Progress in Oceanography Progress in Oceanography 71 (2006) 288–313

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Progress in Oceanography Progress in Oceanography 71 (2006) 288–313 www.elsevier.com/locate/pocean

Structure and function of contemporary food webs on Arctic shelves: A panarctic comparison The pelagic system of the Kara Sea – Communities and components of carbon ﬂow

H.J. Hirche a,*, K.N. Kosobokova b, B. Gaye-Haake c, I. Harms d, B. Meon a, E.-M. No¨thig a

a Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse 1, D-27568 Bremerhaven, Germany b Shirshov Institute of Oceanology, RAS, Moscow, Russia c Centre for Marine and Climate Research, Institute for Biogeochemistry and Marine Chemistry, Hamburg, Germany d Centre for Marine and Climate Research, Institute for Oceanography, Hamburg, Germany

Abstract

After a short introduction to the physical setting and the history of biological research the pelagic ecosystem of the Kara Sea is described. Main emphasis is on regional aspects of the plankton communities and their seasonal dynamics using mostly data collected between 1996 and 2001. In the zooplankton, for which most data were available, four regional aggregations were separated: (1) the rivers and estuaries of the Southern Kara Sea, (2) the south-western and (3) the central Kara Sea, and (4) the northern troughs and slope. The phytoplankton communities had a similar distribution. To provide components for detailed carbon budgets the regional dynamics of bacterial, phytoplankton and zooplankton biomass and production are described and carbon requirements of bacteria and zooplankton are estimated. For completeness a short literature review on higher trophic levels is included. Finally, recent observations of the pelago-benthic coupling are considered. Estimates of the carbon requirements from the plankton and benthos reveal a large underestimation of primary production, which to date, together with seasonal aspects, shows the largest gap in our knowledge. 2006 Elsevier Ltd. All rights reserved.

Keywords: Kara Sea; Pelagic ecosystem; Bacteria; Phytoplankton; Zooplankton; Carbon ﬂux

1. Introduction

The Russian arctic seas can be subdivided into two groups; the Barents and Chuckchi Seas undergo much more inﬂuence from the comparatively warm waters of the Atlantic and Paciﬁc Oceans than do the Kara,

* Corresponding author. E-mail address: [email protected] (H.J. Hirche).

Table 1 Zooplankton nets used for vertical zooplankton tows during cruises of RV Dalnye Selentsye, RV Polarstern, and RV Boris Petrov Platform Net type Area (m2) Mesh (lm) Time RV Dalnye Selentsyea Juday net 0.1 180 October 2000 RV Polarstern Multi net 0.25 155 September 1995/September 1996 RV Boris Petrovb Nansen net 0.44 155 August/September 1997, 1999, 2000, 2001 a Murmansk Marine Biological Institute. b In the text referred to as ‘‘SIRRO’’ Cruises.

Laptev and East Siberian Seas, for which more river runoff determines characteristic features of the carbon cycle (Vetrov and Romankevich, 2004). Recent interest in the fate of anthropogenic pollution, the exploration of natural resources, together with indications for increased river discharge due to climate change (Peterson et al., 2002) has directed increased attention to the Kara Sea (Stein et al., 2003). Klages et al. (2003) have recently presented a first assessment of organic carbon consumption by the macrozoobenthos. However, except for the work of Vetrov and Romankevich (2004), which represents more a generalizing attempt to char- acterize the carbon cycle, the pelagic ecosystem and role in biogeochemical cycles has not been much considered. Here we describe the physical setting, but restrict ourselves to aspects that are directly relevant for ecology. Our main emphasis is on regional aspects of phytoplankton and zooplankton communities and their seasonal dynamics. In addition we provide a short review of the long history of Russian pelagic ecosystem research in the Kara Sea. To provide input for detailed future assessments of carbon budgets we describe the regional dynamis of phytoplankton, zooplankton and bacterial biomass, estimate bacterial and zooplankton production and their food requirements. Finally, we report on recent observations from sediment trap moorings. Due to the lack of quantitative assessments we include only a short review of the populations of fish, birds and mammals. Most of the presented data originate from four SIRRO (Siberian River Runoff, a German–Russian cooperative Project, Fu¨tterer and Galimov, 2003) cruises, two expeditions with RV Polar- stern and one RV Dalnye Selentsye cruise (Table 1). This represents the largest consistent regional data set of recent origin, applying similar mesh sizes for plankton nets, methods of analysis, and taxonomic criteria. Other data were used when appropriate and their origin is mentioned.

2. Geography and physics

The environmental factors with the strongest impact on the arctic marine ecosystem are sea ice, riverine freshwater inﬂow and stratiﬁcation, temperature, and advection. Detailed descriptions and reviews of the hydrography and sea ice of the Kara Sea have been published recently, e.g. Volkov et al. (2002), Stein et al. (2003). Therefore we include here only the aspects relevant for the marine pelagic ecosystems.

2.1. Geography and topography

The Kara Sea is the second largest arctic shelf sea (883,000 km2). According to the oﬃcially adopted boundaries (Fig. 1; Boundaries of the Oceans and the Sea, 1960), the Kara Shelf area comprises 99.4% of the sea area with the shelf water volume comprising 96.5% of the sea volume. More than 40% of the sea area has a depth less than 50 m, yet the average sea depth is 111 m (Volkov et al., 2002). Greatest depths are found in the St. Anna Trough (>500 m) in the north and in the Novaya Semlya Trough (433 m). The estuaries of Ob and Yenisei and adjacent southern and eastern coastal zone are very shallow.

2.2. Ice cover

The Kara Sea is covered by ice for about 9 months per year (Blanchet et al., 1995). Sea ice thickness for ﬁrst year ice ranges from 1.2 m in the southwest to 2 m in the northeast (Barnett, 1991). Ice formation starts in the end of September or beginning of October. Land-fast ice forms along the coasts along the 10–15 m isobath in the south-western region and along the 20–25 m isobath in the north-eastern region (Volkov et al., 2002), 290 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Fig. 1. Station map from seven cruises to the Kara Sea used in this article. where it may extend up to 200 km seaward (Barnett, 1991). Offshore winds create flaw polynyas of up to 100 km width at the seaward edge of the land-fast ice, which act as ice factories during the entire winter (Mar- tin and Cavalieri, 1989). The breakup begins in early to late June (Mironov et al., 1994). The warmer waters of the large rivers accelerate melt initially in the estuaries, followed by an open arc spreading farther seaward. When the seasonal ice minimum is reached by mid-September, the entire sea south of 75N is normally ice- free. In the eastern sector with less river runoff, nearly half of the total area retains some ice through normal summers, although year-to-year variation is great (Barnett, 1991). In September, freeze-up begins in the colder waters of the north. In the south it starts in early October in the estuaries. The regional distribution of the ice compactness is shown in four snapshots for 1997 in Fig. 2, which is derived from SSM/I imagery. The figure illustrates the ice break-up in May and freeze-up in October along the Siberian coast and in the river estuaries. The same satellite data source was used to construct time series of the seasonal ice coverage for the years 1997–2001 (Fig. 3), when SIRRO expeditions took place. It is obvi- ous that the onsets of break-up and freezing vary within a range of approx. 30 days. Also the summer ice extent is highly variable, particularly in the northeastern parts. H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 291

Fig. 2. Snapshots of satellite-derived seasonal ice compactness (in %) in the Kara Sea, for the year 1997 (10 March, 10 May, 10 July and 30 October). Data source: Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data (CD-ROM) (Cavalieri et al., 1996).

Fig. 3. Satellite data derived time series of the seasonal ice coverage in the Kara Sea for the years ’97, ’98, ’99, ’00 and ’01. Data source: Sea ice concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data (CD-ROM) (Cavalieri et al., 1996).

