM, revealed different isotopic signals in the organisms of the food web, and also provided information about the sympagic–pelagic and pelagic–benthic couplings. The marine food web and energy pathways were further determined by fatty acid trophic markers, which to a large extent supported the stable isotope picture of

* Corresponding author. Tel.: +47 77 75 05 22; fax: +47 77 75 05 01. E-mail address: [email protected] (H. Hop).

0079-6611/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2006.09.007 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 183 the marine food web, although some discrepancies were noted, particularly with regard to predator–prey relationships of ctenophores and pteropods. 2006 Elsevier Ltd. All rights reserved.

Keywords: Oceanographic conditions; Sea ice ﬂux; Pelagic food web; Ice biota; Stable isotopes; Lipids

1. Introduction

The Fram Strait, between Greenland and Svalbard, represents the only deep connection to the Arctic Ocean (Fig. 1). The Svalbard archipelago consists of many islands, with Spitsbergen being the largest one facing Fram Strait to the west. The exchange of water masses between the north Atlantic and the Arctic Ocean takes place in two opposing current systems: the West Spitsbergen Current (WSC) heading north along the shelf slope on eastern part of the region and the East Greenland Current (EGC) heading southward along Greenland. The Fram Strait is very important with regard to heat and mass exchange in the Arctic Ocean, and the large quantities of heat carried north by the WSC influence the climate in the Arctic region as a whole. The inflow of Atlantic water into the Arctic Ocean through Fram Strait and the Barents Sea is about 5–10 times larger than the inflow of Pacific water through the Bering Strait (Haugan, 1999; Rudels et al., 1999; Schauer et al., 2002). The export of cold polar surface water and ice by the EGC is even larger, with a net transport southwards for the Fram Strait system. The heat balance is further com- plicated by deep-water formation in the Greenland Sea (>3500 m deep) and associated deep currents (Aag- aard et al., 1985). The extent of the ice cover in the Nordic Seas in spring has decreased since 1860 due to the net thermal effect of the northbound currents (Vinje, 2001). A continuation of this trend is predicted by global circulation models (GCMs; IPCC, 2000). If these predictions are correct, a permanent warming of the climate of the Arctic and a further decrease of the sea ice extent and thickness in the Barents Sea and the Arctic Ocean will occur. Since the first evidence of warming in the Atlantic Water (AW) was found in the Nansen Basin in 1990 (Quadfasel et al., 1991), both observations (Woodgate et al., 2001) and modelling indicate a variable nature of AW flow, with abrupt cooling/warming events. There is general agreement that the Arctic Ocean at present is in a transition towards a new, warmer state (e.g. Polyakov et al., 2005). The cause of these variations are not well understood, but variations in the inflow of AW and outflow of Polar Water (PW) masses and sea ice are shown to be related to the Arctic Oscillation (AO; Rigor et al., 2002; Zhang et al., 2003) and the North Atlantic Oscillation (NAO; Dickson et al., 2000) on inter-annual and decadal scales. These pressure systems are strongly linked to the atmospheric heat balance. Climate changes may thus alter the strength of the large-scale ocean circulation in the region. This would change the relative amount of source waters (PW and AW) that are mixed and subsequently result in modification of the water masses created on the shelf off West-Spitsbergen. The mixing of AW with the coastal Arctic Water (ArW) from the South Cape Current results in Transformed Atlantic Water (TAW). This water mass is advected across the shelf towards the coast (Saloranta and Svendsen, 2001) and subsequently into the fjords on Spits- bergen (Svendsen et al., 2002; Cottier et al., 2005). The advected water masses carry associated Arctic and Atlantic fauna into the fjords (Basedow et al., 2004; Willis et al., 2006). The magnitude of the advection into the fjords varies both seasonally and annually depending on the strength of a geostrophic control mechanism in the fjord mouth. Climate change affecting water mass distribution and sea ice conditions is expected to have large effects on ecosystem functions on different scales. The Kongsfjorden–Krossfjorden fjord system is particularly suitable for studies of effects of climate changes on ecosystems because it lies adjacent to both Arctic and Atlantic water masses (Fig. 1). In addition, a substantial amount of observations is available from this area (reviews, Hop et al., 2002b; Svendsen et al., 2002). The inclusion of these observations and existing time-series for this area is imperative for the detection of changes. In particular, we have a relatively long (10 yrs, since 1996) time series of zooplankton composition for this area. Changes in abundance, size and energy content of zooplankton prey influence the energy flux through the pelagic food web and cascade into ecological consequences for growth and survival of seabirds and marine mammals (Falk-Petersen et al., 1990, 2006; Dahl et al., 2000, 2003). 184 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 1. The Fram Strait region, showing stations sampled for food web structures: Northeast Water Polynya in June/July 1993 (Hobson et al., 1995), Stns. 882 (September 1999), 890 (October 1999) and 1003 (September 2000) from Søreide et al. (in press), and stations south and west of Svalbard (white circles) in January 1999 (Sasaki et al., 2001; Sato et al., 2002). The underlying map was obtained from NOAA (www.ngdc.noaa.gov/mgg/bathymetry/arctic/currentmap.html). Kongsfjorden and Krossfjorden on Spitsbergen (lower panel), the largest island in the Svalbard archipelago, with transect stations for CTD and zooplankton sampling with MPS and WP3 nets (modiﬁed from Svendsen et al., 2002).

The ecosystem components considered here constitute the pelagic and sympagic (ice-associated) systems, which are influenced by different water masses and ice conditions. The physical part focuses on the physical oceanographic conditions and sea ice conditions, whereas the biological parts focus on the lower trophic levels H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 185 of the marine pelagic food web and its energy pathways to middle-to-upper levels. A comparison between the open ocean of Fram Strait and the restricted fjord system of Kongsfjorden–Krossfjorden is performed within limitations. The main problems relate to the lack of data on marine organisms collected simultaneously or semi-simultaneously from each environment during the same season, and the lack of data collected by analogous sampling gear from comparable environments (e.g. equivalent water layers). Both areas generally lack systematic faunistic surveys, even though the record for Kongsfjorden is quite extensive (Hop et al., 2002b). This is particularly surprising in the case of Fram Strait, taking into account its role in the exchange of biomass and energy between the Nordic Seas and the Arctic Ocean. It has often been suggested, however, that the zooplankton faunistic information from these areas can be supplemented by the available information on fauna of the adjacent waters of the Nordic Seas or the Arctic Mediterranean (Smith, 1988; Longhurst, 1998). The main pelagic predators in the system include fishes, marine mammals and seabirds, some of which are associated with ice (e.g. seals and walruses). Their predatory impact on the lower trophic levels has been estimated for the Kongsfjorden system (Hop et al., 2002b). The population numbers of predators in Fram Strait are only known for some species, such as harp seals (ICES, 2004). Consumption by predators in the system has not been estimated, but some indications can be obtained from estimates for the neighbouring Barents Sea (Sakshaug et al., 1994; Wassmann et al., 2006) and the Norwegian Sea (Skjoldal et al., 2004). The ecosystem structure and function in the area of Fram Strait–Kongsfjorden are here revealed by means of stable isotopes of carbon and nitrogen as well as fatty acid trophic markers.

2. Oceanographic conditions of Fram Strait

Numerous studies based on direct observations and modelling of the currents have provided relatively large differences in estimates of southward and northward water volume transport through Fram Strait, ranging from 2.1–13.7 Sv to 1.0–9.5 Sv, respectively (Table 1). However, the net transport estimated though Fram Strait is relatively similar and varying from 1.7 to 4.2 Sv, with the exception of one low estimate by Zhang et al. (2000). The most realistic estimates of the volume transport through Fram Strait have probably been suggested by Fahrbach et al. (2001) based on high-density observations from 14 current meter moorings deployed in Fram Strait from September 1997 to September 1999. Their values for the northward (9.5 Sv), southward (13.7 Sv) and net (4.2 Sv) transports are higher than previous estimates, but are in good agreement with the most recent modelling results (Maslowski et al., 2004). The variations in temperature and current velocities (1997–1998) have a pronounced annual cycle in Fram Strait, except in the southward flow in the western part of the strait where the velocity has no clear annual cycle (Fig. 2). Maximum velocities and relatively high temperatures are observed in the WSC in the eastern part of Fram Strait, whereas maximum velocities in the southward flow and associated low temperatures are observed in the upper layer of the EGC in the western part of Fram Strait.

Table 1 Estimates of volume transport (Sv) through Fram Strait Method Northward transport Southward transport Net transport Author(s) Modelling 1.0 2.7 1.7 Holland et al. (1996) Modelling 3.2 6.4 3.2 Gerdes and Schauer (1997) Modelling 2.4–2.6 2.1–2.4 0–0.5 Zhang et al. (2000) Modelling 1.5 3.4 1.9 Karcher and Oberhuber (2002) Modelling 6.4 8.7 2.3 Maslowski et al. (2004) Observation 8.0 – – Aagaard et al. (1973) Observation 7.0 – – Greisman (1976) Observation 5.6 – – Hanzlick (1983) Observation – 3.0 – Foldvik et al. (1988) Observation 3.0 – – Jonsson (1989) Observation 9.5 13.7 4.2 Fahrbach et al. (2001) 186 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 2. Vertical transects of the potential temperature (left panels) and meridional current velocities (right panels) across Fram Strait. The monthly mean values of temperature and currents were calculated based on records from 14 moorings in Fram Strait during the period September 1997 to August 1998 (data of VEINS Project ‘‘Variability of Exchange in Northern Seas’’). Mooring positions and instrument depths are detailed in Fig. 2 of Fahrbach et al. (2001).

The maximum velocities in the EGC are observed in the upper layer between 2W and 6W, and these are less than velocities in the core of the WSC. Arctic Water, with temperature about 1.3 to 1.75 C, is present near the surface layer of the EGC. The temperature increases relatively fast with depth due to recirculation of AW from the WSC and reaches 1.0–1.3 C during all seasons at 200–300 m. Below 1000 m, the water temperature is negative with a minimum in the bottom layer (0.90 to 0.95 C). The current velocities in the north- H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 187 ward-directed WSC reach 40–50 cm s1 during January–March. From May to July, the currents are significantly weaker, while in August–September, the current velocities have a second maximum (about 20 cm s1), again in northward direction. The annual cycle of both currents and water temperature in the WSC is more pronounced in the upper layer than in the core itself (Fig. 3a). The northward current velocity decreases slightly with depth, and can attain the opposite direction at depths >1500 m (Fig. 2). There is also a sharp decrease of the northward velocity component of the WSC from east to west. The boundary between the northward and southward flow generally occurs at 4–6E in the upper layer, while in the deeper layers, the position of the boundary between the flows varies between months, from 5E to 2–3W. In the upper layer, the monthly mean water temperature in the eastern part of Fram Strait reaches a maximum of 4.5–5.5 C in August–October and a minimum at the beginning of winter. The oceanographic structure of the currents in the deeper layer is generally similar to that in the upper layer, but the maximum temperatures are shifted to winter (Fig. 3b). Between the two major currents on each side of Fram Strait the circulation is characterised by a mesoscale eddy field. Instabilities in the WSC likely contribute to this eddy field (Johannessen et al., 1987), but to what extent is not known. Gascard et al. (1988) suggest that eddies are advected from the east with the recirculation in the strait and that the EGC is dynamically stable and unable to generate eddies, despite the outer fringe of the EGC being dominated by shifts in the position of the East Greenland Front (Holfort and Hansen, 2005). Thus, the baroclinic instability in the polar front, which marks the eastern edge of the EGC, is not a major contributor to the mesoscale eddy field (Foldvik et al., 1988). In the eastern part of Fram Strait, near Spitsbergen, the WSC follows the shelf slope (Hanzlick, 1983; Jons- son et al., 1992; Woodgate et al., 1998; Saloranta and Haugan, 2001) due to conservation of potential vorticity. However, because there is no density front (only a temperature and salinity front) between the warm and saline AW in the WSC and the cold and fresher Arctic-type Water on the West-Spitsbergen shelf, barotropic instabilities in the geostrophically constrained WSC along the slope cause significant onshore exchange (Salo- ranta and Svendsen, 2001). This is not in agreement with Hanzlick (1983) who found that baroclinic instability provides a possible cause of the flow variability. The exchanged water is manifested as numerous remnants of mixed AW and ArW on the shelf and in the fjords on West-Spitsbergen (Saloranta and Svendsen, 2001; Svendsen et al., 2002; Cottier et al., 2005). Related to these remnants, is heat transport from the WSC. The combined effects of topographically trapped vorticity waves along the West Spitsbergen shelf slope and isopycnal eddy diffusion are the main mechanisms causing the heat loss from the core of the WSC, both on-shelf and off-shelf (F. Nilsen, unpubl.). This heat flux is in the order of 1000 W m2 throughout the year, except for the summer months June–July. This is in good agreement with earlier diagnostic estimates by Saloranta and Haugan (2001), who found that the warm core of the WSC loses approximately 1000 W m2 during winter and 300 W m2 during summer. Already at the turn of the last century it was established that a warm subsurface layer of AW was present in the Arctic Ocean (Nansen, 1902). However, even today there is uncertainty about the transport tracks of AW into the Arctic Ocean. Lagrangian float trajectories indicate that the eddy-dominated western part of the WSC recirculates, joining the EGC (Bourke et al., 1988; Gascard et al., 1995; Saloranta and Haugan, 2001). Thus, the major fraction of AW in the Arctic Ocean is likely supplied by the slope-confined eastern part of the WSC (Aagaard et al., 1987; Bourke et al., 1988). The AW is cooled and freshened on its transfer through the Arctic Ocean and is named Modified Atlantic Water (MAW) when returning toward Fram Strait from both the Eur- asian Basin (relatively warm water) and the Canadian Basin (cold water that has been cooled on the long path around the Canadian Basin). The annual variability of the northward volume transport through Fram Strait corresponds to the seasonal changes of sea level in the eastern part of the strait (Fig. 4). The volume transport has two maxima, in Feb- ruary and August, and two minima, in January and June. The similar variability of the sea level from records at the Barentsburg station on West-Spitsbergen (Fig. 4) confirms the conclusions of Morison (1991) and Fahr- bach et al. (2001) about a strong barotropic transport contribution from the WSC. The possible role of the wind field in driving the mesoscale eddy velocity field, as suggested by Manley et al. (1987), was investigated by Jonsson et al. (1992). They analysed current time series observed during the last 50 years and argued that, in at least the central and eastern Fram Strait, most of the observed eddy kinetic energy is generated by wind fluctuations. The mesoscale eddy scales were assumed to be the internal Rossby radius and estimated using hydrographic data to be about 20 km, which is in agreement with the estimates by 188 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 3. Seasonal variability of the potential temperature (C) and currents (cm s1) in (a) the upper layer (41–101 m depth), and (b) the core layer of the West Spitsbergen Current. Graphs are based on temperature and current records from VEINS 14 moorings during September 1997 and August 1998. The upper layer is based on mooring data from 41 to 101 m depth, whereas the deeper layer range is 217–288 m. Red dots on x-axes of a, b are the longitude positions of mooring stations, which are also shown on the map (upper panel). Mooring positions and depths are further detailed in Fahrbach et al. (2001).

Hanzlick (1983). The wind driven circulation on the shelf area off West-Spitsbergen (Fig. 5) indicates little wind effect along the shelf slope, where topographic steering dominates, but strong wind effect over the shelf, especially around the tip of West-Spitsbergen, in trenches and over banks. Simulations with both tide and H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 189

Fig. 4. Seasonal variability (September 1997–August 1998) of the northward monthly mean volume transport (solid line) and sea level at Barentsburg, Svalbard (dashed line). Volume transport was calculated based on VEINS data for the period September 1997–August 1998. Monthly mean sea level data in Barentsburg for the same period was obtained from Permanent Service for Mean Sea Level (PSML: http:// www.pol.ac.uk/psmsl/).

Fig. 5. Simulated surface circulation pattern (without tides) for the eastern Fram Strait, including the shelf and coast of West-Spitsbergen, based on two different wind patterns: northerly 15 m s1 winds (left), and southerly 15 m s1 winds (right). The SINMOD model is used in the simulations (Slagstad, 1987). The numbers on the axes indicate gridpoints, with spacing of 4 km. wind show that the effect of wind dominates completely over the shelf area during windy periods (Ø. Knutsen, H. Svendsen and F. Nilsen, unpubl.). The direct contribution by tides to volume/heat/salt flux through Fram Strait is assumed to be negligible, since their average net energy flux over a tidal period is close to zero (Kasajima and Svendsen, 2002). How- ever, the dynamic response when tides interact with variable topography may influence phenomena on larger scales in the area, and may for instance generate shelf-edge upwelling on the East Greenland shelf (Kasajima and Svendsen, 2002). 190 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 6. Potential temperature in the West Spitsbergen Current, as mean annually values for 1960–2000, in the neighbourhood of 80N, 9E (modiﬁed from Pavlov and O’Dwyer, 2000).

