Physical and ecological processes in the marginal ice zone of the northern Barents Sea during the summer melt period

Journal of Marine Systems 27 Ž2000. 131–159 www.elsevier.nlrlocaterjmarsys

Physical and ecological processes in the marginal ice zone of the northern Barents Sea during the summer melt period Stig Falk-Petersen a,) , Haakon Hop a , W. Paul Budgell a , Else N. Hegseth b, Reinert Korsnes a , Terje B. Løyning a , Jon Børre Ørbæk a , Toshiyuki Kawamura c , Kunio Shirasawa d a

Norwegian Polar Institute, N-9226 Tromsø, Norway Norwegian College of Fishery Science, UniÕersity of Tromsø, N-9037 Tromsø, Norway c Institute of Low Temperature Science, Hokkaido UniÕersity, Kita-19, Nishi-8, Kita-ku, Sapporo 060, Japan d Sea Ice Research Laboratory, Hokkaido UniÕersity, Minamigaoka 6-4-10, Mombetsu, Hokkaido 094, Japan b

Received 9 February 1999; accepted 27 March 2000

Abstract The main physical and ecological processes associated with the summer melt period in the marginal ice zone ŽMIZ. were investigated in a multidisciplinary research programme ŽICE-BAR., which was carried out in the northern Barents Sea during June–August 1995–1996. This study provided simultaneous observations of a wide range of physical and chemical factors of importance for the melting processes of sea ice, from its southernmost margins at about 77.58N to the consolidated Arctic pack ice at 81.58N. This paper includes a description of the oceanographic processes, ice-density packing and structures in cores, optical properties of water masses and the ice, characteristics of the incident spectral radiation and chlorophyll — leading to primary production. Large seasonal and inter-annual variations in ice cover in the MIZ were evident from satellite images as well as ship observations. Even if the annual variation in ice extent may be large, the inter-annual variations may be even larger. The minimum observed ice extent in March, for example, can be smaller than the maximum observed ice extent in September. Oceanographic phenomena such as the semi-permanent lee polynyas found west and south-west of Kvitøya and Franz Josef Land and the bay of open water, the AWhalers BayB, north of the Spitsbergen are structures which can change with time intervals of hours to decades. For example, the polynya south of Franz Josef Land was clearly evident in 1995 but was only seen for a short period in 1996. The observed variability in physical conditions directly affects the primary production in the MIZ. From early spring, solar radiation penetrates both leads and the ice itself, initiating algal production under the ice. Light measurements showed that the melt ponds act as windows, permitting the transmission of incoming solar radiation through to the underlying sea ice, thus, accelerating the melting process and enhancing the under-ice primary production. In June 1995, the N–S transect went through a pre-bloom area well inside the ice-covered part of the Barents Sea to a post-bloom phase in the open waters south of the ice edge. The biological conditions in the later season ŽAugust. of 1996 were considerably more variable. The

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Corresponding author. Tel.: q47-777-50500; fax: q47-777-50501. E-mail address: [email protected] ŽS. Falk-Petersen..

0924-7963r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 7 9 6 3 Ž 0 0 . 0 0 0 6 4 - 6

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longer N–S transect in August 1996 passed through areas with variable ice and oceanographic conditions, and different developmental stages of phytoplankton blooms were encountered. The previously adapted picture of a plankton bloom following the retreating ice edge northwards was not seen. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Arctic marine ecosystems; oceanography; sea ice; remote sensing; phytoplankton; ice algae

1. Introduction Marginal ice zones ŽMIZ. are some of the most dynamic areas in the world’s oceans with large seasonal and inter-annual fluctuations in ice-cover and ice-transport. For example, the location of the ice edge during summer in the Barents Sea can vary by hundreds of kilometres from year to year ŽGloersen et al., 1992. and there is a strong relationship between the North Atlantic oscillation ŽNAO. index and ice edge location ŽVinje, 1997.. These variations reflect the inter-annual dynamics of inflowing Atlantic water. In warm years, the ice edge is further north and the primary production is higher than in cold years. The ice extent is sensitive to atmospheric forcing and currents, and hence, climate change. Wave effects, combined with melting processes produce small ice floes from 10–50 m in the outer part of the ice margin. The wave energy is rapidly dissipated and further inwards from the active wave zone, only long waves penetrate; accordingly, the size of floes increases from several metres to kilometres ŽVinje and Kvambekk, 1991.. The interface between ice and sea water provides a habitat which has been described as an upside-down benthic environment ŽMohr and Tibbs, 1963; Poltermann et al., 2000., although the habitat is more dynamic and may undergo radical changes in structure and composition in response to seasonal melting and freezing as well as physical forcing. The structural under-ice topography, which to a large extent determines the actual distribution and density of ice-fauna, includes both mesoscale structures, such as ridges, flat surfaces and edges, and small scale structures such as brine channels, protruding ice pieces, and other structures related to the melting process ŽGulliksen and Lønne, 1991a,b; Lønne and Gulliksen, 1991; Horner et al., 1992; Hop et al., 2000.. The MIZ in the northern Barents Sea is ecologically important because it represents a highly pro-

ductive area in Arctic water masses north of the polar front. The high bioproduction is due to several factors such as: 1. high annual primary production in close association with the receding ice edge and stratified water column; 2. advection of Calanus finmarchicus from the Norwegian Sea into the Barents Sea; 3. transport of ice fauna by the transpolar drift from the Arctic Ocean into the Barents Sea where organisms are released during the melting process. The primary production consists of three components: Ža. actively growing phytoplankton at the outer edge of the ice margin and in larger leads, Žb. a thick layer of specialised sub-ice algal assemblage in dense pack ice, and Žc. a sub-ice algal assemblage associated with multi-year ice ŽSyvertsen, 1991; Melnikov, 1997; Falk-Petersen et al., 1998; Hegseth, 1998.. The onset of primary production is directly related to the seasonal availability of incident light and melting of the ice ŽSakshaug and Slagstad, 1991.. The high annual primary production is coupled to the spatial variation in ice cover, inflow of warmer, nutrient-rich Atlantic water and the stratification of the water column because of melting processes. Ice melting during the Arctic spring and summer give rise to a strongly stratified and nutrient-rich euphotic zone, with a distinct phytoplankton bloom. The phytoplankton blooms follow the receding ice edge as it melts during the spring and summer ŽSakshaug and Skjoldal, 1989; Sakshaug, 1997. and intensive blooms occur in leads as the MIZ opens up ŽZenkevich, 1963.. This implies that the production sweeps across a large area of the northern Barents Sea into the Arctic Ocean and the result is a relatively large annual production. The mean annual primary production in this area is between 50 and 150 gC my2

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yeary1 , which is comparable with that of the North Atlantic shelf and Norwegian fjords ŽFalk-Petersen et al., 1990; Wassmann and Slagstad, 1993.. The advection of C. finmarchicus into the Barents Sea, related to the persistent north–eastward flowing surface current along the north-western European continental rim, is considered to have a profound impact on the pelagic secondary production in the Barents Sea ŽSkjoldal and Rey, 1989.. The water volume advected into the Barents Sea by the Norwegian Atlantic Current is on average 3 Sverdrup Žs 3 P 10 6 m3 sy1 . ŽBlindheim, 1989.. Pedersen Ž1995. calculated that a total biomass of 660 P 10 3 tons carbon in the form of C. finmarchicus is advected into the Barents Sea during May and June; this is in the same order of magnitude as the endemic production. In addition to the transport of C. finmarchicus, there is a large transport of other species, belonging to the pelagic community, into the Barents Sea. The ice fauna transported into the Barents Sea is closely associated with the general ice-drift in the Arctic. The Fram drift Ž1893–1896., showed the existence of a wind-driven polar ice drift from the Siberian shelf seas over the deepest part of the Arctic Ocean through the Fram Strait ŽNansen, 1897.. The general features of the transpolar drift and the ice motion between the Arctic Ocean and the arctic marginal shelf seas have been substantiated later ŽGordienko and Laktionov, 1969; Romanov, 1995.. The ice fauna found in the Barents Sea can be regarded as existing because of an export from a source population inhabiting the central parts of the Arctic Ocean. Annually, in the order of 7 P 10 5 tons, sympagic fauna are transported through the Fram Strait and then lost as the ice-pack melts ŽLønne, 1992.. How much, in addition to that transported into the Barents Sea, is unknown and will vary between years, depending on factors such as the NAO, the geostrophic wind field and the production of ice in the Arctic Ocean ŽVinje, 1997.. The transport of ice fauna into the Barents Sea, however, must be considerable ŽLønne, 1992.. Even though the range in ice fauna biomass was only 0–2 g my2 in the seasonally ice covered Barents Sea ŽLønne and Gulliksen, 1991., the total biomass becomes considerable because of the large extent of the MIZ. During the melting season, high ice fauna biomass has been recorded in open water off Bjørnøya, on the bottom

