Composition and structure of macrozooplankton and micronekton communities in the vicinity of free-drifting Antarctic icebergs

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Abstract

Recent warming in the Antarctic has led to increased production of icebergs; however, the ecological effects of icebergs on pelagic communities within the Southern Ocean have not been well-studied. We used a 10 m2 MOCNESS to collect macrozooplankton and micronekton in the upper 300 m of the water column near 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). Communities were dominated in all three seasons by Antarctic krill (Euphausia superba) and salps (Salpa thompsoni), which collectively comprised 60-95% of the community wet biomass in most cases. During our spring and summer cruises, mean biomass was elevated by 3.1-4.3x at a distance of 0.37 km from large icebergs vs. 9.26 km away. These differences were not statistically significant, and no trend in biomass with distance was apparent in samples from fall 2008, when total biomass was an order of magnitude lower. Biomass levels near icebergs during Dec 2005 and Mar-Apr 2009 were comparable to values reported from marginal ice zones, suggesting that waters around icebergs support macrozooplankton and micronekton communities comparable in magnitude to those in some of the most productive areas of the Southern Ocean. Sample variance also was significantly higher within 1.85 km of icebergs during Dec 2005 and Mar-Apr 2009, reflecting increased patchiness on scales sampled by the MOCNESS (20–40×103 m3 filtered per sample). This pattern was not significant during Jun 2008. Large predatory medusae were observed within 1.85 km of icebergs and in Iceberg Alley, an area through which icebergs pass frequently, but were virtually absent in areas remote from icebergs. Small euphausiids showed an inverse distribution, with low densities in areas populated by large medusae. A shift in community composition from a near-iceberg assemblage dominated by herbivores to a carnivore-dominated community in Iceberg Alley may reflect a transition from bottom-up to top-down control with increasing distance and time. Body sizes of dominant species varied seasonally but did not show consistent trends with distance from icebergs. Concentrations of photosynthetic pigments in the guts of E. superba and S. thompsoni corresponded broadly to patterns in surface chlorophyll a concentrations and were comparable to maximum gut pigment concentrations measured in animals collected from highly productive marginal ice zones. Our results suggest that the macrozooplankton and micronekton assemblages near free-drifting icebergs can be quantitatively and qualitatively different from those in surrounding, iceberg-free waters, perhaps due to both bottom-up and top-down processes as well as physical forcing by the passage of a large object through the upper ocean.

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

References (66)

  • B.H. Robison et al.

    Algal communities attached to free-drifting icebergs

    Deep-Sea Research II

    (2011)
  • R.M. Ross et al.

    Interannual and seasonal variability in short-term grazing impact of Euphausia superba in nearshore and offshore waters west of the Antarctic Peninsula

    Journal of Marine Systems

    (1998)
  • H.A. Ruhl et al.

    Seabird aggregation around free-drifting icebergs

    Deep-Sea Research II

    (2011)
  • J.N. Schwarz et al.

    Impact of drifting icebergs on surface phytoplankton biomass in the Southern Ocean: Ocean colour remote sensing and in situ iceberg tracking

    Deep-Sea Research I

    (2009)
  • T.J. Shaw et al.

    234Th-based carbon export around free-drifting icebergs in the Southern Ocean

    Deep-Sea Research II

    (2011)
  • R.E. Sherlock et al.

    Boundary layer zooplankton around free-drifting Antarctic icebergs

    Deep-Sea Research II

    (2011)
  • V. Smetacek et al.

    Mesoscale distribution of dominant diatom species relative to the hydrographical field along the Antarctic Polar Front

    Deep-Sea Research II

    (2002)
  • K.L. Smith et al.

    Carbon export associated with free-drifting icebergs in the Southern Ocean

    Deep-Sea Research II

    (2011)
  • K.L. Smith

    Free-drifting icebergs in the Southern Ocean: an overview

    Deep-Sea Research II

    (2011)
  • G.R. Stephenson et al.

    Subsurface melting of a free-floating Antarctic iceberg

    Deep-Sea Research II

    (2011)
  • K.M. Stuart et al.

    Tracking large tabular icebergs using the SeaWinds scatterometer

    Deep-Sea Research II

    (2011)
  • M. Vernet et al.

    Impacts on phytoplankton dynamics by free-drifting icebergs in the NW Weddell Sea

    Deep-Sea Research II

    (2011)
  • D.G. Ainley et al.

    Does prey preference affect habitat choice in Antarctic seabirds?

    Marine Ecology Progress Series

    (1992)
  • K.R. Arrigo et al.

    Ecological impact of a large Antarctic iceberg

    Geophysical Research Letters

    (2002)
  • A. Atkinson et al.

    Natural growth rates in Antarctic krill (Euphausia superba): II. Predictive models based on food, temperature, body length, sex, and maturity stage

    Limnology and Oceanography

    (2006)
  • A. Atkinson et al.

    Oceanic circumpolar habitats of Antarctic krill

    Marine Ecology Progress Series

    (2008)
  • K. Azetsu-Scott et al.

    Influence of melting icebergs on distribution, characteristics and transport of marine particles in an East Greenland fjord

    Journal of Geophysical Research

    (1999)
  • Y. Cherel et al.

    Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean

    Limnology and Oceanography

    (2010)
  • M.A. Collins et al.

    Patterns in the distribution of myctophid fish in the northern Scotia Sea ecosystem

    Polar Biology

    (2008)
  • A.J. Cook et al.

    Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years

    The Cryosphere

    (2010)
  • H.J.W. de Baar et al.

    Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean

    Nature

    (1995)
  • J. Donnelly et al.

    Distribution and abundance of micronekton and macrozooplankton in the NW Weddell Sea: relation to a spring ice-edge bloom

    Polar Biology

    (2006)
  • E.C. Fisher et al.

    Variability of epipelagic macrozooplankton/micronekton community structure in the NW Weddell Sea, Antarctica (1995-1996)

    Marine Biology

    (2004)
  • Cited by (0)

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