Progress in Oceanography Progress in Oceanography 71 (2006) 426–445

Progress in Oceanography Progress in Oceanography 71 (2006) 426–445 www.elsevier.com/locate/pocean Carbon cycling in a high-arctic marine ecosystem – Young Sound, NE Greenland Søren Rysgaard a,*, Torkel Gissel Nielsen b a Greenland Institute of Natural Resources, P.O. Box 570, 3900 Nuuk, Greenland b National Environmental Research Institute, Department of Marine Ecology, Frederiksborgvej 399, DK-4000 Roskilde Abstract Young Sound is a deep-sill fjord in NE Greenland (74°N). Sea ice usually begins to form in late September and gains a thickness of 1.5 m topped with 0–40 cm of snow before breaking up in mid-July the following year. Primary production starts in spring when sea ice algae begin to flourish at the ice–water interface. Most biomass accumulation occurs in the lower parts of the sea ice, but sea ice algae are observed throughout the sea ice matrix. However, sea ice algal primary production in the fjord is low and often contributes only a few percent of the annual phytoplankton production. Following the break-up of ice, the immediate increase in light penetration to the water column causes a steep increase in pelagic primary production. Usually, the bloom lasts until August–September when nutrients begin to limit production in surface waters and sea ice starts to form. The grazer community, dominated by copepods, soon takes advantage of the increased phytoplankton production, and on an annual basis their carbon demand (7–11 g C mÀ2) is similar to phytoplankton production (6–10 g C mÀ2). Furthermore, the carbon demand of pelagic bacteria amounts to 7–12 g C mÀ2 yrÀ1. Thus, the carbon demand of the heterotrophic plankton is approximately twice the estimated pelagic primary production, illustrating the importance of advected carbon from the Greenland Sea and from land in fuelling the ecosystem. In the shallow parts of the fjord (<40 m) benthic primary producers dominate primary production. As a minimum esti- mate, a total of 41 g C mÀ2 yrÀ1 is fixed by primary production, of which phytoplankton contributes 15%, sea ice algae <1%, benthic macrophytes 62% and benthic microphytes 22%. A high and diverse benthic infauna dominated by polychae- tes and bivalves exists in these shallow-water sediments (<40 m), which are colonized by benthic primary producers and in direct contact with the pelagic phytoplankton bloom. The annual benthic mineralization is 32 g C mÀ2 yrÀ1 of which megafauna accounts for 17%. In deeper waters benthic mineralization is 40% lower than in shallow waters and megafauna, pri- marily brittle stars, accounts for 27% of the benthic mineralization. The carbon that escapes degradation is permanently accumulated in the sediment, and for the locality investigated a rate of 7 g C mÀ2 yrÀ1 was determined. A group of walruses (up to 50 adult males) feed in the area in shallow waters (<40 m) during the short, productive, ice- free period, and they have been shown to be able to consume <3% of the standing stock of bivalves (Hiatella arctica, Mya truncata and Serripes Groenlandicus), or half of the annual bivalve somatic production. Feeding at greater depths is neg- ligible in comparison with their feeding in the bivalve-rich shallow waters. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Arctic marine ecosystems; Greenland,Young Sound; Pelagic and benthic processes; Carbon cycle * Corresponding author. Tel.: +299 36 12 00; fax: +299 36 12 12. E-mail address: [email protected] (S. Rysgaard). 0079-6611/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2006.09.004 S. Rysgaard, T.G. Nielsen / Progress in Oceanography 71 (2006) 426–445 427 1. Introduction The research and monitoring station ZERO (Zackenberg Ecological Research Operations) was established in 1995 in the fjord Young Sound, Northeast Greenland, in order to monitor effects of climate changes in a high-arctic environment. The study area is considered to be a sensitive indicator of climate change because of its contact with water masses from the Greenland Sea and direct meltwater flux from the Greenland Ice Sheet. It is believed that the predicted climate changes will dramatically alter conditions in the Greenland Sea, includ- ing the coastal hydrological conditions. A long-term marine monitoring program (MarineBasic Rysgaard et al., 2004) was initiated in 2003 to supplement the ongoing program monitoring the terrestrial environment. Detailed quantitative studies of the marine ecosystem processes have been conducted in the area since 1995 through the research project ‘‘Changes in Arctic Marine Production (CAMP)’’. Data on water discharge