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in Hinlopen and the Fram Strait ŽWerner et al., 1999.. A multidisciplinary research programme named AEcological and physical processes in the marginal ice zones of the northern Barents SeaB ŽICE-BAR. was initiated by the Norwegian Polar Institute in 1995. The overall goal of the programme was to increase our understanding of the importance of the MIZ for the productivity and biodiversity in the northern Barents Sea. This paper describes the programme for the project, the study area and its physical environment in general. This includes a description of the oceanographic processes, ice density packing and structures in ice cores, optical properties of water masses and the ice, characteristics of the incident spectral radiation, nutrients and chlorophyll leading up to primary production. Most of the background data are from the ICE-BAR 1996 cruise ŽLøyning and Budgell, 1996; Falk-Petersen et al., 1997., whereas additional information are from the ICE-BAR 1995 cruise ŽFalk-Petersen and Hop, 1996; Orvik and Kuznetsov, 1996.. 2. Materials and methods 2.1. Programme for the project The sampling programme included studies of ocean-atmospheric CO 2 exchange, optical properties of arctic water masses, and characteristics of the atmospheric boundary layer. The ice physics part included ice density packing, ice structures in cores, under-ice topography and irradiance, melt processes and spectral reflectance properties of the ice and snow surfaces as well as in melt ponds. The physical oceanographic component was designed to provide a general description of the water masses and circulation patterns of the study area, to characterise how topography and ice cover influence water mass distribution and circulation, and to supply information on some of the abiotic factors, such as oceanic mixing and vertical structures, that may affect primary production and biomass distribution. The studies of the primary producers included productivity and distribution of phytoplankton and ice algae, in addition to biodiversity and taxonomy. The secondary producers, zooplankton and ice fauna, were studied with respect to population dynamics,

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distribution and life strategies. In addition, studies were performed on benthic ecology, sea bird distribution in relation to oceanographic fronts, and trophic relationships in terms of food web structures, energy and lipid transfers, isotopic signals and feeding ecology. Thus, the results presented in this paper form the physical background information for different projects within the multidisciplinary ICE-BAR programme, dealt with in auxiliary papers Že.g., FalkPetersen et al., 1998; 1999; Scott et al., 1998; Hop et al., 2000..

2.2. The ICE-BAR cruises Two ICE-BAR cruises with RrV Lance took place during 9–30 June 1995 and 20 July–16 August 1996. On these cruises, three to four ice stations and one open water station were established in an area east of Svalbard ŽFig. 1.. During the ice-stations, the ship was anchored to an ice-floe and the main engine was turned off. All pelagic samples were taken from the ship, whereas SCUBA diving measurements were carried out under ice-floes. At the open water station,

Fig. 1. Topography and main features of current systems in the Barents Sea. Average position of the polar front is indicated with a thick, grey line. Sampling stations are shown on the map.

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the ship was let drift and sampling of the water column ŽCTD casts, chlorophyll, phytoplankton and zooplankton. was done from the ship’s deck.

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malin for later taxonomic identification and enumeration, using an inverted microscope. 2.5. Radiation

2.3. Hydrographical sampling During both years, oceanographic transects were conducted between the main stations. In 1996, this included a long S–N transect from 758N to 81.38N with hydrographical stations spaced at every 10 nautical miles Ž18 km. in order to provide regional coverage of water mass variations. When mesoscale Ž5 km. structures, such as the polar front or ice edge regimes, were anticipated, station spacing was reduced to 2 nautical miles Ž3.6 km. in an attempt to resolve them. The shipboard ADCP was run continuously throughout the cruise. Hydrographical data were obtained with either an OTS-1500 CTD Žin 1995. or a Neil Brown Instruments Mark III CTD Žin 1996.. Pre-cruise calibration was carried out at the Geophysical Institute, University of Bergen ŽLøyning and Budgell, 1996; Orvik and Kuznetsov, 1996.. The CTD sensors were further calibrated against water samples taken during the cruise. A large number of hydrographical stations were performed both years; 92 in 1995 and 217 in 1996 ŽFalk-Petersen and Hop, 1996; Falk-Petersen et al., 1997., and the resulting data sets therefore give a relatively detailed description of water masses and oceanographic conditions, especially for 1996. The northernmost ice station in 1995 was at 78.288N, whereas later in the summer season of 1996, the consolidated pack ice was as far north as 81.68N latitude, although without a clearly defined ice border. 2.4. Algal biomass Water samples for algal biomass Žchlorophyll a. were collected using Niskin Bottles at each station in the upper 100 m from standard depths Ž0–5–10–20– 30–50–100 m.. The samples were filtered through Whatmann GFrC filters. The chlorophyll a concentration was determined fluorometrically ŽHolm-Hansen et al., 1965., using methanol as solvent. Ice-algae samples were taken by either a rectangular mouth hand net, or by an electric suction sampler. Furthermore, samples were preserved with neutralised for-

Radiation measurements were carried out in 1996 both along the main transects and on the ice floes, utilising underwater spectral radiometers for measurements of shortwave Žultraviolet — UV and photosynthetic active radiation — PAR. attenuation profiles in the euphotic zone Žapproximately the upper 50 m of the water column. and a field spectrometer for measurements of surface spectral albedo and under-ice irradiance at the ice floes. The following measurements were carried out: angular distribution of incoming spectral radiance, bi-directional reflectance, spectral albedo at different surfaces of the ice floes Ždry, wet and dirty snow, ice and melt ponds., melt pond bottom albedo and under-ice spectral irradiance. Each melt pond studied was also examined for temperature and salinity profiles by using a portable hand-held CTD sensor. Underwater spectral attenuation was measured with a Licor 1800UW underwater spectroradiometer, performing spectral radiometric scans from 300–850 nm. Secchi depth was also measured at selected stations. An advanced portable spectroradiometer of type FieldSpec FR ŽAnalytical Spectral Devices., measuring the whole wavelength region from 350– 2500 nm, was used for the surface spectral reflectance and under-ice irradiance measurements. In addition to the spectral radiation measurements, continuous measurements of global short-wave radiation were performed from the masthead of Lance with a Kipp and Zonen CM11 pyranometer connected to a Campbell CR10 datalogger. Solar UVB was measured continuously by an SL501 UV-Biometer. The mean surface radiation budget ŽSRB. for the Barents Sea MIZ was produced with the use of the NASArLangley 8-year SRB data-set from July 1983 to June 1991 ŽWhitlock et al., 1995.. 2.6. Sea ice coÕer The annual variability in the extent of sea ice cover in the Barents Sea is substantial. This variation comes in addition to the large seasonal variation in any single year. Ice map statistics are based on ice maps produced routinely Žfour ice charts per month.

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at the Norwegian Meteorological Institute. Maps of ice extent were constructed based on ice statistics from the period of 1966–1989. The ice map series has been compared with US, UK and Icelandic ice maps, and modified to obtain the best possible temporal development. Sea ice distributions were observed on two spatial scales Ždifferent kilometer resolution. based on special sensor microwave imager ŽSSMrI. and NOAA advanced very high resolution radiometer ŽAVHRR. satellite data. Daily mean sea ice concentrations at the largest scale of 25 km resolution were computed from SSMrI data from the Defence Meteorological Satellite Program ŽDMSP. F-13 satellite. Data for sea ice concentrations were provided by the EOS Distributed Active Archive Centre ŽDAAC. at the National Snow and Ice Data Centre, University of Colorado, USA. The NASA Team Algorithm ŽCavalieri et al., 1992. was used in the computation of sea ice concentrations from brightness temperatures. Sea ice distribution at a higher spatial resolution was determined based on AVHRR data with a resolution of 1.1 km at nadir from the NOAA-12 satellite. Sea ice was identified using data from channel 1, the visible band of 560–710 nm. In order to maximise the cloud-free coverage, composite images were constructed from two to four images. The basis for blending the images was a combination of minimum reflectance in the visible band and maximum brightness temperature in the channel 4 thermal infrared band of 10,300–11,600 nm. The AVHRR data were obtained from the NOAA Satellite Active Archive ŽSAA. facility. 2.7. Sea ice cores Sea ice cores were collected, in 1995, with a 75-mm diameter Cold Regions Research and Engineering Laboratory ŽCRREL.-type coring auger at three ice stations. At each station, two sea ice cores were collected at two points 25-cm apart on each floe. Snow depth and sea ice thickness were also measured near those points. One ice core from each floe was cut in lengths of 10 cm, packed in polyethylene bags and melted before measuring salinity with a salinometer ŽSolomat MPM 2000. to 0.01 psu precision. The other ice core samples were packed in