Changes in the water mass properties can also be obtained from historical hydrographic data, at least in the WSC. Pavlov and O’Dwyer (2000) and Falk-Petersen et al. (2006) discussed the inter-annual changes of temperature and salinity in the core of the WSC during the last four decades. The maximum water temperature in summer (>5 C) was observed at a depth of 75 m in the 1960s, and decreased in the 1970s and 1980s (Fig. 6). A sharp increase of water temperature in the surface layer of Fram Strait started at the beginning of the 1990s, and the temperature reached 5.5–6.0 C by the end of the decade. The variability of the maximum water temperature in Fram Strait depends on the intensity of the WSC, which is mainly determined by barotropic factors (Fahrbach et al., 2001) connected to reorganisation of the atmospheric circulation. Dickson et al. (2000) reported that the inﬂow of the AW increases during the peri-

Fig. 7. Maximum temperature (red line) in Fram Strait, 1960–2000, and the NAO winter index (blue line) (Hurrell, 1995). H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 191 ods of strong, positive NAO; this is also conﬁrmed by Schlichtholz and Goszczko (2006). Minimum or maximum values of the NAO winter index (Hurrell, 1995) generally correspond to respective minimum or maximum values of temperature in Fram Strait (Fig. 7). In 1990, when the NAO index reached its highest value, the temperature continued to rise towards its maximum values (in 1998). Apart from meteorological reasons, a northward shift of the recirculation in the Greenland Sea (Fahrbach et al., 2001) can be one reason for this sharp increase of water temperatures in Fram Strait at the end of the 1990s.

3. Oceanographic conditions in Kongsfjorden

Kongsfjorden and Krossfjorden in West-Spitsbergen are open fjords, without sills, and therefore largely inﬂuenced by the processes on the adjacent shelf. The fjords share a common mouth to the adjacent shelf, where the water mass is a mixture of onshore transported warm and saline AW, the colder and fresher Arc- tic-type water on the shelf and freshwater (glacier melt, calving, precipitation). In Svendsen et al. (2002) the mixing product is named Transformed Atlantic Water (TAW), whereas the four other water masses represented in Kongsfjorden are Surface Water (SW), Intermediate Water (IW), Local Water (LW) and Winter Cooled Water (WCW) (Table 2, Fig. 8). The strength of the mechanisms behind the three main sources shifts seasonally, and accordingly also there are changes in the characteristics of shelf and fjord water. Changes are between a state of Atlantic dominance (warm and saline) and one of Arctic dominance (cold and fresh). In years with weak inﬂuence of Atlantic origin water (Fig. 8a) the zooplankton community is represented with

Table 2 Characteristics of water masses identiﬁed in Kongsfjorden (Svendsen et al., 2002) Water mass Acronym Salinity (psu) Temperature (C) Surface Water SW 28.0–34.4 Variable Intermediate Water IW 33.0–34.7 Variable Transformed Atlantic Water TAW >34.7 >1.0 Local Water LW >34.4 <1.0 Winter Cooled Water WCW >34.4 <0.5

Fig. 8. Distribution of water masses in Kongsfjorden, in years with (a) weak influence (data from July 22, 2000), and (b) strong influence (data from July 29 to 30, 2003) of Atlantic origin water. (Data courtesy of Institute of Oceanology, Marine Hydrodynamics Department.) 192 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 more abundant Arctic species, whereas in years of strong Atlantic influence (Fig. 8b) the community shifts toward boreal species (Kwasniewski et al., 2003). In the Arctic dominance period (autumn and winter), SW that represents a mixture of glacial melt water and fjord water formed during late spring and summer, mixes with Atlantic water types below producing IW. In autumn and winter, two other water masses are formed in the fjord; LW <1 C, produced by surface cooling and WCW, with temperatures close to freezing point, devel- oped by cooling and sea ice formation (Svendsen et al., 2002; Cottier et al., 2005). Both, the LW and WCW can persist in the deep basins and depressions in the fjord throughout the year. During the summer, the fjord can undergo an intense and rapid shift from an Arctic-water- to an Atlantic- water-dominated system. As a consequence of a gradual modification of the vertical stratification in the fjord through mixing with warmer and fresher SW, the front separating fjord and shelf water weakens (i.e. the strength of the geostrophic control weakens). This allows TAW to penetrate into the fjord (Cottier et al., 2005). Every summer between 1996 and 2005, TAW was observed in the fjord. In general, TAW is practically limited by a sill between the inner and outer basins in the fjord (Svendsen et al., 2002). The water movements in the fjord are characterised by two flow regimes, which for the greater part of the year is almost de-coupled by a pycnocline; i.e. an upper layer which is mainly driven by local forces (wind and freshwater) and the intermediate layer and deep water that are strongly influenced by processes in the adjacent shelf area (Svendsen et al., 2002; Cottier et al., 2005). Kongsfjorden is a wide fjord (varying between 4 and 10 km). In the period when it is stratified, the fjord width exceeds the baroclinic (internal) Rossby radius of deformation, causing the water movements to be strongly influenced by the Coriolis effect. A down-fjord flow of brackish water which, due to the Coriolis effect, is confined to the northern side of the outer basin, dominates the flow pattern in the upper layer of the fjord. It is a geostrophically controlled flow. The flow is intensified during periods with down-fjord winds. Cyclonic eddies with diameters comparable to the width of the fjord may appear in the outer basin. Up-fjord winds, which are less common, cause up-fjord surface currents and stacking up of water at the head of the fjord that eventually turns the flow down-fjord when the up-fjord wind ceases or the down-fjord pressure gra- dient becomes strong enough to overcome the wind action. The down-fjord advection of brackish water is usually maintained throughout the whole tidal period, although the tide modifies the strength of the flow (Ingvaldsen et al., 2001; Svendsen et al., 2002). The areas near glacier fronts are extremely active with devel- opments of small-scale eddies and vortex filaments within 50–150 m off the glacier face. The direction of the temperature drop along the glaciers in the inner part of Kongsfjorden is clear evidence of the cyclonic circulation which dominates the flow pattern in the inner part of the fjord (Svendsen et al., 2002). The deeper flow regime in the fjord, below the pycnocline, is dominated by the intrusion of TAW in the Atlantic dominance season. As shown by both cross-fjord ADCP-sections and geostrophic currents computed from density fields, the intrusion is marked as a disturbance caused by a Kelvin wave, travelling around the fjord with the coast to the right (Svendsen et al., 2002). Both methods show an up-fjord flow about 4 km wide, comparable to the Rossby radius, along the southern coast, while the opposing down-fjord current occurs along the northern fjord side. There is little information about the circulation and exchange processes in the winter months. It is expected, however, that since the stratification in the fjord is very weak in this period the circulation is dominated by the local forces, among which prevailing down-fjord katabatic winds are dominant.

4. Sea ice conditions and ice ﬂux through Fram Strait

Arctic sea ice plays an important role in the climate system by acting as the boundary between the atmosphere and ocean, and it is modiﬁed in thickness and concentration by dynamic and thermodynamic processes. The ice concentration parameter gives information about the sea ice extent and sea ice edge. The extent of sea ice (i.e. the region with sea ice concentration >15%) is an important indicator in the global climate system. Considerable attention has been given to estimating the Arctic sea ice extent, including its regional, seasonal, decadal and inter-decadal variability detected through analysis of satellite passive microwave data since 1978 (e.g. Parkinson et al., 1999; Parkinson and Cavalieri, 2002; Cavalieri et al., 2003; Stroeve et al., 2005). For instance, Parkinson and Cavalieri (2002) show, over a 21-year period (1979–1999), that the average annual cycle of north polar ice extents ranges from a minimum of 6.9 · 106 km2 in September to a maximum of H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 193

15.3 · 106 km2 in March. For this time period, the ice cover as a whole shows a negative trend in the annual averages of 32,900 ± 6100 km2 yr1, indicating a 2.7 ± 0.5% reduction in sea ice coverage per decade. Based on a 30-year record, Cavalieri et al. (2003) reported that the Arctic sea ice extent decreased by 0.30 ± 0.03 · 106 km2 per decade from 1972 through 2002, and by 0.36 ± 0.05 · 106 km2 per decade from 1979 through 2002, indicating 20% acceleration in the rate of decrease. Satellite passive microwave observations show an overall downward trend in the Arctic sea ice extent and area since 1978. According to recent records (Serreze et al., 2003), Arctic sea ice extent and area in September 2002 reached their lowest levels recorded since 1978. Ice extent in September 2002 was 4% lower than any other previous September since 1978 and 14% lower than the long-term mean (1979–2000). In 2003 and 2004, new extreme September minima were registered (Stroeve et al., 2005). Ice thickness in the Arctic Ocean and the surrounding seas is also an important indicator of climate changes in the Arctic. It integrates many thermodynamic parameters in the atmosphere and ocean. The different types of sea ice can be divided into two basic components, first-year ice (FYI) and multi-year ice (MYI), which includes second-year and older ice. First-year ice represents the ice growth of a single winter, and it comprises up to 40% of the Arctic Ocean’s ice cover (Rothrock and Thomas, 1990; Romanov, 1995). The growth and melt of FYI in the marginal seas is primarily responsible for the large seasonal variability in total ice extent. Away from coastal regions, about 60% of the ice cover is represented by second-year ice, which has survived a summer melt season, and MYI that has survived at least one melt season. Multi-year ice is typically 3–5 m thick, whereas FYI rarely exceeds 2 m (Barry et al., 1993). Information about the spatial–temporal distribution of ice thickness is limited. While the ice extent and ice motion can be obtained from satellite imagery and buoy drifts, the ice thickness must be monitored in situ or from numerical models. The Fram Strait is of great importance to the climatology of the Arctic, because it handles 90% of the heat exchange and 75% of the mass exchange between the Arctic Ocean and the rest of the world oceans (Wad- hams, 1983). Roughly 10% of the total sea ice mass and approximately 20% of the total ice covered area in the Arctic Basin is exported annually through Fram Strait (Barry et al., 1993; Kwok et al., 2004). Thus, Fram Strait is the key area for estimating the net production of ice in the Arctic Ocean because the majority of the ice leaving the Arctic Ocean passes through. This net production of ice also represents the major input of freshwater to the Greenland, Norwegian and Icelandic seas (Vinje and Finneka˚sa, 1986; Barry et al., 1993; Vinje et al., 1998; Kwok et al., 2004). At present there are three main sources of knowledge about sea ice conditions in Fram Strait: (1) field programs (drift stations, mooring stations and drifting buoys); (2) nearly 30- years of satellite records from optical, infrared and passive microwave sensors; and (3) modelling. Based on these studies we can monitor the variability in ice extent, ice area and volume fluxes, and ice thickness, which are important for our understanding of seasonal and inter-annual changes of sea ice conditions in Fram Strait. Features of the sea ice cover, ice edge configuration and ice concentration in Fram Strait are determined by the warm WSC and cold EGC. The intensive heat flux from ocean to atmosphere on the east side of the strait prevents ice formation during winter and promotes the melting of ice, arriving from the Arctic. Therefore, in the course of year, the ice concentration on the east side of Fram Strait is much lower than on the western side, and in the summer months the east side of the strait is practically ice free (Fig. 9). The most intensive contrac- tion of ice extent begins in June, and by August the ice concentration in Fram Strait reaches its minimum (Fig. 9; Falk-Petersen et al., 2000b). From Svalbard to Greenland, at 80N, the ice concentration does not exceed 50% in this period. In November, ice formation starts rapidly and the ice edge becomes similar to winter configuration, with maximum ice extent in April. Approximately 80% of the ice area exiting annually through Fram Strait consists of MYI floes, 2–3 m thick (Gow and Tucker, 1987). The probable travel times for ice from different parts of the Arctic exiting Fram Strait have been estimated by Pavlov et al. (2004). Probability of sea ice export from the area just north of Fram Strait is the highest (88–90%), with travel time not exceeding one year. Sea ice drifting from shelf areas of the Kara and Laptev seas and from the western part of the East Siberian Sea has rather short travel times (3–6 year on average), whereas sea ice from the Beaufort Sea region will reach Fram Strait in 6–7 years, on average. The maximum travel time (10–16 yr) for sea ice to reach the region of Fram Strait, with rather low probabilities (38–65%), apply to ice starting from the areas to the north of the Canadian archipelago. The first successful, year-long ice thickness series was obtained in the East Greenland ice drift at 75Nin 1987–1988 using Upward Looking Sonar (ULS) (Vinje, 1989). The first deployment in Fram Strait was made 194 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 9. Monthly mean ice concentration (April, June, August, November) in Fram Strait, averaged over the period 1978–2000. Coloured scale indicates ice concentrations in 10ths (e.g. 0.9 = 9/10). Sea ice data are derived from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave data. (October 1978–December 2000.) in 1988, but it is only since 1990 that annual series have been obtained regularly, using an improved ULS. The mean ice thickness in the central core of the ice stream for the period 1990–1996 (Fig. 10) shows an annual cycle with amplitude of 1 m, a minimum in September (2.25 m) and a maximum in April–May (3.25 m). The mean thickness of ice (open water excluded) is 3.27 m (Vinje et al., 1998). The ice flux through Fram Strait is important with regard to the balance of ice mass and ocean fresh water, but also because of its associated biomass of organisms. Based on the most recent satellite ice motion observations, the annual cycle and inter-annual variability of the sea ice flux through the strait (81N) have been obtained for the period 1978–2002 (Kwok and Rothrock, 1999; Kwok et al., 2004). Maximum mean monthly areal flux in the winter time (October–May) was observed in December (107,000 km2) and March (108,000 km2), with minimum in May (61,000 km2) over the 24-year period (Fig. 11). The summer months contribute approximately 15% of the ice area to the annual areal export through the strait. The same characteristics

Fig. 10. Monthly mean ice thickness obtained around 79N in Fram Strait (after Vinje et al., 1998). H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 195

Fig. 11. Mean monthly ice area flux through Fram Strait at 81N over the winter (October–May). Solid line results from Kwok et al. (2004) for 1978–2002; broken line results from Pavlov et al. (2004) for 1966–2000. of month-to-month variability in the winter were simulated by Pavlov et al. (2004) using the Statistical Sea Ice Model (ISMO), which established statistical linkages between ice drift and ice concentration, and sea level atmospheric pressure and sea surface temperature from the NCEP/NCAR data set (Fig. 11). The mean annual ice area flux through the strait (81N) over the period 1978–2002 has been estimated to 866,000 km2 yr1 (Table 3; Kwok et al., 2004). The mean winter (October–May) areal sea ice flux over this period is about 754,000 km2 yr1. The winter areal flux ranges from a minimum of 607,000 km2 in 1990– 1991 to a maximum of 952,000 km2 in 1994/1995 (Fig. 12). According to modelling results of Pavlov et al. (2004), the mean annual ice areal flux through the strait (81N) over the 35-year simulated record (1966– 2000) is 639,000 km2 yr1 (Table 3). The mean winter (October–May) areal sea ice flux over this period is 569,000 km2 yr1. The winter area flux ranges from a minimum of 364,000 km2 in 1983/1984 to a maximum of 846,000 km2 in 1994/1995. The correlation (R = 0.71) between the calculated and observed (Kwok and Rothrock, 1999) time series of the monthly area flux is significant. Maximum mean monthly areal flux in the winter time is during December and the minimum in May over the 35-year period. Both observational data and modelling results show a positive trend in the sea ice areal flux in Fram Strait (Fig. 12). Based on submarine-borne upward-looking sonar (ULS) observations in Fram Strait in April–May 1979, Wadhams (1983) calculated a volume flux of 6200 km3 per year (0.29 Sv). This value is much higher than more

Table 3 Estimates of mean sea ice area and volume fluxes through Fram Strait at 79–81N for different periods Method Period (years) Area flux Volume flux Author(s) (103 km2 yr1) (km3 yr1) SSMR and SSM/I data 1978–2002 866 (754) 2218 Kwok et al. (2004) (October–May) ISMO 1966–2000 639 (569) Pavlov et al. (2004) (October–May) IABP observation 1990–1996 1100 2846 Vinje et al. (1998) SSMR data, buoys, ice 1979–1985 830 1900 Thomas and Rothrock (1993) and model Thomas et al. (1996) Observation/ 3100 Aagaard and Carmack (1989) oceanographic budget Oceanographic budget 2450 Rudels (1989) The data are derived from Scanning Multichannel Microwave Radiometer (SSMR), Special Sensor Microwave Imager (SSM/I), Statistical Sea Ice Model (ISMO), and International Arctic Buoy Program (IABP). 196 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 12. The mean winter (October–May) ice area ﬂux through Fram Strait at 81N. Solid line is results from Kwok et al. (2004) for 1978– 2002; broken line is results from Pavlov et al. (2004) for 1966–2000.

