The ecological role of Salpa thompsoni in the Kerguelen Plateau region of the Southern Ocean : a first comprehensive evaluation

• Main Author: Kelly, PS
• Format: Thesis
• Language: English
• Published: 2019
• Subjects: Antarctica; ecosystems; krill; salps; Southern Ocean; Antarctic; The Antarctic; East Antarctica; Kerguelen; Antarc*; Antarctic Krill; Euphausia superba; Sea ice
• Online Access: https://eprints.utas.edu.au/31522/; https://eprints.utas.edu.au/31522/1/Kelly_whole_thesis.pdf

Increasing numbers of gelatinous species may significantly alter the future structure and function of Southern Ocean ecosystems. Most notably, there is evidence of a possible increase in long-term abundance, coupled with a southerly shift in distribution, in the salp Salpa thompsoni. Identifying the effects of a potential shift from a Euphausia superba-based food web, to a salp-based food web is crucial for quantifying potential ecosystem energy-flow changes, and for facilitating robust approaches to ecosystem-based management. A lack of baseline information on Salpa thompsoni biology and ecology (against which to assess change in the ecological role) in the waters off East Antarctica is hampering understanding of the mechanisms underpinning the distribution and dynamics of this species, and therefore preventing researchers from predicting exactly how Southern Ocean salps will behave under forecast environmental change. This thesis examined the abundance and distribution patterns, population structure, environmental drivers, diet, and elemental and nutritional content of Salpa thompsoni populations in the Kerguelen Plateau during the 2016 summer. The Kerguelen Plateau is a hotspot for biological productivity, and a location of valuable toothfish fisheries. Furthermore, this thesis investigated the potential for resource overlap between Salpa thompsoni and Antarctic krill and compared the in-situ energetic potential of the two food sources. This research found that, during the 2016 summer, Salpa thompsoni occurred south of the Southern Boundary of the Antarctic Circumpolar Current (originally considered the southern limit of Salpa thompsoni distributions) in higher abundances than previously reported. Using generalised additive models, large blooms were associated with low-chlorophyll-a concentration and summer sea-ice retreat. A comparison of the distribution patterns of Salpa thompsoni and Euphausia superba identified potential for the habitats of the two species to overlap on both horizontal (latitudinal and longitudinal distribution) and vertical (water-column distribution) spatial scales. Evidence for food source overlap was provided by stable isotope and gut content analyses. While salps were once considered a trophic “dead-end,” improved dietary analysis methods now show Salpa thompsoni to be a valid food source for higher predators, albeit with unclear energetic value. In this research, protein and energy content analyses found Salpa thompsoni to be an energetically inferior food source for Kerguelen Plateau predators, containing significantly less protein and overall energy than Euphausia superba. The findings from this thesis, as well as the resultant datasets on Salpa thompsoni population dynamics, will have wide-ranging applications, including incorporation into ecosystem models and future studies on the changing ecosystem role of Southern Ocean salps.