Trophic Dynamics of Antarctic Shelf Ecosystems - Food Webs and Energy Flow Budgets

Although Bouvet Island is of considerable importance for Southern Ocean species conservation, information on the marine community species inventory and trophic functioning is scarce. Our combined study of stable isotopes and feeding relationships shows that (1) the marine system conforms to the trophic pattern described for other Antarctic systems within the Antarctic circumpolar current (ACC); (2) both the benthic and the pelagic subsystem are almost exclusively linked via suspended particulate organic matter (SPOM); and (3) there is no evidence of a subsystem driven by macroalgae. Bouvet Island can therefore be characterized as a benthic ‘‘oasis’’ within a self-sustaining open ocean pelagic system. Introduction Bouvet Island (Bouvetøya, 54 72¢60S, 3 24¢E) is located just south of the Antarctic Polar Front (APF) and within the Antarctic circumpolar current (ACC, Fig. 1, Foldvik et al. 1981; Perissinotto et al. 1992). Owing to its geographical isolation (i.e. 2,590 km downstream of the South Sandwich Islands, 2,570 km upstream of the Prince Edward Islands and 1,600 km north of Queen Maud Land, Antarctica) the island represents a pristine environment and has been identified as an important case study for the conservation of intact ecosystems (Chown and Gaston 2002). Nevertheless, marked climate change and subsequent invasion of hitherto alien species have already affected other remote systems of the Southern Ocean (e.g. South Georgia; Bergstrom and Chown 1999; Chown and Gaston 2002; Gaston et al. 2003; Frenot et al. 2005) and may thus endanger the ecology of Bouvet Island, too. Unfortunately, to date, next to nothing is known about the Bouvet Island marine community species inventory and the trophic functioning of the system. From existing oceanographic, biogeographic and ecological data we can infer that (1) the pelagic compartment of the Bouvet ecosystem is part of the Antarctic surface water ecosystem (ASW; Perissinotto et al. 1992), i.e. we expect ASW species inventory and trophic structure; (2) the Bouvet inventory of top predatory birds and mammals, albeit enormous in numbers and biomass (Cooper et al. 1984; Croxall 1984; Haftorn 1986; Isaksen et al. 2000; Kirkman et al. 2001; Keith et al. 2002) resembles that of other Sub-Antarctic islands such as the Prince Edward Islands (Perissinotto et al. 1990; Perissinotto and McQuaid 1992; Pakhomov and Froneman 1999); and (3) the benthic compartment of the Bouvet ecosystem should be structured accordingly, with a typical macroalgal depth zonation and a fauna characteristic for systems shaped by strong currents, though species composition may differ from comparable sites such as the South Sandwich Islands (LAMPOS; Jacob et al. U. Jacob (&) Æ T. Brey Æ K. Mintenbeck Æ K. Beyer Æ W.E. Arntz Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, 27515 Bremerhaven, Germany E-mail: ujacob@awi-bremerhaven.de S. Kaehler Southern Ocean Group, Rhodes University, PO Box 94, Grahamstown 6140, South Africa K. Dunton Marine Science Institute, University of Texas, 750 Channelview Drive Port, Austin, TX 78373-5015, USA U. Struck GeoBio-Center at Ludwig-Maximilans-Universität, Richard-Wagner-Strasse 10, 80333 München, Germany I. Fetzer Centre for Environmental Research Leipzig-Halle, Permoserstraße 15, 04318 Leipzig, Germany E.A. Pakhomov Zoology Department, Faculty of Science and Technology, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa E.A. Pakhomov Department of Earth and Ocean Sciences, University of British Columbia, Stores Road, Stores Road, Vancouver, BC, V6T 1Z4, Canada Polar Biol (2005) DOI 10.1007/s00300-005-0071-8 2003; Arntz and Brey 2003) or the Prince Edwards Islands (Kaehler et al. 2000). Here we present the first attempt towards a better understanding of the Bouvet Island food web based on stable isotope signatures of nitrogen and carbon and information on the diet of the most significant species. Material and methods Sampling of marine flora and fauna was conducted near Bouvet Island during RV ‘‘Polarstern’’ cruise ANT XXI/2 in 2003/2004 (Fig. 1, Arntz and Brey 2005). Four Agassiz trawl samples (mouth opening 3 m·1 m, 10 mm·10 mm cod end mesh size, 20 min trawl time) were taken between 100 and 550 m water depth. Abundant taxa were identified to species level whenever possible, and sampled for stable isotope analysis. Small organisms were sampled whole, whereas pieces of body wall or muscle tissue were sampled from macro and megafaunal specimens. A total of four macroalgal samples (Rhodophyta spp.), 96 invertebrate samples of benthic and hyperbenthic invertebrates referring to 15 different taxa (among them decapods, amphipods, bryozoans, crinoids, holothurians, nemerteans, cnidarians, polychaetes, pycnogonids and asteroids) and ten samples of the nototheniod fish Lepidonotothen larseni were collected (Table 1). All samples were kept frozen at 30 C until further analysis. Stable isotope signatures dC and dN signatures serve as proxies of the trophic distance of an organism from the primary food source of the corresponding food chain. dC signatures are commonly used as valuable carbon source tracers (e.g. Lesage et al. 2001), whereas dN values are a useful tool for detecting the trophic position and therefore the trophic hierarchy of the system. Samples were lyophilisated for 24 h in a Finn-Aqua Lyovac GT2E and then ground to a fine powder. Each sample was acidified to remove CaCO3 in accordance with Fry (1988), Cloern et al. (2002) and Jacob et al. (2005) by adding 1 mol l 1 hydrochloric acid (HCl) drop-by-drop until CO2 release stopped, re-dried at 60 C without rinsing to minimize loss of dissolved organic matter and ground again. Stable isotope analysis and concentration measurements of nitrogen and carbon were performed simultaneously with a THERMO/Finnigan MAT Delta plus isotope ratio mass spectrometer, coupled to a THERMO NA 2500 elemental analyzer via a THERMO/Finnigan Conflo IIinterface. Stable isotope ratios are given in the conventional delta notation (dC; dN) relative to atmospheric nitrogen (Mariotti et al. 1984) and PDB (PeeDee Belemnite standard). Standard deviation for repeated measurements of lab standard material (peptone) proved to be better than 0.15& for nitrogen and carbon. Standard deviations of concentration measurements of replicates of our lab standard are <3% of the concentration analyzed. In order to fill gaps in invertebrate and macroalgae species coverage, we added some stable isotope data referring to other sub-Antarctic localities, i.e. Prince Edward Island (Kaehler et al. 2000, 2005) and Anvers Island (Dunton 2001) (Fig. 1). Isotope signatures of Antarctic seabirds, mammals and particulate organic matter (POM) were taken from Rau et al. (1991, 1992), E.A. Pakhomov (unpublished data) and K. Mintenbeck (unpublished data). Data for the pelagic predatory squid Kondakovia longimana were taken from Wada et al. (1987). Fig. 1 Map showing Anvers Island, South Sandwich Islands, Bouvet Island and the Prince Edward Islands. Grey band indicates region of the Antarctic circumpolar current (ACC)