4750 4500 4250 4000 ) 3 3750 3500 3250 3000 2750 2500 2250 2000 Ice Volume Flux (km 1750 1500 90-91 91-92 92-93 93-94 94-95 95-96 96-97 97-98 98-99 Year

Fig. 13. Annual ice volume flux through Fram Strait at 79N. Solid line represents results of Vinje et al. (1998) for the period 1990–1996; broken line represents results of Kwok et al. (2004) for 1991–1999. recent estimates. One reason for the difference was overestimated ice thickness (4.06 m). Vinje et al. (1998) estimated a mean ice volume flux (79N) of 2846 km3 yr1,or0.09 Sv, for the period August 1990 to July 1996, but there is a strong year-to-year variation up to about 130% (Fig. 13). A relatively high transport of 4687 km3 yr1 was estimated during the period August 1994 to July 1995, while the value for August 1990–July 1991 was only 2046 km3 yr1. These estimates are based on velocities derived from satellite images for 1993– 1995, buoy velocities for 1976–1994, and ice maps and ULS ice thickness measurements for 1990–1996. With thickness estimates from ULS moorings (Vinje et al., 1998), Kwok et al. (2004) calculated the mean annual ice volume flux (79N) as 2218 km3 yr1 or 0.07 Sv. The volume flux ranged from a minimum of 1915 km3 yr1 (0.06 Sv) in 1995/1996 to a maximum of 3364 km3 yr1 (0.11 Sv) in 1994/1995 (Fig. 13). The winter (DJFM) volume flux accounts for 50% of the annual ice volume (Kwok et al., 2004). Thus, the main change in ice conditions in Fram Strait is the increase of ice flux from the Arctic Ocean in the previous two decades due to inten- sification of the EGC. The main contribution to the annual sea ice flux is during the winter months.

5. Ice conditions in Kongsfjorden

Data on sea ice extent in Kongsfjorden indicate that the break-up of the fast ice cover occurs between April and July, although there are high inter-annual variations of the position of the ice edge and the timing of melting and break-up (Svendsen et al., 2002). Wind, waves and tides create a highly variable ice situation in the H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 197 middle part of the fjord during spring. Large waves entering the fjord from the west break up the ice cover, and subsequently easterly winds will efficiently remove large amounts of ice in Kongsfjorden. The inner part of the fjord usually remains covered by fast ice until at least May. Some years (e.g. 2001), the sea ice formation starts very late and only the inner part of the fjord becomes covered with ice. In such years, most of Kongs- fjorden may remain open throughout winter and spring. In the unusually warm 2006, there was no ice formation in Kongsfjorden, except for a small area behind the islands in the northern part of the inner bay. The dominant ice type in Kongsfjorden in winter is young ice, in early spring it is first-year fast ice, and after the onset of melting it is a combination of fast ice and drift ice (Gerland et al., 1999). In addition, icebergs and ice pieces from surrounding glaciers can be found frozen into the fast ice or drifting in open water. The latter is a typical situation once all fast ice disappears in summer. The ice thickness at the end of the freezing season is generally 60–70 cm, and the snow layer on top of the ice about 20 cm thick at the onset of the melt. The ice consists of a granular ice layer near the surface (ca. 11 cm) and columnar ice below a 2-cm thick transitional zone. The latter exhibits very long, regular crystals, indicating undisturbed ice growth (Gerland et al., 1999). Early in winter, a large portion of the fjord surface is often covered with young ice, which immediately affects the energy exchange between the atmosphere and ocean. It first forms in the inner part of the fjord during cold and calm weather periods. Specific meteorological conditions combined with tidal currents can lead to large amounts of drift ice in the fjord. Floes typically vary in size between 1 and 10 m, and they originate from either fast ice in Kongsfjorden or Krossfjorden, or from sea ice transported in from Fram Strait.

6. Phytoplankton and primary production

Phytoplankton studies in Fram Strait have mostly been limited to the Greenland Sea (Smith, 1993; Rey et al., 2000; Richardson et al., 2005), including the NE Greenland polynya (e.g. Spies, 1987). In the central Greenland Sea, phytoplankton growth may start in early May and reach a peak in early June (Rey et al., 2000). Diatoms dominate the phytoplankton community during the spring bloom as well as in PW during August (Richardson et al., 2005), although flagellates and dinoflagellates are also common, and blooms of Phaeocystis pouchetii have been recorded near the ice edge in June as well as in open water (Rey et al., 2000; Richardson et al., 2005). The Phaeocystis blooms may also be very intense during spring, April–early May (Smith et al., 1991; Bauerfeind et al., 1994), and some years even the spring bloom may be dominated by this organism (Smith et al., 1991). Later, during the summer and autumn, the plankton community in the Greenland Sea may be dominated by Chaetoceros spp. in the surface layer and Nitzschia spp. and Thalass- iosira spp. deeper (Spies, 1987; Bauerfeind et al., 1994). Nutrient minima are generally correlated with high biomass concentrations in stratified surface waters, but elevated chlorophyll-a values can be associated with abundant nutrients at frontal boundaries given resupply of nitrogen by mixing processes (Spies et al., 1988). The daily production may be relatively high near the polar front, with values >9 g C m2 d1 in June (Legendre et al., 1993). The bacterial biomass develops during the bloom and may reach high cell counts (106 cells cm3) during the peak of the bloom (Noji et al., 1999). The new production in the central Greenland Sea is in the range of 50–60 g C m2 from May to August (Noji et al., 1999; Rey et al., 2000). Diatoms contribute about 25% to new production during spring and 50% on an annual basis (Bauerfeind et al., 1994). The annual production has been estimated to be about 80 g C m2 yr1 for the open Greenland Sea (Richardson et al., 2005). Modelling results support these findings, with an average annual primary production of 68 g C m2 yr1, of which 45 g C m2 yr1 is new production (Slagstad et al., 1999). However, in the eastern part of the Greenland Sea, the average annual new production is >50 g C m2 yr1 (Slagstad et al., 1999). The phytoplankton in Kongsfjorden has been subjected to several studies. A total of 148 phytoplankton taxa have been recorded from Kongsfjorden to date; of these 67 belong to the Bacillariophyta and 46 to the Dino- phyta (Hasle and Heimdal, 1998; Keck et al., 1999; Eilertsen et al., 1989; Wiktor, 1999). Most of the species are of Atlantic and cosmopolitan origin, whereas only 31 (21%) are considered to be Arctic or boreal-Arctic species (Hasle and von Quillfeldt, 1996). The community is most diverse during summer, with 40 taxa listed for July in Okolodkov et al. (2000), although at least 130 taxa have been recorded (Hop et al., 2002b). The spring bloom starts in March–April, peaking in May (Eilertsen et al., 1989; Wiktor, 1999), although blooms may also occur irregularly throughout the summer. The biomass tends to be concentrated in the upper mixed layer and nutrients in this layer become reduced to about half of the winter values (Svendsen et al., 2002). Growth conditions 198 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 deteriorate during summer because of heavy inputs of sediment-loaded glacial water (Svendsen et al., 2002). In the inner part of the fjord, the algae become light limited, whereas grazing tends to decrease algal biomass later in the summer (Eilertsen et al., 1989). The rapidly decreasing day length terminates the growth season; no growth occurs once the polar night starts (October 25). The variability in phytoplankton abundance is reflected in the production rates. The daily production in July ranges between 0.024 and 1.4 g C m2 d1 (Hop et al., 2002b). The annual production is related to open water periods in Arctic and sub-Arctic locations (Rysgaard et al., 1999). The annual primary production in Kongsfjorden is mostly likely about 35–50 g C m2 yr1 (Hop et al., 2002b), which is comparable with production estimates from the northern Barents Sea (Sakshaug et al., 1994; Hegseth, 1998). However, higher estimates, in the range of 120–180 g C m2 yr1, have also been made (listed in Hop et al., 2002b) that are comparable to previous estimates for Spitsbergen fjords (Eilertsen et al., 1989), the Barents Sea (Sakshaug et al., 1994) and fjords in northern Norway (Eilertsen and Taasen, 1984). The Greenland Sea production at 80 g C m2 yr1 is intermediate to these estimates.

7. Zooplankton in the Fram Strait–Kongsfjorden region

7.1. Zooplankton diversity

The summary of zooplankton presented here is biased because of lack of coverage of all faunistic groups in all habitats; the comparisons are made within these limitations. In particular, the summary lacks a detailed account of heterotrophic picoplankton (<5 lm) and nanoplankton (5–20 lm) as well as heterotrophic microplankton (20–200 lm). Recent studies indicate that organisms constituting the above mentioned size categories play important, presumably often crucial, roles in biological sequestering of carbon in high latitudes (Paranjape, 1987; Hansen et al., 1996; Levinsen and Nielsen, 2002; Wassmann, 2002; Sherr et al., 2003; Verity et al., 2002; Pedersen et al., 2005; Møller et al., 2006), similar to the roles they play in other parts of the oceans (e.g. Pomeroy, 1974; Azam et al., 1983; Longhurst and Harrison, 1989; Lenz, 1992; Verity and Smetacek, 1996). The growing amount of data leaves no doubt that also in high latitudes the classical food web (with Calanus as the pivotal grazer) functions only in unison with the microbial food web, in which pico-, nano- and microplankton play key roles. The data suggest, unequivocally, that organisms constituting the classical food web are able to channel a considerable fraction of primary production during a short time if meso- and macrozooplankton grazers match their development to the early part of the Arctic phytoplankton bloom, characterised by diatoms. On an annual basis, though, it is believed that organisms in the classical food web are not able to utilise more than 30% of the total primary production. The remaining 70% of high latitude pelagic primary production is recycled and/or made available for a carbon pathway involving small phytoplankton and small grazers, i.e. pico-, nano- and microzooplankton. Adopting these findings must result in re-evaluation of the paradigms of the functioning of pelagic ecosystems and carbon-cycle. However, a more complete understanding of the biological dynamics of both food webs and, thus, a more holistic approach to studies of ecosystem function are still ahead. Faunistic information concerning the key consumers in the pelagic food webs, normally does not include identification of the smallest sized zooplankton below the taxonomical/size categories of picoflagellates, nano- flagellates, choanoflagellates and ciliates. Representatives of microprotozooplankton are sometimes identified to species/genus level. They encompass dinoflagellates, such as various species of Amphidinium, Gyrodinium,or Protoperidinium, tintinnid ciliates e.g. Parafavella denticulata, Ptychocylis obtusa, Leprotintinnus pellucidus, Acanthostomella norvegica as well as other ciliates e.g. Balanion, Didinium, Laboea strobila, Lohmaniella, Scu- ticutiliates, Strombidium and Strobilidium (Paranjape, 1987; Boltovskoy et al., 1991; Hansen et al., 1996; Sherr et al., 1997; Levinsen et al., 1999; Rysgaard et al., 1999; Sherr et al., 2003; Møller et al., 2006). Other unicel- lular microzooplankton include foraminiferans and radiolarians, taxa that are often omitted or treated with less systematic accuracy in studies of pelagic ecosystems due to sampling biases and identification problems. They are persistent in sediments and important in paleo-oceanographic studies (Be, 1967; Kellog, 1976; Ratk- ova and Wassmann, 2002; Cortese et al., 2003; Risebrobakken et al., 2003; www.radiolaria.org). Among the most typical foraminiferan species in high-latitude waters there are: Neogloboquadrina pachyderma, Globige- rina bulloides, Globigerina quinqueloba, Globigerinita glutinata and Globorotalia inflata. Examples of radiolar- H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 199 ians common in northern pelagic ecosystems include: Actinomma boreale, Actinomma leptoderma, Spongotro- chus glacialis, Amphimelissa setosa, Lithomelissa setosa and Plectacantha oikiskos. Analogous underestimates exist regarding faunistic diversity and role in the pelagic ecosystem of high latitudes of multi-cellular microzooplankton. The most common taxa in this category include, first of all, minute copepods and their developmental stages, exemplified by O. similis, several species of Oncaea (O. englishi, O. parila = O. notopus), Triconia borealis or Microsetella norvegica, as well as Rotifera (Synchaeta, Keratella) and some planktonic larvae of benthic organisms (Heron et al., 1984; Hansen et al., 1994; Gallienne and Robins, 2001; Arashkevich et al., 2002). This size category may also include juvenile stages of mesozooplankton taxa. Comparisons of abundance of small taxa obtained from different sampling tools (nets vs. pumps or water bottles) show that a typical zooplankton net of 0.180 mm mesh size samples only 10% of the small copepods (exemplified by 0.800 mm length and 0.270 mm width of Oithona). As a result it is suggested that biomass of the small sized metazoan zooplankton may be underestimated by one-third and production by two-thirds. The current presentation mainly summarises the classical food webs characteristic for Fram Strait and Kongsfjorden. The review of published information (e.g. Smith et al., 1985; Smith, 1988; Diel, 1991; Long- hurst, 1998; Werner et al., 1999; Walkusz et al., 2003; Broms et al., 2004), supplemented with our own observations, yields a total of 83 taxa (including 69 species and genera) of both holo- and mero-zooplankton recorded in the pelagic zone of the Fram Strait area (Table 4). The review for Kongsfjorden yields a total of 84 taxa or 64 species (Weslawski et al., 1991; Hop et al., 2002b; Kwasniewski et al., 2003; Schulz and Kwas- niewski, 2004; Walkusz et al., 2004). In Fram Strait, approximately 33% of the 253 taxa (species, genera and higher) known from the pelagic zone of the Nordic Seas and the Arctic Mediterranean have been encountered (Wiborg, 1954; Wiborg, 1955; Østvedt, 1955; Grainger, 1965; Brodskii et al., 1983; Groendahl and Hernroth, 1986; Grainger, 1989; Mumm, 1993; Richter, 1994; Mumm et al., 1998; Auel and Hagen, 2002). The list of taxa known from Kongsfjorden includes nearly 75% of the 110 taxa ever recorded on various locations within the Svalbard archipelago (Stott, 1936; Digby, 1961; Koszteyn and Kwasniewski, 1989; Kwasniewski, 1990; Weslawski et al., 1990; Karnovsky et al., 2003; Prestrud et al., 2004). Typically for polar latitudes, zooplankton in Fram Strait and Kongsfjorden is predominated by calanoid copepods (24 vs. 25 species, respectively). Other crustaceans relatively rich in species are Amphipoda (10 vs. 6) and Euphausiacea (3 vs. 4). Of the 103 taxa recorded in both regions there are 64 that are in common and 19 and 20 found exclusively in Fram Strait or Kongsfjorden, respectively. The key zooplankton components of both areas are, first of all, Copepoda such as three Calanus species (C. finmarchicus, C. glacialis, and C. hyperboreus), Metridia longa, Pseudocalanus (P. minutus and P. acuspes), Microcalanus (M. pusillus and M. pygmaeus) and Oithona similis. Of other taxa, the most important species appear to be the amphipods Them- isto libellula and T. abyssorum, the euphausiids Thysanoessa inermis and T. longicaudata, the pteropods Lima- cina helicina and Clione limacina, the ctenophores Mertensia ovum and Beroe¨ cucumis, and the chaetognaths S. elegans and Eukrohnia hamata. The Fram Strait zooplankton fauna differs by the presence of meso- and bathypelagic copepods such as Augaptilus glacialis, Heterorhabdus compactus, Scaphocalanus brevicornis,or ostracods Boroecia borealis and B. maxima. The Kongsfjorden zooplankton fauna includes, for example, hyperbenthic copepods Bradyidius similis, Mesaiokeras spitsbergensis, Xantharus siedleckii, Neoscolecithrix farrani and the neritic euphausiid Thysanoessa raschii. Intense research in the past few years in Kongsfjorden has shown that even in such a relatively well studied place there are still undiscovered species in the pelagic (Schulz and Kwasniewski, 2004) as well as in the benthic realms (Kuklinski and Hayward, 2004), and new species may invade due to climate warming.

