Variability of upper-ocean vertical structure along 80°E and implications for nutrient supply and phytoplankton dynamics in the East Antarctic

Supervisory Team:

Primary supervisor: Dr Annie Foppert, AAPP/IMAS

Co-supervisor: Dr Sophie Bestley, IMAS/AAPP

Additional supervisors: Karen Westwood, AAD/AAPP

Brief project description:

The marine system offshore from Australia’s Antarctic Territory plays a vital role in supporting Antarctic food-webs; the area in the vicinity of 80°E is known to support high primary production, positively influencing Antarctic krill and dependant predators. To better understand the physical environment that supports the marine ecosystem in this region, several large-scale multidisciplinary research voyages have been undertaken (Nicol et al. 2000, Nicol et al. 2010, Constable and Swadling 2020, Kelly et al. 2021). The surface mixed layer (Sallée et al. 2021) plays an important role in shaping marine ecosystems by hosting most of the ocean’s primary production, hence changes in mixed layer properties have the potential to restructure the seasonal ecosystem dynamics. This project will construct and examine a time-series of the vertical structure of the upper water column, determine links with physical drivers, and ascertain how these factors may influence nutrient supply and phytoplankton dynamics (Westwood et al. 2010, Wright et al. 2010) in this region.

Repeat surveys have provided multiple occupations of the 80°E transect line (GO-SHIP line I08S) from 1994 through to 2021. Here, the southernmost branches of the Antarctic Circumpolar Current merge as the flow steers south of the southern Kerguelen Plateau and approaches the Princess Elizabeth Trough (Williams et al. 2010, Bestley et al. 2019). To the south, the West Ice Shelf supports persistent sea ice and the whole region is seasonally ice-covered from March through to October. Consequently, the regime is dynamic, with some transects sampling mesoscale eddy features, and has a strong seasonal dependence.

The timing of seasonal sea-ice advance and retreat, duration of ice coverage, and exposure of the sea surface to wind, all play an important role in determining upper-ocean structure. The consequences for the upper ocean stratification, supply of nutrients from depth, and oceanic characteristics that may drive phytoplankton growth, are of key interest here. This project aims to:

  1. Construct a time-series of the vertical structure of the upper water column (seasonal mixed layer, Winter Water, permanent pycnocline) using data available from repeat hydrographic surveys along 80°E
  2. Examine links between key water column properties (e.g. seasonal mixed layer depth, stratification, Winter Water characteristics: temperature and thickness) and relevant physical drivers (e.g. sea ice cover, wind fields)
  3. Ascertain the importance of these physical characteristics in driving nutrient supply to surface waters and governing phytoplankton dynamics

Skills students will develop during this research project:

The student will gain expertise with acquiring and handling ship-based hydrographic data; conducting analyses in a programming language such as MATLAB or R; generating quantitative summaries of key oceanographic variables; and the production of informative figures and tables to clearly communicate results. As this project will contribute to the larger Australian Antarctic Program Partnership, the student will also gain experience working in this broader collaborative effort.

Authorised by the Executive Director, Institute for Marine and Antarctic Studies
September 9, 2021