Progress in Oceanography Progress in Oceanography 71 (2006) 314–330 www.elsevier.com/locate/pocean Trophic pathways and carbon flux patterns in the Laptev Sea Michael K. Schmid a,*, Dieter Piepenburg a,b, Alexander A. Golikov c, Karen von Juterzenka a, Victor V. Petryashov c, Michael Spindler a a Institut fu¨r Polaro¨kologie der Universita¨t Kiel, Wischhofstr. 1-3, Geb. 12, 24148 Kiel, Germany b Akademie der Wissenschaften und der Literatur Mainz, c/o Institut fu¨r Polaro¨kologie der Universita¨t Kiel, Wischhofstr. 1-3, Geb. 12, 24148 Kiel, Germany c Zoological Institute of RAS (ZIN), 199034 St.-Petersburg, Universitetskaja nab., 1, Russia Available online 9 November 2006 Abstract The Laptev Sea is a high-Arctic epicontinental sea north of Siberia (Russia) that is one of the least understood regions of the world’s ocean. It is characterized by a shallow and broad shelf plateau, high influx of river water, sediments and nutrients during summer, long-lasting sea-ice cover from October to May, and the formation of a narrow flaw-lead poly- nya off the fast-ice edge during winter. Here, we describe results of a German–Russian research project (1993-present), presenting the distribution patterns and dynamics of its marine flora and fauna, as well as pathways and processes of coupling between sea-ice, water-column and sea-floor biota. Three ecological zones are distinguished along a combined east–west and Lena-impact gradient, differing in the compo- sition of pelagic and benthic communities. In general, high Chl a concentrations in the sediments indicate a tight coupling between sympagic and pelagic primary production and nutrient supply to the benthos throughout the entire Laptev Sea. However, there were pronounced regional differences between the ecological zones in magnitude of primary production and trophic dynamics. Primary production during the ice-free summer was highest in the estuarine zone most strongly influ- enced by the Lena River (210 mg C mÀ2 dayÀ1). The western and northeastern Laptev Sea yielded 55 and 95 mg C mÀ2 dayÀ1, respectively. Moreover, the zones differed in the partitioning of carbon flux between zooplankton and benthic food webs. In the Lena zone zooplankton carbon demand was about 31 mg C mÀ2 dayÀ1 whereas in the western zone it was 21 mg C mÀ2 dayÀ1 and in the eastern zone 4 mg C mÀ2 dayÀ1. Total benthic carbon demand was 32 mg C mÀ2 dayÀ1 for the Lena zone, 56 mg C mÀ2 dayÀ1 in the western zone and 100 mg C mÀ2 dayÀ1 in the northeastern zone. A carbon budget constructed for the Laptev Sea indicates that (1) a high proportion of primary production is chan- nelled through the benthic trophic web, bypassing the pelagic trophic web, and (2) autochthonous primary production in the northeastern and western Laptev Sea might not be sufficient to fuel both pelagic and benthic secondary production and, hence, input of allochthonous organic carbon is required to balance the overall carbon demand. Ó 2006 Elsevier Ltd. All rights reserved. Regional Index Terms: Arctic; Russia; Siberia; Laptev Sea Keywords: Cryo–pelago–benthic coupling; Production; Carbon flux; Climate change; Benthos; Zooplankton; Phytoplankton * Corresponding author. E-mail address: [email protected] (M.K. Schmid). 0079-6611/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2006.09.002 M.K. Schmid et al. / Progress in Oceanography 71 (2006) 314–330 315 1. Introduction 1.1. The Laptev Sea Initially called the ‘Nordenskjo¨ld Sea’ for the Swedish explorer Nils Adolf Nordenskjo¨ld, it was renamed in honour of Khariton Prokofievitch Laptev and Dmitri Yakovlevitch Laptev, two Russian Arctic explorers of the second Bering expedition (1733–1743) sent out by Czar Peter the Great. The Laptev Sea is a truly high-Arctic region off northern Siberia (Russia) that largely consists of a shal- low shelf plateau (Anonymous, 1985). Due to its isolated central position along the Eurasian continental margin, it is minimally affected by Atlantic and Pacific influences in terms of meteorology, oceanography and biogeography (Zenkevitch, 1963). Consequently, it is characterized by extreme ecological conditions, featuring permanently low temperatures, sea ice cover lasting from October to May, as well as pro- nounced salinity fluctuations caused by the seasonally pulsed riverine inflow of freshwater (Timokhov, 1994). Since the early 1990s, the Laptev Sea has been the target of a number of intensive research efforts, because it has been recognized that the Arctic in general – and the Laptev Sea in particular – are key regions that heavily impact large-scale processes shaping the global climate (Macdonald, 1996). Considering the extremes in location and climate of the Laptev Sea, it is not surprising that comparatively less is known about its ecology than about that of other arctic marginal seas, such as the Barents and Bering Seas, and it arguably ranked as one of the least known sea regions of the world. However, this by no means implies that the Laptev Sea was an absolute terra incognita in ecological