polyethylene bags and stored in a y158C cold room on the ship, and subsequently shipped under the same conditions to the Sea Ice Research Laboratory at Hokkaido University in Japan. In the laboratory, each core was split lengthways to obtain 0.5-cm-thick vertical sections along the entire core length. Horizontal thick sections were also produced at depth intervals of 10–20 cm. These thick-sections allowed us to examine bubble and brine layer distributions under scattered light. Then, the sections were smoothed by planing to a thickness less than 0.1 cm and they were illuminated under polarised light to identify individual grains and their structures. The entire length of the cores was classified into ice types in accordance with Eicken and Lange Ž1989. and then cut at 5–10-cm intervals along the structural boundaries. In 1996, sea ice cores were also collected with a 75-mm-diameter CRREL-type coring auger at three ice stations. At each station, one sea ice core was collected, packed and stored as previously described before being shipped to Japan. Similar to the 1995 core analysis, each core was analysed for horizontal and vertical sections. The cores were then cut at 4–10 cm intervals along the structural boundaries to be able to measure the density of each structure. Samples were melted at 208C before measuring the salinity with a salinometer. The ratio of isotopically heavy oxygen molecules Ž18 O. to light Ž16 O. molecules in the ice cores was determined in 1996. This oxygen isotope analysis, together with textural analysis, provides information on whether the ice was formed directly from the seawater or from a mixture of snow and sea water. The so-called d18 O value is the ratio of these two isotopes in the sample relative to the same ratio in standard mean ocean water Že.g., Lange et al., 1990.. The oxygen isotope analysis was made with a mass spectrometer ŽFinnigan MAT Delta E, precision of 0.1 ppt. at the National Institute of Polar Research, Japan. 3. Results 3.1. The study area The investigated area was located in the northern Barents Sea, between 758N and 828N and between

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Spitsbergen and 358E ŽFig. 1.. The northern Barents Sea is a continental shelf sea with moderate depths, generally - 300 m. The banks of Storbanken, Spitsbergenbanken and Sentralbanken have depths of 120–200 m. Deep trenches Ž300–500 m. penetrate into the continental shelf both from south and north. From the south, the Hopen Trench penetrates north to Storbanken between Sentralbanken and Spitsbergenbanken. From the north, trenches penetrate between Nordaustlandet and Kvitøya, and between Kvitøya and Victoria Island. The deep trenches from north and south are separated by a narrow sill Ž200 m. in the Storbanken area. 3.2. Annual Õariation in incident light The incoming light changes dramatically during the year at high latitudes ŽFig. 2.. After 4.5 months of polar night, from the middle of October to the end of February, the solar radiation increases sharply during spring with increasing solar elevation over the northern Barents Sea. The midnight sun in the MIZ at 81825X N lasts from 12 April to 31 August, and the global solar radiation can reach maximum midsummer values as high as 400 W my2 under clear sky conditions. The global radiation is significantly reduced under cloudy skies ŽFig. 2., and a large variation in incident radiation is caused by the large variability in the weather and surface conditions.

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The integrated albedo ŽØrbaek, the Norwegian Polar Institute, personal communication. of the MIZ shows representative values of 20–60%, illustrating the variable ice conditions and ice densities. Highest albedo values are seen in early spring when the ice extent is at its maximum, whereas the minimum values appear when the ice density is low during late summer. The downward longwave radiation shows little variation over the year, with slightly higher values during the summer and early autumn reflecting the higher atmospheric temperature and the increased cloud cover during this period. Whereas the net longwave radiation is almost constantly negative with approximately y50 W my2 , the net shortwave radiation is highest after solstice because of the lower albedo of the more open waters. The total net radiation is thus, positive only during the 4 months of May–August. The weather conditions at the ICE-BAR 1996 cruise were mostly cloudy with global radiation levels ranging from cloudy at 100–200 W my2 to clear sky at approximately 400 W my2 . 3.3. Annual Õariation in sea ice conditions The dynamic and thermodynamic processes of the area determine the sea ice conditions in the Barents Sea. There is also a transport of sea ice into the Barents Sea from the Arctic Ocean between Nordaustlandet and Franz Josef Land and from the Kara

Fig. 2. SRB of the Barents Sea MIZ, 81.248N, 24.548E. The data is averaged over the period 1983–1991 and monthly mean radiation components are derived from NASA Langley SRB Data set Ž1983–1991.. The plot shows the clear sky shortwave downward ŽSW Down Clear Sky. radiation and the corresponding all sky radiation ŽSW Down All Sky. together with the similar longwave ŽLW. components. Shortwave net ŽSW Net All Sky., longwave net ŽLW Net All Sky. and total net radiation ŽTotal Net Rad. are also plotted.

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Sea between Novaja Zemlja and Franz Josef Land. The ice-covered area in the Barents Sea and the exchange of sea ice between the Arctic OceanrKara Sea and the Barents Sea show great interannual variation.

The ice extent in the Barents Sea has its maximum during late winterrspring and a minimum in late autumn. This gives an indication of the response time due to transport of heat through the air and radiation, which has its maximum in midsummer.

Fig. 3. Frequency of ice coverage in the Barents Sea 1966–1989: Ža. January to June, Žb. July to December. White area indicates that the ice extent always has covered this area. The lightest grey tone similarly indicates that the ice extent has covered this area more than 90% of the time Žbut not always.. The second lightest grey tone indicates that the ice extent has covered this actual area 90–80% of the time. The third lightest grey tone indicates that the ice extent has covered this area 80–70% of the time, etc. Black indicates that the ice extent has covered this area at least one time during 1966–1989 but less than 10% of the time. Blue indicates the ice extent never has covered this area. ŽAfter geophys.npolar.nor;reinertrmapsrmaps.html..

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Fig. 3 Ž continued ..

The inter-annual variations of the sea ice extent in the Barents Sea can exceed annual variations. The minimum observed ice extent in March, for example, can be smaller than the maximum observed ice extent in September ŽFig. 3.. During the June 1995 cruise, heavy ice conditions existed east of Spitsbergen ŽFigs. 4a and 5a.. A well-defined, distinct ice edge was found south of Hopen, close to 768N and ice concentrations of up to

10r10 existed in Storfjorden and the area between Edgeøya and Storbanken. Open water was found north and west of Svalbard, as well as in the eastern Barents Sea south of Franz Josef Land. The well known semi-permanent polynya west off Kvitøya can also be seen on the image and is associated with a band of loosely packed ice southwards along 358E. The increase in size of the ice floes northwards can bee seen clearly on the AVHRR image ŽFig. 4a.. The

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open lead in the pack ice north of Svalbard, called the AWhalers BayB by the early whale hunters of the 17th century, is also visible. During the 1996 cruise, in July and August, there was a dramatic retreat of the ice as shown from the

SSMrI and AVHRR ŽFigs. 4b–f and 5b–f.. On 19 July, unconsolidated ice existed between Edgeøya and Franz Josef Land, south to 778N, with ice concentrations of 3r10 to 5r10. A ring of 4r10 to 5r10, ice centred south-west of Kong Karls Land,

Fig. 4. AVHRR visible band images of Svalbard and the northern Barents Sea. Sampling stations and ship track are marked in red and depth contours are in blue. Ža. 19 June 1995, 1411 UTC image, Žb. 19–20 July 1996, 1347 UTC and 0646 UTC images combined, Žc. 28 July 1996, 1351 UTC image, Žd. 3 August 1996, 1320 UTC image, Že. 5 August 1996, 0740, 1235, 1417 UTC and 6 August 1354 UTC images combined, Žf. 8 August 1996, 1310 UTC image.

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Fig. 4 Ž continued ..

extended from 768N to just north of Kong Karls Land and from Edgeøya to 358E. This ring persisted through 28 July but was mostly melted by 3 August. The semi-permanent polynya off Kvitøya as well as the AWhalers BayB north-west off Spitsbergen can

also be seen. An ice tongue stretching down from the Arctic pack ice south-west of Franz Josef Land can be seen during the whole period. In the beginning of August, the ice edge had retreated to Kvitøya, and the area around Kvitøya

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Fig. 5. Ice concentration from SSMrI of the northern Barents Sea. Ice concentration, or fractional areal ice cover, varies from 0 Žopen water. to 1 Ž100% ice cover.. Ža. 19 June 1995, Žb. 19 July 1996, Žc. 28 July 1996, Žd. 3 August 1996, Že. 5 August 1996, Žf. 8 August 1996.