2.3. River runoﬀ

The Kara Sea receives more than 40% of the total arctic river runoff (Fu¨tterer and Galimov, 2003). The rivers Ob and Yenisei together with many medium and small rivers discharge roughly 1200 km3 of water (Aagaard and Carmack, 1994) and >220 million tons of suspended particulate (SPM) and dissolved organic 292 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 matter (DOM) annually (Ivanov, 1976; Gordeev et al., 1996). About 30–40% of the DOM and >90% of SPM are deposited in the so-called ‘‘marginal filter zone’’ (Lisitzin, 1995; Lisitzin et al., 2000) in front of the river estuaries, where flocculation and coagulation of particles enhance sedimentation (Lisitzin, 1995). Only 0.47 million tons of SPM are believed to leave the shelf (Schlosser et al., 1995). In contrast, most of the riverine DOC seems to transit the shelf and enter the surface pool of the Arctic Ocean (Schlosser et al., 1995). The rivers contribute a considerable amount of heat, which accelerates the ice-melt in the estuaries and has an accelerating effect on the metabolic activity. Thus in summer surface temperatures are several degrees higher in the inner estuaries than farther north. Eighty percent of the run-off takes place in early summer (Fig. 4), however, the onset of the runoff varies within a range of 30 days. The peak discharge of the Yenisei may exceed 100,000 m3 s1 which leads to a pronounced vertical stratification in its estuary. In summer the plume of riverine water (defined arbitrarily by the 25 psu isohaline) covers ca. 600,000 km2 (Stephantsev and Shmelkov, 2000). The spreading of the plume is under the influence of the prevailing

Fig. 4. Monthly mean river runoﬀ from Ob and Yenisei for the years ’97, ’98, ’99, ’00 and ’01. Source: A Regional Hydrometeorological Data Network for the pan-Arctic Region (R-ArcticNET, http://www.r-arcticnet.sr.unh.edu/abstract.html). H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 293

Fig. 5. Simulated climatological surface and bottom water salinity in the Kara Sea at the end of May, June, July and August. atmospheric circulation and can thus move in opposite directions (Ivanov, 1995, Fig. 5). During summer the salinity at the bottom is generally higher than at the surface. Highly saline waters may penetrate as a counter current into the estuaries (Harms et al., 2003), depending on the atmospheric conditions. How- ever, the strong seasonal pycnocline between 5 and 10 m restricts the exchange of nutrients and matter between diﬀerent water layers. The nutrient regime of the Kara Sea is shaped by Atlantic water with silicate concentrations elevated due to riverine import in the southern part (Holmes et al., 2000). Unfortunately, most historic nutrient records for the Kara Sea are not reliable. Especially ammonium concentrations are particularly erroneous and uncorrectable (Holmes et al., 2000). The traditional concept of a large riverine nutrient supply, be it directly or through regeneration of organic matter has been challenged recently by reports of low nutrient concentrations in Arctic rivers (Dittmar and Kattner, 2003) and more detailed studies of POM and DOM. Thus Amon and Benner (2003) estimated that only about 2% of the DOM in the surface waters is of labile nature. Riverine particulate organic matter is constituted primarily of refractory compounds derived from vas- cular plant detritus, whereas phytoplankton and living bacterial biomass is low in all Arctic rivers (Cauwet and Sidorov, 1996; Sorokin and Sorokin, 1996). It seems to be biogeochemically stable in the estuaries and shelves and, therefore, does not substantially support productivity of the pelagic community. 294 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Fig. 6. Simpliﬁed scheme of the simulated surface circulation in the Kara Sea. (a) Spring and summer, (b) autumn and winter (Harms et al., 2000).

2.4. Hydrography

Recent modelling studies (Harms et al., 2000; Harms and Karcher, 1999) have shown a pronounced seasonal variability of currents and hydrography, caused by the wind field, freshwater runoff and ice formation in the Kara Sea. Two circulation schemes were identified (Fig. 6). The spring and summer situation is char- acterized by weak winds from north-east and strong river runoff. The general circulation pattern is anticy- clonic with strong currents very close to the estuaries (Fig. 6a). In fall and winter the circulation is almost reversed and strongly intensified, showing a flushing from south-west to north-east (Fig. 6b). Strong winds from south-west and a large extent of the river plume cause a very pronounced coastal current at the Siberian coast. It is noteworthy that the vast majority of plankton observations were made during this latter period. As a consequence of the reversal of the circulation, the variable habitat of the estuarine communities underlies their extreme seasonal variability. Apart from river run-off, the hydrography of the Kara Sea is strongly influenced by lateral fluxes of water masses derived from inflow of modified Atlantic water through the Barents Sea (Pavlov and Pfirman, 1995). Atlantic waters enter the Kara Sea through three entrances, and accordingly their effect is variable in different parts (Karcher et al., 2003): (1) a branch from the Barents Sea entering through the Kara Strait influences the southern and western Kara Sea (Pavlov and Pfirman, 1995; Harms and Karcher, 1999); (2) Atlantic water passing between Franz Josef Land and Novaya Semlya influences the northern Kara Sea; and (3) an inflow of subducted eastward bound Atlantic water that follows the shelfbreak enters from the north along the western slope of St. Anna Trough. Part of this inflow is recirculated south and westwards after passing Franz Josef Land (Schauer et al., 2002).

3. History of Kara Sea plankton studies

Phytoplankton studies in the Kara Sea started with the Swedish Proven expedition in the Yenisei Bay 1875 and Nordenskjo¨ld’s Vega navigation of the Northeast Passage 1878–1880. The data collected during the Rus- sian Polar expedition in 1900–1903 and by the Russian research vessels Persey, Sedov and Litke in the 1920s and 1930s supplied the ﬁrst information on the pelagic ﬂora and fauna of the Kara Sea (Vetrov and H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 295

Romankevich, 2004). From this material Usatchev (1968) compiled the first list of phytoplankton species of the Kara Sea where diatoms comprised >50% and peridineans 40% of the species number, the rest was composed by flagellates, silicoflagellates, coccolithophorids, green and blue green algae and nanoflagellates. Dia- toms accounted for 87% of the total phytoplankton biomass, followed by peridineans (6%), flagellates (5%) and green algae (1%) (Usatchev, 1947). In the beginning of the 1980s the Murmansk Marine Biological Insi- tute (MMBI) initiated hydrobiological observations in the Kara Sea during ice-free period (Matishov et al., 2000). The phytoplankton species composition was studied in the western and central part (Makarevich, 1995; Makarevich and Druzhkov, 1994), and biomass and chlorophyll a distribution were measured (Bobrov et al., 1989; Druzhkov and Makarevich, 1996). In the 1990s phytoplankton was also collected during several multidisciplinary MMBI summer surveys in the western and central sea; winter observations were carried out along the Northern route aboard the Russian nuclear icebreakers (Matishov et al., 2000). The Biological Atlas of the Arctic Seas: Plankton of the Barents and Kara Seas (Matishov et al., 2000) presents the latest compi- lation of data on phytoplankton collected from 1994 to 1999. From this data Druzhkov and Makarevich (1999) composed a detailed floristic list of diatom and dinoflagellate assemblages of the Kara Sea. In the northern Kara Sea phytoplankton composition and biomass were studied in the ice marginal zone at a transect across the St. Anna Trough during RV Polarstern expedition in 1996 (Druzhkov and Druzhkova, 1999). The first report on sea-ice algae of the southern Kara Sea was provided by Druzhkov et al. (2001). First measurements of the photosynthesis rate were carried out by Shirshov (1982) in 1933. In August and September 1981 primary production at the sea surface was measured in the western Kara Sea by Bobrov et al. (1989). Detailed data on primary production based on modern methods were obtained in the south-western, southern and central Kara Sea during the 49th cruise of RV Dmitry Mendeleev (Vedernikov et al., 1995) at the end of the summer season. Russian zooplankton investigations between 1920 and 1940 covered large parts of the Kara Sea. These early studies dealt mostly with faunal composition, ecological and biogeographical aspects (Linko, 1913; Jas- chnov, 1927; Bernshtein, 1931, 1934; Khmiznikova, 1935, 1936). Especially the role of indicator species for waters advected into the Kara Sea was investigated (Bernshtein, 1934; Khmiznikova, 1936; Virketis, 1944; Bogorov, 1945; Zenkevitch, 1963). This led to detailed descriptions of biogeographical groups (Zenkevitch, 1963) and species lists (Matishov et al., 2000; Sirenko, 2001). Only recently the indicator species concept was replaced by community cluster analysis (Fetzer et al., 2002; Deubel et al., 2003). Jaschnov (1940), Bogorov (1945) and Ponomareva (1957) obtained first quantitative data on zooplankton biomass. Leshinskaya (1962) and Pirozhnikov (1985) worked on distribution of zooplankton in the Ob and Yenisei Bays and in near shore areas. Chislenko (1972a,b) carried out seasonal observations between 1955 and 1957 in the Yenisei Bay and near Dikson Island. Data on abundance, biomass distribution and life cycles of several macroplankton species were collected in the southwestern and western Kara Sea by Timofeev (1983, 1985, 1990), Fomin et al. (1984), Fomin and Petrov (1985), Fomin (1989), Zubova (1990). The necessity of ecological monitoring in areas of radioactive disposal and planned exploitation of gas fields initiated studies of the formation and transforma- tion of particulate matter and its flux through the ecosystem (Novoselov, 1993; Timofeev, 1989; Fomin, 1989). These studies were often conducted in the framework of international programs (Lisitzin and Vinogradov, 1995; Stein et al., 2003). Only recently sampling from nuclear icebreakers provided some insight into the distribution of phyto- and zooplankton during winter and early spring in the open Kara Sea (Matishov et al., 2000; Vinogradov et al., 2001). Information on fish biology and distribution in the Kara Sea is insufficient. Species composition has been studied since early European expeditions in the end of the 19th century, beginning with Nordenskjo¨ld’s expedition in 1887–1879 (Essipov, 1952). The most complete list with 54 marine and eight freshwater species was published by Andrijashev (1954) based on data obtained during various Russian expeditions between 1894 and the 1930s.

