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 con- sidered. 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. Ó 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). 0079-6611/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2006.09.010 H.J. Hirche et al. / Progress in Oceanography 71 (2006) 288–313 289 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 consid- ered. 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 zooplank- ton 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 75°N 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).