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Fig. 5 Ž continued ..

was nearly ice-free on 8 August. However, an area of low ice concentration can be seen in the area

around Kong Karls Land during the entire period. This is an area of intense melting. Between 28 July

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Fig. 5 Ž continued ..

and 3 August, a strong wind from the north pressed the ice border southwards. The ice tongue was forced

towards Kvitøya and south into the eastern part of the Barents Sea. This tongue is clearly evident in

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both AVHRR and SSMrI images ŽFigs. 4d and 5d.. A bay of ice-free water, similar to that found in 1995, opened up south-west of Franz Josef Land. 3.4. Physical oceanography The large scale water masses and currents in the northern Barents Sea ŽFig. 1. are related to one branch of the Norwegian Atlantic Current entering the Barents Sea and flowing eastward along the southern edge of the Bear Island Trough into the Hopen Trench where it partly submerges under the lighter Arctic water ŽLoeng, 1991.. At the sill between Nordkappbanken and Sentralbanken, this current splits, with one branch continuing over the sill and the other turning northwards towards the Hopen Trench and eventually exiting the Barents Sea to the west. This latter branch, which can be seen mainly during autumn is bounded to the east and north by the sill, roughly at the 250 m isobath ŽGawarkiewicz and Plueddemann, 1994.. It flows west of Spitsbergen and forms the North Atlantic Water recirculation. Arctic surface water flows southwards in the Barents Sea through two passages, between Spitsbergen and Franz Josef Land and between Franz Josef Land and Novaja Zemlya. The polar front is identified as the transition zone between the Atlantic and Arctic water masses, roughly following the 250 m isobath along the Hopen Trench. The warm Atlantic water is characterised by salinity above 35 psu, whereas the properties of the intermediate Arctic water, found between 20 and 150 m depth, has temperatures close to the freezing point Žbelow y1.58C. and salinity between 34.4 and 34.6 psu. A striking feature of the hydrography in the MIZ in spring and summer is a 20-m-thick layer of melt water due to melting of the first-year ice ŽFigs. 6–8.. Later in summer ŽAugustrSeptember., the ice edge retreats further north in the Barents Sea and exposes a large area of open water to light, setting the stage for plankton blooms. The oceanographic conditions in June 1995 were spring-like, with ongoing ice melt ŽFig. 6.. All three ice stations showed equally stratified density fields with a weak pycnocline in the 15–20 m layer, which was defined by a halocline produced by dilution of the surface layer Ž0–15 m. by melting ice. The temperatures were close to the freezing temperature

Fig. 6. Temperature Ž8C., salinity Žpsu. and chlorophyll a concentration Žmg ly1 . in the water column during June Ž1995. at sampling stations, in the northern Barents Sea.

Žy1.48C to y1.68C. for water with this salinity at the ice stations. At depths down to 100 m there was

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seasonal thermocline. Further down to the bottom, we detected Barents Sea water Ž0–1.08C.. The pycnocline and the seasonal thermocline are both subjected to periodic disturbances, most probably produced by the semi-diurnal tidal cycle. The oceanographic conditions at open water stations, situated only a few kilometres from the ice edge, differed from those at the ice stations in that an upper-layer thermocline was well defined in open water and that the temperature was considerably higher Ž0.78C.. A commonly used spacing between hydrographic stations in the Barents Sea has been 10 nautical miles, which cannot resolve dynamic features of scales less than 20 nautical miles, as explained by the Nyquist frequency. This means that we may not be able to explain all the variations observed, particularly on the smaller scales. However, the physical environment in relation to the primary production and the marine ecosystem may be adequately described. Fig. 8 shows isolines of temperature, along a transect from south to north, across Storbanken and passing by Kvitøya. We clearly see a warm water mass ŽT ) 08. penetrating southwards from the north. This water mass is of Atlantic origin. Above this water mass is a colder water mass, with a cold core above Storbanken. This water mass is of the Arctic Water type, modified by the late winter freezing and summer melting. Results from the analysis of observations from 1996 revealed dynamic features in the water masses on a much smaller spatial scale than had been observed before, i.e., down to two–three nautical miles ŽLøyning, 2000.. 3.5. Sea ice stations and ice coring

Fig. 7. Temperature Ž8C., salinity Žpsu. and chlorophyll a concentration Žmg ly1 . in the water column during July and August Ž1996. at sampling stations in the northern Barents Sea.

a layer produced by winter convection, and under this layer down to a depth of 140 m, there was a

Ice cores were collected at three different ice stations in 1995 ŽFig. 1. and the characteristics of these cores are summarised ŽTable 1, Fig. 9.. Ice station 1 was situated in an area of dense first-year ice with ice concentration of 8r10–9r10. The ship was anchored to a large ice floe of the size 5 = 10 km. Ice cores showed that the ice thickness varied from 1.1 to 1.6 m. The ice core analysed for stratigraphy in Japan was 143 cm in situ, but the 24-cmthick topmost layer was fragile and lost during sampling. The ice core from 53-cm thickness to the bottom was distorted and melted from 7.5 to 6.0–6.5 cm in diameter, presumably as a result of melting

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Fig. 8. Potential temperature Ž8C. from south to north along 358E Žsection A., July 1996, in the northern Barents Sea Žafter Løyning 2000.. The thick zero-temperature contour line, separates the warm subsurface water mass of Atlantic origin, penetrating from the north, and the colder arctic water mass on top.

during transportation to Japan. Stratigraphy of the entire ice core indicated that the granular ice g and columnar ice c alternated. The ice core between the

24–53 cm was composed of columnar ice, then the granular ice appeared between the 53–90 cm, and again the columnar ice appeared from 90-cm thick-

Table 1 Snow and ice parameters at ice stations during June 1995, in the northern Barents Sea Station

1 2 3

Latitude, longitude

X

78804.8 N, X 34816.9 E X 77839.6 N, X 34816.9 E X 77839.2 N, X 34856.5 E

Data of sampling

Ice concentration

Snow depth Žcm.

Ice core for salinity

Ice core for stratigraphy

Structural component

Thickness Žcm.

Salinity Žpsu.

Thickness Žcm.

Granular ice Ž%.

Columnar ice Ž%.

15 June

8r10–9r10

10.0–11.0

132

4.4

143

31

69

19 Jun

7r10

2.5–4.0

134

3.7

148

77

23

23 Jun

6r10–7r10

2.5–3.5

79

3.4

72

7

93

148 S. Falk-Petersen et al.r Journal of Marine Systems 27 (2000) 131–159

Fig. 9. Vertical profiles of salinity and stratigraphy of the ice cores obtained at ice station 1, 2 and 3 in June 1995 Ž g indicates granular ice and c indicates columnar ice. from the northern Barents Sea.

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ness to bottom. The mean salinities measured on board the ship were 3–5 psu with no depth trends. Ice station 2 was situated in thick first-year hummocked ice, with concentration of 7r10. The ice floes were much smaller than those at the former station, but the ice cores showed that the ice thickness was still between 1.35 and 1.5 m. The length of the ice core used for stratigraphy was 148 cm in situ, but considerable portions of the top layer were fragile and melted, and only the remaining 88-cm ice core was kept for analysis. The top layer consisted of granular ice, whose grain size generally was several millimeter with a maximum of 20 mm. The middle and lower sections of the core were composed of columnar ice, crystallographically connecting completely down to the bottom. This suggests that icecontaining columnar features might be formed and grown in calm conditions. The salinity increased with depth, from 1.5 psu for the 15-cm thick topmost layer to 5 psu at the bottom of the core. Ice station 3 was located in an area of first-year ice, with ice concentration of 6r10, where small floes up to 50 m in diameter prevailed. Some of the ice floes were strongly affected by melting processes, so that the ice could be characterised as rotten. The station was located close to the ice edge, and the swells were penetrating from the open water. Characteristics of two ice cores collected on 23 June showed that the core was distorted and partly melted, and the diameter of the core was reduced to 6.0–6.5 cm when it was analysed. The 5-cm-thick topmost layer consisted of granular ice with large grains of 0.5–1.0 cm in diameter. Below this layer, there was a 50-cm-thick layer consisting of columnar ice, which continued crystallographically to the bottom of the

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core. The salinity increased from 1 psu at the top to 4.5 psu at 40-cm thickness. Snow depths in 1995 on the first-year ice in this study area ranged from 2.5 to 11 cm. Similar values were reported for a summer snow cover on first-year ice in the Fram Strait with a mean of 8 cm at a maximum of 20-cm deep snow ŽTucker et al., 1987.. At station 1, the granular ice was observed in the middle of the core, sandwiched between the upper and lower columnar ice. This sandwiched granular ice might be associated with folding ice or buried snow-ice formed during rafting andror ridging processes ŽJeffries et al., 1997.. Although the sandwiched granular ice was not identified to be formed from snow ice or frazil ice in this study, it is presumed that the ice floes at station 1 might be grown in calm conditions and thereafter affected by dynamic atmospheric and oceanic processes. The average ice salinity were 4.4, 3.7 and 3.4 psu at stations 1, 2 and 3, respectively. These values were similar to 1–5 psu recorded in the first-year ice elsewhere in the Arctic ŽCox and Weeks, 1974; Overgaard et al., 1983; Tucker et al., 1987.. Characteristics of the ice cores collected at three stations in 1996 are summarised in Table 2 and Fig. 10. Ice stations 0 and 1 were situated in an area of dense multi-year ice, with ice concentration of 8r10–9r10, close to the perennial ice pack at 81.308N, whereas the other stations were situated in bands of loosely packed ice ŽFig. 4b.. Ice core stratigraphy from ice station 0 shows that the granular and columnar ice appears alternately through the entire core. The orientation of the columnar ice was slanted on the vertical at three layers in the core. This suggests that the ice might be formed

Table 2 Snow and ice parameters at ice stations during August 1996, in the northern Barents Sea Station

1A 2A 3A

Latitude, longitude X

81830.9 N, X 33815.3 E X 79826.8 N, X 32815.4 E X 78832.0 N, X 35849.4 E

Data of sampling

Ice concentration

7 August

7r10–8r10

5 August 8 August

Snow depth Žcm.

