May 14, 2019
Oceanography & Hydrochemistry in the Great Australian Bight
Posted by larryohanlon
By Jessica Bolin (University of the Sunshine Coast), Sophie Dolling (University of Adelaide), Luke Walker (Macquarie University)
Welcome on board RV Investigator, Australia’s blue water research vessel. We’re currently sailing from Hobart to Fremantle as part of the 2019 Collaborative Australian Postgraduate Sea Training Alliance Network (CAPSTAN) voyage, a hands-on training experience for marine science students. This year, our voyage is led by CAPSTAN director April Abbott, Chief Scientist Leah Moore, and Voyage Manager Matt Boyd. We’re the hydrochemistry and oceanography student team for the voyage. The three of us have been working closely with one of our trainers, Veronica Tamsitt, to understand the physical processes at our study site.
Waters on the continental shelf in the eastern region of the Great Australian Bight (South of Australia), support Australia’s largest fishery by weight: the Australian Sardine Fishery and the valuable Commonwealth Southern Bluefin Tuna Fishery. They also support the highest densities of seabirds and marine mammals in the Australian region, some of which can be found nowhere else on the planet. These important fisheries exist here due to unique ocean currents forced by wind and sea floor bathymetry, driving upwelling of cold and nutrient-rich waters and promoting blooms of small phytoplankton; tiny plants that that live in the water column. The presence of these tiny organisms forms the base of the food chain in the Southern Ocean, and are the reason for the biologically rich and diverse waters south of Australia.
To sustain this ecosystem and continue benefiting from its functions, it is essential that we deepen our understanding of the complex interactions between the biological, geological and oceanographic processes at play in this unique part of the ocean. This was the goal of the oceanography team for the CAPSTAN voyage aboard the RV Investigator, as we explore the Great Australian Bight from Hobart to Fremantle.
But first, as a part of the science communication focus on this voyage, our group decided to try an exercise called “up-goer five” which required us to explain a complex topic using only the most common ten-hundred English words (one thousand was too rare to use). In regular terms, a CTD stands for Conductivity, Temperature and Depth. The CTD is an instrument that is used to analyse these parameters in the water column as well as collect water samples from nominated depths. For this activity, we defined a CTD as:
“A thing that shows how hot or cold water is at different points in the big water around land.”
We also attempted to explain Sophie’s thesis in the same fashion. She described her thesis as “developing a technique using analytical chemistry to find traces of plastic byproducts in fish tissues”. This was explained using the most common words as:
“Small water animal eats bad small bits of not real stuff. Not real stuff gets in body. I find way to see not real stuff in body.”
The goal of our oceanography/hydrochemistry team was to analyse data recorded by the CTD at each station along the canyon, as well as the continuous data recorded by the RV Investigator’s underway data system and shipboard ADCP (acoustic doppler current profiler). An ADCP is used to measure current velocity as a function of depth along the ship’s track in real-time by measuring the Doppler effect of sound waves backscattered through the water column. We compared these sets of data to circulation models of the Southern Ocean, to examine the current oceanographic processes at play in the Bonney upwelling region off the south coast of Portland, Victoria.
The CTD (conductivity, temperature, depth) is a series of up to 36 bottles loaded onto rosette that is lowered to the bottom of the ocean by our winch crew, where it collects water samples from different levels in the water column. From these samples, we get a snapshot of the state of the water column in regard to conductivity, temperature and depth/pressure, as well as dissolved oxygen, fluorescence and backscatter (which give us an idea of how much phytoplankton growth is in the water). Once the CTD was retrieved, we all geared up in our wet weather clothes and stylish gumboots to collect smaller sub-samples from each bottle. These were later analysed to give the same snapshot of the water column, however this time it was to calibrate the oxygen and salts measured by the CTD and to measure the amount of nutrients present in the water.
The ship’s underway data system and ADCP both run 24/7 from the operations room and records salinity, temperature, fluorescence, and dissolved oxygen at the surface and current velocity as a function of depth, respectively.
As you can imagine from what you’ve read, our team has ended up with a colossal amount of data, which we have processed into a number of graphs. Many of these graphs display the expected outcomes of the research, while others have left us scratching our heads in long discussions about what the data is suggesting and why, but that’s part of the fun! We are now tasked with piecing together our data to develop an idea of what questions our team will answer and discuss in our report. Some questions we have in mind are: how does our hydrochemical data relates to plankton, mammal, and bird observations at each station and across the transits? What does the LADCP tell us about currents in the submarine canyon and current fronts in underway data? How do hydrochemical properties vary from the shelf to the deep?
The next day and a half will be spent compiling our different parts of the report into a finished product (in between eating Arnott’s Shapes and listening to music) before we begin to present our findings to the rest of the CAPSTAN participants. We are all looking forward to being able to combine information from all groups (oceanography, sediments, plankton, operations/geophysics, and marine bird and mammal identification) to piece together a complete picture of the voyage that shows how a multidisciplinary approach is essential to marine science research. The topic of marine science is intrinsically linked to biology, geology and environmental science, meaning that it is impossible to study one of these disciplines without considering the interactions with other aspects of the marine system.
Hear more about CAPSTAN from our director’s blog (aprilabbott.wordpress.com) and our individual student blogs (voyage9181.wordpress.com). Learn more about the CAPSTAN program or sign up for our mailing list at https://www.mq.edu.au/about/about-the-university/faculties-and-departments/faculty-of-science-and-engineering/departments-and-centres/department-of-earth-and-planetary-sciences/study-with-us/capstan