Distribution, abundance and seasonal flux of pteropods in the Sub-Antarctic Zone W.R. Howard a,n , D. Roberts a , A.D. Moy a , M.C.M. Lindsay b , R.R. Hopcroft c , T.W. Trull a,b,d , S.G. Bray a a Antarctic Climate & Ecosystems Cooperative Research Centre, University of Tasmania, Private Bag 80, Hobart, Tasmania, Australia b Institute of Antarctic & Southern Ocean Studies, University of Tasmania, Hobart, Tasmania, Australia c University of Alaska Fairbanks, Fairbanks, AK, USA d Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research, Castray Esplanade, Hobart, Tasmania, Australia article info Available online 30 May 2011 Keywords: Sub-Antarctic Zone Southern Ocean Thecosomata Pteropod Heteropod Ocean acidification abstract Pteropods were identified from epipelagic net and trawl samples in the Sub-Antarctic Zone during the 2007 mid-summer (January 17–February 20) Sub-Antarctic Zone Sensitivity to Environmental Change (SAZ-Sense) voyage, as well as in a moored sediment trap in the same region. Overall pteropod densities during SAZ-Sense were lower than those reported for higher-latitude Southern Ocean waters. The four major contributors to the Sub-Antarctic Zone pteropod community during the SAZ-Sense voyage, Clio pyramidata forma antarctica, Clio recurva, Limacina helicina antarctica and Limacina retroversa australis, accounted for 93% of all pteropods observed. The distribution of the two dominant pteropods collected in the Sub-Antarctic Zone, L. retroversa australis and C. pyramidata forma antarctica, is strongly related to latitude and depth. L. retroversa australis is typical of cold southern (50–541S) polar waters and C. pyramidata forma antarctica is typical of shallow (top 20 m) Sub-Antarctic Zone waters. A moored sediment trap deployed to 2100 m at 471S, 1411E in 2003/04 showed the pteropod flux in the Sub- Antarctic Zone had late-Spring and mid-summer peaks. The diversity, abundance and distribution of pteropods collected during SAZ-Sense provide a timely benchmark against which to monitor future changes in SAZ ocean pteropod communities, particularly in light of predictions of declining aragonite saturation in the Southern Ocean by the end of the century. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Increasing amounts of anthropogenic carbon dioxide have been entering the global ocean since the industrial revolution, decreasing the carbonate-ion concentration and pH of the surface ocean. The ecological effects of this process, known as ocean acidification, are uncertain, with a variety of studies suggesting the ability of marine calcifying organisms to form carbonate shells may be reduced (e.g. planktonic foraminifera: Spero et al., 1997; Bijma et al., 1999, 2002; Moy et al., 2009, corals: Gattuso et al., 1998; Kleypas et al., 1999; Gattuso and Buddemeier, 2000 and some coccolithophorids: Riebesell et al., 2000; Zondervan et al., 2001), while others suggest marine calcifiers may benefit from increasing carbon dioxide (e.g. some coccolithophorids: Iglesias-Rodriguez et al., 2008). The importance of calcification in the survival of shell-making organisms may vary among different groups and taxa (e.g. Fine and Tchernov, 2007). However, calcification plays an important role in the marine carbon and alkalinity cycles, with aragonite precipitation and dis- solution being particularly prominent components of the upper- ocean alkalinity cycle (Betzer et al., 1984; Gangstø et al., 2008). Quantifying the distribution, abundance and seasonal flux of calcifying organisms in the Sub-Antarctic Zone (SAZ) is an important and current issue, as these calcifiers will be among the first organisms to respond to the impacts of ocean acidifica- tion as it spreads throughout the global ocean. One important marine group of calcifiers in the Sub-Antarctic Southern Ocean are the thecosomatous (shelled) pteropods, planktonic gastropods (Lalli and Gilmer, 1989) that produce aragonitic shells. Aragonite is a metastable mineral phase of calcium carbonate under oceanic conditions, more vulnerable to dissolution at depth than calcite, and potentially more vulnerable to ocean acidification than calcite in terms of shell formation. Not only are pteropods some- times important in terms of carbonate flux to the deep-sea, but also when abundant they can be significant grazers on phyto- plankton and smaller mesozooplankton (reviewed in Lalli and Gilmer, 1989). Given their biogeochemical and ecological impor- tance, there is a particular urgency in determining the present distribution and abundance of SAZ pteropods, as it is likely that parts of the Southern Ocean will begin to become undersaturated Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsr2 Deep-Sea Research II 0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.05.031 n Correspondence to: Office of the Chief Scientist, GPO Box 9839, Canberra, ACT, Australia. Tel.: þ61 2 6213 6776; fax: þ61 2 6213 6558. E-mail address: will.howard@chiefscientist.gov.au (W.R. Howard). Deep-Sea Research II 58 (2011) 2293–2300