Ice core

Structural components

Thickness Žcm.

Salinity Žpsu.

Density Žkg m y3 .

Granular ice Ž%.

Columnar ice Ž%.

7

465

1.3

893

52

48

2r10

10

428

1.1

875

52

48

3r10

4

362

1.3

901

51

49

150 S. Falk-Petersen et al.r Journal of Marine Systems 27 (2000) 131–159 Fig. 10. Vertical profiles of salinity, density, stratigraphy and, isotope composition Ž d18 O. of the ice cores obtained at ice station 0, 2 and 3, in August 1996 Ž g indicates granular ice, c indicates columnar ice, clg indicates mixed columnar and granular ice and cr g indicates intermediate columnar and granular ice. from the northern Barents Sea.

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and grown during rafting andror ridging processes. The salinity of the core was almost consistent with depth at 0 psu for the 60-cm-thick topmost layer, then increased with depth from 0.5 psu at the 60-cm thickness to 2.5 psu at the 1-m, and below this layer fluctuated around 2 psu down to the bottom of the core. The density of the core was between 840 and 880 kg my3 for the 60-cm-thick topmost layer of granular ice, and then about 900 kg my3 down to the bottom of the core. The d18 O values were about 0 ppt throughout the entire core, although slightly negative Žy1.0 ppt. at the topmost layer. Characteristics of the core stratigraphy at ice station 2 were similar to those of station 0 with granular ice and columnar ice alternating throughout the entire core. The orientation of the columnar ice was also slanted here on the vertical at four layers through the 4.28-m thick core. The salinity of the core was almost 0 psu for the 70-cm-thick topmost layer, and below this layer it fluctuated between 1 and 3 psu down to the bottom of the core. The density of the core was between 790 and 900 kg my3 for the 70-cm-thick topmost layer and then 850 and 910 kg my3 for the underlying part of the core. The d18 O values were negative, about y20 ppt, at the topmost layer and increased steeply to y2.7 ppt down to about 30-cm depth. Below this depth the values ranged from y1.3 to y3.3 ppt. The core collected at ice station 3 was similar to the other ice cores with alternating granular and columnar ice. The slanted orientation of the columnar ice was also seen at three layers near the bottom part of the core. The salinity of the core was almost 0 psu for the 50-cm-thick topmost layer, and below this layer, it fluctuated between 0.5 and 2.2 psu down to the bottom of the core. The density of the core increased with depth from 762 kg my3 at the topmost layer to 898 kg my3 at 34-cm thickness, and below this layer it was between 840 and 910 kg my3 with no relationship to depth of the core. The d18 O values were almost constant at about 1.5 ppt from the topmost to the middle of the core at 2.0 m depth. Below this depth at the minimum of 0.28 ppt, the values increased gradually to about 2.2 ppt at the bottom of the core. Snow thicknesses ranged from 4 to 10 cm and were similar to those obtained in 1995. However, the ice thickness, salinity and structural components for

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the 1996 cores ŽTable 2. were quite different from those in 1995 ŽTable 1.. The ice thickness in 1996 was 3.6–4.7 m, several times thicker than in 1995 when the stations were located farther south. The thick ice in 1996 appears to be multi-year ice floes, since similar thickness of multi-year ice have been observed in other parts of the Arctic ŽSchwarzacher, 1959; Overgaard et al., 1983; Tucker et al., 1987; Eicken et al., 1995.. One of the uncommon characteristics of the 1996 ice cores was a thin layer consisting of a few submillimeter to a few millimeter grains with vertically oriented c-axes at the top of the cores at stations 2 and 3 and at the middle of the cores, at stations 0 and 3. Similar layers were observed in the ice collected in the Sea of Okhotsk, suggesting that those layers might be formed during the initial growing stage of nilas in calm conditions ŽT. Toyota and T. Kawamura, personal communication.. Another characteristic was a layer of irregularly oriented crystal structure at the bottom of the core at station 2. The crystal structure appeared to be similar to the platelet ice, reported in Antarctic sea ice ŽEicken and Lange, 1989; Lange et al., 1989; Jeffries et al., 1993. and also in the Arctic sea ice ŽEicken, 1994; Jeffries et al., 1995.. Several mechanisms of platelet ice growth have been proposed, but mechanisms which contribute to this irregular structure remain unresolved. 3.6. Radiation measurements The underwater radiation profiles in 1996 showed large variations in the optical properties of the water masses in question. Profiles of the attenuation coefficients for PAR from ice stations 0, 1, 2 and 3 are shown in Fig. 11. Secchi depths varied considerably Ž9, 16 and 25 m at stations 1, 2 and 3, respectively., and the 1% level of the surface irradiance varied from approximately 15 m at ice station 0, 25 m at ice station 1 and it was 40 m at the very clear water at ice station 2. The PAR profiles for ice stations 1 and 2 showed a strong absorbing layer between 20–30 and 20–40 m, respectively, whereas the profile for station 0 showed a more homogeneous absorbing layer over the whole region from 5 to 25 m. These layers corresponded well to the distribution of phytoplankton in the water column. The highest density was found just above the pycnocline.

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Fig. 11. Underwater photosynthetic active radiation ŽPAR. profiles, ŽPAR in percent of surface radiation. for stations 0, 1, 2, and 3 during July–August 1996, in the northern Barents Sea.

Albedo measurements at different melting snow surfaces and melt ponds illustrate the large variation in optical absorption properties of different surfaces in the MIZ ŽFig. 12.. The spectral albedo Žat 450 nm. of two melt ponds, one clean and one dirty pond, varied from 0.1 to 0.5. The albedo of both ponds was very low above 700 nm, and the main difference between the two occurred below 580 nm. For the snow surfaces, the albedo changed from about 0.4 for wet snow to above 0.8 for dry snow at

450 nm. The more blue- and grey-coloured surfaces had intermediate values of about 0.75, with similar spectral shape over the spectral range. A dirty snow surface turned out to be similar to the dry snow albedo, above 1000 nm, but was significantly reduced below 1000 nm with a spectral albedo of about 0.3 at 450 nm. The variability of the under-ice radiation field was greater than expected. The under-ice surface was a mosaic of bright and dark areas, even under ice with

Fig. 12. Surface spectral albedo of clean snowrice, dirty snowrice, clean melt pond, hidden melt pond, and dirty melt ponds of ice floe surfaces at Station 0, 1, 2 and 3, during August 1996, in the northern Barents Sea.

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Table 3 Dominating phytoplankton species during the summers of 1995 and 1996, in the northern Barents Sea Date

Area

Dominating species

June 1995

Ice station 1 Ice station 2 Ice station 3 Open water Polar Front Storbanken Kvitøya Ice station 0, 2 Ice station 2, 3 Open water

Heterocapsa rotundata, small flagellates Fragilariopsis oceanica, small flagellates Thalassiosira antarctica var. borealis, Chaetoceros socialis, Dinobryon balticum Phaeocystis pouchetii, T. antarctica var borealis, T. graÕida, C. socialis, D. balticum P. pouchetii Small dinoflagellates and flagellates T. nordenskioeldii, T antarctica var borealis, C. wighami, C. fragilis, F. oceanica T. antarctica var borealis, T. nordenskioeldii, T. bioculata, C. socialis C. socialis Žsome with resting spores., P. pouchetii, flagellates C. socialis, P. pouchetii, D. balticum, flagellates

July 1996

August 1996

homogeneous snow surfaces. This was induced by all the cracks and the multitude of different ice cubes frozen together, refracting the light path, as well as the optical window effect of the melt ponds that results from the large difference in albedo from pond to snowrice surfaces. Some of the dark areas may also have been ice algae and detritus lumps. 3.7. Biomass of primary producers During the 1995 cruise, the phytoplankton biomass was concentrated in the upper layers at ice station 1 ŽTable 3, Fig. 6.. The low chlorophyll values Žmax. 0.5 mg ly1 ., and a plankton population dominated by a small dinoflagellate, H. rotundata, along with other small flagellates, indicated a pre-bloom phase. The phytoplankton conditions at ice station 2 were comparable to that of ice station 1, and the chlorophyll biomass was still fairly low Žmax. 0.9 mg ly1 .. A dominance of the early spring diatom Fragilariopsis oceanica in the upper layer populations indicated a bloom in its early phase. At ice station 3, the phytoplankton biomass exhibited a small maximum at 20 m Žmax. 1.9 mg ly1 ., indicating a bloom just about to peak and sink out of

the euphotic zone. This picture was further confirmed by populations dominated by typical spring species, such as T. antarctica var. borealis and F. oceanica in the biomass maximum, and of the chrysophyte D. balticum, a typical summer species in the Barents Sea, dominating above. The phytoplankton conditions at the open water station were those of a post-bloom stage with a well developed chlorophyll maximum at about 50-m depth Ž2–2.5 mg ly1 ., typical of a summer situation in large parts of the Barents Sea. The biomass maximum was dominated by P. pouchetii and the diatoms T. antarctica var. borealis and T. graÕida, whereas D. balticum dominated in the upper 30 m. In 1995, the ice algae were mainly found either as balls and lumps that were loosely attached under the ice, or as long strands that were attached to the ice ŽTable 4.. At ice station 1, strands of ice algae of up to 2-m length were found under first-year ice floes. These strands were formed by the dominant diatom M. arctica and three epiphytic diatom species on the Melosira strands: A. septentrionalis, Pseudogomphonema cf. septentrionale and Synedra hyperborea. At ice station 2, the dominant and subdominant species were N. frigida, NaÕicula karianar

