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Scientists unlock secrets of the Great Australian Bight

The largest specimens of nano-phytoplankton barely equal the width of the finest strand of human hair, while impossibly small pico-phytoplankton is 10-100 times tinier again.

Although virtually invisible, the two self-feeding microscopic organisms represent the beginning of the food chain for the entirety of Earth's aquatic mammal population including that which thrives in Australia's prime marine habitats.

Yet seemingly like every other component of the natural world, they are under threat from ozone depletion and global warming.

Enter a team of South Australian scientists determined to learn whether something can be done about it.

The focus of their multi-year study has been to establish precisely how pico- and nano-phytoplankton underpin the so far stable stock of animal- or zooplankton species that support the vast and diverse food web within the Great Australian Bight.

Think extensive seagrass beds and kelp forests to exotic animal species such as the protected leafy seadragon, great white sharks and mighty blue whales.

It's long been thought the Bight is low in plankton biomass except during summer when nutrients supplied by upwelled ocean currents trigger phytoplankton blooms in the shelf and coastal waters west of Kangaroo Island.

However the research team has discovered the region in fact produces enough pico- and nano-phytoplankton year-round to help maintain vital zooplankton species in relatively constant supply.

"Our findings bust the myth of the Bight as an oligotrophic ocean (deficient of plant food)," according to Flinders University oceanographer Associate Professor Jochen Kaempf.

"Instead, the findings point to a year-round supply of nutrients fuelling the marine food web, most likely related to a high degree of nutrient recycling of the region."

Project co-author and PhD student Michelle Newman said the research featured analysis of phytoplankton pigment markers, something which required a work-intensive and costly process of collecting water samples.

To this end, Dr Mark Doubell of the South Australian Research and Development Institute has led more than 100 cruises employing the capabilities of the federally funded Integrated Marine Observing System (IMOS).

The program uses underwater vehicles, moorings, research vessels, satellites and radars to capture information about the ocean and deliver high quality scientific data.

The phytoplankton project was made possible by IMOS investment, Dr Doubell said.

"Every drop of seawater contains thousands of individual plankton.

"The analysis of over 10 years of water samples undertaken in this study has deepened our knowledge of the composition of the plantonic ecosystems fundamental to the health and productivity of our marine ecosystems and the fisheries they support."

Prof Kaempf said ongoing research was critical so as to "not miss the opportunity to understand possible climate-change impacts on this important marine ecosystem such as the enhanced risk of marine heatwaves".