4. Structure and seasonal development of the pelagic system

4.1. Phytoplankton communities and seasonal development

The phytoplankton in the Kara Sea is exposed to a strong seasonality in light regime and sea ice cover. Additionally, a severe signal of fresh water, heat and matter supply originating from rivers during spring 296 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Fig. 7. Phytoplankton species number (a) and composition of the main phytoplankton and protozooplankton groups (b) along a transect from the Yenisei to the open Kara Sea in September 2000 (modified from Deubel et al., 2003). and summer shapes the phytoplankton distribution and plays a significant role in phytoplankton development. Within the SIRRO project, investigations carried out during September 2000 revealed a drastic change in composition of phytoplankton communities along a south–north transect starting in the Yenisei estuary (Deubel et al., 2003; No¨thig et al., 2003). While bluegreen algae dominated in the rivers, diatoms and dino- flagellates dominated populations found in the open sea (Fig. 7). The endosymbiotic algae-carrying ciliate Merionecta rubrum presented an important component of the autotrophic population in more northern areas. Differences were also found in September 1999 between species compositions in the Ob and Yenisei estuaries. Diatoms dominated by Thalassiosira cf. punctigera and Chaetoceros spp. in the brackish/marine environment and the freshwater diatom Aulacosira spp. in the regions influenced by freshwater. In the other years (1997, 2000, 2001) freshwater species, bluegreens and chlorophytes were abundant in both rivers. Similar patterns were reported by Larionov and Makarevich (2001) and Makarevich et al. (2003). In accordance with these observations four zones were distinguished for phytoplankton distribution in the southern Kara Sea: Ob and Yenisei Shallows, south-western/western, central, and northern zones (Deubel et al., 2003). Arctic-boreal species dominated together with cosmopolitan species in the south, west and north (No¨thig et al., 2003; Mak- arevich et al., 2003). Boreal species decreased from west to east (No¨thig et al., 2003; Makarevich et al., 2003). Despite solid ice cover in spring, phytoplankton seems to develop early under and in the ice. Thus, in the southwestern Kara Sea an initial phytoplankton bloom was observed under 30 cm sea ice already in mid-April (Pautova and Vinogradov, 2001). Dominant species consisted of mainly cryophilic algae, 80% was made up by Fragilariopsis oceanica, Achnanthes taeniata and F. cylindrus. The phytoplankton samples collected by nuclear icebreakers from 1996 to 1999 also showed a dominance of diatoms throughout the northern (east of Novaya Semlya) and south-western Kara Sea between February and May (Matishov et al., 2000). Arctic-boreal species dominated followed by cosmopolitan and a few boreal species. Most of the species were neritic forms. H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 297

Unfortunately, data on early blooms of algae and their biomass in and under the ice are still very scant, and biomass data are not available. During this early growth phase a part of the nutrients might be taken up before the main phytoplankton increase starts by the end of June when both melting of sea-ice and river-runoff cause stabilisation of the upper layers. In the southern Kara Sea a main centre of an early spring bloom (from late March to early April) forms at the Ob–Yenisei Shallows (Makarevich and Matishov, 2000; Makarevich et al., 2003). In summer (August– September), there is also a high phytoplankton biomass in this area (No¨thig and Kattner, 1999; Larionov and Kodina, 2000). In contrast, phytoplankton growth in the central deep-water areas of the south-western Sea phytoplankton is restricted to the ice-edge (Makarevich et al., 2003). In the rest of the coastal south-western Kara Sea two spring biomass maxima were observed. The first bloom took place directly after the ice melt, while the second was fuelled by riverine runoff. After depletion of these nutrients, intense primary production ended in the areas not influenced directly by runoff of the Ob and Yenisei rivers (Druzhkov and Mak- arevich, 1996; Druzhkov et al., 2001). A model of the pelagic ecosystem of the Kara Sea (Lebedeva et al., 1995) based on published data and results of the Dimitry Mendeleev 49th cruise also suggested two phytoplankton maxima in the northern and south-western Sea: a high peak of biomass at the end of June/early July and a smaller one from mid-July to mid-August. According to this model, the decline of nutrients after the stratification and zooplankton grazing are the main reasons for the decline of phytoplankton productivity (Lebedeva et al., 1995).

4.2. Zooplankton communities

In accordance with the results of earlier biogeographical studies (Bernshtein, 1934; Khmiznikova, 1936; Virketis, 1944; Bogorov, 1945; Zenkevitch, 1963) and cluster analysis based on SIRRO data (Fetzer et al., 2002; Deubel et al., 2003), four large geographical regions can be distinguished in the Kara Sea.