Table 4 Dominating species of ice algae during June 1995, in the northern Barents Sea Date

Area

Dominating species

June 1995

Ice station 1 Ice station 2

Melosira arctica, Attheya septentrionalis, Pseudogomphonema arctica, Synedropsis hyperborea Nitzschia frigida, N. promare, Fossula arctica, M. arctica, A. septentrionalis, S. hyperborea

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Fig. 13. Chlorophyll a concentration Žmg ly1 . isopleths between 76. 348N and 81. 138N along 358E Žsection A., during July–August, 1996, in the northern Barents Sea.

transitans, N. septentrionalis, M. arctica, F. oceanica, S. hyperborea and A. septentrionalis. Much slime with bacteria, protozoans and empty frustules of diatoms were included. The cruise track in 1996 started at the Polar Front in an area with a deep chlorophyll maximum of 4.5 mg ly1 ŽFigs. 7 and 13, Table 3., dominated by P. pouchetii. Further north over Storbanken, the phytoplankton populations were extremely scarce Ž0.2–0.4 mg ly1 ., consisting primarily of flagellates and small dinoflagellates. The open water area around Kvitøya exhibited higher chlorophyll biomass Ž2–3 mg ly1 ., particularly in the upper layers, and the populations were dominated by diatoms such as Thalassiosira ŽT. nordenskioeldii, T. antarctica., Chaetoceros Ž C. wighami, C. fragilis . and F. oceanica. Biomass decreased north of Kvitøya, although some of the same diatoms were still present. At the stations furthest north, scattered blooms were encountered, with high biomass both in the surface layers Žice station 0, max. 3–4 mg ly1 . and at greater depths Žice station 1, max. 16 mg ly1 . between areas of low biomass. The blooms were dominated by the diatom

T. bioculata. In the western part of the cruise area, between Spitsbergen and Storbanken, deep chlorophyll maxima of 2–5 mg ly1 were often encountered, consisting of diatom species such as T. nordenskioeldii, T. antarctica and C. sosialis with resting spores, together with Phaeocystis flagellates and Dinobryon sp.

4. Discussion Pelagic and sympagic ecosystems in the MIZ are exposed to extreme variability in light climate, ice cover, insulation, melt water formation, surface salinities, and sedimentation. They are also exposed to low, but variable temperature. These fluctuations can have a time span of hours, days, seasons, and years to decades or even longer. The MIZ of the Barents Sea is situated above a large continental shelf were water mass modifications are due to strong interactions between the ice cover, the water column and the bottom topography. The observations described herein confirm the descriptions of water

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mass distributions of Pfirman et al. Ž1994. and Loeng Ž1991. and the ice characteristics described by Vinje and Kvambekk Ž1991. and Korsnes Ž1993, 1994.. However, very few simultaneous measurements of such a wide range of physical and biological variables have been taken previously from the northern Barents Sea. Oceanographic phenomena such as the semi-permanent lee polynyas found west and south-west of Kvitøya and Franz Josef Land ŽVinje and Kvambekk, 1991. and the bay of open water, the AWhalers BayB north of the Spitsbergen, are structures which can change with time intervals of hours to decades. For example, the polynya south of Franz Josef Land was clearly evident during ICE-BAR 1995 and during the late stages of ICE-BAR 1996, but not during the first two weeks of ICE-BAR 1996. Sea ice conditions changed dramatically during ICE-BAR 1996, largely because of the occurrence of northerly winds and the resultant southward transport of Arctic pack ice into the central and eastern Barents Sea. The location of the ice edge during the summer in the Barents Sea can vary by hundreds of kilometres from year to year and there is a strong relationship between the NAO index and the location of ice edge ŽVinje, 1997.. The ice cover is also significantly reduced in the Nordic Seas in high, as compared to low, NAO years. For the period of 1960 to present, it is found that high NAO years bring more winter storms, higher vapour flux, higher precipitation and relatively warmer water into the Barents Sea. This causes a northward shift in the MIZ of the Barents Sea. An extreme northern ice edge location in April was recorded in 1995 and the northerly distribution of the MIZ in the period 1989–1995 was associated with a high NAO index ŽVinje, 1997; Vinje, Norwegian Polar Institute, personal communication.. Sea ice generally consists of granular ice in the upper part and columnar ice in the lower part, with columnar ice being the predominant structure in the Arctic. The columnar ice tends to grow downwards and its crystal structure is continuous ŽTucker et al., 1987; Meese, 1989; Eicken et al., 1995.. Sea ice cores taken in 1995, showed that the ice thickness was 1.3–1.5 m at stations 1 and 2, whereas it was about half of that on station 3. This thickness indicated that the ice was first-year ice ŽOvergaard et al., 1983.. The granular layer in the top part of the ice

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constituted 7% and 18% of the total length of cores, taken at ice stations 3 and 2, respectively. This is similar to what has been observed in other Arctic areas. Ice from the Fram Strait contained an upper layer of about 5% granular ice ŽTucker et al., 1987., whereas 5–10% granular ice has been recorded for Arctic near-shore conditions ŽMartin, 1979.. However, in 1996, granular and columnar ice alternated throughout the entire ice cores at all three stations, and a slanted orientation of the columnar ice was also apparent. The three cores had similar mean density values, but variable density profiles Ž760–910 kg my3 .. The large variation in density profiles suggests that the ice floes had been formed and grown during multiple rafting events andror ridging processes. The average salinity for the 1996 cores was 1.32, 1.10 and 1.25 psu at stations 1, 2 and 3, respectively. The salinity values were about 60% lower for the 1995 cores. This was likely due to difference in age; first-year ice 1995 vs. multi-year ice in 1996. The salinity was almost zero for the topmost 60 cm of the ice at all stations in 1996. This indicates desalination during previous freezing events ŽTucker et al., 1987; Eicken et al., 1995., which is consistent with other observations for multi-year sea ice. ŽSchwarzacher, 1959; Cox and Weeks, 1974; Overgaard et al., 1983; Tucker et al., 1987.. In 1996, d18 O values in the topmost layer of the ice at station 2 were negative, about y20 ppt, whereas ice below 0.3 m had only slightly negative values. This suggests that the topmost layer was formed from the snow cover and that the underlying layers were formed from a mixture of snow and sea water. The shape of the d18 O profile at station 2 was similar to that of Antarctic sea ice Že.g., Lange et al., 1990; Jeffries et al., 1994., which has large negative values at the top of the ice core. The d18 O profiles at stations 1 and 3 were uniform throughout the ice cores with zero to slightly positive values. This suggests that the ice was formed from a mixture of snow and sea water or directly from the freezing of sea water. The observed variability in physical conditions had a great impact on the biology because of the close coupling between primary producers and the physical environment. In 1995, the cruise track intersected an area covering a pre-bloom phase well