terms. Dayton’s (1990) general statement that the history of the scientific exploration of high-Arctic regions is not as short and fragmentary as one might suppose because of their remoteness, difficult accessibility and extreme climatic conditions applies particu- larly to the Laptev Sea. In the course of a number of field studies, quite a wealth of information on various ecological aspects has been collected. In fact, the first ecological investigations can be dated back to Nordenskjo¨ld’s Northeast Passage in 1878 (Sirenko and Piepenburg, 1994). Later, there were several Russian (1912–14) and Soviet expeditions, primarily during the 1930s and after the Second World War. The main purposes of these research efforts were to collect information to ensure the navigation via the Northern Sea Route and to sup- port the exploration of mineral resources (Pivovarov et al., 2004). In 1973, Golikov and co-workers started a first inventory of the littoral habitats of the Laptev Sea, using a scuba-diving approach (Golikov, 1990). However, compared to other Arctic seas, the Laptev Sea remained one of the less known regions, particu- larly to the Western scientific community. Russian seas were closed to international exploration for a num- ber of decades and most research reports published in Russian journals remained obscure because they were not easily available to foreign researchers. That changed in the 1990’s when international interdisciplinary research programmes focussed on this rather secluded Arctic sea, because of its significance as one of the major ‘ice factories’ in the Arctic. They led to a considerable increase in our knowledge about the Laptev Sea system (Pivovarov et al., 2004). The ‘Scientific Programme on Arctic and Siberian Aquatorium’ (SPASIBA) expeditions were conducted in 1989–1991 as part of the ‘Joint Global Ocean Flux Study’ (JGOFS). These were the first international field studies in the Laptev Sea after the Soviet regions were opened to foreign workers. The investigations focused on the impacts of the riverine influx on the Laptev Sea and yielded a variety of novel findings, inter alia, on the biogeochemistry of the Lena river outflow (Cauwet and Sidorov, 1996), microbial activities (Saliot et al., 1996), distribution and sedimentation of phytoplankton and detritus (Heiskanen and Keck, 1996), as well as plankton and primary production (Sorokin and Sorokin, 1996). In the winter of 1992, the Russian–German ‘East Siberian Arctic Region Expedition’ (ESARE) studied the significance of the eastern Laptev Sea for Arc- tic sea-ice formation and transpolar sediment flux (Dethleff et al., 1993). In the following years, there was a series of 10 ‘TRANSDRIFT’ expeditions conducted within the frame- work of the Russian–German interdisciplinary project ‘Laptev Sea System’ (1993–2003) (Thiede et al., 1999). This comprehensive research programme combined, for the first time, a suite of coordinated studies (field expeditions, laboratory experimental work, and modelling efforts) on a broad range of scientific issues, both marine and terrestrial. Main goals have been to elucidate and understand the modern processes determining 316 M.K. Schmid et al. / Progress in Oceanography 71 (2006) 314–330 the land–ocean interactions in the study region, to reconstruct the history of its paleoclimate from geological records and, eventually, to model and predict the mechanisms and effects of future climatic and ecological changes. To address these ambitious goals, the project comprised extensive investigations on the sea ice, water column, and sea floor of the Laptev Sea shelf, as well as on soils, permafrost, and vegetation of its coastal hinterland. The marine fieldwork was not confined to the summer only, but it included studies in spring during river break-up (TRANSDRIFT IV, 1996; Kassens et al., 1998) and in autumn during freeze-up (TRANS- DRIFT III, 1995; Kassens et al., 1997). Overall, this project, as well as a concomitant intensification of Rus- sian field research work (Gukov, 1989, 1991, 1992a; Sidorov and Gukov, 1992; Gukov, 1995, 1996, 1997, 1998; Gukov et al., 1999), yielded remarkable progress in our knowledge of the ecology of the Laptev Sea (Pivova- rov et al., 2004). 1.2. Carbon flux patterns, cryo–pelago–benthic coupling and carbon import A fundamental conclusion of the ecological field studies conducted in Arctic regions in the recent past is that there is not just one typical marine ecosystem, but rather a wide variety of them in distinct regions and depth zones (shelf, slope, and basin). This prominent variation is caused by differences in, e.g., water depth, geographical setting, biogeographic history, water current and advection regime, river runoff, ice cover, seafloor composition, and food availability. The ecological effects of these factors are often interrelated (Piep- enburg et al., 2001). River runoff, for instance, strongly affects sea-ice dynamics and oceanic circulation patterns, the pelagic productivity regime, and ultimately the benthic food supply.