4.2.1. Southern Kara Sea: rivers and estuaries The pelagic system of the southern Kara Sea is shaped predominantly by freshwater input. The strongly pulsed river runoff (Fig. 4) changes the functional size of the estuaries tremendously (Fig. 5). Before the ice melting, estuaries are small with a strong salinity gradient (Chislenko, 1972a). In contrast, in summer the layer of brackish water extends far to the north (Vinogradov et al., 1995a). Regional distribution of the plankton communities in late summer–autumn mirrors closely the spreading of the river plumes in a northeastern direction with the Yamal Current (Burenkov and Vasilkov, 1995; Pavlov and Pfirman, 1995) with salinity as the structuring force (Fetzer et al., 2002; Deubel et al., 2003). Cluster analysis based on a dense station grid of SIRRO expeditions (Fig. 1) separated six zooplankton assemblages in the southern sea along the salinity gradient from freshwater, estuarine, brackish to marine, with differences between the rivers Ob and Yenisei in the freshwater and estuarine assemblages (Fetzer et al., 2002). Mainly freshwater expatriates including rotatoria, copepods and cladocerans inhabit the inner parts of the estuaries of Ob and Yenisei (Khmiznikova, 1936; Bogorov, 1945; Chislenko, 1972a; Vinogradov et al., 1995a,b; Fetzer et al., 2002; Deubel et al., 2003). Abundance and biomass are quite low in the inner estuaries (Table 2). Differences between the assemblages in Ob and Yenisei found during SIRRO (Fetzer et al., 2002; Deubel et al., 2003) might originate in different nutrient regimes from different drainage regions due to different bottom topography (No¨thig and Kattner, 1999; Larionov and Kodina, 2000). In the outer estuaries and adjacent coastal areas, brackish water overlies marine water masses, resulting in two communities, which are, however, not strictly vertically separated. Especially the brackish water species seem to cross the pycnocline easily (Fetzer et al., 2002). The copepods Limnocalanus macrurus, Drepanopus bungei, Pseudocalanus major, Jaschnovia tolly, Senecella siberica, and the hydromedusa Halitholus yoldia-arctica dominate above the pycnocline (Khmiznikova, 1936; Virketis, 1944; Bogorov, 1945; Vinogradov et al., 1995b; Fetzer et al., 2002; Hirche et al., 2003) and are all able to tolerate a wide salinity range. Thus, P. major was recorded at salinities between 0.9 and 33 psu, D. bungei between 1 and 31 psu, and L. macrurus between 0 and >33 psu, respectively (Chislenko, 1972a; Vinogradov et al., 1995a; Abramova, 2000; Hirche et al., 2003). Below the pycnocline abundances are usually 2–4 times higher. Due to the presence of both marine and brackish forms, species numbers are higher than in the upper layers (Vinogradov et al., 298 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Table 2 Average relative abundance (Ab) and biomass (Bi) (%) of zooplankton in different areas of the Kara Sea Group Region South-west South Central Northern troughs and slope 2000 1997 1999 2000/01 1995/96 Depth (m) 60–200 m 7–40 m 45–300 m 90–1000 m Number of stations 6 20 24 11 8 Abundance/biomass Ab Bi Ab Bi Ab Bi Ab Bi Ab Bi Calanoida Acartia longiremis 0.2 <0.1 0.1 <0.1 – – <0.1 <0.1 – – Calanus glacialis 2.8 53.3 1.4 17.9 2.3 5.3 5.2 48.7 9.1 31.9 C. finmarchicus 0.2 1.9 0.1 0.6 0.1 0.1 0.8 3.6 2.5 17.9 C. hyperboreus <0.1 0.2 <0.1 0.4 <0.1 0.2 0.3 3.9 0.5 9.7 Diaptomus sp. – – 0.4 0.2 0.9 <0.1 – – – – Drepanopus bungei 1.1 0.1 58.7 45.7 44.7 14.8 <0.1 <0.1 – – Eurytemora sp. – <0.1 0.2 0.1 <0.1 <0.1 – – – – Gaetanus tenuispinus – – – – – – 0.1 0.1 Heterorhabdus norvegicus – – – – – – – – <0.1 0.1 Jaschnovia tolli – – 0.5 1.2 1.7 <0.1 – – <0.1 – Limnocalanus macrurus <0.1 – 0.5 1.6 10.6 64.9 ––– – Metridia longa 0.3 0.5 0.1 0.3 <0.1 <0.1 2.5 3.8 9.3 13.3 Microcalanus pygmaeus 2.3 1.1 1.4 0.3 0.6 0.1 3.9 1.4 6.7 1.2 Paraeuchaeta glacialis <0.1 0.4 <0.1 0.2 <0.1 0.1 <0.1 0.6 0.3 1.5 P. norvegica <0.1 – <0.1 0.1 – – <0.1 0.1 <0.1 0.9 Pseudocalanus acuspes/minutus 6.5 1.6 7.1 9.0 3.5 0.1 10.1 1.0 3.8 0.3 P. major 0.2 0.3 3.5 12.6 12.6 0.5 1.4 1.6 –– Scolecithricella minor – – – – – – – – 0.1 <0.1 Cyclopoida Cyclops strenuus –– 3.0 1.3 4.3 7.9 ––– – Oithona similis 41.8 8.4 6.2 0.6 7.3 0.2 44.4 6.9 34.2 2.6 O. atlantica – – – – – – – – 0.1 <0.1 Poecilostomatoida Oncaea borealis 0.1 <0.1 0.9 0.1 0.4 <0.1 0.9 0.1 4.7 0.2 Harpacticoida Microsetella norvegica 12.3 10.8 <0.1 <0.1 <0.1 <0.1 0.8 0.1 0.2 <0.1 Harpacticidae spp. <0.1 <0.1 <0.1 <0.1 0.2 0.1 <0.1 <0.1 <0.1 <0.1 Nauplii Nauplii Copepoda 18.7 3.0 13.6 0.7 7.7 0.2 9.1 1.1 13.5 0.8 Ova Ova Copepoda 0.8 0.1 – – – – 1.3 0.1 4.5 0.1 Cladocera Bosmina sp. – – 0.1 * 0.4 <0.1 – – – – Mysidacea Mysis sp. – – <0.1 0.6 0.2 2.3 <0.1 <0.1 <0.1 – Ostracoda Ostracoda <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.5 1.0 Amphipoda Hyperiidae <0.1 – <0.1 <0.1 <0.1 <0.1 <0.1 0.1 0.1 0.4 Gammaridae – – <0.1 <0.1 0.1 0.2 <0.1 <0.1 – <0.1 Tintinnina Tintinnina spp. 2.8 * – – <0.1 * ––– * Foraminifera Foraminifera – – – – <0.1 * –– 3.2 * Hydromedusa Aeginopsis laurentii 0.1 * <0.1 * <0.1 * 0.1 * 0.1 * Homoeonema platygonon – – –––––– 0.1 * Dimophyes arctica <0.1 * – – – – <0.1 * 0.2 * Pteropoda Limacina helicina 3.6 4.0 0.1 <0.1 <0.1 <0.1 0.2 0.5 <0.1 0.1 Clione limacina 0.5 3.9 0.2 0.5 <0.1 <0.1 0.2 0.9 0.5 1.0 Bivalvia Bivalvia larvae 0.8 * 0.1 * ––––– * Polychaeta Polychaeta larvae & juv. 0.6 0.3 0.5 <0.1 0.2 <0.1 <0.1 0.8 0.1 0.3 Rotatoria Rotatoria spp. <0.1 * <0.1 * 0.8 * ––– – Echinodermata Echinodermata larvae 0.8 * 0.5 * <0.1 * – – 0.2 * Appendicularia Oikopleura vanhoeffeni 1.8 4.8 0.1 <0.1 0.1 0.3 7.6 8.3 3.2 4.7 Fritillaria borealis 1.0 1.3 0.1 <0.1 <0.1 <0.1 11.0 10.4 1.5 0.8 Chaetognatha Parasagitta elegans 0.7 3.7 0.2 5.9 0.2 2.4 0.3 5.8 0.1 2.2 Eukrohnia hamata – – <0.1 <0.1 – – – – 0.2 8.5 Species with P1% relative abundance/biomass in bold type. * not included in biomass calculations.

1995a,b; Fetzer et al., 2002). The typical marine fauna below the pycnocline is represented by Oithona similes, Pseudocalanus spp., Calanus glacialis, and juvenile Asteroidea (Chislenko, 1972a; Vinogradov et al., 1995a; Fetzer et al., 2002). H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 299

4.2.2. South-western Kara Sea The southwestern Kara Sea is strongly influenced by waters from the Pechora Sea that enter through the Kara Strait (Vinogradov et al., 1995b). The zooplankton community here (Table 2) consists of a mixture of cosmopolitan forms (e.g. the copepods Oithona similes, Microsetella norvegica, Microcalanus pygmaeus), marine species widely distributed throughout the Arctic (e.g. the copepods Calanus glacialis, Pseudocalanus minutus, the appendicularians Fritillaria borealis, Oikopleura vanhoeffeni, the chaetognath Parasagitta elegans, the pteropods Limacina helicina and Clione limacina) and Atlantic indicators (e.g. the hydromedusa Rathkea octo- punctata, the copepods C. finmarchicus, O. atlantica, Centropages hamatus, Temora longicornis, Acartia longiremis, the cladoceran Evadne nordmanii, the euphausiid Thysanoessa neglecta)(Virketis, 1944; Vinogradov et al., 1995b). According to Vinogradov et al. (2001) zooplankton abundance and biomass are very low in this area in winter, and small copepod species (Oithona similes, Microcalanus pygmaeus, Pseudocalanus minutus/acuspes) dominate. Already in March large numbers of Cirripedia nauplii appeared, and one month later, with the phytoplankton bloom, larvae of polychaets and echinoderms, euphausiid eggs and numerous O. similis with egg sacs were observed (Vinogradov et al., 2001). In summer (August 1993) Vinogradov et al. (1995b) recorded high numbers and biomass of Calanus and large carnivores (Parasagitta elegans, Themisto libellula and Paraeuchaeta glacialis) in the same region. During our studies in October 2000 zooplankton biomass was also high, mainly due to the presence of Calanus glacialis (Table 2, Fig. 8). Small species were also abundant, and two pteropods contributed almost 8% of the abundance. The absence or low abundance of brackish-water species indicate that there is a poor hydrographic connection between the southern and southwestern Kara Sea.

4.2.3. Central Kara Sea While the southwestern Kara Sea is mostly influenced by waters from the Pechora Sea, the Central Kara Sea community is shaped by the Barents Sea waters entering north of Novaya Semlya and by Arctic water (which in fact is modified Atlantic water) entering the shelf from the north via deep troughs. In terms of the species composition, this community is similar to that of the southwestern area, however, the shares of major components differ. Both during SIRRO (Stein et al., 2003) and the RV Mendeleev expeditions (Vinog- radov et al., 1995a) Calanus glacialis made up half of the biomass. Pseudocalanus minutus and Metridia longa were abundant in addition to small species Microcalanus pygmaeus and Oithona similis. Appendicularians were also numerous (Table 2). In the deep waters below 50–100 m true cold-water oceanic species dominated: C. hyperboreus, Paraeuchaeta glacialis, M. longa, Conchoecia borealis, Themisto libellula, Dymophies arctica and Clione limacina.

4.2.4. Northern troughs and slope The zooplankton composition of the northern troughs and shelf slope is strongly influenced by the advection from the west of modified Atlantic intermediate and Arctic waters from the Arctic Ocean and the Barents Sea. The upper layer zooplankton has the most oceanic character compared to other parts of the Kara Sea, and the community is similar to that found all around the northern Eurasian slope, e.g. north of the Barents and Laptev Seas (Mumm, 1993; Kosobokova et al., 1998). The three Calanus species dominated the biomass strongly (up to 60%) and Metridia longa contributed about 13% (Table 2). Among chaetognaths Eukrohnia hamata was more important than Parasagitta elegans, which is more typical of arctic shelves. Atlantic mid- water species (the copepods Scaphocalanus acrocephalus, Chiridius obtusifrons, Paraeuchaeta norvegica, Het- erorhabdus norvegicus, the euphausiid Thysanoessa longicaudata, the amphipod Themisto abyssorum, the hydromedusa Homoenema platygonon, and the appendicularian F. polaris) were advected from the north with intermediate water. Typical brackish-water and neritic components of the communities of the southern sea were almost (Pseudocalanus minutus/acuspes) or entirely (Drepanopus bungei, Limnocalanus macrurus, P. major) absent (Table 2), indicating a big difference between the southern and northern sea.