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within the ice-covered parts of the Barents Sea to a post-bloom phase in the open waters to the south. A moderate increase in phytoplankton biomass, but no typical ice edge bloom, was found in the MIZ, indicating that such blooms are not always present in spite of apparently favourable physical conditions. A build-up of a bloom requires 2–3 weeks of open water, relatively stable ice cover and stratification of the water column. During the first week of June, the ice cover was pushed southwards, and by the time of our sampling Žmid June., it was rapidly retreating northwards. Hence, the period of open water prior to our sampling was obviously too short for a bloom to develop. The biological conditions in 1996 were considerably more variable as the cruise track passed through areas of very different ice and oceanographic conditions. All developmental stages of phytoplankton blooms were encountered more or less simultaneously, and the previously adapted picture of a bloom following the retreating ice edge northwards was never seen ŽSakshaug and Skjoldal, 1989; Strass and Nothig, 1996; Sakshaug, 1997.. No typical ice edge ¨ was found in the northern Barents Sea and melting occurred all over the area, creating a number of weak melt water fronts, none of which were suitable for creating bloom conditions. Production was going on from the Polar Front and north to 81.38N, but different physical conditions created highly different phytoplankton biomass and production patterns. The Polar Front area had been ice-free for about 1 month by the beginning of August, hence, any previous blooms were long gone. The phytoplankton biomass was here concentrated as a deep maximum dominated by Phaeocystis, typical for the Barents Sea summer conditions in the southern part. Cold water masses produced during freezing the previous winter over Storbanken were characterised by extremely low productivity and low biomass, predominated by small flagellates ŽHegseth, 1998.. Further north, however, phytoplankton conditions similar to those in the front area were found. Melting in these areas had started 2–3 weeks earlier, in the beginning of July, creating sufficient time for blooms to develop. These blooms could be seen as deep chlorophyll maxima at or below the pycnocline, with moderate production rates ŽHegseth, 1998.. The conditions closely resembled those of the 1991 sum-

mer ŽStrass and Nothig, 1996., and might, thus, be ¨ representative of this part of the northern Barents Sea. In the lee polynya around Kvitøya and in the AWhalers BayB north of Spitsbergen, the biomass was concentrated in the upper water layers, and diatoms indicating a spring scenario dominated the phytoplankton populations. However, this area had been open for 2 months and the special physical conditions seemed to nourish an ongoing phytoplankton bloom, probably lasting for as long as the area stayed open and the light conditions were suffiŽ1996. also noted the encient. Strass and Nothig ¨ hanced biomass in the Kvitøya polynya in the 1991 summer. The AWhalers BayB which is kept open by upwelling of Atlantic water flowing eastwards north of Spitsbergen, obviously offered good conditions for algal growth. According to ice maps, this area opened up mid May and closed by mid November, long after the polar night had started. Furthest north, close to the perennial ice zone and the shelf break of the Arctic Ocean, conditions resembling an ice edge bloom, with high production and diatom-dominated biomass, were found in the upper water layers. Zenkevich Ž1963. described late summer phytoplankton blooms in the northern Barents Sea and the Arctic Ocean, which he attributed to the gradual opening of leads in the ice as melting progressed during the summer and autumn. It is possible that the northernmost blooms observed during ICE-BAR 1996 are consistent with the blooms described by Zenkevich. The dynamics of the ice zone has a large impact upon the melt process and, hence, the primary production. From early spring, solar radiation penetrates both leads and the ice itself, initiating algal production under the ice. Light measurements show that the melt ponds act as windows, permitting the transmission of incoming solar radiation through to the underlying sea ice, and, thus, accelerating the melting process and enhancing the under-ice primary production. Factors such as snow cover, ice stratigraphy, brine channels, sediment and water content of the ice are also important for the melt process and primary production. The production is dependent upon the amount of light penetrating the ice, and ice algal production is initiated 1–2 months earlier than the pelagic production. Brown layers have been ob-

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served under the ice from early March, indicating growing algal populations ŽHegseth, 1992.. Because ice algaes are attached to the microblade ice crystals on the under-surface of the ice, they will loosen from the ice as soon as melting starts. Therefore, ice algal vegetation will mainly be found in spring ŽMarch– MayrJune. in the central parts of the Barents Sea. During our cruise in June 1995, melting had started and parts of the algal layers were gone. This was even more evident in 1996 where nothing but remnants of the ice flora were found during the summer, even farthest north. The rapid increase in solar radiation in March is the prime factor for the annual onset of primary production. The retreat of the ice edge, the opening of leads, the penetration of light through the ice and the density stratification of the water column create the short bloom events characteristic of the MIZ. These blooms fuel the energy flow in the arctic ice edge ecosystem and the carbon fixed by the algae is transferred up the food chain, mainly as lipids ŽFalk-Petersen et al., 1990; Scott et al., 1998.. The main characteristics of MIZs are, as shown in this paper, the large temporal and spatial variability of the ice-ocean system. This variability is a critical factor, which structures the arctic marine environment and determines its biodiversity. The variability in the ice-ocean system, as well as natural and anthropogenic changes in climate, can effect the biological systems in the MIZ, in such ways as, Ža. loss of ice-associated habitats, Žb. regional and seasonal shift in prey availability, Žc. alternation of energy flow in the marine food chains, and Žd. impacts on biochemical and physiological mechanisms of arctic organisms. Process-oriented studies, which can examine and monitor both biotic and abiotic processes, will be of importance for the management of Arctic waters.

Acknowledgements We wish to thank Mitsuo Ikeda and Gen Hashida for his assistance in the field and data processing. We also wish to express our gratitude for funding for this work provided by the National Institute of Polar Research, Tokyo and the Norwegian Research Council Žproject no 112497r410.. This study was par-

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tially supported by the partners of the Barents Sea Production Licences 182, 225, 228; Norsk Hydro, Statoil, Chevron, Enterprise, Fortum, Agip and SDØE. We thank the captain and crew of RrV Lance for ship logistics and assistance.

References Blindheim, J., 1989. Cascading of Barents Sea bottom water into the Norwegian Sea. Rapp. P.-V-Reun. — Cons. Int. Explor. ´ Mer. 188, 49–58. Cavalieri, D.J., Crawford, J., Drinkwater, M., Emery, M.J., Eppler, D.T., Framer, L.D., Goodberlet, M., Jentz, R., Milman, A., Morris, C., Onstott, R., Schweiger, A., Shuchman, R., Steffan, K., Swift, C.T., Wackerman, C., Werner, R.L., Weaver, R.L., 1992. NASA sea ice validation program for the DMSP SSMrI: final report. NASA Technical Memorandum 104559. National Aeronautics and Space Administration, Washington, DC, 126 pp. Cox, G.F.N., Weeks, W.F., 1974. Salinity variations in sea ice. J. Glaciol. 13, 109–120. Eicken, H., 1994. Structure of under-ice melt ponds in the Central Arctic and their effect on the sea-ice cover. Limnol. Oceanogr. 39, 682–694. Eicken, H., Lange, M.A., 1989. Development and properties of sea ice in the coastal regime of the southern Weddell Sea. J. Geophys. Res. 94, 8193–8206. Eicken, H., Lensu, M., Lepparanta, M., Tucker, W.B. III, Gow, A.J., Salmela, O., 1995. Thickness, structure, and properties of level summer multiyear ice in the Eurasian sector of the Arctic Ocean. J. Geophys. Res. 100, 22697–22710. Falk Petersen, S., Hop, H., 1996. Ecological processes in the Marginal Ice Zone of the northern Barents Sea, ICE-BAR 1995, cruise report. Nor. Polarinst. Rapportser 93, 240 pp. Falk-Petersen, S., Hopkins, C.C.E., Sargent, J.R., 1990. Trophic relationships in the pelagic Arctic food web. In: Barnes, M., Gibson, R.N. ŽEds.., Trophic Relationships in the Marine Environment. Aberdeen Univ. Press, Aberdeen, pp. 315–333. Falk-Petersen, S., Hop, H., Pedersen, G., 1997. Ecological and physical processes in the Marginal Ice-Zone during the summer melt period: the ICE-BAR 1996 cruise in the northern Barents Sea. Nor. Polarinst. Rapportser 102, 47 pp. Falk-Petersen, S., Sargent, J.R., Henderson, J., Hegseth, E.N., Hop, H., Okolodkov, Y.B., 1998. Lipids and fatty acids in ice algae and phytoplankton from the Marginal Ice Zone in the Barents Sea. Polar Biol. 20, 41–47. Falk-Petersen, S., Pedersen, G., Hop, H., Hegseth, E.N., Kwasniewski, S., 1999. Spatial distribution and life-cycle timing of zooplankton in the marginal ice zone of the Barents Sea during summer melt season in 1995. J. Plankton Res. 20, 1249–1264. Gawarkiewicz, G., Plueddemann, A.J., 1994. Where the Arctic meets the North Atlantic: exploring the Barents Sea Polar Front. Oceanus 37, 17–20. Gloersen, P., Campbell, W.J., Cavaliere, D.J., Comaso, J.C.,

