Oceanographers have discovered how the bight supports a year-round abundance of marine predators, including different whale species and white sharks.

For the first time, experts from Flinders University and the South Australian Research and Development Institute have described substantial subsurface phytoplankton layers deep beneath the surface, even when surface phytoplankton blooms disappear.

Using satellite data and a marine observation system, they uncovered layers of substantial phytoplankton at water depths of between 30 and 70 metres.

Oceanographer Jochen Kaempf said the new study highlighted how the marine food chain survived even in years when the surface phytoplankton upwelling didn't occur.

''Our observation of subsurface phytoplankton layers is the missing key explaining the high productivity of the region," he said.

"This study demonstrates how little we actually know about the functioning of marine ecosystems, and how important real field observations are."

Ordinarily, the seasonal upwelling of nutrient-rich water, which extends about 800 kilometres from Ceduna on SA's west coast to Portland in Victoria, produces one of Australia's most productive marine ecosystems.

It attracts large marine mammals including whales, seals and sea lions, as well as valuable fish populations such as the southern bluefin tuna.

"To conserve this important region, and prepare for climate change, we need to further understand these systems and food sources," researcher Alex Shute said.

Australian Associated Press