4.2.5. Seasonal cycle The seasonal change in ice cover, light regime, current direction (Fig. 6), and fresh water runoﬀ (Fig. 4) strongly aﬀect the seasonal distribution and development of the pelagic communities of the Kara Sea. 300 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Fig. 8. Distribution of total zooplankton dry mass in the Kara Sea from seven cruises (see Table 1). Numbers in the respective geographic boxes give mg m2 (top), mg m3 (middle) and box number (bottom).

Unfortunately, information on complete annual cycles in the pelagic is very limited. Fresh-water zooplankton species are present only in summer and displaced northwards reflecting the spreading of the river plumes (Chislenko, 1972a; Fetzer et al., 2002; Deubel et al., 2003). In contrast, the majority of the brackish-water and marine species are found all year round (Chislenko, 1972a). The mechanisms of retention for the brackish-water ones are unknown. Resting eggs were not found in sediment incubations in the southern Kara Sea (Engel, 2005), therefore most species seem to overwinter as older copepodites as in other high latitudinal seas. Similarly to the adjacent Laptev Sea (Abramova, 1999, 2000), large lipid deposits were observed in the overwintering CV and adult stages of the brakish-water copepod Drepanopus bungei (Chislenko, 1972a,b). Young stages and reproducing females were found from February to August. Pseudocalanus acuspes and P. minutus overwinter as CIII to CV. In the Ob and Yenisei Bays females with spermatophores and egg sacs were found between February and April (Vinogradov et al., 2001; H.J. Hirche, unpublished data). All stages were present throughout the summer (Fetzer et al., 2002), suggesting a long reproductive season and gradual maturation of different cohorts. Pseudocalanus major was present as CV and adults between October and April in plankton nets and sediment traps (H.J. Hirche, E.M. No¨thig, unpublished data). Females with egg sacs appeared first in December, pointing to the onset of winter reproduction. Near H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 301

Dikson and in the Yenisei Bay the proportion of females with egg sacs and mature ovaries reached 75% of total female abundance in February (K.N. Kosobokova, unpublished data). Nauplii and CI were also present. By the end of summer the proportion of young (CI–II) stages of P. major became quite low (Fetzer et al., 2002). Calanus glacialis overwinters as CIII to adults in the Kara Sea and in the Barents and Laptev Seas, where conditions are similar (Kosobokova and Hirche, 2001; Hirche and Kosobokova, 2003). Females were ready to spawn in early spring (K.N. Kosobokova, unpublished data), when egg production and growth of young stages could be fuelled by ice algae (Runge and Ingram, 1988) or early phytoplankton blooms in polynyas (Hirche and Kwasniewski, 1997; Kosobokova and Hirche, 2001). As an exception Limn- ocalanus macrurus moves through all copepodite stages to adult without diapausing in the Kara Sea (Chis- lenko, 1972b), Laptev Sea (Abramova, 1999, 2000), and in fresh water lakes (Vanderploeg et al., 1998). Survival of long periods of unfavourable conditions is made possible by large lipid reserves, mostly wax esters (Vanderploeg et al., 1998; Hirche et al., 2003) and increased carnivory in the older stages. The presence of nauplii, all copepodite stages and adult males in Oithona similis and Microcalanus pygmaeus over much of the year and the absence of cohorts suggests sustained reproduction in the Kara Sea as well as in the Arctic Ocean (Kosobokova, 1983; Ashjian et al., 2003).

4.3. Higher trophic levels

4.3.1. Fish The Kara Sea is usually rated as ‘‘fishless’’ (Zenkevich, 1963) due to a scarcity of fish populations in comparison to the arctic seas to the west. Benthic fish are most numerous, pelagic and bottom-dwelling, coastal, shallow-water fish are very few (Antonov, 1989; Antonov and Chernova, 1989). A list of 19 of the more abundant species together with information on size and trophic level is provided by SAUP, University of British Columbia (http://saup.fisheries.ubc.ca/lme). The distribution range of the atlantic-boreal and boreal-arctic fish is restricted to the southwestern part of the Kara Sea. They almost do not penetrate into the eastern sea due to severe environmental conditions there. In the brackish coastal waters the species feeding on fish and crustaceans prevails; in the open sea the majority feeds on benthos. Some fish are planktivorous with low feeding selectivity. The marine fish fauna is very sparse and so difficult to access. No commercial fishing takes place in the open parts of the sea, except for the westernmost Kara Sea. In the coastal area the fishery is restricted to the rivers and estuaries, where the main species caught are anadromous whitefish (Novoselov and Chuksina, 1999). Eight species of this family have been recorded, of which six species make up 70–90% of the total recorded landings from the area: Coregonus nasus (Broad Whitefish), C. autumnalis (Omul), C. muksun (Muksun), C. peled (Peled), C. sardinella (Siberian Cisco) and C. lavaretus (Humpback Whitefish). Data on fish landings of the most important fish species indicate a significant decline in landings of whitefish in four areas. This tendency is most evident in the western part of the Kara Sea. In the Ob Bay, the landings of whitefish declined by 42% between 1990 and 1994. In the lower Yenisei river, the decline was 35% during the same period. The recorded landings of whitefish from Ob Bay in 1994 were only 46% of the landings recorded 10 years earlier (http://na.nefsc.noaa.gov/lme/text/lme58.htm#fish).

4.3.2. Mammals Greenland seal (Phoca groenlandica), ringed seal (P. hispida), bearded seal (Erignathus barbatus), walruses (Odobenus rosmarus), and narwhals (Monodon monoceros) breed and rest on the coastal areas of the Kara Sea. Polar bears (Ursus maritimus) hunt for seals in the marginal ice zone. White whales (Delphinapterus leucas) appear in the Kara Sea in April–May in the Ob and Yenisei region where ﬁsh populations are most numerous (Klumov, 1936). Most whales leave this region by autumn to overwinter in the Barents Sea (Boltunov and Belikov, 2002). On the other hand, there is evidence that some whale families can stay in the Ob polynya and in the eastern sea throughout the winter (Ivashin et al., 1972; Kleinberg et al., 1960). White whales and Greenland and ringed seals are known to be ﬁsh-feeding, while walrus and bearded seal are feeding on benthos (Mishin et al., 1989). The distribution, reproduction, and feeding of these mammals are rather well studied (Matishov et al., 1998). However, reliable assessments of their numbers are missing. 302 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

4.3.3. Birds No special investigations on the seabirds in the Kara Sea are available except for a few studies on distribution and breeding of ivory gull (Pagophila eburnea) in Severnaya Semlya (Volkov and Pridatko, 1994; Vol- kov and de Korte, 1996) and occasional observations on seabirds during the Russian ship-going surveys (Matishov et al., 1998). The species list of seabirds is short, their abundance is low and seasonally restricted. Most populations can be found over the western part of the Kara Sea during the ice-free season in spring and fall. They mostly penetrate in this region from the west through the Kara Strait and from the north around the Novaya Semlya archipelago. Taking into account that kittiwake and thick-billed murres feed on zooplankton and pelagic ﬁsh, one may assume that seabirds’ distribution in the western Kara Sea follows the distribution of their pelagic food objects. A few species encountered in the Kara Sea during their seasonal migrations are ful- mar (Fulmarus glacialis), glaucous gull (Larus hyperboreus), kittiwake (Rissa tridactyla) and thick-billed murres (Uria lomvia) (Y.V. Krasnov, personal communication). The two latter ones appear regularly from the colonies on the western coast of Novaya Semlya after termination of the breeding season. In winter seabirds are mostly found in polynyas and ﬂaw leads. Small groups of euryphagous Larus hyperboreus are observed there most often. The numbers of kittiwake and thick-billed murres overwintering in the polynyas are consid- erably lower.

5. Components of the carbon cycle

Due to low seasonal and regional coverage the data presented for different components of the carbon flux are only rough estimates. However, they may help to direct future activities to fill the gaps. For regional and interannual comparison, the carbon content of phytoplankton and zooplankton was calculated for all stations and cruises shown in Fig. 1, applying a factor of 50 to convert chlorophyll a to carbon (own data and Saksh- aug (2003) but note that: phytoplankton carbon:chlorophyll a ratio is acclimation-dependent, and varies between different phytoplankton groups; majority of data in arctic and subarctic waters ranges between 25 and 100 w:w; cited in Sakshaug (2003)) and assuming a 50% carbon content of zooplankton dry mass (own data). In order to identify regional differences, the Kara Sea was divided into boxes covering 5 longitude and 2 latitude. Mean carbon concentration was calculated per m2 and m3 for every box where samples were available (Fig. 8).