158

S. Falk-Petersen et al.r Journal of Marine Systems 27 (2000) 131–159

Parkinson, C.L., Zwalley, H.J., 1992. Arctic and Antarctic sea ice, 1978–1987. Satellite passes: microwave observations and analysis. Scientific and technical program, NASA, Washington, 290 pp. Gordienko, P.A., Laktionov, A.F., 1969. Circulation and physics of the Arctic Basin waters. Ann. Int. Geophys. Year 46, 94–112. Gulliksen, B., Lønne, O.J., 1991a. Sea ice macrofauna in the Antarctic and the Arctic. J. Mar. Syst. 2, 53–61. Gulliksen, B., Lønne, O.J., 1991b. Distribution, abundance, and ecological importance of marine sympagic fauna in the Arctic. Rapp. P.-V- Reun. ´ — Cons. Int. Explor. Mer. 188, 133–138. Hegseth, E.N., 1992. Sub-ice algal assemblages of the Barents Sea: Species composition, chemical composition, and growth rates. Polar Biol. 12, 485–496. Hegseth, E.N., 1998. Primary production of the northern Barents Sea. Polar Res. 17, 113–123. Holm-Hansen, O., Lorentzen, C.J., Holmes, R.W., Strickland, J.D.H., 1965. Fluorometric determination of chlorophyll. J. Cons. Int. Explor. Mer. 30, 3–15. Hop, H., Poltermann, M., Lønne, O.J., Falk-Petersen, S., Korsnes, R., Budgell, W.P., 2000. Ice-amphipod distribution relative to ice density and under-ice topography in the northern Barents Sea. Polar Biol. Žin press.. Horner, R., Ackley, S.F., Dieckmann, G.S., Gulliksen, B., Hoshiai, T., Legendre, L., Melnikov, I.A., Reeburgh, W.S., Spindler, M., Sullivan, C.W., 1992. Ecology of sea ice biota: 1. Habitat, terminology, and methodology. Polar Biol. 12, 417–427. Jeffries, M.O., Weeks, W.F., Shaw, R.A., Morris, K., 1993. Structural characteristics of congelation and platelet ice and their role in the development of Antarctic landfast sea ice. J. Glaciol. 39, 223–238. Jeffries, M.O., Shaw, R.A., Morris, K., Veazey, A.L., Krouse, H.R., 1994. Crystal structure, stable isotopes Ž d18 O., and development of sea ice in the Ross, Amundsen, and Bellingshausen seas. Antarct. J. Geophys. Res. 99, 985–995. Jeffries, M.O., Schwartz, K., Morris, K., Veazey, A.D., Krause, H.R., Cushing, S., 1995. Evidence for platelet ice accretion in Arctic sea ice development. J. Geophys. Res. 100, 10905– 10914. Jeffries, M.O., Worby, A.P., Morris, K., Weeks, W.F., 1997. Seasonal variations in the properties and structural composition of sea ice and snow cover in the Bellingshausen and Amundsen seas. Antarct. J. Glaciol. 43, 138–151. Korsnes, R., 1993. Quantitative analysis of sea ice remote sensing imagery. Int. J. Remote Sens. 14, 295–311. Korsnes, R., 1994. Some concepts for precise estimation of deformationsrrigid areas in polar pack ice based on time series of ERS-1 SAR images. Int. J. Remote Sens. 15, 3663–3674. Lange, M.A., Ackley, S.F., Wadhams, P., Dieckmann, G.S., Eicken, H., 1989. Development of sea ice in the Weddell Sea. Ann. Glaciol. 12, 92–96. Lange, M.A., Schlosser, P., Ackley, S.F., Wadhams, P., Dieckmann, G.S., 1990. d18 O concentrations in sea ice of the Weddell Sea. Antarct. J. Glaciol. 36, 315–323. Loeng, H., 1991. Features of the physical oceanographic conditions of the Barents Sea. Polar Res. 10, 5–18.

Lønne, O.J., 1992. Distribution, abundance and trophic role of the sympagic macro-fauna in Svalbard Waters, Dr. Scient. Thesis, Univ. of Tromsø. Lønne, O.J., Gulliksen, B., 1991. On the distribution of sympagic macro-fauna in the seasonally ice covered Barents Sea. Polar Biol. 11, 457–469. Løyning, T.B., 2000. Hydrography in the north-western Barents Sea, July–August 1996. Polar Res. Žsubmitted for publication.. Løyning, T.B., Budgell, W.P., 1996. Physical oceanography data report from the ICEBAR cruise 1996. Nor. Polarinst. Rapportser 95, 48 pp. Martin, S.A., 1979. A field study of brine drainage and oil entrapment in sea ice. J. Glaciol. 22, 473–502. Meese, D.A., 1989. The chemical and structural properties of sea ice in the southern Beaufort Sea. CRREL Cold Reg. Res. Eng. Lab., pp. 25–89, Hanover, N.H. Rep. Melnikov, I., 1997. The Arctic Sea Ice System. Gordon and Beach Science Publishers, Amsterdam, 204 pp. Mohr, J.L., Tibbs, J., 1963. Ecology of ice substrates. Proc. Arctic Basin Symp., pp. 245–248. Nansen, F., 1897. AFarthest NorthB: Being the Record of a Voyage of Exploration of the Ship Fram 1893–96: and a Fifteen Months’ Sleigh Journey by Dr Nansen and Lieut. Johansen P Westminster: Archibald Constable. 2 volumes. Orvik, K.A., Kuznetsov, V., 1996. Ecological processes in the Marginal Ice-Zone of the northern Barents Sea. ICE-BAR 1995, CTD observations. Nor. Polarinst. Rapportser 94, 55 pp. Overgaard, S., Wadhams, P., Lepparanta, M., 1983. Ice properties in the Greenland and Barents Seas during summer. J. Glaciol. 29, 142–164. Pedersen, G., 1995. Factors influencing the size and distribution of the copepod community in the Barents Sea with special emphasis on Calanus finmarchicus ŽGunnerus.. PhD Thesis, Norwegian College of Fishery Science, University of Tromsø. Pfirman, S.L., Bauch, D., Gammelsrød, T., 1994. The northern Barents Sea: water mass distribution and modification. In: Johannessen, O.M., Muench, R.D., Overland, J.E. ŽEds.., The Polar Oceans and their Role in Shaping the Global Environment. Geophys. Monogr. 85, pp. 77–93. Poltermann, M., Hop, H., Falk-Petersen, S., 2000. Life under Arctic sea ice — reproduction strategies of two sympagic Žice-associated. amphipod species, Gammarus wilkitzkii and Apherusa glacialis. Mar. Biol. Žin press.. Romanov, I.P., 1995. Atlas of Ice and Snow of the Arctic Basin and Siberian Shelf Seas. Backbone Publishing, 277 pp. Sakshaug, E., 1997. Biomass and productivity distributions and their variability in the Barents Sea. ICES J. Mar. Sci. 54, 341–350. Sakshaug, E., Skjoldal, H.R., 1989. Life at the ice edge. Ambio 18, 60–67. Sakshaug, E., Slagstad, D., 1991. Light and productivity of phytoplankton in marine ecosystems: a physiological view. Polar Res. 10, 69–85. Schwarzacher, W., 1959. Pack-ice studies in the Arctic Ocean. J. Geophys. Res. 64, 2357–2367. Scott, C.L., Falk-Petersen, S., Hop, H., Sargent, J.R., Lønne, O.J., Poltermann, M., 1998. Lipids and trophic interactions of ice

S. Falk-Petersen et al.r Journal of Marine Systems 27 (2000) 131–159 fauna and pelagic zooplankton in the marginal ice zone of the Barents Sea. Polar Biol. 21, 65–70. Skjoldal, H.R., Rey, F., 1989. Pelagic production and variability of the Barents Sea ecosystem. In: Sherman, K., Alexander, L.M. ŽEds.., Biomass Yields and Geography of Large Marine Ecosystems. AAAS Selected Symposium 111. Westview Press, CO, USA, pp. 241–286. Strass, V.H., Nothig, E.M., 1996. Seasonal shifts in ice edge ¨ phytoplankton blooms in the Barents Sea related to the water column stability. Polar Biol. 16, 409–422. Syvertsen, E.E., 1991. Ice algae in the Barents Sea: types of assemblages, origin, fate and role in the ice-edge phytoplankton bloom. Polar Res. 10, 277–288. Tucker, W.B. III, Gow, A.J., Weeks, W.F., 1987. Physical properties of summer sea ice in the Fram Strait. J. Geophys. Res. 92, 6787–6807. Vinje, T., 1997. Ice fluxes through Fram Strait. Proceedings of the WCRP Conference on Polar Processes and Global Climate,

159

3–6 November, ACSYS Project Office, Norwegian Polar Institute, Oslo. ˚ Vinje, T., Kvambekk, A.S., 1991. Barents Sea drift ice characteristics. Polar Res. 10, 59–68. Wassmann, P., Slagstad, D., 1993. Seasonal and annual dynamics of carbon flux in the Barents Sea: a model approach. Polar Res. 13, 363–372. Werner, I., Auel, H., Garrity, C., Hagen, W., 1999. Pelagic occurrence of the sympagic amphipod Gammarus wilkitzkii in ice-free waters of the Greenland Sea — dead end or part of life-cycle? Polar Biol. 22, 56–60. Whitlock, C.H., Charloek, T.P., Staylor, W.F., Pinker, R.T., Laszlo, I., Ohmura, A., Gilgen, H., Konzelman, T., DiPasquale, R.C., Moats, R.C., LeCroy, S.R., Ritchey, N.A., 1995. First global shortwave surface radiation budget dataset. Bull. Am. Meteorol. Soc. 76, 905–922. Zenkevich, L.A., 1963. Biology of the Seas of the USSR. Georg Allen & Unwin, London, 955 pp.