7.2. Zooplankton abundance

The majority of information on zooplankton abundance and biomass in Fram Strait regards the large herbivorous calanoid copepods C. finmarchicus, C. hyperboreus and C. glacialis, and considers usually the epipe- lagic (<200 m) layer during the summer season (June–August). Data covering the entire extent of the oceanic pelagic environment and other seasons are scarce. Available references indicate that the maximum abundance of C. finmarchicus is 50 ind. m3 (van Aken et al., 1991) to 180 ind. m3 (Smith et al., 1985), although it may be as high as 210 ind. m3 (Smith, 1988) (stages combined, values approximate, recalculated based on the fig- ures in the referred papers). Abundance of C. hyperboreus is 10 ind. m3 (van Aken et al., 1991) to 22 ind. m3 200 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Table 4 List of zooplankton encountered in Fram Strait and Kongsfjorden Species/genus or taxon Fram Strait Kongsfjorden Reference Augaptilus glacialis +1 Heterorhabdus compactus +1 Heterorhabdus norvegicus + + 10; 3 Metridia longa + + 8; 13 Metridia lucens + + 4; 3 Pleuromamma robusta +4 Acartia longiremis + + 10; 13 Limnocalanus macrurus Own obs. Temora longicornis Own obs. Calanus ﬁnmarchicus + + 9; 13 Calanus glacialis + + 9; 3 Calanus hyperboreus + + 9; 13 Rhincalanus nasutus + + 10; 11 Aetideus armatus +3 Bradyidius similis +3 Chiridius obtusifrons + + 10; 11 Gaetanus (=Gaidius) brevispinus Own obs. + 11 Gaetanus (=Gaidius) tenuispinus + + 10; 3 Microcalanus pygmaeus + + 14; 13 Microcalanus pusillus + + 14; 3 Pseudocalanus acuspes + + 1; 13 Pseudocalanus minutus + + i;3 Paraeuchaeta glacialis + + 5; 11 Paraeuchaeta norvegica + + 5; 13 Mesaiokeras spitsbergensis +7 Scaphocalanus brevicornis +1 Scaphocalanus magnus (=Amallophora magna) + + 10; 11 Scolecithricella minor + + 10; 13 Xantharus siedleckii +7 Neoscolecihrix farrani (=Oothrix borealis=O. bidentata)+13 Oithona atlantica (=O. spinirostris=O. plumifera) + + 14; 13 Oithona similis (=O. helgolandica) + + 14; 13 Triconia (=Oncaea) borealis + + 14; 13 Oncaea spp. + 8 Cyclopoida indet. +3 Microsetella norvegica (=M. atlantica) + + 14; 11 Harpacticoida indet. + + 10; 3 Mormonilla polaris +1 Monstrilloida indet. +3 Evadne nordmanni Own obs. Own obs. Podon leuckarti Own obs. Own obs. Boroecia (=Conchoecia) borealis +5 Boroecia (=Conchoecia) maxima +1 Discoconchoecia (=Conchoecia) elegans + Own obs. 5 Ostracoda indet. + + 10; 13 Cirripedia Balanida nauplii and cypris + + 5; 13 Boreomysis arctica Own obs. + 3 Mysis oculata + + 10; 13 Pseudomma truncatum + Own obs. 10; 6 Mysidacea indet. +2 Hyperia galba + + i;3 Hyperoche medusarum +3 Themisto (=Parathemisto) libellula + + 5; 13 Themisto (=Parathemisto) abyssorum + + 5; 3 Scina borealis +1 Apherusa glacialis + + 10; 3 Cyphocaris bouvieri +1 Eusirus holmi +1 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 201

Table 4 (continued) Species/genus or taxon Fram Strait Kongsfjorden Reference Gammarus wilkitzkii + + 12; 3 Onisimus (=Pseudalibrotus) glacialis +10 Onisimus (=Pseudalibrotus) nanseni +10 Gammaridea indet. + 11 Isopoda Bopyridae indet. + + 10; 11 Tanaidacea indet. +11 Leucon sp. + Own obs. 1 Cumacea indet. +11 Meganyctiphanes norvegica Own obs. + 3 Thysanoessa inermis + + 5; 3 Thysanoessa longicaudata + + 5; 3 Thysanoessa raschii +3 Hymenodora glacialis +1 Pandalus borealis larvae + Own obs. 10 Sabinea septemcarinata zoea + 3 Decapoda larvae Own obs. + 13 Aglantha digitale + + 5; 3 Botrynema ellinorae +1 Catablema vesicarium +11 Halitholus cirratus +11 Homoeonema platygonon +1 Sarsia princeps +3 Sarsia tubulosa +2 Sarsia sp. + 10 Hydrozoa medusae indet. + + 10; 13 Dimophyes arctica (=Diphyes arctica)+ + 5;3 Siphonophora indet. + 1 Beroe¨ cucumis + + 5; 3 Mertensia ovum + + 5; 3 Nematoda indet. +11 Clione limacina + + 5; 13 Limacina (=Spiratella) helicina + + 5; 3 Limacina retroversa + + 10; 11 Bivalvia larvae + + 10; 13 Gastropoda larvae (not Clione or Limacina) Own obs. Own obs. Polychaeta larvae indet. + + 10; 13 Polychaeta indet. + Own obs. 5 Echinodermata larvae + + 10; 11 Bryozoa cyphonautes larvae + + 1; 11 Eukrohnia hamata + + 5; 13 Sagitta elegans + + 5; 13 Fritillaria borealis (F. b. acuta) + + 5; 13 Oikopleura vanhoeﬀeni + + 5; 11 Oikopleura spp. + + 10; 13 Fish larvae Own obs. + 13 References selected as the primary source of faunistic information for studies in Fram Strait and Kongsfjorden: 1. Broms et al. (2004);2. Digby (1961);3.Hop et al. (2002b);4.Longhurst (1998);5.Mumm (1993);6.Prestrud et al. (2004);7.Schulz and Kwasniewski (2004);8. Smith (1988);9.Smith et al. (1985); 10. Walkusz et al. (2003); 11. Walkusz et al. (2004); 12. Werner et al. (1999); 13. Weslawski et al. (1991); 14. Wiborg (1955).

(Smith, 1988). Abundance of C. glacialis, usually considered the least abundant Calanus sp. in the oceanic environment of Fram Strait, is 10 ind. m3 (Smith, 1988) to 40 ind. m3 (Smith et al., 1985). However, lower abundance values of Calanus species in the area of Fram Strait are reported by Mumm et al. (1998), as 85, 4 and 9 ind. m3, for C. ﬁnmarchicus, C. hyperboreus and C. glacialis, respectively. All studies indicate close association of individual species with their original water masses. The boreal C. ﬁnmarchicus is associated with AW of the WSC. The two other species are associated with cold Arctic water masses, i.e. PW of the Arctic 202 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Ocean and ArW of the Greenland Sea, although C. glacialis is regarded a shelf species, whereas C. hyperboreus is an oceanic one inhabiting primarily deep water sea basins (Smith et al., 1985; Hirche et al., 1991; Falk-Pet- ersen et al., 2006). Thus, sea depth is another factor that strongly influences Calanus species distribution and abundance. There are a few other zooplankton species for which different accounts of abundance from Fram Strait area are available. Metridia longa, another large, cold-water, omnivorous copepod, attains abundance of 11– 19 ind. m3 (Smith, 1988; van Aken et al., 1991; Mumm et al., 1998). Small copepods are generally more abundant, such as Pseudocalanus with 20–27 ind. m3 (Smith et al., 1985; Smith, 1988) and Triconia (=Oncaea) borealis with 23 ind. m3 (Smith, 1988). The most numerous zooplankton component in this area is the ubiq- uitous Oithona similis, although published information on the abundance and biomass of this species from the Fram Strait area is vague. According to Auel and Hagen (2002), O. similis was the predominant zooplankton component in the Arctic Ocean, with up to 254 ind. m3 in the upper 50 m, whereas Walkusz et al. (2003) observed this species north of Fram Strait in densities up to 695 ind. m3 in the upper 300 m. In Kongsfjorden during summer (July), where the zooplankton was sampled at five reference stations (Fig. 1) with a Multiple Plankton Sampler (MPS, 0.180 mm mesh) during 1996–2002, the mean water column abundance of C. finmarchicus varied from 50 to 600 ind. m3 (Hop et al., 2002b; S. Kwasniewski, unpubl. data). The abundance of C. glacialis with 20–330 ind. m3 was also higher than in Fram Strait. At the same time, the abundances of C. hyperboreus and M. longa varied over a much wider range, 2–110 ind. m3 and 1– 100 ind. m3, respectively. Similar accounts of smaller copepods suggest that the predominant zooplankton species also attained higher abundances in Kongsfjorden than in the open sea area of Fram Strait. The abundance ranges of Pseudocalanus, O. similis and Triconia borealis in Kongsfjorden were 35–1370 ind. m3, 75– 3930 ind. m3, and 3–180 ind. m3, respectively. The distribution along environmental gradients and less ordered natural variability (patchiness) are also important in glacial fjords such as Kongsfjorden. Some patterns in distribution have been revealed, such as evident limitation of occurrence of Metridia longa to deeper parts of the fjord (Hop et al., 2002b), while Cir- ripedia nauplii seem to have a very patchy occurrence during spring with up to 3900 ind. m3 observed in one patch and <400 ind. m3 in the surrounding waters, at one of the outer stations in Kongsfjorden (S. Kwas- niewski, unpubl. data).

7.3. Zooplankton biomass

Hitherto, studies have revealed that zooplankton biomass (dry mass) in Fram Strait may vary from 0.02 to 13 g DM m2 (Smith et al., 1985; Smith, 1988; Diel, 1991; Hirche, 1991; Hirche et al., 1991; Hirche and Kwas- niewski, 1997; Mumm et al., 1998; data recalculated to allow direct comparison). This estimate, however, includes various sources of biomass variability, both natural and methodological. Zooplankton biomass in Fram Strait varies with time or season, up to 10-fold (0.2–2.5 g DM m2) for early spring and summer on the northeast Greenland shelf (Hirche and Kwasniewski, 1997), or with depth; 2.1 ± 1.4 and 8.4 ± 2.2 g DM m2 (means ± SD) for the upper 100 m and 500 m water layers, respectively (Mumm et al., 1998). It also varies in relation to distribution of sea ice and water masses. In the areas predominated by AW, that is in the WSC around the MIZ, the biomass of the dominant C. finmarchicus ranges from 5.2 g DM m2 (Smith, 1988)to5.7gDMm2 (Hirche et al., 1991). In the areas predominated by PW from the Arctic Ocean or by ArW in the Greenland Sea, cold-water species form the main zooplankton biomass, with C. hyperboreus ranging up to 5.5 g DM m2 (Hirche et al., 1991) or 7.4 g DM m2 (Smith, 1988). As the Fram Strait proper is not the typical habitat for C. glacialis, the maximum biomass of the species found there is only 1.1 g DM m2 (Diel, 1991) to 2.8 g DM m2 (Smith et al., 1985). The habitat preferences of typical zooplankton components, coupled with complex hydrological regimes, result in different biomass distribution patterns across frontal zones in Fram Strait (Smith et al., 1985; Smith, 1988; Hirche et al., 1991). Additional natural causes of variability that may influence zooplankton biomass estimates relate to the physiological condition of zooplankton organisms (Smith, 1988; Diel, 1991) and to population age-structure of the species studied. Most of the zooplankton biomass data for Fram Strait regards only Calanus or Metridia longa, i.e. the mesozooplankton herbivore/omnivore component. Virtually nothing is known on biomass of small zooplankton in Fram Strait and the main source of crude data on biomass of H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 203 macrozooplankton or carnivorous taxa is Hirche et al. (1994). According to their measurements, macrozooplankton biomass in Fram Strait was 1.5 g DM m2 in ice free waters and 0.6–1.0 g DM m2 in the Northeast Water Polynya. Furthermore, they reckoned that macrozooplankton biomass was about one tenth that of mesozooplankton. The biomass of zooplankton in Kongsfjorden has been estimated from two summer seasons (1996, 1997) as 8.8 ± 5.1 g DM m2 (Hop et al., 2002b). Comparison between fjord’s basins revealed that the biomass was higher in the outer (11.5 ± 4.3 g DM m2) than in the inner (4.3 ± 2.2 g DM m2) fjord. The most likely cause of this difference is freshwater runoff with heavy sediment loads from tidal glaciers in the inner basin, but other factors, such as water depth and the presence of a sill near the inner bay limiting water exchange and advection of zooplankton, also affect the community composition and biomass (Basedow et al., 2004).

7.4. Calanus life history

Of particular importance for understanding the role of zooplankton in functioning of the ecosystem in Fram Strait and Kongsfjorden is the knowledge of biological dynamics of the zooplankton community. The basic issues in this respect concern zooplankton vertical distribution and seasonal dynamics, both of which are related to the life histories of zooplankton organisms (Falk-Petersen et al., 2006). There are no results from exactly the Fram Strait area, but in the neighbouring waters of the Greenland Sea or the Arctic Ocean the bulk mesozooplankton components, Calanus species, are distributed unevenly with depth and the distribution pattern is changing seasonally as a result of pronounced seasonal migrations (Richter, 1994, 1995; Auel and Hagen, 2002). Typically, populations of a Calanus species overwinter at great depths, reproduce in the spring time close to the surface and the offspring stays in the productive zone (epipelagial) until gaining sufficient energy resources for continuing or completing the life cycle. Each of the three Calanus species has its individual life cycle strategy (Conover, 1988; Smith, 1990; Diel, 1991; Hirche, 1991, 1997; Hirche and Kwasniewski, 1997) allowing for maximum utilisation of available resources, and the resulting differences in spatial and temporal distribution of the developmental stages may be one of the important constituents of the functional biological diversity of the ecosystem. In fjords of northern Norway and northwards, the boreal C. finmarchicus has a 1-year life cycle (Tande, 1982; Tande et al., 1985), and this has also been suggested for this species in Kongsfjorden (Scott et al., 2000). The spawning time of the species coincides with the period of maximum phytoplankton spring bloom (Tande, 1982; Tande et al., 1985). The Arctic shelf species C. glacialis has a 2-year life cycle within its original range (Tande et al., 1985; Eilertsen et al., 1989), with spawning taking place before or during the algal bloom (Smith, 1990; Hirche and Kwasniewski, 1997; Falk-Petersen et al., 1999; Kosobokova, 1999). It dominates among Calanus in waters north of the Polar Front around Svalbard, and also on the northeast Greenland shelf (Unstad and Tande, 1991; Hirche and Kwasniewski, 1997). The deep water Arctic species C. hyperboreus has a 3–5-year life cycle depending on the food availability (Scott et al., 2000; Falk-Petersen et al., 2006). It overwin- ters mainly as CIII to CV (Hirche, 1997; Scott et al., 2000) and spawns at in deep water, in October–March, prior to the spring bloom (Hirche and Niehoff, 1996). Its main centre of distribution is the deep Greenland Sea (Hirche, 1997), and it is considered an expatriate to the Kongsfjorden (Kwasniewski et al., 2003).

7.5. Other zooplankton groups

A few studies have described other zooplankton groups (Smith, 1988; Hirche et al., 1994; Mumm et al., 1998; Walkusz et al., 2003) and provided qualitative information regarding the role of these taxa in the pelagic ecosystem (Smith, 1988; Mumm et al., 1998). The most abundant zooplankton taxa in Kongsfjorden have been summarised (Hop et al., 2002b; Kwasniewski et al., 2003), and more detailed studies have been conducted on some species, such as the pteropods Limacina helicina and Clione limacina (Falk-Petersen et al., 2001; Boër et al., 2005; Gannefors et al., 2005), the ctenophores Mertensia ovum and Beroe¨ cucumis and (Falk-Petersen et al., 2002; Lundberg, 2006), and the hyperiid amphipod Themisto libellula (Dale et al., 2006). Still largely unresolved questions involve biological diversity and dynamics of gelatinous filtrators (Appendicularia) and gelatinous predators (Hydromedusae, Ctenophora), large size filtrators and predators capable of swimming and avoiding nets (Euphausiacea, Hyperiidea, Chaetognatha and fish larvae) and mero- 204 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 plankton components (larval stages of Cirripedia, Polychaeta, Bivalvia or Echinodermata). Parallel to studies concerning biological dynamics of key zooplankton components on a yearly/seasonal basis, it will be crucial to establish a long-term monitoring program that will approach climate change related questions of zooplankton dynamics in a way that considers short-term natural variability (e.g. Willis et al., 2006).

7.6. Zooplankton seasonality and inter-annual variability

The seasonal variations in the zooplankton community as well as of individual species are very pronounced in fjords in Svalbard. Different patterns of the dynamics are illustrated by comparing average water column abundance of selected taxa, calculated for five reference stations in Kongsfjorden, for three seasons represented by May, July and September 2002 (W. Walkusz unpubl. data). Pelagic nauplii stages of Cirripedia peaked in spring (380 ind. m3) but were hardly found in summer or autumn (1 ind. m3). The abundance of C. finmarchicus was low in spring (6 ind. m3), the highest during summer (170 ind. m3) and reduced in autumn (90 ind. m3). A third type of seasonal abundance changes was observed, for example, for O. similis, L. helicina or larvae of Bivalvia. Each of these taxa had the lowest abundance in spring and the highest in autumn. The most dynamic abundance changes were probably those recorded for O. similis with 90, 1820 and 3190 ind. m3, respectively for May, June and September. Slightly less dramatic changes were noticed for L. helicina (<1, 81 and 165 ind. m3, for the respective months) and larvae of Bivalvia (<1, 60 and 165 ind. m3). Indications of such a strong seasonal dynamics have been observed previously, but only for summer and autumn seasons (Hop et al., 2002b). Year-round observations of variations in abundance of Pseudocalanus and O. similis in Kongsfjorden (Lischka and Hagen, 2005) corroborate with our results and indicate that the small taxa reach their maxima in autumn (November). Some aspects of inter-annual variability in zooplankton in the vicinity of Fram Strait can be discussed based on the results of research conducted in Kongsfjorden. Year-to-year changes in composition of the zooplankton community and in the abundance of selected taxa are shown on a fjordwide basis, with data repre- senting mean zooplankton abundance at five reference stations sampled with MPS during July 1996–2002 (Hop et al., 2002b; S. Kwasniewski, unpubl. data). The bulk of the zooplankton in Kongsfjorden in summer consisted of Copepoda, with 92–64%, or 98–70% including nauplii (Table 5). Each year the four dominating copepod species, C. finmarchicus, C. glacialis, Pseudocalanus and O. similis, consistently made up 56–86% of the zooplankton (or 72–82% of the Copepoda). Other contributing taxa included larvae of Echinodermata, Fritillaria borealis, and larvae of Bivalvia. The inter-annual changes in relative composition of the zooplankton community mainly reflected changes in abundance of numerically predominant taxa. The most striking abundance change was the increase in abundance observed for Oithona similis, but similar increasing trends were observed for a few other taxa as well (Fig. 14).