from the catchment area, bathymetry, sea ice and hydrographical conditions have been presented by Rysgaard et al. (2003). Seasonal measurements of primary production by phytoplankton (Rysgaard et al., 1996, 1999), sea-ice microalgae (Ku¨hl et al., 2001; Rysgaard et al., 2001; Glud et al., 2002a), brown algae (Borum et al., 2002; Kra- use-Jensen et al., in press), benthic microalgae (Glud et al., 2002b), and coralline red algae (Roberts et al., 2002) have been reported. Furthermore, seasonal measurements of biogeochemical cycling in sea ice (Rysg- aard and Glud, 2004), water column (Rysgaard et al., 1999; Levinsen et al., 2000; Nielsen et al., in press) and in sediments (Rysgaard et al., 1998; Glud et al., 2000; Berg et al., 2001, 2003) have been conducted, along with estimates of vertical export (Rysgaard and Sejr, in press). Finally, the abundance and production of dominant infauna (Sejr et al., 2000, 2002 Sejr and Christensen, in press) and walrus abundance and predation (Born et al., 1997, 2003) have been investigated in Young Sound. Based on these earlier studies, a carbon bud- get can be established for this high-arctic fjord. A depth transect in the outer parts of Young Sound (Fig. 1)is the focus of the present chapter, in which we establish two annual carbon budgets: one representing the shallow coastal part of the fjord where light penetrates to the bottom, i.e. water depths <40 m, and one for the deeper (unlit) central parts of outer Young Sound. For the transect considered, the mean depth representing the shallow coastal part is 15 m, and 100 m for the deeper central part of the fjord. 2. Study area Young Sound (74°180N, 20°180W) is a deep-sill East Greenland fjord similar to numerous other large, deep fjords in the region, often penetrating hundreds of km inland. It covers an area of 390 km2 and is 90 km long and 2–7 km wide (Fig. 1). The fjords are typically U-shaped valleys incised into high mountain plateaus and continuing westward beneath the Greenland Ice Sheet (Sugden, 1974). With lengths of up to 300 km and depths to 1600 m, the East Greenland fjords are among the longest and deepest in the Northern Hemisphere (Funder et al., 1998; Cofaigh et al., 2001). The Young Sound–Tyrolerfjord system is narrow and steep-walled in its interior (Tyrolerfjord) and wider and more shallow toward the mouth (Young Sound) (Fig. 1). This topographical contrast is typical of glacially eroded fjords (Bennet and Glasser, 1996) but has become more prominent in the Young Sound–Tyrolerfjord system due to differences in bedrock geology between the inner part of the fjord, which consists of hard Caledonian geneisses, and the outer part of the fjord, which consists of Cretaceous and Tertiary basalts and sandstones (Escher and Watt, 1976). Long-term climatic records (1958–1999) are available from the military outpost Daneborg, situated in the outer part of the fjord (Cappelen et al., 2001), and from Zackenberg Ecological Research Operations (1995– present) located farther inside the fjord (Meltofte and Thing, 1996, 1997; Meltofte and Rasch, 1998; Rasch, 1999). Midnight sun and polar night prevail in the area for 101 days and 81 days, respectively, and the max- imum incoming radiation during summer is 800 W mÀ2. Winters are generally very cold, with mean monthly temperatures as low as À22 °C(Fig. 2). Mean temperatures (1961–1999) are below freezing 9 months of the year, and only the months June to August have positive mean air temperatures of up to 4.0 °C. During summer the outer area of Young Sound is often affected by fog from the ice-filled sea, the temperature of the fog being only a little above 0 °C. The mean annual wind velocity (1958–1999) in the area is 4.9 m sÀ1, most fre- quently coming from the north. However, during June–August (1958–1999) the dominant wind direction is easterly, with a mean wind velocity of 4.1 m sÀ1. Annual precipitation varies from 214 to 320 mm, of which approximately 75% falls as snow (Ohmura and Reeh, 1991; Soegaard et al., 2001). Snow cover usually forms 428 S. Rysgaard, T.G. Nielsen / Progress in Oceanography 71 (2006) 426–445 a A.P. Olsen Land b Zackenberg River Zero. Wollaston Forland Payer Land .Daneborg Clavering Ø Daneborg Greenland Sea Godthåb Gulf 0 24 km le c irc 40-160 m 0-40 m ic c Arct Basalt ø Wollaston Forland 0 0 0 0 5 2 8 0 0 Sandøen 0 6 4 2 8 ) m 0 ( 0 0 n r 2 e Clavering Ø 4 h 2 t r 8 o N 0 ) 0 0 m 0 ( -40 8 h 3 t -80 2 p 8 e -120 D -160 0 0 0 0 0 0 0 0 4 0 0 0 0 0 3 0 6 0 0 0 0 8 2 0 1 0 0 0 0 0 1 8 2 0 5 0 0 2 4 0 5 2 0 6 0 5 2 8 5 0 2 0 5 2 2 5 3 5 3 We 5 stern (m) 5 Fig.

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