5.1. Phytoplankton

5.1.1. Phytoplankton biomass The assessments of phytoplankton biomass in the Kara Sea presented in almost all Russian publications were up to the present date based on calculation of phytoplankton wet weight (biovolume) from algal cell volume and species numbers (Usatchev, 1947; Druzhkov and Makarevich, 1996; Makarevich, 1995; Makarevich et al., 2003). Most investigations did not consider small solitary living nanoflagellates and ciliates. As an exception, Vedernikov et al. (1995) measured chlorophyll a concentrations ranging between 0.2 and 5.5 lgL1 in the surface with one exception in the inner Ob Bay (22 lgL1) in September. Generally concentrations decreased towards the open Kara Sea (<1.0 lgL1). During SIRRO, chlorophyll a values ranged from 0.2 to 5.0 lgL1. The highest values were found usually in the surface (No¨thig et al., 2003; Deubel et al., 2003). Only in 1999 was a phytoplankton bloom with maximum chlorophyll a values of 13 lgL1 observed in the Ob estuary and in the open Kara Sea. Values below the pycnocline were generally <0.5 lgL1. Integrated biomass from bottom to surface showed large regional and interannual variability (Table 6). Biomass was higher In the rivers than in the estuaries or the open Kara Sea, but in 1999 it was also very high in the brackish water (boxes 10 and 11; Fig. 7). In the northernmost region biomass was in the same range as in the central sea, but measurements were from different years. Espe- cially in 1995 a relatively high biomass was encountered in box 20 (Table 6). There was great interannual variability and highest concentrations were found in 1999. Thus in box 10 phytoplankton carbon was higher in 1999 than in other years by a factor of 8, and in box 11 even by a factor of 19. Differences in timing and amount of freshwater outflow may have caused the observed interannual variability (Fig. 5). On a transect across the St. Anna and Voronin Troughs (ARK XII in 1996) chlorophyll a concentrations between 0.4 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 303 and 1.7 lgL1 were found in the last week of July,which may reflect the end of the spring increase in the northern Kara Sea (E.M. No¨thig, unpublished data).

5.1.2. Primary production The growth season begins under single-year ice in early spring. Within the coastal shallows blooms were observed already in the water column long before the ice breaks up (Makarevich and Matishov, 2000). When the ice starts to break up in May (Fig. 3), ice edge blooms should develop similar to those in the Barents Sea (Sakshaug, 2003). Another source of enhanced primary production might be polynyas, which are known to be highly productive areas. The growth season ends when the nutrients are depleted, light becomes reduced, and the formation of sea ice begins, usually in October (Fig. 3). So far Vedernikov et al. (1995) presented the most detailed data on primary production in the Kara Sea. It averaged 80 mg C m2 d1 in the southern sea up to 76N in September and varied from 107 to 312 mg C m2 d1 in the Yenisei inlet, from 25–63 in the Ob inlet, and from 20 to 359 in the southwestern open sea. Low production was attributed to deficiency in nutrients, low temperature, turbidity (especially in the Ob estuary), and low insolation. Vinogradov et al. (2000) using satellite data and field measurements estimated an annual primary production of 20 · 106 t C (ca. 23 g C m2) for the Kara Sea. A slightly lower value (14 · 106 tCa1;ca.16gCm2 a1) was used by Vetrov and Roman- kevich (2004). They suggest that primary production of macrophytobenthos is negligible in the Kara Sea; however, the contribution of microphytobenthos may be significant, reaching up to 11% of the overall phytoplankton production. Sakshaug (2003) suggested an average primary production of 35 g C m2 for the Sibe- rian shelf seas, which is much higher than the other estimates. There is certainly need for more direct measurements, covering both seasonal and regional variability adequately.

5.2. Zooplankton

5.2.1. Zooplankton biomass The sampling period for the zooplankton data presented in Fig. 8 was between end of August and mid- October (Table 1), when most of the species had reached their overwintering state (Fetzer et al., 2002)and biomass probably was highest. Regional zooplankton concentrations ranged from 3 to 57 mg C m3. Lowest concentrations were found at the southernmost Yenisei stations, followed by the Ob and Kara Straits. At the deeper stations north of 76N, concentrations were also low. Surprisingly, on the slope (box 21), concentrations were relatively high despite great sampling depth. Highest concentrations were found in the river estuaries (boxes 7 and 8) and in the Novaya Semlya Trough (box 5). A west-to-east gradient. as reported by Jaschnov (1940) and Bogorov (1945), was not detected in our data. The mean of all boxes up to 78N (1.2 g C m2) was higher by 70% than the 0.7 g m2 estimate by Vinogradov et al. (1995a). For some regions estimates from several years (Table 3) allow an interannual comparison. There were often considerable differences among years; for example, zooplankton biomass in 1999 was more than twice that in the other years. Phytoplankton biomass was also highest in 1999. Large interannual changes were also observed in zooplankton species composition in the southern Kara Sea (Fetzer et al., 2002). As zooplankton species occupy different trophic levels, for future determination of the potential carbon flow it was necessary to separate secondary (herbivorous) and tertiary (carnivorous) production. Therefore, all species were assigned to three trophic groups, and the biomass for each group was calculated for the four zones mentioned before (Section 4.2). For classification, published information especially from fatty acid composition (Peters et al., 2004) was applied. General rather than specialized feeding behaviour was envis- aged, as in some species in addition to a broad spectrum of food items of particular stages there were onto- genetic changes in diet, as, for example, forLimnocalanus macrurus (Warren, 1985). Classification of the trophic state of each species and contribution of each trophic group to total biomass are shown in Table 4. The portion of herbivorous species is least in the southern region, where they are replaced by omnivores. Carnivores are most important in the slope waters. Mean biomass m2 of the three trophic groups was calculated for the whole Kara Sea after extrapolation for missing boxes (Table 5). Our values are higher every- where in the Kara Sea than the 0.04–0.35 g dry mass reported by Vinogradov et al. (1995a) for the eastern Kara Sea and the Yenisei estuary. The total zooplankton biomass in the Kara Sea during our study was 1.43 · 106 tC. 304 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Table 3 Interannual variability of zooplankton dry weight (mg m3) in diﬀerent geographic boxes (see Fig. 8) of the Kara Sea Box 1997 1999 2000 2001 3 18.5 (1) 4 10.8 (4) 2.8 (7) 7 80.6 (7) 156.6 (7) 40.6 (1) 8 62.2 (6) 181.0 (8) 25.1 (1) 9 33.5 (6) 110.3 (2) 30.1 (3) 33.2 (2) 10 60.3 (2) 11.8 (2) 11 39.4 (1) 80.0 (5) 21.8 (1) 32.6 (3) 12 21.5 (6) 29.8 (2) 15 18.9 (1) 24.5 (6) 16 52.6 (3) 23.4 (2) 17 27.4 (1) 35.2 (7) Number of stations in brackets.

Table 4 Trophic composition of zooplankton biomass (%) in different areas of the Kara Sea Region South-west South South Central Troughs and slope 2000 1997 1999 2000/01 1995/96 Number of stations 6 20 24 11 8 Predominant Calanus glacialis 53.3 17.9 5.3 48.7 31.9 herbivores C. finmarchicus 1.9 0.6 0.1 3.6 17.9 C. hyperboreus 0.2 0.4 0.2 3.9 9.7 Pseudocalanus acuspes/ 1.6 9 0.1 1 0.3 minutus P. major 0.3 12.6 0.5 1.6 – Nauplii Copepoda 3 0.7 0.2 1.1 0.8 Limacina helicina 4 <0.1 <0.1 0.5 0.1 Oikopleura vanhoeffeni 4.8 <0.1 0.3 8.3 4.7 Fritillaria borealis 1.3 <0.1 <0.1 10.4 0.8 Total 70.4 41.2 6.7 79.1 66.2

Omnivores Metridia longa 0.5 0.3 <0.1 3.8 13.3 Microcalanus pygmaeus 1.1 0.3 0.1 1.4 1.2 Polychaeta larvae & juv. 0.3 <0.1 <0.1 0.8 0.3 Drepanopus bungei 0.1 45.7 14.8 <0.1 – Jaschnovia tolli – 1.2 <0.1 – – Limnocalanus macrurus – 1.6 64.9 –– Microsetella norvegica 10.8 <0.1 <0.1 0.1 <0.1 Oithona similis 8.4 0.6 0.2 6.9 2.6 Mysis sp. – 0.6 2.3 <0.1 – Total 21.2 50.3 82.3 13.0 14.8 Carnivores Paraeuchaeta glacialis 0.4 0.2 0.1 0.6 1.5 P. norvegica – 0.1 – 0.1 0.9 Cyclops strenuus – 1.3 7.9 –– Clione limacina 3.9 0.5 <0.1 0.9 1 Parasagitta elegans 3.7 5.9 2.4 5.8 2.2 Eukrohnia hamata – <0.1 – – 8.5 Total 8 8 10.4 7.4 14.1