Table 5 Relative abundance (%) and inter-annual variability of zooplankton taxa in Kongsfjorden in summer (July) Year/taxon 1996 1997 1999 2000 2001 2002 Calanus ﬁnmarchicus 25.5 22.4 13.9 14.5 12.4 4.5 Calanus glacialis 24.4 5.6 20.3 14.4 6.0 1.5 Pseudocalanus 22.3 13.2 13.2 22.2 11.3 7.6 Oithona similis 14.2 28.3 25.7 20.3 48.4 42.4 Microcalanus 1.9 4.3 8.0 2.6 1.0 1.3 Metridia longa 1.2 6.8 4.5 1.7 0.9 1.1 Calanus hyperboreus 0.6 1.5 0.9 0.6 1.5 0.4 Oithona atlantica 0.1 0.2 0.6 0.3 0.4 0.9 Triconia (=Oncaea borealis) 0.7 2.2 4.9 1.0 4.2 4.0 Other Copepoda 1.0 0.6 0.1 0.4 0.2 0.2 Copepoda nauplii 6.1 11.5 5.3 10.9 4.0 6.1 Bivalvia veliger 0.0 0.0 0.1 1.6 3.6 1.0 Limacina helicina 0.1 0.2 0.1 0.7 0.2 2.0 Echinodermata larvae 0.6 0.4 0.2 0.6 3.9 24.2 Fritillaria borealis 0.0 2.3 0.6 6.9 1.5 2.5 Other non-Copepoda 1.3 0.6 1.4 1.2 0.6 0.4 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 205

Fig. 14. Abundance (ind. m3) of mesozooplankton in Kongsfjorden (with Oithona similis speciﬁed).

Because the abundance of most of the predominant taxa showed an increasing tendency, the abundance of the entire zooplankton was increasing, especially in the years 2001–2002 (Fig. 14). Other taxa showing increasing abundance during the same period were Pseudocalanus, Triconia borealis and Oithona atlantica (Fig. 15). Higher abundance in the years 2000–2002 was also observed for F. borealis and L. helicina, but their abundances were more variable (Fig. 16). In contrast to the taxa described above, the abundances of C. finmarchicus and C. glacialis did not show an apparent change over the years (Fig. 17). In 1997, as well as in the latest two years, there was a shift in abundance pattern with high abundance of C. finmarchicus when C. glacialis had low abundance. The available data do not allow for accurate description of the Calanus species abundance oscillation, although as a first approximation the time between years of predominance of a given species may be 4–5 years. Thus, during the observation time there were two seasons with the advantage of C. finmarchicus in Kongsfjorden (1997 and 2001–2002) and one with the advantage of C. glacialis (1999) separated by intermediate phases (1996, possibly also 1998 and 2000). An oscillating pattern of abundance changes can also be observed for the Arctic T. libellula and the more boreal T. abyssorum, although only smaller size fractions of the respective populations are included due to the limitations of the sampling method (vertical hauls with 0.180 mm mesh size MPS) (Fig. 18). With the limitations of the data set in mind, it is interesting to notice that in the years of predominance of C. finmarchicus over C. glacialis, the advantage of T. libellula over T. abyssorum decreased (1997 and 2001) or the latter species even dominated (2002).

Fig. 15. Abundance (ind. m3)ofOithona atlantica, Triconia borealis and Pseudocalanus in Kongsfjorden. 206 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 16. Abundance (ind. m3)ofLimacina helicina and Fritillaria borealis in Kongsfjorden.

Fig. 17. Abundance (ind. m3)ofCalanus ﬁnmarchicus and C. glacialis in Kongsfjorden.

Fig. 18. Abundance (ind. m3)ofThemisto abyssorum and T. libellula in Kongsfjorden. H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 207

The notion of importance of advection for Calanus (and other zooplankton) distribution in Kongsfjorden was suggested by Hop et al. (2002b) and Kwasniewski et al. (2003), and further evaluated by Basedow et al. (2004) and Willis et al. (2006). Dependence on advection, and as a consequence of plankton distribution, on hydro-climatic forcing, has been advocated for Kongsfjorden by Svendsen et al. (2002) and further shown by Cottier et al. (2005). These findings are consistent with results of much earlier research documenting close con- nections and an integrated response of the ecosystem, zooplankton in particular, to ocean-atmosphere inter- actions on long time scales (e.g. Fromentin and Planque, 1966; Planque and Taylor, 1998; Greene and Pershing, 2000; Ottersen et al., 2001; Greene et al., 2003; Beaugrand and Reid, 2003; Beaugrand and Ibanez, 2004, and citations therein). Our time series of zooplankton data is by far too short and limited to allow for- mulating indisputable predictions regarding the entire community. However, we believe it suggests a plausible scenario of long-term (inter-decadal) changes in Kongsfjorden zooplankton. We speculate, therefore, that with the persistence of increasing Atlantic influence, the Kongsfjorden pelagic ecosystem will be subjected to reorganisation, including an increasing role of C. finmarchicus and possibly of T. abyssorum and euphausiids. We expect also growing importance of small taxa such as Psudocalanus and Oithona similis. These changes, because they involve key elements of the trophic structure, will result in restructuring of the trophic links and will have far-reaching consequences for the entire Kongsfjorden ecosystem, similar to the ones described for the Svalbard marine ecosystem by Weslawski et al. (2000), Karnovsky et al. (2003) or Stempniewicz (2005). Planktonic larvae of boreal benthic organisms are also likely to show increased abundance as well as set- tlement on the coast of Svalbard. The blue mussel (Mytilus edulis) has recently appeared in Isfjorden on Sval- bard (Berge et al., 2005). This might be another indication of the anticipated climate warming and stepwise, northward advancement of Atlantic fauna, as suggested by Weslawski et al. (1997) after finding live blue mussels on Bjørnøya. The recent extension of the blue mussel distribution range was made possible by the unusually high northward mass transport of warm AW resulting in elevated sea-surface temperatures in the North Atlantic and along the west coast of Svalbard in 2002. Other boreal species will likely follow, given that the warming trend persists.

8. Ice biota

8.1. Ice ﬂora and microorganisms associated with ice

Different types of ice algal assemblages are regularly found in the Arctic (Horner et al., 1988; Syvertsen, 1991). The most abundant species observed in the ice of the Northeast Water Polynya include Nitzschia frigida, Chaetoceros socialis, Melosira arctica, Pleurosigma stuxbergii and Fragilaria sp. (von Quillfeldt, 1997). Diatoms are normally the main primary producers (60% of algal biomass), but flagellated cells may also contribute significantly to the algal biomass (Gradinger et al., 1999). Chlorophyll-a concentrations of 0.5–1.5 lgl1 have been measured in new ice (Gradinger and Ika¨valko, 1998), and integrated chlorophyll-a concentration in pack ice from the Greenland Sea ranges from 0.1 to 3.3 mg Chl-a m2 (Gradinger et al., 1999). Ice algae may contribute 40% of the biomass, followed by bacteria (30%), heterotrophic flagellates (20%) and meiofauna (4%) (Gradinger et al., 1999). Among the meiofauna, ciliates, nematodes, acoel turbellarians and crustaceans are dominant (Gradinger and Ika¨valko, 1998; Gradinger et al., 1999). Calculated potential ingestion rates of meiofauna (5.5 g C m2 d1, during 120 days) are on the same order of magnitude (660 mg C m2 during the productive season) as annual primary production estimates suggested for Arctic MYI (600 mg C m2; Legendre et al., 1992).

8.2. Ice fauna

The ice fauna, or sympagic biota, represents organisms that live in close association with sea ice (Gulliksen and Lønne, 1991; Horner et al., 1992). The ice fauna consists of microorganisms and meiofauna (Gradinger et al., 1999), but also amphipods and fish (Lønne and Gulliksen, 1989a, 1991c; Hop et al., 2000). The most conspicuous autochthonous or permanently ice-residing fauna are the amphipods Gammarus wilkitzkii, Aphe- rusa glacialis, Onisimus glacialis and O. nanseni (Poltermann, 1998; Lønne and Gulliksen, 1991a,b; Hop et al., 2000). Another gammarid amphipod, Gammaracanthus loricatus, may also be present in the ice as well as a 208 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 polychaete (Hermanthoinae) (Lønne and Gulliksen, 1991c) and the copepod Jaschnovia brevis (Scott et al., 2002). The sympagic amphipods appear to be the main grazers in Arctic sympagic environments and feed primarily on ice algae, but also on zooplankton and ice fauna as well as on detritus derived from the ice community (Poltermann, 2001; Werner and Auel, 2005). They are all omnivorous in general, but show distinct differences in mouthparts indicating different feeding strategies (Poltermann, 2001; Arndt et al., 2005). The small Apherusa glacialis is primarily a grazer on ice algae. Gammarus wilkitzkii also graze ice algae, although the larger individuals tend to be carnivorous and all age-groups feed on detritus. The two Onisimus species, as well as Jaschnovia brevis, seem to have a more mixed diet, including ice algae, detritus and copepods (Scott et al., 1999; Scott et al., 2002). The polar cod (Boreogadus saida) is the main fish species living in close association with sea ice, which is made possible because of antifreeze compounds in its blood (Osuga and Feeney, 1978). Polar cod feeds primarily on planktonic copepods and amphipods, as well as ice-associated amphipods (Bain and Sekerak, 1978; Craig et al., 1982; Bradstreet et al., 1986; Lønne and Gulliksen, 1989a). It is a key species in Arctic marine pelagic food webs (Welch et al., 1992). The sympagic fauna in the Arctic is considered to have an important function both as trophic link between the sympagic and pelagic systems, and as a base for food chains culminating in seabirds and seals (Bradstreet and Cross, 1982; Lønne and Gabrielsen, 1992). The composition of the sympagic fauna is determined largely by the age and structure of sea ice, its history (drift pattern), and also by characteristics of the underlying water masses (Horner et al., 1992; Hop et al., 2000; Borga˚ et al., 2002). Studies on the distribution of sympagic amphipods related to the age and structure of the ice undersides have shown a large variation in density and biomass of ice amphipods both east and west of Svalbard (Lønne and Gulliksen, 1989b; Lønne and Gulliksen, 1991a,b,c; Poltermann, 1998; Hop et al., 2000). Biomass values from MYI were 10–100 times higher than corresponding values from FYI (Lønne and Gulliksen, 1991a,c). Both G. wilkitzkii and A. glacialis are found in MYI, whereas A. glacialis tends to be more common in FYI. The highest biomass values, involving the large Gammarus wilkitzkii, are associated with MYI floes or pieces of MYI frozen into FYI (Lønne and Gulliksen, 1991c; Hop et al., 2000; H. Hop, diving obs.). The abundance of Apherusa glacialis is closely related to the under-ice water properties and ice-algal biomass, whereas the abundance of G. wilkitzkii and Onisimus spp. are more influenced by the under-ice morphology (Hop et al., 2000; Werner and Gradinger, 2002). The reasons for different distribution patterns of the dominant amphipod species under Arctic sea ice are probably related to different requirements of the species, especially for food, shelter and physiological conditions (Hop et al., 2000).

8.3. Ice-associated biomass transport

Ice flora and fauna, as well as sediments, drift with the sea ice into Fram Strait. This transport with subsequent melting represents a large loss of biomass, but then also a large input of biomass into the Greenland Sea. The annual biomass transported and lost through Fram Strait has been estimated to be 7 · 105 ton (Lønne and Gulliksen, 1991c), equivalent to 1–2 · 105 tCyr1 (Werner et al., 1999). This estimate was based on amphipod biomass density of 5 g m2 and 140,000 km2 ice flux based on the Transpolar Drift Stream entering Fram Strait at a speed of 0.5 km d1. The high biomass value for Fram Strait, compared to the Barents Sea (generally <2 g m2)(Lønne and Gulliksen, 1991a; Hop et al., 2000) reflects that the Fram Strait ice has more MYI, with higher biomass of G. wilkitzkii (Arndt and Lønne, 2002). The combined wet biomass values for ice-amphipods typically vary from 0.1–1 g m2 for FYI and 0–10 g m2 for MYI (Lønne and Gulliksen, 1989b). Seasonal variation in abundance (and biomass) indicates lower abundance during winter (Werner and Auel, 2005), when the sea ice area flux is the greatest (Fig. 11), but we have not applied this in our bulk estimate below. If approximate values of 0.5 and 5 g m2 are adopted for FYI and MYI respectively, and 80% of the ice enters Fram Strait as MYI (Gow and Tucker, 1987), the ice-associated biomass transport can be calculated based on the mean annual ice area flux of 866,000 km2 yr1 (1978–2002) of Kwok et al. (2004). A bulk value for mean biomass export of ice fauna is then about 3.55 · 106 ton wet weight annually. This is almost an order of magnitude higher than the previous estimate by Lønne and Gulliksen (1991c), and basically reflects the higher, and improved, estimates of annual ice flux. Based on wet/dry weight percentage (±SE) of 28.3 ± 0.0% for G. wilkitzkii, 33.2 ± 0.1% for A. glacialis, 39.7 ± 1.2% for O. glacialis, and 32.1 ± 1.7% for O. nanseni (H. Hop, unpubl.) and their relative importance in terms of biomass, 80% for G. wilkitzkii, 12% for A. glacialis and 8% for Onisimus spp (Lønne and Gulliksen, 1991c), the overall mean wet/dry weight percentage for ice amphipods is about 29.5%. A dry H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 209 weight/carbon factor of 40%, which is widely used value for marine crustaceans (e.g. Omori, 1969), can be used to convert the above biomass valued to 4.2 · 105 tCyr1. The annual loss of ice fauna through Fram Strait represents a large drain on the populations of ice-amphipods in the Arctic Ocean. They cannot be maintained in the open water masses and eventually sink to the bottom as they run out of energy (Werner et al., 1999). Their potential recolonisation of ice during the next season (Poltermann, 1998) is likely not possible because of the depth of Fram Strait. A continuous loss of MYI due to climate warming will likely dramatically reduce this important food source, because most of the biomass of G. wilkitzkii, is associated with this ice habitat. The ice-associated biomass transport of ice algae and microorganisms is probably also substantial, although the ice algae are less abundant in the predominately MYI of Fram Strait than in FYI of the Barents Sea (Hegseth, 1992). The biomass transport of ice macrofauna in Fram Strait (0.5 g C m2, based on 4.1 g m2 for combined FYI and MYI) is likely double that of ice flora, meiofauna and microorganisms combined, with 0.2 g C m2 estimated for the Greenland Sea pack ice (Gradinger et al., 1999).

9. Middle-to-upper trophic levels

The middle trophic levels are represented by some plankton species as well as ﬁshes. The ﬁsh fauna in Fram Strait and Kongsfjorden consists of a mixture of boreal and Arctic species, but only few are pelagic and most of the benthic species are Arctic residents. The upper trophic levels in Fram Strait–Kongsfjorden are represented by a variety of marine mammals (seals, walruses, whales and polar bear) and seabirds. Many species are migratory and only reside in the Arctic during their breeding and subsequent feeding periods.

9.1. Fishes

Some of the commercial, boreal fish species, such as Atlantic cod (Gadus morhua), herring (Clupea haren- gus), blue whiting (Micromesistius poutassou), redfish (Sebastes mentella), and capelin (Mallotus villosus) extend their northern distribution range into Fram Strait, particularly associated with the WSC along Sval- bard (Skjoldal et al., 2004). Except for blue whiting, these species also occur in Kongsfjorden (Hop et al., 2002b). Their ranges overlap with Arctic species, of which polar cod is the most abundant fish in Kongsfjorden (Hop et al., 2002b) and in fjords on Greenland (Christiansen, 2003). The polar cod is the main fish species that is associated with sea ice (Lønne and Gulliksen, 1989a), but in some areas off the coast of Greenland, such as the Northeast Water Polynya, another ice-associated gadid, Arctogadus glacialis, is also abundant (Su¨fke et al., 1998). Capelin and herring occur in Kongsfjorden, presumably in larger numbers during warm years, such as the unusually warm spring of 2006 when some were caught in bottom trawls in the inner part of the fjord (50 capelin and 110 juvenile herring; H. Hop, unpubl. data). Other Arctic pelagic fishes include the deepwater redfish (Sebastes mentella), and also larvae of some benthic fish species, such as the daubed shanny (Leptoclinus maculatus). Most fish species in Fram Strait as well as in Kongsfjorden are small, benthic Arctic species, with notable exceptions such as the Greenland halibut (Reinhardtius hippoglossoides) and the Greenland shark (Somniosus microcephalus). Only 5–6 species in Kongsfjorden are pelagic, whereas the benthic fish community probably counts of about 30 species in total (Hop et al., 2002b).

