Deep Sea Research Part II: Topical Studies in Oceanography
Composition and structure of macrozooplankton and micronekton communities in the vicinity of free-drifting Antarctic icebergs
Introduction
Icebergs are conspicuous features of the Southern Ocean, arising from glaciers and ice shelves attached to the Antarctic continent and ranging in largest dimension from meters to over 300 km. Increased production of large Antarctic icebergs has been associated with regional warming and the breakup of large ice shelves around the continent (e.g., Scambos et al., 2000, Long et al., 2002, Cook and Vaughan, 2010). The impact of this increasing iceberg population on Southern Ocean ecosystems has not been characterized, although the available evidence suggests a range of effects across multiple trophic levels (Arrigo et al., 2002, Smith et al., 2007, Schwarz and Schodlok, 2009).
Early evidence suggested that icebergs may release mineral-rich terrestrial material as they break up and melt (e.g., Azetsu-Scott and Syvitski, 1999, Smetacek et al., 2002, Raiswell et al., 2006, Raiswell et al., 2008). In addition, icebergs with deep keels have the potential to mix nutrients from below the pycnocline into surface waters and thus enhance local primary production (Neshyba, 1977, Sancetta, 1992, Schwarz and Schodlok, 2009). Elevated concentrations of chlorophyll a (Smith et al., 2007), nanoplankton (de Baar et al., 1995) and acoustic targets likely to be macrozooplankton or micronekton (Kaufmann et al., 1995) have been found in the vicinity of free-drifting icebergs, and top predators such as penguins and flying seabirds have been observed in high abundance on and near icebergs (e.g., Joiris, 1991, Ribic et al., 1991, Veit and Hunt, 1991).
In contrast, the influence of icebergs on macrozooplankton and micronekton communities has not been well studied. If icebergs enhance primary production, elevated densities of herbivorous zooplankton might be expected in areas with richer food resources. Zooplankton could graze down phytoplankton stocks and enhance the sinking flux of organic carbon through the production of fecal material. In addition, zooplankton could provide a food source for higher-level predators such as fishes, predatory gelatinous animals, seabirds and marine mammals.
The goal of this study was to examine the abundance and species composition of macrozooplankton and micronekton in the vicinity of free-drifting Antarctic icebergs compared to assemblages in surrounding, iceberg-free areas. The physical characteristics of dominant species were investigated to determine whether animals in proximity to icebergs were different from conspecifics at greater distances away.
Section snippets
Study sites
Macrozooplankton and micronekton were collected at various distances from free-drifting icebergs in the Atlantic sector of the Southern Ocean during three seasons: December 2005 (late spring), June 2008 (late fall) and March-April 2009 (late summer) (Fig. 1). In all, five icebergs were studied, ranging from 2.0 to 39 km in maximum dimension (Table 1). In Dec 2005 and Mar-Apr 2009, sampling was conducted in the NW Weddell Sea, whereas the Jun 2008 cruise took place in the Scotia Sea (Fig. 1). In
Environmental characteristics
In general, water column structure varied between icebergs within a single sampling season and among MOCNESS deployments. Distinct differences sometimes were observed with distance from an iceberg, although this was not true in all cases.
In Dec 2005, water column structure varied between icebergs W-86 and A-52 and among MOCNESS deployments. Surface temperatures near (≤1.85 km from) W-86 ranged from −0.5 to −0.8 °C (Fig. 2). During some descents, the water column was nearly isothermal to 300 m
Discussion
Macrozooplankton and micronekton exhibited variability in biomass, density and species composition over space and time in relation to icebergs. A portion of the variability may have been due to differences in geographic location and season among sampling locations. Studying free-drifting icebergs requires locating such icebergs during scheduled cruises (i.e. opportunistically) rather than in a specific geographic area. In addition, icebergs must be sufficiently remote from land and the edge of
Acknowledgments
We are grateful to personnel from Raytheon Polar Services and Edison Chouest Offshore, as well as the captains and crews of the ARSV Laurence M. Gould and RVIB Nathaniel B. Palmer for their support over the course of this study. Assistance in the collection and processing of trawl samples was provided by S. Bush, D. Garcia, C. Huffard, J. Kinsey, C. Koehler, L. Lovell, S. Lowery and K. Noble. Special thanks to A. Townsend, L. Lovell and G. Matsumoto for their assistance with the identification
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