Total zooplankton biomass in the northern shelf and slope region is on the high end of biomass estimates in various regions of the Arctic Ocean (see review in Kosobokova and Hirche, 2000; their Table 3). Many concentrations in the southern and central Kara Sea (Fig. 8) were much higher than the highest concentration of 13 mg C m3 in the Laptev Sea (Peters, 2001). For the seasonally ice-covered northern Barents Sea, biomass varied between 1 and 7 g m2 carbon between 1986 and 2000 (Dalpadado et al., 2003; assuming 50% carbon H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 305

Table 5 Mean biomass and annual production for herbivorous, omnivorous and carnivorous zooplankton (for classiﬁcation see Table 4) in the Kara Sea calculated per m2 and the total area Biomass (g C m2) Production Production 106 (t Corg a1) Carbon required 106 (t Corg a1) (g C m2 a1) Herbivorous 1.280 0.920 0.79 Omnivorous 0.197 0.171 0.14 Carnivorous 0.193 0.102 0.09 Total 1.672 1.193 1.01 Secondary production 1.006 0.860 8.6 Tertiary production 0.188 0.160 1.6 Secondary production = herbivorous + 50% omnivorous, tertiary production = 50% omnivorous + carnivorous. content of dry mass), which is in the same range as observations during our study in the northern Kara Sea, where depth is similar to the Barents Sea (Fig. 8).

5.2.2. Zooplankton production As no direct measurements of zooplankton growth and production were available for the Kara Sea, we used a simple approach to estimate the minimum annual zooplankton production. We assumed that our biomass measurements represent the maximum annual biomass, which had been produced during the last one or two years according to a species’ generation time, ignoring mortality. Furthermore we used a single-year life cycle for all smaller species (Kosobokova, 1983), and two-year life cycles for C. glacialis (Slagstad and Tande, 1990; Kosobokova, 1999), C. hyperboreus, Paraeuchaeta spp. and two chaetognaths. Their biomass share was estimated separately for each of the four zones and then applied to each box (Table 2). We ignored the fact that at least C. hyperboreus has a longer life cycle (Hirche, 1997), as the proportion of this species, especially its older stages, was rather low (Table 2). The annual production (g C m2) for each box is presented in Fig. 8. Regional patterns of production were very similar to biomass distribution (see above) as the share of species with one and two year life cycles was very similar in most areas except for the rivers, where hardly any species with a two year life cycle was encountered. For the whole Kara Sea (855,639 km2) a total zoooplankton production of 1.01 · 106 tCa1 corresponding to a mean of 1.2 g C m2 a1 was estimated after extrapolation to boxes without data (Table 5). For calculation of secondary production we pooled the herbivorous and 50% of the omnivorous biomass, for tertiary production carnivorous and 50% omnivorous biomass were pooled. Mean secondary production was 1.006 g C m2 a1 and carnivorous production was 0.188 g C m2 a1 (Table 5). These values are smaller than the 1.6 · 106 tCa1 reported by Vetrov and Romankevich (2004), but it is difficult to compare values of different origin and derived by different approaches. Vinogradov et al. (1995a) stated that the Kara Sea was a low-productive basin when compared to the Barents Sea, while the estuaries and adjacent waters were more productive, but their biomass values were much lower than the ones reported here for this area. This may relate to the fact that they collected samples mostly by water bottles.

5.2.3. Carbon requirements of zooplankton Carbon requirements of both herbivorous and carnivorous zooplankton were estimated from mean annual production applying 10% growth efficiency for the entire year, rather than the frequently used 30% (Ikeda and Motoda, 1978) that should represent only the active growth period (Table 5), taking into account that more than half of the year is spent in dormancy or under starvation conditions. To understand regional aspects and short-term processes for the period of some SIRRO cruises the effect of short term grazing impact on phytoplankton was studied (Table 6). The organic carbon demand necessary to maintain optimum growth of the zooplankton stock was determined assuming maximum growth rate at 2 C (production/biomass = 0.027; from Huntley and Lopez, 1992) and a growth efficiency of 30% (Ikeda and Motoda, 1978). Herbivorous biomass was calculated as Cherbivores + 0.5 · Comnivores. During the SIRRO expeditions the portion of phytoplankton carbon thus required varied from 0.1% to 30.4% (Table 6). The high phytoplankton concentrations and relatively low herbivorous biomass in the 306 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Table 6 Carbon content (mg m2) of herbivorous zooplankton (CHZ) and Phytoplankton (CPP), integrated from top to bottom, and grazing impact (GI; % of CPP d1 by CHZ) in different years calculated for geographical boxes of the Kara Sea Box 1995/6 1997 1999 2000 2001 CHZ CPP GI CHZ CPP GI CHZ CPP GI CHZ CPP GI CHZ CPP GI 3 89 5224 0.1 4 49 2445 0.2 12 4723 0.0 6 244 1045 1.9 7 313 677 3.7 575 4285 1.1 121 392 2.5 8 284 1449 1.6 764 1790 3.4 186 1317 1.1 9 237 971 2.0 409 2458 1.3 152 660 1.8 127 642 1.6 10 342 5820 0.5 427 387 8.8 11 523 6820 0.6 435 365 9.5 350 411 6.8 12 298 1410 1.7 362 1693 1.7 13 935 631 11.9 14 1337 854 12.5 15 630 510 9.9 1749 1568 8.9 16 1332 350 30.4 1189 1704 5.6 17 756 650 9.3 18 488 355 11.0 19 1048 720 11.6 20 2079 1580 15.4 For position of boxes see Fig. 8; details of calculation in the text. southern Kara Sea were reflected in very low requirements. In contrast, the large herbivorous species in the central sea and lower phytoplankton concentrations there resulted in requirements around 10% of standing phytoplankton stock. Interannual variability was only moderate within the respective boxes. Our data from the Kara Sea are well within the range of data on copepod herbivory from various Arctic locations compiled recently by Saunders et al. (2003). The data show that, at least during late summer/early autumn, phytoplankton carbon was more than sufficient for the requirements of copepods. Hence there was no necessity to feed on detritus with probably low nutritive value (see Section 5.1). In contrast, high concentrations of refractory particles may hinder filtration efficiency of copepods. Nowhere was there an indication of grazing controlling primary production. Assuming a conservative doubling rate of algal cells of 0.5 day1 for the environmental conditions of the Kara Sea, only a fraction of primary production is consumed by zooplankton. These conclusions are in contrast to earlier assumptions by Vinogradov et al. (1995a), who suggested detritus would be the main food for marine zooplankton at the observed low abundance and productivity of phytoplankton.

5.3. Bacteria

5.3.1. Bacterial production In addition to zooplankton, heterotrophic bacteria are a major sink for organic carbon produced by phytoplankton (Nagata, 2000). While there are some older reports on bacterial abundance in the Kara Sea (e.g. Mitskevich and Namsaraev, 1995, and references therein), so far only one study directly measured bacterial production and addressed the interaction of dissolved organic carbon (DOC) and bacteria (Meon and Amon, 2004). Pooled according to surface salinities, the average bacterial production in August and September, 2001, was highest in the surface waters of the rivers (13.5 lgCL1 d1) and signiﬁcantly decreased towards the estuaries (5.8 lgCL1 d1) and the open Kara Sea (2.4 lgCL1 d1)(Table 7). Production also decreased with depth at stations where a pronounced pycnocline separated the surface layer and the bottom layer. Bacterial production close to the bottom on average constituted about 25% of that measured near the surface. Decreas- ing bacterial production towards the open Kara Sea was mirrored in bacterial numbers averaging 1.51 · 106 cells ml1 and 1.93 · 106 cells ml1 in the rivers and estuaries, respectively, but decreasing to <0.5 · 106 cells ml1 in the surface waters of the open Kara Sea (Meon and Amon, 2004). In summer 2001, the distribution of heterotrophic bacterial activity in the Kara Sea was controlled by the availability of DOC. The ultimate source of the DOC pool is release by phytoplankton cells directly (e.g. cell lysis, exudates) H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 307

Table 7 Bacterial production (BP) and bacterial carbon demand (BCD) in the rivers Ob and Yenisei and the Kara Sea (August–September 2001) for surface waters and integrated over depth (Areal); average ± SD Location Surface Areala BPb (lgCL1 d1) BCDc (lgCL1 d1)BPb (mg C m2 d1) BCDc (mg C m2 d1) Rivers (0 psu) 13.5 ± 7.0 (n = 8) 49.9 ± 26.1 (n = 8) 183 ± 36 676 ± 133 Estuary (5–15 psu) 5.8 ± 1.9 (n = 7) 22.8 ± 9.7(n = 7) 68 ± 10 252 ± 37 Kara Sea (>20 psu) 2.5 ± 1.5 (n = 13) 9.2 ± 5.3 (n = 13) 80 ± 35 296 ± 129 a Integrated over the depth at the respective sampling stations using bacterial production data from the surface, pycnocline and bottom layer. b Based on a conversion factor for leucine uptake of 1.15 · 1017 cells mol1 (Kirchman, 1992) and 20 fg C cell1. c Based on a bacterial growth eﬃciency of 0.27. or indirectly (e.g. grazing zooplankton, viral lysis). The tight temporal and spatial coupling of primary production and bacterial activity in the Kara Sea system was corroborated by the signiﬁcant correlation of chlorophyll a and bacterial production (r = 0.78, p < 0.001), and low concentrations of labile substrates such as free glucose and amino acids (<50 nM), combined with fast turnover rate constants (up to 2.8 d1) of these substrates (Meon and Amon, 2004). In contrast, allochthonous DOC from the rivers Ob and Yenisei did not enhance the growth of bacterial communities, indicating the refractory nature of the riverine dissolved organic material. This was also supported by the conservative behavior of riverine dissolved organic material and its chemical properties during estuarine mixing (Ko¨hler et al., 2003; Amon and Meon, 2004).