9.2. Marine mammals

The pinnipeds include the true Arctic species ringed seal (Phoca hispida), bearded seal (Erignathus barbatus) and walrus (Odobenus rosmarus). These also occur in Kongsfjorden at low abundances, with ringed seal as the most numerous (Lydersen and Gjertz, 1986; Hop et al., 2002b). Walruses occur near the west coast of Spits- bergen outside their breeding season (Gjertz and Wiig, 1994, 1995), and the harbour seal (Phoca vitulina) has a local population of <1000 individuals on Prins Karls Forland on Spitsbergen (Lydersen and Kovacs, 2005). Seal species with more sub-arctic distribution ranges are also present in Fram Strait but rarely in Kongsfjor- den. Of these, the harp seal (Pagophilus groenlandicus) is most abundant, with the West Ice stock of about 400,000 animals in the pack ice off the east coast of Greenland (Lavigne and Kovacs, 1988; ICES, 2004). The hooded seal (Crystophora cristata), which is less common, has overlapping distribution with harp seal 210 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 in Fram Strait, but wanders more widely (Lavigne and Kovacs, 1988). Most of the seals prey on polar cod as well as other fish species present, squid and the pelagic amphipod Themisto libellula (e.g., Nilssen et al., 1995; Falk-Petersen et al., 2004; Haug et al., 2004), whereas walruses feed on benthos, particularly clams (Gjertz and Wiig, 1992). Both bearded seals and ringed seals may also utilise the benthic food base (Gjertz and Lydersen, 1986; Hjelset et al., 1999). Whales in Fram Strait include the Arctic species, white whale (Delphinapterus leucas), narwhal (Monodon monoceros) and bowhead whale (Balaena mysticetus), but only the white whale is commonly sighted in Kon- gsfjorden (Lydersen et al., 2001). Of the oceanic whale species, which frequent Fram Strait as well as the Barents Sea during their annual seasonal migrations (Wassmann et al., 2006), only the minke whale (Balae- noptera acutorostrata) is regularly seen in Kongsfjorden (Hop et al., 2002b). This species is also the most abundant whale species in the North Atlantic, with stock size in the 100,000 range (Skaug et al., 2004). Fin whales (Balaenoptera physalus) also forage on occasion in Kongsfjorden, but these and other large baleen and odon- tocete whales that frequent Svalbard waters tend to remain in coastal and offshore waters as opposed to mov- ing inside the fjords (Hop et al., 2002b). The toothed whales mostly prey on fish, such as white whales preying on polar cod (Boreogadus saida) and capelin (Mallotus villosus)(Dahl et al., 2000). The large baleen whales are generally zooplankton feeders, whereas the minke whale’s diet consists of a wide variety of crustaceans and fish (Lydersen et al., 1991; Haug et al., 2002). The polar bear (Ursus maritimus) has a circumpolar distribution which includes both Fram Strait and West- Spitsbergen, although it is not frequently observed in Kongsfjorden. The Fram Strait represents an area of overlap in ranges for the sub-populations in Svalbard-Franz Josef Land and East-Greenland (Lunn et al., 2002). Most of the polar bears in Fram Strait are from East-Greenland (Born et al., 1997; Wiig et al., 2003), whereas the Svalbard bears have their centre of distribution in the eastern part of the archipelago across to Franz Josef Land (Wiig, 1995). The polar bear represents a top predator in the Arctic pelagic system, mainly preying on ringed seals and harp seals, but also on bearded seals (Derocher et al., 2002; Grahl-Nielsen et al., 2003).

9.3. Seabirds

The seabirds in Fram Strait breed primarily on West-Spitsbergen or the east coast of Greenland, but are also observed offshore during their seasonal migrations or outside their breeding periods. Seabird surveys in Fram Strait–Barents Sea resulted in 22 species (Mehlum, 1989). Of these, the little auk (Alle alle), northern fulmar (Fulmarus glacialis), black-legged kittiwake (Rissa tridactyla) and Bru¨nnich’s guillemot (Uria lomvia) accounted for 95% of the number of birds seen. It is estimated that about 150,000 pairs of nine seabird species breed in the Kongsfjorden area, in numbers >50 pairs, and the common eider (Somateria mollissima) is one of the most abundant in addition to black-legged kittiwakes (Hop et al., 2002b). The alcids are pursuit divers, which prey on fish or zooplankton (Mehlum and Gabrielsen, 1993; Anker-Nilssen et al., 2000). The most important food items in sea ice covered waters are polar cod and pelagic and sympagic amphipods (Lønne and Gabrielsen, 1992). The smallest alcid, the little auk, preys on the large calanoid copepods C. glacialis and to a lesser degree on C. finmarchicus, C. hyperboreus and other planktonic prey (Karnovsky et al., 2003). The gulls generally feed near the surface, on pelagic fish and invertebrates, whereas the glaucous gull (Larus hyperboreus) is a top predator with a diverse diet (Anker-Nilssen et al., 2000). The common eider is a benthic feeder on invertebrates (Anker-Nilssen et al., 2000; Dahl et al., 2003).

10. Pelagic–benthic coupling

Pelagic–benthic coupling encompasses various processes, of which the most important is the sinking of organic, bio-available matter through the water column to the benthic system (i.e. vertical particle ﬂux), where it can be grazed, buried or advected. Simulated, annual carbon ﬂux of CO2 from the atmosphere is 53 g C m2 yr1, which sums up to 2.6 · 106 tCyr1 for the whole Greenland Sea (Slagstad et al., 1999). Of this, 9 g C m2 yr1 is exported by sinking particles, 6 g C m2 yr1 by migrating zooplankton (mainly C. hyperboreus), and 38 g C m2 yr1 by advection. The sympagic contribution by ice algae and ice fauna was not part of this model. H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 211

The sedimentation of particulate organic carbon (POC) from the upper 200–300 m in the central Greenland Sea, however, is found to be rather low (<2 g C m2 yr1)(Noji et al., 1999). The daily rates of sedimentation of particulate organic material may range up to 19 mg m2 d1 during autumn, but the total particulate material (TPM) may range as high as 250 mg m2 d1 during spring or autumn (Noji et al., 1999). Seasonally, the fluxes in the Greenland Sea increase in April, given the onset of the algal bloom, and reach maximum in May– June, with total flux rates around 100 mg m2 d1 at 500 m depth (Bauerfeind et al., 1994). The faecal pellet flux increases after the May–June event due to grazing by predominately copepods (Bauerfeind et al., 1994), but also by other zooplankters (e.g. appendicularians, krill and amphipods). The lithogenic flux is highest near the ice margin in Fram Strait due to release of ice-rafted material by melting (Hebbeln and Wefer, 1991). On the permanently ice covered Greenland continental shelf, the sedimentation rate is similar, varying from 52 to 229 mg m2 d1, with the highest fluxes during February–March (Bauerfeind et al., 2005). However, ice transported lithogenic matter tends to dominate during winter (73%), whereas biogenic matter dominates during the summer (58%), with POC accounting for 13 mg m2 d1 (Bauerfeind et al., 2005). The ice alga Melo- sira arctica may contribute significantly to the vertical particle flux during the ice melt in June, in the Northeast Water Polynya, and may even dominate the flux of organic carbon during this period (Bauerfeind et al., 1997). In the glacially influenced Kongsfjorden, the rate of sedimentation of TPM is considerably higher, ranging from 7.8 g m2 d1 in May to 107 g m2 d1 in July, of which particulate organic matter (POM) is 1.5– 7.4 g m2 d1, respectively (Svendsen et al., 2002). Sinking phytoplankton biomass is an important component of organic carbon and has been found to range from 1.3 mg C m2 d1 during spring, under fast ice, to 5mgCm2 d1 during the summer in open water (Wiktor, 1999; Keck, 1999; Keck et al., 1999). Another important source of sinking organic matter is faecal pellets (Wassmann et al., 1996). Their sedimentation is higher in summer than in spring, and may increase to 0.75 g C m2 d1 in the outer part of Kongsfjorden, presumably because of high grazing activity due to higher zooplankton abundance and biomass in summer (Walkusz et al., in review). The glacier outflow enhances the input of high amounts of organic material into the inner fjord where it sinks rapidly to the bottom in shallow areas (most of the inner part of the fjord is <100 m deep). The freshwater input to Kongsfjorden, mainly limited to the summer and autumn, modifies the oceanographic conditions of the inner basin to a great extent and also influences the middle part of the fjord. Zooplankton may die of osmotic shock when they come into contact with water of salinity <9 psu in the vicinity of the glacier front (Weslawski and Legezynska, 1998; Zajaczkowski and Legezynska, 2001). Because this effect persists throughout the melting season (100 days), as much as 85 ton of zooplankton may be removed from the water column during this period, constituting 15% of the zooplankton biomass in the fjord (Zajaczkowski and Legezynska, 2001). The glacial run-off thus provides an important mechanism for tight pelagic–benthic coupling in the inner part of the fjord by increasing the vertical flux of organic matter, both as flocculated material and dead zooplankton.

11. Pelagic and sympagic food web structures, indicated by stable isotopes

Trophic relationships and energy pathways in ecosystems can be investigated in an objective and quantita- tive manner by measuring naturally occurring stable isotopes of carbon and nitrogen in organisms (e.g. Hob- son and Welch, 1992; Fry and Sherr, 1998). The stable isotope ratio of carbon (d13C) changes relatively little (0–1& per trophic level) upwards in the food web and provides information on organisms’ carbon sources in ecosystems with two or three isotopically distinct carbon sources present (Fry and Sherr, 1998; Post, 2002; Søreide et al., 2006). The stable isotope ratio of nitrogen (d15N) undergoes a predictable step-wise enrichment (3–4&) between prey and consumer tissues and is a useful tool to estimate an organism’s trophic level (TL) (Hobson and Welch, 1992; Post, 2002; Søreide et al., 2006).

11.1. Primary producers: food web baseline

Ice algae and phytoplankton are the two major carbon sources, off-shore, in the Fram Strait region (e.g. Birgel et al., 2004; Bauerfeind et al., 2005). Terrestrial organic carbon, incorporated in sea ice and transported 212 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 off shore contributes little to the total organic carbon pool, although its relative importance increases, as the primary production decreases, towards the central Arctic Ocean (Gosselin et al., 1997; Schubert and Calvert, 2001; Birgel et al., 2004). To be able to estimate the importance of ice algae and phytoplankton food sources for lower trophic levels in the Fram Strait region, representative stable isotope values for these two major carbon sources need to be determined. Ice algae, phytoplankton and terrestrial organic matter have distinctly different d13C composition, with ice algae being more enriched in 13C than phytoplankton, and phytoplankton being more enriched in 13C than terrestrial organic matter (Hobson et al., 1995, 2002; Goñi et al., 2000; Schu- bert and Calvert, 2001). Several factors lead to distinctly different d13C values of primary producers in sea ice, open water and land habitats, such as differences in CO2 concentration, light, temperature, boundary layers and the type of carboxylating enzymes (Peterson and Fry, 1987; France, 1995; Fry, 1996; Kennedy et al., 2002). Ice algae are normally represented by samples of particulate organic matter (POM) from the bottom of the sea ice (Ice-POM), and phytoplankton by particulate organic matter suspended in the upper water column (Pelagic-POM) (e.g. Hobson et al., 1995; Søreide et al., 2006). Taxonomical investigations of the POM samples are therefore important, since the POM samples may consist of much material other than algae. The algal species composition is also necessary to know, since ice algae (e.g. Nitzschia frigida and Melosira arctica) released from the sea ice may contribute significantly to the total algae suspended in the water column particularly at the onset of sea ice melting (Bauerfeind et al., 1997). Pelagic-related algae (e.g. Thalassiosira spp. and Chaetoceros spp.) can grow temporarily on the underside of thin and young sea ice (Syvertsen, 1991; Hegseth, 1992), and such assemblages can have similar isotopic composition to phytoplankton (Tremblay et al., 2006; Søreide et al., 2006). In the East-Greenland/Fram Strait region, Ice-POM d13C values range from 21.0& to 18.3&, while Pelagic-POM has d13C values from 28.4& to 22.0 & (Hobson et al., 1995; Notholt, 1998; Schubert and Cal- vert, 2001; Søreide et al., 2006). The depleted Pelagic-POM values (d13C < 26.0&) suggest that much terrestrial material is present (Goñi et al., 2000), although no taxonomical data are available to confirm this. Lipids are depleted in 13C relative to proteins and carbohydrates (DeNiro and Epstein, 1977; van Dongen et al., 2002; Sotiropoulos et al., 2004). Algae can, at the end of their growth season, build up large lipid stores in response to nutrient stress (Fahl and Kattner, 1993; Lindqvist and Lignell, 1997), and high algal lipid content may result in relatively low d13C values (van Dongen et al., 2002; Søreide et al., in press). Removal of lipids prior to stable isotope analyses are commonly performed on animal tissue samples in Arctic marine food web studies in order to reduce differences in d13C values due to variations in body lipid content (e.g. Hobson et al., 1995; Tamelander et al., 2006; Søreide et al., 2006). Removal of lipids from POM samples has not normally been done prior to stable isotope analysis, except for in a few recent studies (Tamelander et al., 2006; Søreide et al., 2006). In the Fram Strait region, the d15N values of Pelagic-POM range from 3.9& to 5.7& during July–October (Hobson et al., 1995; Søreide et al., 2006), which is comparable to the d15N values of Pelagic-POM collected from productive waters elsewhere in the Arctic (Hobson and Welch, 1992; Iken et al., 2005; Tamelander et al., 2006; Tremblay et al., 2006). In winter, when there is very little algae and much detritus, Pelagic-POM is more enriched in 15N (6.7 ± 1.6&)(Sasaki et al., 2001; Sato et al., 2002). Tremblay et al. (2006) found that Pelagic- POM was most depleted in 15N at the onset of the bloom (4.0&) in the North Water Polynya, Canada. The stable nitrogen values of sedimented matter on the east Greenland shelf show a gradual increase from summer (3.4&) to winter (7.2&)(Bauerfeind et al., 2005). Bacterial degradation of algae and increased microbial activity can increase the d15N values of POM (Owens, 1985; Rolff, 2000). Ice-POM collected from the underside of sea ice may be similarly or more enriched in 15N than Pelagic-POM in the Fram Strait region (Hobson et al., 1995; Schubert and Calvert, 2001; Søreide et al., 2006), although Ice-POM from meltwater ponds can be strongly depleted in 15N(4.2& to 4.5&; Schubert and Calvert, 2001).

11.2. Trophic structures and sympagic–pelagic coupling

Few stable isotope studies of consumers exist from the Fram Strait region. Hobson et al. (1995) measured the stable isotope composition of ice fauna, zooplankton and benthic animals to determine sources of primary production, pelagic–benthic coupling and trophic structures in the Northeast Water Polynya off Greenland in summer (Fig. 1). Søreide et al. (2006) studied the stable isotope composition of several macrozooplankton and H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 213 ice fauna species (>1 mm) in the Marginal Ice Zone (MIZ) west and north of Svalbard, and east of Greenland in autumn, and Sasaki et al. (2001) and Sato et al. (2002) measured the stable isotope composition in copepods during winter, west of Svalbard (Fig. 1). Søreide et al. (2006) used a two-source food web model to determine trophic levels and carbon sources of macrozooplankton and ice fauna (>1 mm). Since this model was based on stable isotope values of POM and consumers in the Barents Sea MIZ and not from the Fram Strait region, we only consider the d13C and d15N values and not the model-estimates here. To get an overview of the trophic structures and sympagic–pelagic coupling in the Fram Strait region, we pooled macrozooplankton and ice fauna taxa with similarly enriched d13Candd15N values (ANOVA, p > 0.05) from the study of Søreide et al. (2006) and made a scatter plot of the mean values (Fig. 19). The pooled samples were categorised according to their most likely carbon source: pelagic (P), sympagic (S) or a mixture (M) of these, based on the assumptions that d13Candd15N values were max- imally enriched by 1& and 3.4&, respectively from one trophic level to the next (Post, 2002; Søreide et al., 2006), and that the Pelagic- and Ice-POM collected in the Fram Strait region in the study of Søreide et al. (2006) were representative for phytoplankton and ice algae, respectively, in this region. The trophic enrichment factors for d13C and d15N values were roughly estimated from the plot (Fig. 19), as 0.50–1& and 2.4–3.4&, respectively. Hobson et al. (1995) did not estimate trophic enrichment factors in their study from the Northeast Water Polynya. They used a higher step-wise enrichment factor (d15N = 3.8&), taken from Hobson and Welch (1992), to determine trophic levels, which resulted in about three trophic levels (TL = 1.7–2.5) within the Polynya plankton community (Fig. 20). Similar ranges in d13C and d15N values were found for plankton organisms and polar cod in the studies of Hobson et al. (1995) and Søreide et al. (2006), but since a lower enrichment factor was estimated for d15N, the planktonic food web was considered to be longer, roughly comprising four TLs in the study of Søreide et al. (2006) (Table 6, Fig. 19). Hobson et al. (1995) concluded that Pelagic-POM was the major carbon source for the lower marine food web in the Northeast Water Polynya, since zooplankton and ice fauna were relatively depleted in 13C and 15N compared to Ice-POM (Fig. 20). However, in their study Pelagic-POM was much more depleted in 13C than zooplankton and ice fauna. The much depleted d13C values in Pelagic-POM (27.7& to 28.4&) compared to herbivorous zooplankton (>23.0&) may partly be explained by removal of lipids from animals and not from POM (Søreide et al., 2006). A significant proportion of terrestrial POM may also have been present in the Pelagic-POM fraction since Hobson et al. (1995) sampled POM relatively close to Greenland. Søreide et al. (2006) sampled Pelagic-POM relatively far from land and removed lipids from all POM samples prior to stable isotope analysis, and found a better fit between POM and animal d13C values (Fig. 19).

