Graduation Year

2012

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Marine Science

Major Professor

Joseph J. Torres

Keywords

Assemblage, Continental Shelf, Distribution, Fishes, Krill

Abstract

ABSTRACT

Micronekton and macrozooplankton were sampled from the Western Antarctic Peninsula (WAP) and eastern Ross Sea regions. Samples were collected over the course of six research cruises to the Southern Ocean. Four of those cruises were conducted in the Marguerite Bay region of the WAP during the austral fall and winter of 2001 and 2002. A fifth cruise sampled faunal assemblages at nine sites, ranging from Joinville Island at the northern tip of the WAP to Charcot Island near the southern extent of the WAP, during austral fall 2010. A sixth cruise was conducted in the pack ice within the offshore, continental slope, and continental shelf zones of the Eastern Ross Sea during austral summer 1999/2000. The purpose of this dissertation was to describe the macrozooplanktonic/ micronektonic faunal assemblages present in each of those regions and relate differences in species composition and distribution patterns to local bathymetry, hydrography, and physical conditions. A variety of multivariate techniques were used to identify unique multispecies assemblages and to quantify the contributions of both oceanic and neritic species to the assemblages within each study region.

The invertebrate micronekton/macrozooplankton communities found in the Marguerite Bay region of the WAP during the 2001/2002 cruises were a mixture of oceanic and neritic fauna: a direct result of local hydrographic conditions. Near the shelf break and in the outer reaches of the Marguerite Trough, a deep canyon transecting the shelf in a south-southeast direction, the communities were more diverse, dominated by oceanic species such as the euphausiid, Euphausia triacantha, the salp, Salpa thompsoni, and, an amphipod, Themisto gaudichaudii. The assemblages present in the nearshore fjords exhibited lower diversity and were dominated by neritic species such as the euphausiid, E. crystallorophias, and the mysid, Antarctomysis ohlinii. At the mid-shelf and mid-trough locations, the assemblages were composed of a variable mixture of oceanic and neritic fauna. The faunal mixing and overall species composition in those areas is the result of episodic Circumpolar Deep Water (CDW) intrusions onto the shelf via deep bathymetric features such as the Marguerite Trough.

Distinct multispecies assemblages were identified at Joinville, Croker Passage, Marguerite Bay, Charcot Island, and from a region that included samples from sites near Anvers Island, Renaud Island and the Marguerite Trough. Assemblages at Joinville and Croker Passage were both dominated by E. superba and S. thompsoni, but hydrographic conditions at Joinville favored a neritic assemblage, underscored by the substantial numbers of the nototheniid fish, Pleuragramma antarcticum. The assemblage at Croker Passage was more oceanic in nature with major inputs from the myctophid fish, Electrona antarctica and the amphipod, T. gaudichaudii.

Marguerite Bay and Charcot Island were well-mixed assemblages with strong representation by both neritic and oceanic fauna. The mid-peninsula assemblage was also oceanic in character, being overwhelmingly dominated by the euphausiid, Thysanoessa macrura, and T. gaudichaudii. Pleuragramma antarcticum were captured at five sites: Joinville, Croker Passage, Marguerite Bay, and the two sites near Charcot Island. They were completely absent at the two sites near Anvers Island, at Renaud Island, and in the Marguerite Trough.

In the eastern Ross Sea, cluster analyses identified three primary groups, which were characterized as oceanic, mixed, and neritic assemblages due partly to their geographical location, but mostly to their faunal composition. The oceanic assemblage contained the highest number of taxa and was dominated by oceanic fauna, such as the hydrozoans Diphyes antarctica and Calycopsis borchgrevinki, and the scyphozoan Atolla wyvillei. Top contributors in the mixed assemblage included those species that dominated in the oceanic assemblage as well as substantial contributions from E. superba and the tunicate, Ihlea racovitzai. The neritic assemblage was overwhelmingly dominated by E. crystallorophias and E. superba. The physical environment strongly impacted micronektonic/ macrozooplanktonic distributions and densities in the eastern Ross Sea. Changes in faunal composition were directly related to temperature differences encountered at the shelf break and the subsequent filtering out of oceanic fauna from cold, shelf waters where endemic fauna were most prevalent.

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