News Archive

Biological and physical oceanographers from the Oceans Institute, led by Winthrop Professor Chari Pattiaratchi, have teamed up with the CSIRO to study wintertime continental slope and shelf processes along south-west WA.

One aspect of these processes is ‘dense shelf water cascades' - a term which describes the flow of water from shallow areas inshore of the Leeuwin Current to deeper water offshore.

This occurs as high evaporation of the shallow water in summer leads to increased salinity, and therefore increased density. In winter the water cools, becomes even denser, and is pulled down by gravity to deeper waters offshore.

Dense shelf water cascades (DSWC) have been known to exist off south-west WA since 1993, but the recent biological sampling component of the study, led by Dr Christine Hanson, will be the first step in assessing how they influence phytoplankton populations. Phytoplankton are vital photosynthetic organisms that form the foundation of the oceanic food chain.

Eventually the research will also help to develop an understanding of the role of DSWC in carbon sequestration along Australia's continental margin.

Departing from Fremantle on July 29, the team began their 11-day voyage aboard the CSIRO's Marine National Facility:  the RV Southern Surveyor . They conducted cross-shelf transects between Jurien Bay and Cape Naturaliste and sampled over 100 oceanographic stations.

Data from recent ocean glider surveys by the Australian National Facility for Ocean Gliders (ANFOG) helped to determine appropriate sampling periods and locations.

The team used Niskin bottles to collect seawater from just below the ocean's surface to as deep as 3000m, to assess phytoplankton communities and nutrient levels.

Continuous measurements of water column parameters, including temperature and conductivity, were also taken using oceanographic profiling instruments .

Anton Kuret, an Honours student at UWA, will be analysing chlorophyll from the seawater samples. "Chlorophyll is a ubiquitous pigment found in all phytoplankton, so I'm using it as a proxy to measure phytoplankton density," Anton explains. "The data will help to explain the impact cascades have on the winter phytoplankton bloom."

Anton's Honours project is one small part of a much larger study, and samples taken on the trip are being sent to various centres around Australia and overseas for analysis.

The WA Biogeochemistry Centre will analyse samples for nitrogen uptake rates; the Institute of Ocean Sciences in Canada will examine phytoplankton abundance and species composition; and CSIRO Marine and Atmospheric Research in Hobart will analyse phytoplankton pigment composition.

The multi-faceted sampling program has yielded one of the most detailed winter time oceanographic datasets yet collected from the region. Once fully analysed, we will have a deeper understanding of WA's oceans and the recently discovered cascades.

Page 4: The UWA Oceans Institute December Newsletter

Media references

Maryann Evetts (The UWA Oceans Institute), (+61 8) 6488 8116