Fig. 19. Trophic characterisation of the lower pelagic and sympagic food web structure in the Fram Strait region, based on stable isotope data (mean ± SE) collected in September/October 1999 and 2000 (Søreide et al., 2006). Trophic levels (TL) were estimated from d15N values, assuming a maximum TL-enrichment of 3.4&. Major carbon sources were estimated from d13C values, assuming a maximum TL- enrichment of 1&. Numbers refer to species listed in Table 6. 214 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Fig. 20. The plankton food web structure, as deﬁned by stable isotopes of carbon and nitrogen (mean ± SD), of the Northeast Water Polynya in June/July 1993 (redrawn after Hobson et al., 1995).

From a total of 85 zooplankton and 39 ice fauna samples analysed from the Fram Strait region in the study of Søreide et al. (2006), the d13C values indicated that only four out of 17 zooplankton species (i.e. Calanus hyperboreus, Paraeuchaetha glacialis, Themisto abyssorum, Mertensia ovum) utilised exclusively Pela- gic-POM source pathways, while two (Apherusa glacialis and Onisimus glacialis) out of five ice fauna species utilised primarily Ice-POM carbon sources (Table 6). The others assimilated energy from both carbon sources, which suggests a tight sympagic–pelagic coupling in the Fram Strait region. In areas with extensive ice cover, ice algae may not only be a crucial food source for ice fauna, but also an important seasonal food source for zooplankton (e.g. Bradstreet and Cross, 1982; Tremblay et al., 1989; Runge and Ingram, 1991; Runge et al., 1991; Michel et al., 1996; Auel et al., 2002; Fortier et al., 2002). Vertical particle flux studies on the northern shelf of Spitsbergen during summer show rapid recycling of phytoplankton and ice algal- derived matter in the upper water column by zooplankton (Andreassen et al., 1996). Suspended and sedimented particulate matter was strongly dominated by faecal pellets, while algal cells only made up 1–6% of the total particulate organic carbon (Andreassen et al., 1996). Extensive zooplankton grazing has also been found elsewhere in the Fram Strait region. In the Northeast Water Polynya, almost half of the spring and summer primary production may be utilised by copepods (Daly, 1997), and in the upper 200 m of the Green- land Sea a substantial recycling of sinking particles occurs from spring to autumn (Noji et al., 1999). This implies that the flux of POM to benthos may consist of much faecal matter and detritus, and less of fresh algae material.

11.3. Feeding strategies of macrozooplankton

The relatively depleted d15N values of C. glacialis and C. hyperboreus in summer, in the Northeast Water Polynya, indicated a herbivorous diet (Hobson et al., 1995). In autumn, at the end of the vegetative season, Calanus spp. showed large variations in d13C(23.9& to 20.9&)andd15N (6.4–9.5&) values, which indicated that Calanus spp. can switch from being mainly a herbivore (C. ﬁnmarchicus and C. glacialis) to mainly a detritivore/omnivore (C. glacialis and C. hyperboreus)(Table 6). Calanus is the key herbivore group in the Arc- tic, and is known to descend to depths and go into a resting stage (diapause) to survive the long unproductive winter (Hagen, 1999). However, high abundances of Calanus spp. have been found in the upper water column (0–200 m) south and west of Svalbard in winter, and the stable isotope values of these specimens (particularly of C. glacialis) suggested active feeding on detritus (Sato et al., 2002). The large copepods Paraeuchaeta norvegica and P. glacialis are known to be carnivores (Harding, 1974; To¨nnesson et al., 2006), and their heavy d15N values (10.1–11&) conﬁrmed this (Table 6). H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 215

Table 6 Stable carbon (d13C) and nitrogen (d15N) values of participate organic matter from open waters (Pelagic-POM) and underside of the ice (Ice-POM), and zooplankton (z) and ice fauna (i) from the Fram Strait region in September (Stn. 882, 1003) and October (Stn. 890) (Søreide et al., in press) Trophic level Species/taxa (dominating taxa) Size/stage # Samples # ind. Station d13C d15N TL =1 P1.0 Pelagic-POM (diatom-mix) 3 1003 24.6 ± 0.3 3.9 ± 0.4 P1.0 Pelagic-POMa (Chaeotoceros spp.) 8 890 23.5 ± 0.2 5.7 ± 0.2 S1.0 Ice-POM (Nitzschia spp.) 3 882 21.0 ± 0.2 4.2 ± 0.1 TL =2 P2.1 Themisto abyssorum z <9 2 14 882 23.1 ± 0.1 6.6 ± 0.1 Thysanoessa inermis z 16–19 mm 3 3 882 23.1 ± 0.8 6.9 ± 0.8 Thysanoessa longicaudata z 12–13 mm 1 2 882 23.7 6.6 M2.1 Calanus ﬁnmarchicus z CV & CVIF 5 147 890, 1003 22.0 ± 0.1 6.4 ± 0.2 M2.2 Calanus glacialis z CV & CVIF 6 91 882 21.0 ± 0.5 7.4 ± 0.3 S2.1 Apherusa glacialis i 5–13 mm 7 70 882, 890, 1003 20.6 ± 0.3 5.7 ± 0.2 Gammarus wilkitzkii i 7–35 mm 6 29 1003 20.9 ± 0.5 5.9 ± 0.3 S2.2 Onisimus nanseni i <10 mm 2 8 890 20.0 ± 0.0 6.0 ± 0.3 Gammarus wilkitzkii i 12–27 mm 3 8 890 20.6 ± 0.3 6.4 ± 0.1 Onisimus glacialis i 7–12 mm 4 9 890 20.0 ± 0.5 6.8 ± 0.4 TL =3 P3.1 Mertensia ovum z 15–45 mm 2 15 882 22.5 ± 0.6 8.1 ± 0.4 Themisto abyssorum z 8–12 mm 3 12 890 22.4 ± 0.1 8.3 ± 0.3 Thysanoessa longicaudata z 14–16 mm 2 8 882 23.4 ± 0.6 8.6 ± 0.2 Clione limacina z 25–35 mm 2 2 882 22.8 ± 0.1 8.6 ± 0.3 Limacina helicina z 8–10 mm 3 4 882 22.6 ± 0.2 8.7 ± 0.2 P3.2 Themisto abyssorum z 10–15 mm 3 6 882 23.5 ± 0.4 8.8 ± 0.1 Calanus hyperboreus z CV & CVIF 6 111 882 24.0 ± 0.1 8.9 ± 0.2 P3.3 Themisto libellula z 13–26 mm 6 9 890 23.0 ± 0.5 9.6 ± 0.4 Paraeuchaeta spp. z CV 3 33 882, 890 22.5 ± 0.3 10.1 ± 0.3 Paraeuchaeta glacialis z CVIF 6 28 890 22.9 ± 0.4 10.6 ± 0.3 M3.1 Gammarus wilkitzkii i 35–62 mm 2 2 890 22.0 ± 0.3 8.0 ± 0.8 Themisto libellula z 10–20 mm 3 6 882 21.5 ± 0.1 8.5 ± 0.6 Eukrohnia hamata z 15–30 mm 5 31 882, 890 21.5 ± 0.2 8.6 ± 0.2 Thysanoessa longicaudata z 14–17 mm 3 15 890 21.7 ± 0.5 8.6 ± 0.6 M3.2 Calanus hyperboreus z CV & CVIF 6 890 21.2 ± 0.2 9.4 ± 0.4 Calanus glacialis z CV & CVIF 9 890, 1003 21.3 ± 0.2 9.5 ± 0.2 S3.1 Onisimus nanseni i 20–21 mm 2 2 882 19.1 ± 0.9 8.9 ± 0.4 Beroe¨ cucumis z 70–80 mm 2 2 882 20.0 ± 0.6 9.0 ± 0.1 Thysanoessa inermis z 25–29 mm 3 4 890 20.0 ± 0.3 9.1 ± 0.1 TL =4 P4.1 Onisimus nanseni i 18 mm 1 1 890 21.9 12.8 M4.1 Paraeuchaeta norvegica z CVIF 3 12 882 22.0 ± 0.4 10.7 ± 0.6 Boreogadus saida (Polar cod) i 99–112 mm 3 3 882 22.0 ± 0.3 11.3 ± 0.2 M4.2 Sagitta elegans z 26–30 mm 1 4 890 21.0 12.6 Taxa were, from their stable isotope values, categorized into four trophic levels and their most likely carbon source: Pelagic-POM (P), Ice- POM (S) or a mixture (M) of these two (see Fig. 19). a POM sampled from the underside of the sea ice were pooled with Pelagic-POM since no diﬀerences in algal species or stable isotope composition were found.

The stable isotope values in the two common Thysanoessa species, T. inermis and T. longicaudata, suggested diﬀerent feeding strategies dependent on body size. Medium sized T. inermis (16–19 mm) and T. longicaudata (10–13 mm) mainly grazed on Pelagic-POM, whereas larger individuals of T. inermis (>22 mm) and T. longicaudata (>13 mm) switched to a more omnivorous and carnivorous diet. The feeding strategy of the hyperiid amphipod Themisto abyssorum was also found to be dependent on its body size. Small individuals of T. abyssorum (<9 mm) grazed mainly on Pelagic-POM, whereas larger specimens became more omnivorous (Table 6). 216 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

The d15N values (8.5–9.96&) in the larger amphipod T. libellula (13–29 mm) suggested similar, or slightly higher TL compared to large specimens of T. abyssorum (8.3–8.8&). In the Northeast Water Polynya, T. libellula was slightly more enriched in 15N(d15N = 10.4&)(Hobson et al., 1995) than T. libellula sampled farther east in the Fram Strait region in the study of Søreide et al. (2006), but Calanus spp., which are considered to be an important food items for T. libellula (Scott et al., 1999), was also slightly more enriched in d15N in the Northeast Water Polynya than the most depleted Calanus spp. in the study of Søreide et al. (2006) (d15N = 7.6–8.2& vs. 6.4– 7.4&). The chaetognath S. elegans (17–29 mm) was found to be one of the planktonic top-predators (Table 6, Fig. 19). It had much higher d15N values (12.6&) than the other arrow worm Eukrohnia hamata (d15N = 8.6&), which seemed to be predominantly omnivorous (Table 6). The ctenophore Beroe¨ cucumis (70–80 mm) is suggested to feed on the cydippid ctenophore Mertensia ovum (Falk-Petersen et al., 2002). How- ever, B. cucumis had only slightly more enriched d15N value (9.0&) than M. ovum (8.1&), so such a predator– prey relationship was not supported by stable isotope analysis. The pteropod Clione limacina (25–35 mm) is considered to feed monophageously on the other pteropod L. helicina (shell B 10 mm) (Conover and Lalli, 1974). However, similar stable isotope signatures were found in these two pteropods (Table 6), indicating that both species were primarily omnivorous (Søreide et al., 2006). Fatty acid analysis has also not revealed a clear predator–prey relationship between these two species (Falk-Petersen et al., 2001; Bo¨er et al., 2005).

11.4. Feeding strategies of ice fauna

The relatively small ice amphipods A. glacialis (5–13 mm) and O. glacialis feed predominantly on Ice-POM (Table 6). The large ice amphipod Gammarus wilkitzkii (7–35 mm) grazed also on Ice-POM, but the largest specimens (35–62 mm) switched to a more omnivorous diet (Table 6). Hobson et al. (1995) only analysed the stable isotope composition in one ice amphipod species, G. wilkitzkii, and determined it to mainly feed on Pelagic-POM (TL = 2.2) in the Northeast Water Polynya. However, G. wilkitzkii from the Polynya had a higher d15N value (9.2&) than the G. wilkitzkii analysed in the study of Søreide et al. (2006). Poltermann (2001) investigated the gut contents of Arctic ice-amphipods, and concluded that all were omnivorous, but that A. glacialis was herbivorous–detritivorous and that G. wilkitzkii was a detritivorous–carnivorous– necrophagous–suspension–feeder. Studies of fatty acid trophic markers in ice-amphipods also support these ﬁndings (see below). The trophic position of the relatively large Onisimus nanseni varied also with body size. Individuals with body sizes <10 mm fed mainly on Ice-POM, whereas larger individuals (>15 mm) switched to a more omnivorous and carnivorous diet (Table 6). Juvenile (ages 1 and 2) polar cod use the sea ice as a feeding ground and refuge to avoid predators (Lønne and Gulliksen, 1989a). The stable isotope data of macrozooplankton and ice fauna (>1 mm) from the Fram Strait region suggest that the chaetognath S. elegans, the predatory copepod P. norvegica, and the polar cod are exclusively carnivorous, whereas the other species switch between herbivorous-omnivorous and carnivorous feeding strategies dependent on specimens body sizes and/or algal food availability.

11.5. Upper trophic levels and pelagic–benthic coupling

There are only few studies of stable isotopes in upper trophic levels in the Fram Strait–Kongsfjorden region, one on seabirds in Kongsfjorden (Dahl et al., 2003), and the Hobson et al. (1995) study on the food web structure and pelagic–benthic coupling in the Northeast Water Polynya. In shallow areas, the sources of carbon become important, as shown for pelagic versus benthically feeding seabirds. Both isotope values and fatty acid composition indicated strong pelagic links for the black-legged kittiwake (Rissa tridactyla) and northern fulmar (Fulmarus glacialis), whereas the common eider (Somateria mollissima) feeds on the benthic system. The isotopic signal in benthos shows that grazing crustaceans and filter-feeding bivalves rely on pelagic POM, and tight benthic–pelagic coupling was also confirmed for some other benthic organisms, with notable exceptions for predatory and deposit-feeding echinoderms. Sea urchin (Stongylocentrotus droebachiensis)is a main prey for eiders (Bustnes and Lønne, 1995), which may explain the benthic isotopic signal in this seabird species. The stable isotope signatures of upper trophic levels and pelagic–benthic coupling have been further described in the Barents Sea (Hop et al., 2002a; Tamelander et al., 2006), and some of these findings can likely also be applied to Fram Strait. H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 217

12. Lipids and energy pathways

The zooplankton stocks in the Arctic pelagic zone as well as the ice fauna transfer energy from the seasonal pulse of primary production upwards through the marine food web (Falk-Petersen et al., 1990; Hagen, 1999), and this lipid-driven energy ﬂux is a key specialisation in Arctic bio-production (Falk-Petersen et al., 2006). The use of fatty acid trophic markers (FATM) to trace energy transfer from phytoplankton to top predators is based on the observation that primary and some secondary producers synthesise characteristic fatty acids and alcohols that are conservatively transferred through the food chain (Dalsgaard et al., 2003; Falk-Petersen et al., 2004).

12.1. Phytoplankton and ice algae

Diatoms in general tend to be rich in 20:5(n3), 16:1(n7) and C16 PUFA (polyunsaturated fatty acids) but deﬁcient in C18 PUFA. Phaeocystis pouchetii, which is part of both early and late blooms in polar waters, is rich in C18 PUFA, especially 18:4(n3) and 18:5(n3). Dinoﬂagellates are rich in 20:5(n3) and 22:6(n3), but not in 16:1(n7) and C16 PUFA (Sargent et al., 1985; Hamm et al., 2001). Ice algal assemblages are generally dominated by diatoms such as Nitzschia frigida, but large mucilaginous masses of Melosira arctica may also be present (Falk-Petersen et al., 2000b). Melosira assemblages had higher percentages of C16 PUFA (polyunsaturated fatty acids), especially 16:4(n1) and 20:5(n3) than Nitzschia assemblages (Falk-Petersen et al., 1998). The ice associated diatoms have high nutritional value and represent a source of 20:5(n3) as well as 22:6(n3) in higher trophic levels.