5.3.2. Bacterial carbon demand A conservative estimate of the annual bacterial carbon demand (BCD) of the Kara Sea is 44 · 106 tCa1 (Table 7), based on low bacterial production measurements in the northern part of the Kara Sea and a bacterial growth eﬃciency of 27% (Meon and Amon, 2004). To approximate the bacterial activity during the winter period in the Kara Sea, a BCD measured by Sherr and Sherr (2003) during winter in the central Arctic Ocean was applied. For comparison, Vetrov and Romankevich (2004) report an annual bacterial net production in the Kara Sea of 7 Tg C. However, methods and source data leading to this estimate are not speciﬁed. The SIRRO data show that among the heterotrophic organisms in the Kara Sea the bacterial community represents a major sink for organic carbon produced by phytoplankton.

5.4. Vertical ﬂux

Variations in the vertical flux are the expression of differences in primary production, as well as retention and export food chains (Wassmann, 1998). In several arctic regions, river discharge and resuspension may influence primary production and vertical export. Pelagic-benthic coupling and vertical flux in many arctic regions are strongly dominated by episodic events on daily, weekly, seasonal and yearly time scales (Wass- mann et al., 1996). There exist only very few measurements on the vertical flux of POC in the Kara Sea (Table 8). In September 1999 two short-term sediment trap moorings collected sinking particles in bottom waters off the Ob and Yen- isei river mouths (Gaye-Haake et al., 2003). Diatoms and copepod faeces dominated the identifiable particles in the trap material. As a phytoplankton bloom was situated north of the Ob estuary, sedimentation rates were generally higher off the Ob than off the Yenisei. The Ob trap collected fresh, surface-derived particulate matter. Particles from the Yenisei trap were more degraded and might partly have been derived from resuspended material. POC fluxes from the short-term traps (Gaye-Haake et al., 2003), as well as a long-term trap deployment off the Yenisei, were around 150 mg C m2 d1 during the growth season. The short-term moorings in late summer showed an episodic event (Table 8): sedimentation of a late summer bloom in the Ob estuary, as was mentioned by Wassmann et al. (2003). The only other exposure of sediment traps was carried out by Wassmann et al. (2003). They reported low vertical POC export (1–10 mg C m2 d1) in the open Kara Sea at the beginning of autumn (August/September). Amorphous aggregates dominated vertical export of particulate matter. Faecal pellets of crustaceans were abundant in a few cases. In the outer parts of the Ob and 308 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313

Table 8

Total ﬂux of total and organic carbon (Corg) and portion of organic carbon during deployment of sediment traps in the Kara Sea

Station No. Latitude Longitude Water depth Trap depth Deployment Duration Total ﬂux Corg ﬂux Corg Reference N E (m) (m) (date) (days) (mg m2 d1) (mg m2 d1) (%) 4395 7413.900 7300.080 30 15 15-Sep-93 10.31 9 1.05 11.68 1 20 16.2 3.29 20.29 1 4396 7500.000 7301.220 35 15 15-Sep-93 9.5 62.6 0.71 1.14 1 20 22.9 1.51 6.58 1 4400 7414.330 7958.170 35 15 17-Sep-93 5.34 9.4 1.58 16.81 1 4401 7400.150 7956.640 34 14 17-Sep-93 4.95 109.2 7.25 6.64 1 19 54.4 6.29 11.57 1 4402 7332.330 7057.320 40 20 17-Sep-93 4.34 158.5 1 25 856 1 4403 7300.100 7955.610 25 10 18-Sep-93 3.78 103.4 14.78 14.3 1 15 227.2 11.79 5.2 1 4404 7031.450 7949.510 16 8 18-Sep-93 3.15 9937 1 4405 7138.500 8326.900 17 9 18-Sep-93 1.71 22,156 368 1.66 1 4415 7245.800 7328.330 32 22 26-Sep-93 3.31 1321 26.7 2.02 1 BP-1 7353.30 7310.50 32 22 13-Sep-97 9.88 109 19.1 2.7 2 BP-2 7359.80 7925.20 30 17 16-Sep-97 4.45 5.8 0.5 8.6 2 BP-3 7250.00 8010.10 18 9 18-Sep-97 1.32 60.7 2.55 4.2 2 Ob-01 7429.940 7459.650 35 30 15-Aug-99 22 (2) 1375.9–2916.5 391.2–1351.8 16.1–46.4 3 3 Yen-01 7400.080 7958.630 36 31 26-Aug-99 4 (1, 3) 336.0–3908.5 47.7–450.6 11.5–14.2 3 3 Yen 02 7400.280 8000.450 31 20 16-Sep-00 28 (7, 14) 909.8–2898.7 81.9–148 10.1–5.1 This work This work Kara 01 7612.080 7545.30 73 54 28-Jun-02 41 (14, 28) 807.4–1320.9 42.9–77.1 5.31–5.84 This work This work Sampling intervals in brackets. 1 Lisitzin et al., 1995. 2 Wassmann et al., 2003. 3 Gaye-Haake et al., 2003.

Yenisei estuaries vertical flux was also small, but the total flux was greater than in the open sea. The dominance of terrigenous matter was probably related to resuspended material. In the zone of the marginal filter (sensu Lisitzin, 1995), where riverine and sea water are mixing, POC fluxes were as high as 370 mg C m2 d1, and more comparable to the rates measured by Gaye-Haake et al. (2003). These two studies show that flux rates on the shallow shelf can be relatively high for an arctic system in summer due to sedimention events or strong sedimentary reworking in the highly energetic river mouth. In contrast, the vertical flux data from the open Kara Sea reflect the oligotrophy of the system.

6. Conclusions

Our study has shown that some of the current views of the ecology of the Kara Sea need revision. Thus zooplankton production is not very low, and biomass is actually rather high in the northern Kara Sea. Fur- thermore, the contribution of the riverine input of nutrients to primary production and of detritus to nutrition of zooplankton is rather limited, at least in summer, while bacterial production decreases from the rivers to the open sea, obviously controlled by the availability of DOC from primary producers. It seems that the current estimates of primary production are either too low or other sources of dissolved organic carbon have to be considered, if the data for the requirements of zooplankton and bacteria from this study and for the benthos from Klages et al. (2003) are realistic. These authors estimated a carbon requirement of 5.06 · 106 tCa1 for the macrozoobenthos communities of the Kara Sea based on biomass measurements during the SIRRO cruises. One of these sources could be the inflow of water from the Barents Sea via the St. Anna Trough and the Kara Strait. Modeling results indicate that the volume flow through the Kara Sea is subject to strong interannual variations (Karcher et al., 2003). On average it is more than an order of magnitude higher than the H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 309 combined river discharge of Ob and Yenisei. According to Fransson et al. (2001) the Barents Sea exports about 9.6 Tg C a1 of relatively fresh DOC that could support bacterial growth. Ice algae production was not considered here, but may contribute significantly to overall primary production. Furthermore the role of the microbial loop has not yet been studied at all. As a consequence the high demand of carbon by the various consumers makes the export of surplus production from the Kara Sea to neighbouring coastal seas and the central Arctic Ocean unlikely.

Acknowledgements

Thanks to Stefan Kern and Udo Hu¨bner who prepared the graphical output of the satellite data. Ingo Fet- zer and Marcus Engel helped with maps and ﬁgures. Yury Krasnov, MMBI and Natalia Nikolaeva, WWF (Russia) supplied unpublished information on sea-birds. The original data used here were collected during the Bundesministerium fu¨r Bildung und Forschung (BMBF) Grant No. 03G0539-SIRRO. The work of KNK was supported by RFBR Grant No. 03-05-64871.

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