12.2. Herbivorous zooplankton

The generally herbivorous zooplankton C. hyperboreus, C. glacialis and C. finmarchicus have high levels of diatom trophic markers 16:1(n7) (18–23%) and 20:5(n3) (10–15%) and moderate to low levels of the dinoflagellate markers C18PUFA (3–6%) and 22:6(n3) (1–3%), indicating diatoms to be their main food source (Table 7). The krill species T. inermis and T. raschii, which are regarded as a key component of the herbivorous community (Mauchline, 1980; Falk-Petersen et al., 2000a), show some interesting Arctic adaptations in that their lipids show clear signs of carnivory, with 18:1(n9) and copepod 20:1 and 22:1 FATM. The pteropod Limacina helicina has very high level of the dinoflagellate FATM C18PUFA (24%) and 22:6(n3) (9%) together with moderate to high levels of 18:0 (2%), 20:1(n9) (5%) and 18:1(n9) (10%) indicating detrital input of both plant and animal origin (Falk-Petersen et al., 2001). This confirms the understanding that L. helicina is an omnivore, feeding on particulates of the right size for its mucous trapping net (Gilmer, 1972; Gilmer and Harbison, 1991).

12.3. Ice fauna

The fatty acid trophic markers of the ice amphipod A. glacialis are dominated by 16:1(n7) fatty acids, followed by the substantial components 20:5(n3) and 18(n9) (Table 8). Minor, but quite significant components include 22:6(n3) and C16 PUFA. These data are consistent with a major part of the diet of A. glacialis being ice algae, mainly diatoms. Gammarus wilkitzkii differs from A. glacialis in that 16:1(n7) and 18(n9) are present, respectively, in lesser and greater amounts in G. wilkitzkii, and this species also has substantial amounts of 20:1(n9) and 22:1(n11) which are not present in A. glacialis. The 20:5(n3) is the major PUFA in G. wilkitzkii with 22:6(n3) being a minor component. The fatty acid profiles of Onisimus nanseni and Jaschnovia brevis are similar to that of G. wilkitzkii in containing substantial amounts of 20:1(n9) and 22:1(n11), and also 18:1(n9) and 16:0, indicating that both species are omnivores.

12.4. Carnivorous zooplankton and plankton eating ﬁshes

The three most important predators, Mertensia ovum, Themisto libellula, and polar cod, together with juvenile stages of daubed shanny (Leptoclinus maculates) all have high levels of Calanus FATM and 20:1 218 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Table 7 Fatty acid and fatty alcohol composition of the neutral lipids (WE = wax esters, TAG = triacylglycerols) in herbivorous zooplankton Calanus ﬁnmarchicus, C. glacialis, C. hyperboreus, Thysanoessa inermis, T. raschii and Limacina helicina Calanus Calanus Calanus Thysanoessa Thysanoessa Limacina ﬁnmarchicus glacialis hyperboreus inermis raschii helicina (% of WE) (% of WE) (% of WE) (% of WE) (% of WE) (% of TAG) Fatty acids 14:0 9.1 6.7 3.3 0.8 4.1 2.9 16:0 7.1 6.4 2.5 3.0 16.7 14.0 16:1(n7) 23.0 18.0 20.6 10.5 7.5 5.5 C16 PUFAs 4.6 2.6 5.3 0.0 0.4 0.2 18:0 0.4 0.7 0.3 1.0 2.7 2.4 18:1(n9) 2.6 5.6 2.4 55.3 5.1 9.5 18:1(n7) 1.5 1.0 1.5 11.8 4.9 3.2 18:2(n6) 0.8 1.3 1.1 1.1 0.2 4.8 18:3(n3) 0.6 0.9 0.6 0.3 0.3 4.1 18:4(n3) 2.7 6.0 6.0 0.5 0.4 15.1 20:1(n9) 14.5 16.4 16.2 1.8 4.0 4.5 20:5(n3) 11.4 12.4 12.6 4.4 2.0 10.8 22:1(n11) 9.7 9.9 14.9 1.0 2.3 0.4 22:6(n3) 1.4 2.6 3.0 0.4 0.3 9.4 Fatty alcohols 14:0 2.0 1.9 2.9 0.0 0.0 16:0 8.2 9.2 7.6 54.1 24.5 16:1(n7) 6.5 3.5 2.7 13.1 9.8 18:1(n7) 2.4 2.0 1.0 0.3 1.1 20:1(n9) 35.5 40.6 27.9 0.9 13.2 20:1(n7) 1.5 1.1 2.8 0.0 0.4 22:1(n11) 37.8 30.7 50.3 2.3 19.7 22:1(n9) 0.6 2.8 1.4 0.0 0.0 Data from Scott et al. (2002) and Falk-Petersen et al. (2000a, 2001).

Table 8 Fatty acid composition in ice-associated fauna: Apherusa glacialis, Gammarus wilkitzkii, Onisimus sp. and Jaschnovia brevis Apherusa glacialis Gammarus wilkitzkii Onisimus spp. Jaschnovia brevis (% of TAG) (% of TAG) (% of TAG) (% of TAG) Fatty acids 14:0 3.3 4.9 4.4 10.1 16:0 16.2 12.3 12.9 15.9 16:1(n7) 40.8 31.7 30.2 23.9 16:4 1.8 0.9 0.7 0.5 18:0 0.9 0.5 11.2 1.0 18:1(n9) 7.5 13.6 3.2 5.6 18:1(n7) 2.1 3.3 0.8 3.2 18:3(n3) 0.6 0.8 2.9 1.2 18:4(n3) 2.3 3.4 0.2 3.7 20:0 0.3 0.0 5.1 0.4 20:1(n9) 0.7 3.3 1.1 7.7 20:4(n3) 0.6 0.6 15.7 0.4 20:5(n3) 11.7 12.6 2.1 8.5 22:1(n11) 0.2 2.5 0.5 6.2 22:5(n3) 0.7 0.3 2.6 0.4 22:6(n3) 2.6 3.1 0.4 3.3 Data from Scott et al. (1999, 2002). and 22:1 alcohols conﬁrming the dominant role of Calanus in their diet (Table 9). Large T. libellula are known to prey on all zooplankton forms, but the dominance of Calanus in their diet is conﬁrmed by fatty acid analyses from northern Fram Strait and the central Arctic Ocean (Auel et al., 2002), in Svalbard waters H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 219

Table 9 Fatty acid composition in carnivorous zooplankton, Themisto libellula, Mertensia ovum and Clione limacina, and the plankton-eating ﬁshes polar cod (Boreogadus saida), snake blenny (Lumpenus lampretaeformis) and daubed shanny (Leptoclinus maculatus) Themisto Mertensia Clione Boreogadus Mallotus Lumpenus Leptoclinus libellula ovum limacina saida villosus lampretaeformis maculatus (% of WE) (% of WE) (% of TAG) (% of TAG) (% of TAG) (% of TAG) (% of TAG) Fatty acids 14:0 5.1 13.1 2.7 2.6 5.1 3.1 4.7 16:0 4.4 8.2 15.0 13.1 21.5 12.6 6.9 16:1(n7) 13.6 7.4 8.8 11.6 12.5 18.9 13.9 18:0 0.8 0.5 1.6 2.2 1.8 2.6 0.9 18:1(n9) 14.3 11.7 6.7 12.3 20.9 15.2 3.7 18:1(n7) 1.7 0.7 4.5 5.1 8.2 7.9 2.0 18:2(n6) 2.2 1.1 1.4 0.8 1.1 0.7 0.9 18:3(n3) 1.4 2.2 1.6 0.5 0.5 0.2 0.5 18:4(n3) 9.4 19.3 3.3 2.0 3.2 0.9 3.5 20:1(n9) 16.7 7.8 3.9 12.4 3.3 2.0 26.0 20:1(n7) 1.0 0.0 2.5 2.0 0.3 1.1 0.6 20:5(n3) 7.1 8.3 8.6 9.2 10.4 11.0 7.1 22:1(n11) 4.7 7.1 1.2 6.3 2.7 0.3 17.5 22:6(n3) 4.8 3.4 11.7 10.3 4.3 2.9 3.9 Fatty alcohols 14:0 3.2 0.0 16:0 10.1 14.4 16:1(n7) 5.2 2.5 18:1(n9) 1.4 5.3 20:1(n9) 35.9 33.0 20:1(n7) 0.0 0.7 22:1s 37.5 32.2 Data on M. ovum and C. limacina are from Falk-Petersen et al. (2001), whereas data on T. libellula, B. saida, L. lampretaeformis, and L. maculatus are unpublished (S. Falk-Petersen).

(Scott et al., 1999) and on the Norwegian coast (Falk-Petersen et al., 1987). Mertensia ovum is considered an opportunistic predator (Swanberg and Ba˚mstedt, 1991) feeding on a range of small crustaceans. The FATM show, however, that Calanus copepods are the dominant energy and food source for this ctenophore (Falk- Petersen et al., 2002). Clione limacina is supposed to feed monophageously on L. helicina (Conover and Lalli, 1972), so a close similarity between their lipids may be expected. However, there is little similarity between the fatty acid compositions of the neutral lipids in the two species. This can be accounted for by the large ability of C. limacina for de novo biosynthesis of 1-O-alkyldiacylglycerol ethers (DAGE), with fatty acids 15:0, 17:0, and 17:1(n8) from non-lipid dietary precursors (Phleger et al., 1997; Kattner et al., 1998; Boër et al., 2005). Nonetheless, the stable isotope data indicated that C. limacina and L. helicina were on a similar trophic level. The two arctic fish species polar cod and daubed shanny are known to ingest large amounts of Calanus copepods (Falk-Petersen et al., 1986; Hop et al., 1997) in contrast to the bottom dwelling fish snake blenny (Lumpenus lampretaeformis).

12.5. Top predators

The importance of Calanus in the pelagic Arctic marine food web is very visible in the top predators (Table 10). Calanus FATM are dominating in northern fulmar (28%), black-legged kittiwake (17%), white whale (14%) and B. cucumis (16%). The only top predator with no signs of Calanus FATM was the common eider, a benthic feeder (Dahl et al., 2003). The high level of Calanus FATM in northern fulmar, black-legged kittiwake and white whale is probably a result of secondary ingestion of 20:1 and 22:1 fatty acids via polar cod, capelin and larger zooplankton predators that have Calanus as a dominant part of their diet (Dahl et al., 2000, 2003). Beroe¨ cucumis is assumed to feed primarily on M. ovum, based on the similarity in fatty acid proﬁles of 220 H. Hop et al. / Progress in Oceanography 71 (2006) 182–231

Table 10 Fatty acid composition in the top predators Beroe¨ cucumis, common eider (Somateria mollissima), black-legged kittiwake (Rissa tridactyla) and northern fulmar (Fulmarus glacialis) and white whale (Delphinapterus leucas) Beroe¨ cucumis Somateria mollissima Rissa tridactyla Fulmarus glacialis Delphinapterus leucas (% of WE) (% of TAG) (% of TAG) (% of TAG) (% of TAG) Fatty acids 14:0 10.6 0.9 4.3 2.8 4.9 16:0 6.8 23.6 21.7 14.2 6.3 16:1(n7) 10.0 6.0 9.0 7.7 22.5 18:0 0.5 7.5 6.5 4.6 1.1 18:1(n9) 12.2 29.3 24.6 22.8 22.6 18:1(n7) 0.7 6.1 3.9 5.0 3.5 18:2(n6) 1.6 2.6 1.4 1.5 1.2 18:3(n3) 2.0 1.1 0.5 0.6 0.5 18:4(n3) 18.7 0.8 0.8 0.9 0.7 20:1(n9) 8.5 1.3 10.7 16.4 11.7 20:5(n3) 9.1 6.5 2.9 3.7 3.1 22:1(n11) 7.6 0.2 6.1 10.0 2.8 22:5(n3) 0.8 1.2 0.4 0.7 1.5 22:6(n3) 3.1 3.9 3.1 4.1 3.2 Fatty alcohols 14:0 16:0 15.4 16:1(n7) 4.0 18:1(n9) 5.1 18:1(n7) 1.9 20:1(n9) 31.6 22:1(n11) 31.3 Data from Dahl et al. (2003).

their lipids (Falk-Petersen et al., 2002), which explains its high level of Calanus FATM. However, this predator–prey relationship was not supported by the stable isotope data, which indicate similarity in diet rather than predation. Another interesting aspect is the high level of diatom FATM 16:1(n7) and 20:5(n3) raging from 10% in northern fulmar to 25% in white whale compared to the low levels of dinoﬂagellate FATM 22:6(n3) and C18 PUFAs, conﬁrming the importance of diatoms and Calanus at the base of the Arctic pelagic marine food web.

13. Concluding remarks

Fram Strait represents a connection between the North Atlantic and the Arctic Ocean, with a two-way transport of upper layer water masses (WSC and EGC), associated heat and biological energy. These currents are somewhat connected with water masses circulating as deep return currents both in the Arctic Ocean and in the deep part of Fram Strait. This makes it possible for long-lived plankton species, such as C. hyperboreus,to complete its life cycle in a loop in the Arctic Ocean and return to Fram Strait and Greenland Sea in the EGC. In addition, there is a one-way transport of ice in Fram Strait, with net transport southwards. The distributions of water masses and ice are climate driven, and the export of Atlantic water directly into the Arctic Ocean though Fram Strait represents a major heat transport into the ice covered sea. The magnitude of the Atlantic water masses in the branch entering the Arctic Ocean will likely play a major role in decrease of ice extent in the Arctic Ocean in future climate warming. The West Spitsbergen Current largely influences the west coast of Svalbard, and directly influences the open, sill less, fjords along the Spitsbergen coast. Local forcing by winds, freshwater runoff and tidal currents are important, especially for the water masses in the fjords, but the larger current systems in Fram Strait also influence the hydrographical conditions over the shelves and in the fjords. Advection of warm water masses during late autumn and winter, together with prevailing wind patterns and air temperatures, may prevent ice H. Hop et al. / Progress in Oceanography 71 (2006) 182–231 221 formation in the fjords. In the winter-spring of 2006, Kongsfjorden was almost completely ice free, except for a small area of fast ice close to the islands in the inner bay. Kongsfjorden, which is directly connected to waters over the shelf and in the deep Fram Strait, provides a more detailed archive of the larger oceanographic situation due to long-term monitoring records that includes both physical and biological measurements. The Kongsfjorden–Krossfjorden system can be used as an indicator for the larger climate driven processes in Fram Strait. The fjords receive variable Arctic/Atlantic climatic signals between years, with measurable effects on the physical and biological systems. The biological responses may be fast for the pelagic system and slow or delayed in the benthic system. The physical transport of water masses and ice through Fram Strait has an associated biological transport of organisms and carbon. Transport of sediments, detritus and ice-associated organisms, incorporated in the southward ice flux, fuels the Greenland shelf and sea with a large annual input of carbon. Reduction in ice thickness and extent in the Arctic Ocean, due to climatic warming, will likely decrease this southward biomass transport substantially. The northward transport of biological matter, such as zooplankton associated with the water masses, is currently unknown but must also be large. Some long-lived zooplankton species may complete a loop in the Arctic Ocean, but most boreal zooplankton transported into the Arctic Ocean are probably lost to their populations, similarly to ice-amphipods transported southward with the ice through Fram Strait, which are lost to their core population that likely resides in the ice gyre of the high-Arctic Canada Basin. The pelagic systems of both Fram Strait and Kongsfjorden are highly influenced by advection, and, thus, may be regarded as transitional ecosystems for most species. However, some zooplankton species, such as C. hyperboreus and meso- and bathypelagic copepods, reside in the deep Greenland Sea. Also, some Arctic species from all trophic levels are considered residents of Kongsfjorden, given that they reproduce there, although most will not complete their entire life cycle in this system. The transitional pattern is mainly brought about by advection of boreal species, which are distributed seasonally into both systems during the spring and summer. The balance between boreal vs. Arctic zooplankton and pelagic fishes depends on advection of Atlantic vs. Arctic water masses in which they occur. Marine mammals and seabirds also increase in population numbers during the Arctic spring and summer season due to breeding and subsequent feeding on the lipid rich marine food sources. The trophic structure of the food web has traditionally been determined by analysing stomachs to determine feeding habits. Stable isotopes and lipids provide a complementary way of looking at food web structure and energy transfer, and have thus provided some new insight into Arctic marine food webs. The marine food web and energy pathways determined by fatty acid trophic markers have largely supported the stable isotope picture of the Arctic marine food web, but also provided new information on the trophic relationships for some species. As more gaps are filled in our understanding of the functioning of Arctic marine food webs, particularly with regard to seasonal and annual variability, it will become more feasible to predict how climate change will influence the production, structure and system-couplings in these food webs.

Acknowledgements

This publication was supported by the Research Council of Norway NORKLIMA projects Carbon flux and ecosystem feedback in the northern Barents Sea in an era of climate change (CABANERA, project 155936-700), and On thin ice: Climatic influence on energy flow and trophic structure in Arctic marine ecosystems (Project 150356-S30).

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